steel selection guide

Guide to selection of engineering steels

Steel selection guide

The object of this guide is to provide assistance in selection of a suitable engineering steel for your application. The grades included are those to be found in our extensive stock programme. Of course, alternative grades and executions exist but we have chosen to focus on materials available from stock with short lead times.

If you consider that your application requires an engineering steel other than those in our stock programme, then do not hesitate to contact us even at the design stage. By so doing, we will be able to help in steel selection for just your project and also give advice on analysis, properties, execution and if nec essary, heat treatment.

This guide does not pretend to be a reference work with answers for

everything but should be used to provide a preliminary indication. With engineering steels, it is especially important to try and achieve a correct balance between the demands of the application and available properties. Once again, you are welcome to contact Tibnor if your project has special material requirements outside those which are considered here.

In what follows, we have chosen to refer to engineering-steel grades by their designation in EN-standards. Older Swedish Standard designations (SS-) are presented in parentheses.

ExampleS355JR (SS 2172)34CrNiMo6 (SS 2541-03/05)

CONTENTS

High standards allow an optimal steel selection . . . . . . . . 4

Choosing the right steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Alloying elements in steel and their effects . . . . . . . . . . . . 8

Steel selection based on stipulation of requirements . . . 10

Dimensions and tolerances . . . . . . . . . . . . . . . . . . . . . . . . . 18

Bar tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Tube tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Standard constructional steels; micro-alloyed constructional steels. . . . . . . . . . . . . . . . . . 23

Case-hardening steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Quenched-and-tempered steels . . . . . . . . . . . . . . . . . . . . 28

Yield and tensile strengths for quenched-and-tempered steels . . . . . . . . . . . . . . . . . . 30

Spring steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Bearing steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Hard chrome bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Free-machining steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

M-steels for machinability . . . . . . . . . . . . . . . . . . . . . . . . . .40

Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

Cold forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Fatigue and how the risk for fatigue failure can be lessened . . . . . . . . . . . . . . . . . . . . . . 47

Reducing weight of components and constructions . . . 50

Standards for steel grades . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Colour coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Translation of seventh editionof Tibnor’s Stålvalsguidepublished March 2012 (in Swedish).

Our suppliers improve their products continuallyand we therefore cannot assume responsibility forchanges to data given in this document. For the mostrecent up-dates and current product catalogues,see www.tibnor.se.

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HIGH STANDARDS ALLOW AN OPTIMAL STEEL SELECTIONOur aim through collaboration with both customers and suppliers is to develop the best solutions for material selection, logistics and manufacture. Material that is to be processed by machining must have close tolerances and consistent quality so that it behaves in exactly the same way from one delivery to the next. It is for this reason that we have taken extra care in formulating specifications for just engineering steels.

Since machining often represents a large portion of the cost to manu facture a part, another important facet of our offer from a customer perspective is to increase efficiency; for example, via increased automation or by supplying material with improved machinability.

A quality-control system certified according to SS-ISO 9001:2008 and the environmental-control system certified in accord with SS-ISO 14001:2004 constitute the touchstone of our activities and behaviour. More information can be found on www.tibnor.se.

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CHOOSING THE RIGHT STEELA correct steel selection necessitates that you can stipulate the requirements of the application for your part or construction. Some requirements are easy to define while others may be less tangible. For example, it is sometimes difficult to exactly specify the loading that a part is subjected to and thereby the mechanical properties which are necessary as a basis for material selection.

In many instances, one relies on experience and makes the same material choice as in similar com-ponents which previously have functioned satisfactorily. For the most part, this philosophy works well but does not give consideration to new steel grades or executions which may be more suitable or cheaper or both.

In order to use this guide, you will need to spipulate the requirements for the part or construction of interest. Once you have done this, a suggestion as to how to proceed can be found on pages 10-17. First however, we will by way of background look briefly at the most important properties of steel from the standpoint of its behaviour both in manufacturing and in service.

AvailabilityWhen considering your steel selection, availability is a primary concern and it is obviously advantageous if the material of your choice is both available and can be delivered quickly. This is seldom an issue since Tibnor’s stock programme encompasses the most extensive range of grades, properties and executions all of which can be supplied with minimum lead times. Should the material of your choice not be available from stock, then please feel free to contact us for more information on the product, lead times, minimum quantities etc.

Hardness/wear resistanceFor the most common engineering and constructional steels, increasing hardness is synonymous with greater resistance to wear. High-carbon steels hardened to 60-62 HRC show best wear resistance. However, steels through hardened to such high hardness are rather brittle and surface hardening, which combines a hard, wear-resistant surface with a softer tougher core, may be a preferable alternative. Examples of such surface-hardening methods are induction hardening, case hardening and nitriding.

The concept of hardness and ways of measuring it will be discussed in more detail later on in this guide.

HardenabilityHardenability defines the ease with which a given steel can be hardened by rapid cooling from high temperature. A steel with low hardenability, must be cooled more quickly if it is to be hard. On the other hand, if hardenability is higher, cooling need not be so rapid and larger dimensions can be hardened. Hardenability increases with increasing content of carbon and alloying additions.

Of course, a successful hardening operation depends not only on the hardenability of the steel but also on the method of cooling. Common cooling media are water, polymer-water mixtures, oil and even air. Very rapid cooling, in water for example, engenders an effective hardening but also results in greater dimensional changes and increased risk for cracking.

Hardened steel is normally tempered which improves toughness and relieves stresses from hardening. The tempering temperature can lie anywhere between 200 and 700°C. Quenched-and-tempered steels in Tibnor’s stock programme have been tempered at higher temperatures (500-700°C), which admittedly reduces hardness but improves toughness dramatically.

Strength-yield stress/tensile stressThe yield strength or yield stress determines the load that a component can be subjected to without plastic deformation resulting in a permanent change of dimension. The tensile strength or ultimate tensile stress on the other hand, relates to the maximum tensile load a component can withstand without breaking.

Grossly oversimplified, one can say that the yield and tensile strengths of steel increase with carbon content. For a given carbon content, some

alloy additions also elevate yield stress in particular. For example, the yield strength of steel S355JR (SS 2172) with 0.15 % C and 1.5 % Mn is more or less the same as that for C45E (SS 1672) with 0.45 % C. So-called micro-alloy additions, such as niobium and vanadium, are particularly effective in raising yield strength since even very small amounts (<0.1 %) give rise to a strong contribution from precipitation hardening.

Heat treatment, in particular hardening and tempering, has a pronounced effect on both yield and tensile strength. Quenched-and-tempered steels develop their highest strength when alloying with carbon is combined with additions of chromium (Cr), nickel (Ni) and molybdenum (Mo) in amounts between 0.2 - 2 %.

For a given content of carbon and alloying elements, the achievable strength is very dependent on dimension with larger dimensions having lower strength than smaller ones. Hence, it is necessary to select a steel with greater alloy content if a certain level of strength needs to be retained over a wide dimensional range.

Fatigue strength is an important property which defines how the steel can stand up to variable or pulsating loads. This quantity, which is intim-ately coupled to tensile strength, is discussed in more detail in a separate section of this guide.

Toughness/ductilityBy toughness is meant the resistance of a material to the initiation and propagation of cracks upon loading which can cause failure of a component. A material is tough if such cracking requires considerable energy whereas a brittle material breaks very easily with the expenditure of very little energy. Toughness can be measured in a number of different ways and some are technically rather complicated.

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Impact testing constitutes a method that is relatively simple and cheap, and by far the most widely used testing method is Charpy-V (KV). KV-testing of most steels is char acter-ised by a transition from ductile to brittle fracture, see the diagram below.

The ductile-brittle transition temperature can vary between 50 and -100°C. Generally speaking, toughness decreases (i.e. the transition temperature is raised) with increasing hardness and strength, even if there are exceptions to this rule. A fine microstructure is positive for both strength and toughness and quenching and tempering is an example of a means to optimise the combination tensile strength-toughness.

The concept ductility relates to the ability of a material to undergo plastic deformation without the development of cracks or complete failure. The parameters defining ductility that can be measured in a tensile test are elongation to fracture (A

5) or the reduction in area at fracture,

Z; these are normally expressed as a percentage of the original sample length or cross-sectional area. Ductility and toughness are in many respects similar and there exists a clear correlation between KV-values at higher temperatures where the failure is ductile and, for example, the reduction in area in a tensile test.

With some exceptions, ductility is lowered as strength increases. Steel cleanliness is also an important factor and large amounts of inclusions in the steel are negative in relation to ductility.

MachinabilityGenerally speaking, higher hardness is commensurate with poorer machinability. However, softer low-carbon steels have a tendency to stick to the cutting tools with negative consequences in relation to surface finish and tool life. Steels with hard-ness in the range 180-220 HB and which give short chips machine best.

Machinability is often acceptable when the hardness is less than about 300 HB, even if steels with hardness up to 450 HB can be machined satisfactorily if the cutting speed is lowered. Working of even harder steels necessitates grinding or machining in stable machines with special tooling.

Deliberate addition of certain elements such as sulphur or lead, results in markedly enhanced machin-ability, although this improvement is most often achieved to the detriment of other properties. Machinability is also improved by Si/Ca-treatment, sometimes termed M-treatment (M-steels are discussed later in this guide).

WeldabilityWith the correct technique and consumables, all of the steels referred to in this guide can be welded, at least if the sole aim of welding is to join together. However, if welding procedures shall not be too complicated and requirements are placed on weld mechanical properties, the carbon content should be limited to < 0.25 % and other alloy additions should not be too high either. This means that steels with high strength and wear resistance are more difficult to weld.

You will find more detailed information on welding later on.

Cold formabilityParts made of steel are often shaped by cold-forming operations such as bending, upsetting, cold drawing, deep drawing etc. Cold formability determines the extent to which the steel can undergo such plastic forming without cracking. Cold formability is thus strongly correlated to ductility. In consequence, cold formability decreases with increasing strength but certain high strength steels can be cold formed using simpler methods such as bending or upsetting without problem. See later for more detailed information on cold forming of engineering steels.

Protection against corrosionOne of the major disadvantages of steel is that the element iron corrodes (rusts) rather easily. Steel will rust in the atmosphere outdoors (especially close to the sea or if humidity is high), in oxygenated water or if buried. In all these cases, electrochemical cells are created in which iron is dissolved to react with oxygen thereby forming a corrosion product (rust).

The principle of corrosion protection is to by some means limit this electrochemical reaction. For example, the surface of the steel can be painted or oiled in order to prevent physical contact with the external environment. Galvanising involves covering the steel surface with a layer of zinc, which is a metal that corrodes even more easily than iron. So long as the zinc remains and corrodes prefer-entially, the steel will stay protected.

In some cases, one can coat the surface of the steel with another metal having better corrosion resistance such as chrome, tin or nickel. Chrome plating brings the added advantage of increasing the wear resistance of the surface. Surface treatment by nitriding, and especially by ion nitriding, also gives increased protection from corrosion as well as improved resistance to wear.

Schematic impact transition curve from Charpy-V testing of a low-carbon steel. In this case, the transition temperature, e.g. the temperature corresponding to an absorbed energy of 27J, is -28°C.

Brittle

80400

50

100

150

200

250

-40 0-80

Ductile/fibrous

27J

Absorbed energy, (J)

Test temperature (°C)

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ALLOYING ELEMENTS IN STEEL AND THEIR EFFECTSSteel is a unique constructional material. No other metal can achieve such a broad array of mechanical properties ranging from soft and formable to hard, strong and wear resistant. This outstanding versatility is coupled to the transformation of iron between different states depending upon temperature and the influence of alloying with carbon on this transformation. Metallic elements other than iron exist which undergo similar transformations but the positive effect of carbon is exclusive to iron.

Carbon (C)Alloying with carbon constitutes the basis of all steel (with the exception of some special alloys and for most types of stainless steel where carbon is considered an unwanted impurity). Carbon increases the strength of steel but to the detriment of ductility and toughness. Carbon is essential if steel is to be hardened (pure iron cannot be hardened) and the achievable hardness and wear resistance increases with carbon content (see diagram below).

De-oxidants/manganese(Mn),silicon (Si), aluminium (Al)& sometimes calcium(Ca)During manufacture, the steel melt becomes contaminated with oxygen (from air) which is negative for properties and must be removed. This so-called de-oxidation is effected by

additions which have a higher affinity for oxygen than iron. Manganese and silicon are almost always present but if a more complete de-oxidation is required then aluminium is added as well. The reaction between these de-oxidants and oxygen results in the formation of slag particles, manganese silicate and/or aluminium oxide. These are lighter than the steel and are eliminated from the melt by flotation. However, a small fraction remains in the finished steel as non-metallic inclusions. Control of the content of inclusions is important since they affect the properties of the steel, most often negatively. De-oxidation with calcium in conjunction with silicon gives rise to inclusions of a specific type which have a positive effect on machinability (see separate present ation on M-steels).

Manganese (Mn)Virtually all steels contain manganese which fulfils a number of different functions. Apart from its effect as a de-oxidant (see above), manganese refines the microstructure of the steel which is positive for both strength and toughness. For example, the higher strength of S355JR (SS 2172) in comparison with S235JR (SS 1312) derives completely from the difference in manganese content (typically 1.5 % and 0.7 % respectively).

In combination with sulphur, manganese gives rise to manganese sulphide inclusions which improve machinability (see section on free-machining steels).

Dependence of hardness on carbon content for carbon steels in the as-hot-rolled and as-hardened condition.

00

200

400

600

800

0.4 0.6 1.00.2 0.8 1.2 1.4

Hardened

Hot-rolled

43 HRC

60 HRC

65.5 HRC

Hardness, HB

Wt. % Carbon

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Additions which increase hardenability/manganese (Mn),chromium (Cr), nickel (Ni),molybdenum (Mo), silicon (Si)Carbon increases the hardenability of steel but its effect is insufficient if anything other than small dimensions is to be hardened. Larger dimensions require alloying additions to supplement and enhance the hardening derived from carbon. As is clear from the diagram, manganese, chromium and in particular molybdenum have a strong positive effect on hardenability whereas the influence from nickel is weaker. Nickel is, however, desirable for toughness in quenched-and-tempered steels.

In the quenched-and-tempered condition, steels alloyed with chromium, molybdenum and nickel are characterised by an outstanding combination of strength and toughness even in larger dimensions. Furthermore, NiCrMo-steels have sufficient hardenability that effective hardening of larger dimensions is possible even when cooling is slow (in oil or even air) with reduced risk for dimensional changes and/or cracking.

The hardenability raising effect of silicon is limited. Even so, certain grades of spring steel have high silicon content.

Boron (B)Very small amounts of boron, as little as 0.001 %, exert a marked positive influence on hardenability. To a certain degree, boron steels, which apart from boron are often alloyed with manganese and chromium, offer a

cost-effective alternative to more alloyed heat-treatable steels. Boron’s hardenability-raising effect is, however, limited and disappears more or less completely when the carbon content exceeds 0.4 %. Boron steels find extensive application for wear parts which are hardened in water and used un-tempered. Toughness is often not especially good but sufficient for this type of application.

Micro-alloying additions/niobium (Nb), vanadium (V),titanium (Ti), aluminium (Al)For weldable low-carbon steels, grain refinement is the sole means whereby strength as well as toughness can be increased simultaneously. Grain refiners are added in small quantities between 0.01-0.1 % (micro-alloying) in order to counteract microstructural coarsening in connection with hot working, heat treatment or welding. These micro-additions have the common characteristic that that they have a strong affinity for carbon or nitrogen or both (nitrogen from air is absorbed by a steel melt).

Furthermore, niobium, vanadium and titanium all give rise to sub-microscopic particles of nitrides and/or carbides which make an additional contribution to strength via so-called precipitation hardening. Micro-alloying with vanadium is particularly favourable in this respect and its effect is more or less independent of carbon content. Vanadium micro-alloyed steels attain high strength even after hot rolling since precipitation takes place during subsequent cooling. Such grades are

cost-efficient in that they do not require heat treatment in order to achieve high strength.

Additions that enhance machinability/sulphur (S), lead (Pb), calcium (Ca)Deliberate addition of sulphur to a steel alloyed with manganese results in small manganese sulphide inclusions which give improved machinability especially when using high-speed steel tooling. Otherwise sulphur is generally regarded as an undesirable impurity.

Another addition for enhancing machinability is lead. Free-machining steels containing lead and/or sulphur do not have particularly good mechanical properties since the inclusions of lead and/or manganese sulphide have a negative influence on both ductility and toughness.

Treatment of a steel melt with silicon plus calcium (often called Si/Ca-treatment) has a very favourable influence on machinability without too negative repercussions for other properties. More information is given in the section on M-steels later in this guide. The benefit of Si/Ca treatment is most prevalent at high machining speeds as can, for example, be achieved with coated carbide tooling. Processing with Si/Ca necessitates careful control in steelmaking; otherwise the beneficial effect for machinability can vary from heat to heat or in the worst scenario be absent altogether.

Illustrating the effect of various alloy additions on hardenability.

00

1.5

3.0

4.5

6.0

0.5 1.0 1.5 2.0 2.5

Mo

Mn

Cr

NiSi

Hardenability factor

Weight %

10

STEEL SELECTION BASED ON STIPULATION OF REQUIREMENTSThe requirements on an engineering steel to manufacture a specific component or construction can conveniently be divided into three categories:

1. Economic requirementsExamples are low cost for starting materials, no extra expenses in manufacturing arising from special precautions, good material yield and low scrap rates, minimal risk for claims and payment of compensation. Of course, availability from stock permitting supply of exact quantities with short lead times is also a requirement with an economic dimension.

2. Manufacturing requirementsThese include all necessary steps involved in production of the part or construction – welding, machining, cold forming, heat treatment etc. The steel selected should be amenable to cost-efficient, trouble-free processing using the machine park which is available.

3. Requirements on satisfactory service performanceThese are requirements coupled to the application in which the part or construction will serve. Examples are stiffness, strength, fatigue resistance, toughness and resistance against wear.

It is not always the case that requirements from all three categories are compatible. For example, the highest level of service performance is seldom achievable in parity with uncompli-cated manufacturing and low material costs.

Please now attempt to specify the requirements that are to be met by the engineering steel in your application. You can categorise the requirements as above if you wish or use any other system which better suits your needs. The requirements should then be rated according to the following:- absolute requirement (level 5),- very important requirement

(level 4),- rather important requirement

(level 3).

On the next page, you will find a list with various types of engineering steel and their properties with emphasis on the more positive characteristics. Tibnor’s stock programme comprises a number of grades from all of these steel groups, each of which is thereby available in exact quantities with short lead times. Every property has been assigned a rating where 1 is worst and 5 best. Try to find the steel type which best fits the requirements profile for your application bearing in mind that some degree of compromise may be necessary. In some instances, attention is drawn to the fact heat treatment may be required in order that the given property rating is achieved. Heat treatment will always involve extra costs even if carried out “in-house”.

When you have come to a decision as to the steel type which conforms most closely to your requirements profile, the next step is to refine the selection making use of the detailed property specifications for all engineering steel grades in Tibnor’s stock programme. You will find these listed on pages 12-17. In this instance, even more negative properties with rating 1 or 2 are included so that you are made aware of any negative repercussions coupled to your selection. Moreover, details of the profiles, surface finishes and tolerances which are available are specified on pages 18-21.

For the reader needing more information on a specific steel type and its characteristic features and properties, a more detailed description for each group can be found later on in this guide.

If after following these guidelines, you still have difficulty in finding a grade which matches the require-ments profile for your application or if there is a property of interest which has not been covered, then you are most welcome to contact us at Tibnor. We will see to it that you receive all the information and help you need.

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– Strength (4–5)–Fatigue strength (4–5)–Toughness (4–5)–Price (2–3)

– Strength (5)–Fatigue strength (4)–Wear resistance (4)– Spring properties (5)–Toughness (1–2)–Price (3)

–Machinability (5)– Strength (2)–Toughness (1)– Tolerance of product in stock (4)–Price (3)

– Strength (4)– Weldability (4)– Toughness (4)– Cold formability (3)– Price (3-4)

– Strength (5)–Fatigue strength (5)–Wear resistance (5)–Surface hardness (5)–Edge strength (4)–Toughness (2)–Price (2)

– Strength (2)– Weldability (5)– Toughness (3)– Cold formability (3)– Price (5)

–Wear resistance (5)–Fatigue strength (bend/impact) (5)–Surface hardness (5)–Toughness (3)–Price (3)

CONSTRUCTIONALSTEELS

CASE-HARDENINGSTEELS (*)

MICRO-ALLOYEDSTEELS

BEARING STEELS (*)QUENCHED-AND-TEMPERED STEELS

SPRING STEELS (*)

FREE-MACHINING STEELS

Ratings for properties: Ratings for price:(5) – very good (5) – low(4) – good (4) – medium-low(3) – quite good (3) – medium(2) – less good (2) – medium-high(1) – rather poor (1) – high

* Does not apply to delivery condition – must be heat treated to attain these ratings.

12

– Strength - less good (2)– Weldability - very good (5)– Impact toughness - quite good (3)

– Cold formability - quite good (3)

– Price - low (5)

S235JRC+C Compressed axle (*)(SS 1312-06) Cold-drawn flats(*)

S235JR Hot-rolled or peeled/turned (*) rounds(SS 1312) Hot-rolled squares and profiles

S355J2 Hot-rolled or peeled/turned (*) rounds(SS 2172) Hot-rolled squares and profiles

E355+SR Cold-drawn tubes, skived/roller-burnished (**)

CONSTRUCTIONAL STEELSwith properties profile:

* Tolerances on product in stock - good (4) ** Tolerances on product in stock - very good (5)

13

– Strength - good (520M (3)) (4)

– Weldability - good (4)– Impact toughness - good (4)– Machinability - good (520M) (4)– Cold formability - quite good (3) – Price - medium-low (4)

– Strength - quite good (3)– Machinability - very good (5)– Weldability - rather poor (1)– Impact toughness - quite good (3) – Price - medium (3)

– Strength - good (4) – Impact toughness - less good (2) – Weldability - good (4)– Machinability - good (4)– Tolerances on product in stock - good (4)– Price - medium-high (2)

– Strength - good (4) – Impact toughness- less good (2)– Machinability - very good (5)– Weldability - rather poor (1)– Tolerances on product in stock - good (4)

– Price - medium-high (2)

– Strength - very good (5) – Impact toughness - less good (2)– Weldability - good (4)– Machinability - quite good (3)– Tolerances on product in stock - good (4)


S450J0/280 Hot-rolled or peeled (*) rounds(SS 2142) Centerless-ground bars (**)

OVAKO 280 Hot-rolled seamless tubes

E470 Hot-rolled seamless tubes

S355J2/520M Hot-rolled or peeled/turned (*) rounds

520MW+ Hot-rolled or peeled (*) rounds

Hydax 25 Hot-rolled flats and squares

550M Cold-drawn rounds

S355JRC+C Cold-drawn rounds

550MW+ Cold-drawn rounds

280D Cold-finished seamless tubes

MICRO-ALLOYEDCONSTRUCTIONALSTEELSwith properties profile:


14

– Wear resistance - very good (5)– Fatigue resistance (bending, impact) - very good (5)

– Surface hardness - very good (5) – Impact toughness - quite good (3) (*)

– Weldability - quite good (3) (*)– Machinability - good (4) (*)– Price - medium (3)

16NiCrS4 Hot-rolled or peeled/turned (*) rounds(SS 2511)

CASE-HARDENINGSTEEL

with properties profile:

* Tolerances on product in stock - good (4) * As supplied or core after case-hardening

15

– Strength - less good/ quite good (2-3) (*)

– Impact toughness - less good (2)– Weldability - less good (2)– Machinability - good (4)– Potential for induction hardening - good (4)

– Price - medium-low (4)

– Strength - good (4)– Impact toughness - good (4)– Weldability - quite good (3)– Machinability - good (4) (if M-steel)

– Price - medium (3)

– Strength - very good (5)– Fatigue strength - good (4)– Impact toughness - good (4)- Weldability - rather poor (1)- Machinability - good (if M-steel). (4)- Wear resistance - quite good (3)- Price - medium (3)

– Strength - very good (5)– Fatigue strength - very good (5)– Impact toughness - very good (5)– Weldability - rather poor (1)– Machinability - quite good (3) (if M-steel) – Wear resistance - good (4)– Price - medium-high (4)

SS-EN C45R Hot-rolled or peeled/turned (*) rounds(SS 1672) Centerless-ground bars (**)

SS-EN C45E Hot-rolled flats and squares(SS 1672)

SS-EN C45E+N Forged and turned (*) rounds(SS 1672)

SS-EN 25CrMoS4 Hot-rolled or peeled (*) rounds(SS 2225) (quenched-and-tempered)

SS-EN 42CrMoS4 Hot-rolled or peeled (*) rounds(SS 2244) (quenched-and-tempered)

SS-EN 34CrNiMo6 Hot-rolled or peeled (*) rounds(SS 2541) (quenched-and-tempered)

QUENCHED-AND-TEMPERED STEELS

with properties profile:


* As-rolled (2); quenched-and- tempered (3)

16

100Cr6 Peeled (*) rounds(Ovako 803) Hot-rolled seamless tubes

100CrMo7 Hot-rolled seamless tubes (Ovako 824)

100CrMo7-3 Peeled (*) rounds(Ovako 825)

– Strength - very good (5)– Fatigue strength - very good (5)– Wear resistance - very good (5) – Surface hardness - very good (5)– Edge strength - good (4)– Impact toughness - less good (2) – Machinability - quite good (3) (*)– Price - medium-high (4)

SS-EN 56Si7 Hot-rolled flats(SS 2090-00)

SS-EN 51CrV4 Peeled (*) rounds(SS 2230-02M)

– Strength - very good (5)– Fatigue strength - good (4)– Spring properties (resilience) - good (4)

– Impact toughness - less good (2)– Machinability - quite good (3) (*) – Wear resistance - good (4)– Price - medium (3)

BEARINGSTEELSwith properties profile:

* Tolerances on product in stock - good (4) * As supplied (full annealed)

SPRING STEELSwith properties profile:

* Tolerances on product in stock - good (4) * As supplied. Other properties relate to hardened/tempered execution

17

S450J0/280X Hard-chrome-plated round bars (SS 2142) Hard-chrome-plated tubes

Ovako 482 Induction hardened hard-chrome-plated round bars (*)

– Wear resistance - good (low friction) (4)

– Corrosion resistance - quite good (3)

– Strength - good (4)– Weldability - good (4)– Impact toughness - good (4)– Machinability - quite good (3)– Tolerances on product in stock - very good (5)


– Machinability - very good (5)– Strength - less good (2)– Impact toughness - rather poor (1) (*)

– Tolerances on product in stock - good (4)

– Price - medium (3)

11SMnPb30 + C Cold-drawn rounds, squares and hexagons (SS 1914) (amenable to case-hardening)

MACH 50 Cold-drawn rounds and squares (S + Pb leg.) (amenable to case-hardening and induction hardening) 520MW+ Peeled rounds (amenable to case-hardening and nitriding)

550MW+ Cold-drawn rounds (amenable to case-hardening and nitriding)

HARD-CHROME- PLATED BARS with properties profile

FREE-MACHINING STEELS with properties profile:

* Resistance to external impact - very good (5)

* 520MW+ has quite good impact toughness (3)

18

DIMENSIONS AND TOLERANCESFor engineering components, the correct choice of execution is just as important for manufacturing costs as an optimum steel selection. Tibnor’s stock programme comprises a number of different executions such as hot-rolled, peeled or turned, centre-less ground and cold drawn.

The dimensional tolerances for the various executions are for the most part standardised. On occasion, products may have special tolerances which have been tailored for a specific project or application.

D = Nominal diameterDy = Nominal outside diameter Di = Nominal inside diameter Wall = Nominal wall thickness

Round bar: weight/metre D2 X 0.006165 kg/m

Tube: weight per metre (Dy2 – Di2) X 0.006165 kg/m

T = Nominal edge length or thicknessB = Nominal width

Square bar: weight/metreT2 X 0.00785 kg/m

Flat bar: weight/metre T X B X 0.00785 kg/m

Hexagonal bar: weight/metre N2 x 0.006798 kg/m

A = 1.155 X N

All dimensions in mm. The density of steel is 7.85 kg/dm3.

D

Round bar

T

Square bar

Di

Dy

Wall

Tube

B

Flat bar

T

Hexagonal bar

A

N

19

BAR TOLERANCESDimensional tolerances– Hot-rolled bar is usually

characterised by a ± tolerance on D, T and B. Since the bar contracts during cooling after hot rolling,

the tolerance achieved is very dependent on process control during this operation.

– Peeled rounds have as a rule a minus tolerance on D. The standards allow a variation between h12 and h15 depending on diameter, with larger diameters having the widest tolerance range.

– Rough-turned rounds can have either an h or a ± tolerance. Very large diameters are generally turned to tolerances similar to hot-rolled but tighter, +3/-0 mm is common.

– Centre-less ground or cold-drawn rounds have as standard an

h-tolerance, normally h8 for ground and h10 for cold drawn.

– Hard-chrome-plated bar (round) has tolerance f7 as standard but this tolerance range is also covered by h9. The tolerance range f is minus/minus while h is zero/minus.

OvalityThe ovality of a round bar is defined as the difference between two mutually perpendicular diameter measurements. This difference is as a rule expressed as a percentage of the tolerance range for diameter.– Hot-rolled: The ovality shall be less

than 75 % of the tolerance range for D.– Peeled or turned:The ovality shall be

less than 50 % of the tolerance range for D.

– Ground: The ovality shall be less than 33 % of the tolerance range for D.

– Cold-drawn: The ovality shall be less than 100 % of the tolerance range for D.

As an example, consider a ground bar with D = 40 mm. The tolerance h8 means that the actual diameter shall be between 39.961 and 40.000 mm. The tolerance range is hence 0.039 mm and the ovality shall be less than one third of this or 0.013 mm.

StraightnessStraightness, which is especially important for bars that have to be machined by turning, is measured as the greatest gap between the bar and a horizontal flat surface upon which it is placed. For a regular hot-rolled bar with normal straightness, this gap should be less than 0.004 x length. In other words for a standard 6-metre bar, the maximum allowed gap is 6000 x 0.004 = 24 mm. For other executions, the straightness is often better, for example 0.002 x L for peeled bar and 0.001 x L for cold-drawn or ground bar.

Surface defectsSteel is a mass-produced commodity and as such surface defects will always be present to some degree. Larger defects such as scale, cracks, flakes, laps, decarburisation etc. must be detected and remedied at the steel mill. Even so, for the product supplied to the market, especially when in the hot-rolled condition, smaller defects inevitably remain and this should be taken into consideration when assessing machining allowance. Of course, surface defects will be fewer for executions such as peeled or ground, in which the hot-rolled surface has been removed. On the other hand, defects originating from hot rolling can remain on drawn bars. The extent, type and size of surface defects that are allowed on steel products are regulated in the standard, SS-EN 10221.

Internal defectsCavities, larger inclusions, excessive segregation, pipe etc. are controlled by the steel manufacturer either through testing of cast material or via ultrasonic examination performed on finished products. Some degree of segregation and smaller inclusions are inevitable features of all steel products and in most instances are of no consequence for service performance.

It is important that in the event of special requirements as regards freedom from internal defects, the requested levels should be clearly specified in any enquiries preferably with reference to appropriate standards (for example SS-EN 10247:2007 for defining allowable inclusion levels).

20

Hot-rolled rounds and squares

D or T, mm Tolerance

above up to and including

5.5 10 ±0.4 10 15 ±0.4 15 25 ±0.5 25 35 ±0,6 35 50 ±0.8 50 80 ±1.0 80 100 ±1.3 100 120 ±1.5 120 160 ±2.0 160 200 ±2.5 200 270 ±3.0

Hot-rolled flats and universal bars

B, mm Tolerance Thickness tolerance

above up to and including T ≤20 >20 – 40 >40

– 40 ±0.7 ±0.5 ±1.0 – 40 80 ±1.0 ±0.5 ±1.0 ±1.3 80 100 ±1.5 ±0.5 ±1.0 ±1.3 100 120 ±2.5 ±0.5 ±1.0 ±1.3 120 150 ±2.5 ±0.5 ±1.0 ±1.3 150 200 ±2 % ±0.5 ±1.0 ±1.3 200 275 ±2 % ±0.5 ±1.0 ±1.3 275 400 ±2 % -0.4/+0.8 -0.7/+1.1 -1.0/+1.4

All tolerances are in mm unless stated otherwise.

Peeled rounds for further machining

D, mm D, mm

Tolerance limits, mm Tolerance limits, mm

Nominal dia.

Toler-ance upper lower kg/m Nominal

dia.Toler-ance upper lower kg/m

20.8 h12 20.8 20.59 2.67 102 h13 102 101.46 64.1 22.8 22.8 22.59 3.20 107 107 106.46 70.6 25.8 25.8 25.59 4.10 112 112 111.46 77.3 28.8 28.8 28.59 5.11 117 117 116.46 84.4 30.8 30.8 30.55 5.85 122 122 121.37 91.7 32.8 32.8 32.55 6.63 127 127 126.37 99.4 36.0 36.0 35.75 7.99 132 132 131.37 107

39.0 39.0 38.75 9.37 138 h14 138 137.00 117 41.0 41.0 40.75 10.4 143 143 142.00 126 43.0 43.0 42.75 11.4 148 148 147.00 135 46.0 46.0 45.75 13.0 153 153 152.00 144 49.0 49.0 48.75 14.8 163 163 162.00 164 51.2 51.2 50.90 16.2 173 173 172.00 184 53.2 53.2 52.90 17.4 184 184 182.85 209 56.2 56.2 55.90 19.5 194 194 192.85 232 59.2 59.2 58.90 21.6 204 204 202.85 256 61.2 61.2 60.90 23.1 214 214 212.85 282 63.2 63.2 62.90 24.6 224 224 222.85 309

66.2 66.2 65.90 27.0 235 h15 235 233.15 340 69.2 69.2 68.90 29.5 245 245 243.15 370 71.4 71.4 71.10 31.4 255 255 252.90 401 73.4 73.4 73.10 33.2 285 285 282.90 501 76.4 76.4 76.10 36.0 306 306 303.90 577 79.4 79.4 79.10 38.8 326 326 323.70 655 81.4 81.4 81.05 40.0 356 356 353.70 781 86.4 86.4 86.05 46.0 386 386 383.70 918 91.4 91.4 91.05 51.5 406 406 403.50 1016 96.4 96.4 96.05 57.3

Cold drawn and ground rounds

Tolerance h6 - h11

Nominal dia, mm h6 h7 h8 h9 h10 h11

above up to and including Upper limit always +/-0. Lower limit as below, mm

1 3 –0.007 –0.009 –0.014 –0.025 –0.040 –0.060 3 6 –0.008 –0.012 –0.018 –0.030 –0.048 –0.075 6 10 –0.009 –0.015 –0.022 –0.036 –0.058 –0.090 10 18 –0.011 –0.018 –0.027 –0.043 –0.070 –0.110 18 30 –0.013 –0.021 –0,033 –0.052 –0.084 –0.130 30 50 –0.016 –0.025 –0.039 –0.062 –0.100 –0.160 50 80 –0.019 –0.030 –0.046 –0.074 –0.120 –0.190 80 120 –0.022 –0.035 –0.054 –0.087 –0.140 –0.220 120 180 –0.025 –0.040 –0.063 –0.100 –0.160 –0.250

21

TUBE TOLERANCES– Straightness: The maximum

allowed gap depends on execution but is at most 1 mm over a length of 1 metre (1000 x 0.001).

– Ovality: The ovality is at most 65 % of the tolerance range for Dy (does not apply for ISO-tubes).

– Internally skived/roller-burnished cylinder tubes are characterised by an H-tolerance (a plus/zero tolerance) on Di with H8 as standard. The Dy-tolerance conforms to EN 10305-1.

– Maximum finished dimension depends on whether the tube is centred internally or externally when machining. The finished dimension which is guaranteed is coupled to a machined length which is dependent on Dy.

– Cold drawn tubes have a Dy tolerance of +0.6/-0 mm and have straightness corresponding to a gap which is less than 1mm over a length of 1 metre (1000 x 0.001).

Wall thickness Maximum variation< 6 mm 0.7 mm6-8 mm 0.8 mm> 8 mm 0.9 mm

– Surface defects: Tubes shall have smooth surfaces. Local high and low points along with shallow longitudinal cracks are allowed so long as they lie within the limits for diameter tolerance.

Tolerances Ovako 280 hot-rolled

Grade Outside diameter Machined length for finish dimension

280 Hot-rolled Dy ≤80 mm ±0.4 mm 3 x Dy (applies for all Dy)280 Hot-rolled Dy >80 mm ±0.5%

Wall

280 Hot-rolled <12 mm ±0.7 mm280 Hot-rolled >12 mm ±(5% x wall + 0.1 mm)

Tolerances SS-EN 10294-1:2005 hot-rolled

Grade Outside diameter Machined length for finished dimension

E470 Hot-rolled Dy ≤75 mm ±0.5 mm 3 x Dy (applies for all Dy)E470 Hot-rolled 75 <Dy ≤180 mm ±0.75%E470 Hot-rolled Dy >180 mm ±1 %

Wall

Grade Dy ≤180 mm Dy >180 mm

E470 Hot-rolled ≤15 mm ±12.5% ≤30 mm ±12.5% or 0.4 mm* E470 Hot-rolled >15 mm ±10% >30 mm ±10%

Tolerances cold-finished tubes 280

Grade Outside diameter Wall Machined length for finished dimension

280D Cold finished ±0.2 mm ±0.2 mm 2.5 x Dy

Tolerances E355+SR skived/roller-burnished tubes

Tolerance H8

Nominal Di, mm

upper up to and including Upper limit as below, mm. Lower limit always +/-0.

30 50 +0.039 50 80 +0.046 80 120 +0.054 120 180 +0.063 180 250 +0.072

*The greater alternative is the one which applies.

23

STANDARD CONSTRUCTIONAL STEELS;MICRO-ALLOYED CONSTRUCTIONAL STEELSStandard constructional steels can be used for components and constructions with moderate strength requirements irrespective of whether or not their manufacture involves welding. If higher strength is needed, micro-alloyed steels are more suitable. The carbon and alloy content of both types is adapted so that under normal circumstances, they can be welded without pre-heating.

Both yield and tensile strength are increased as a result of micro-alloying which means that in many cases, smaller dimensions can be used without having to compromise in relation to strength requirements thereby reducing the weight of the construction (weight saving is discussed in more detail in a later chapter of this guide). A micro-alloyed constructional steel has just as high or even higher yield strength than a medium-carbon steel but is much easier to weld.

Machining of these steel types can give difficulties in the shape of built-up edge formation leading to excessive tool wear, poor surface finish and long chips, which may prove troublesome in unmanned operations. In order to improve machinability, this type of steel is available from Ovako Imatra in “M-steel” execution. M-steel treat-ment confers improved machin ability without significant negative influence on other properties. More information on M-steels is given in the section devoted to machinability.

Bars in standard constructional steels and micro-alloyed steels are usually as-hot-rolled with straightening as the only finishing operation. However, larger dimensions can be heat-treated by normalising so as to refine the microstructure and improve toughness.

Cold-finished material is normally used in the condition in which it is supplied. Cold drawing or cold rolling engenders increased strength as well as improving the tolerance of both bars and tubes. Cold working is to some degree also positive for machinability.

Cold working gives rise to residual stresses which are normally greatest closer to the surface and decrease towards the centre of the section. Hence, machining of such material can result in dimensional changes; in particular, long components can become crooked. Welding of cold-worked steel requires some thought in that the strength can be reduced in the heat-affected zone adjacent to the weld; it is therefore prudent to locate welds in areas where the service loading is low.

24

Grade Mechanical properties

Tensile test SS-EN 10002-1

Hardness SS-EN ISO 6506-1

Impact test

SS-EN ISO 148-1

Typical analysis % Execution

Dim-ension **

mm

Re*

N/mm2 min

Rm

N/mm2A5%

min HB

KVmin 27 J

at °C

C 0.18 Hot rolled bar -16 355 470-630 22 140-200 -20 Si 0.30 >16-40 345 470-630 22 140-200 -20 Mn 1.50 > 40-63 335 470-630 21 140-200 -20 S 0.050 > 63-80 325 470-630 20 140-200 -20 > 80-100 315 470-630 20 140-200 -20 > 100-150 295 470-630 18 140-200 -20 > 150-200 285 450-630 17 140-200 -20 > 200-250 275 450-630 17 140-200 -20 Normalised bar -250 300 470-620 21 140-200 -20 >250- 500 260 470-610 20 140-200 -20 Hot-rolled tube -5 340 470-610 21 140-200 -20 > 5-16 320 470-610 21 140-200 -20 > 16-40 300 470-610 21 140-200 -20 Cold-drawn bar 5-16 490 600-850 9 190-250 > 16-40 460 600-820 9 185-240 > 40-63 400 580-800 10 180-230 > 63-80 375 550-740 11 175-220

C 0.18 -00 Hot rolled bar -80 450 580-750 19 180-230 Si 0.35 > 80-160 410 580-750 19 180-230 Mn 1.50 > 160-185 380 580-750 19 180-230 V 0.10 -01 Normalised bar -16 390 490-630 20 140-200 -20 S 0.015- > 16-35 380 490-630 20 140-200 -20 0.035 > 35-50 370 490-630 20 140-200 -20 > 50-70 360 490-630 20 140-200 -20

C 0.19 Hot-rolled tube ≤ 25 500 670 min 20 ≈ 225 +20 Si 0.38 > 25 470 640 min 20 ≈ 220 +20 Mn 1.53 Normalised tube ≤ 15 430 600 min 25 ≈ 190 -40 V 0.10 >15-25 400 580 min 25 ≈ 185 -40 S 0.020- > 25 380 560 min 25 ≈ 180 -40 0.035 Quenched-and- tempered tube ≤ 30 600 700 min 20 ≈ 260 -40

C 0.20 Hot-rolled tube ≤ 16 470 650 min 17 ≈ 225 Si 0.40 17-≤ 25 460 620 min 17 ≈ 220 Mn 1.60 26-≤ 40 430 600 min 17 ≈ 190 Cr 0.25 41-≤ 50 430 550 min 17 ≈ 180 V 0.12 S 0.035

Cold-finished tube 740 760 min 10 ≈ 250

C < 0.20 Hot-rolled bar 20-70 380 490-630 22 ≈ 170 -20 Si < 0.55 > 70-180 350 490-630 20 ≈ 165 -20 Mn < 1.60 > 180-200 285 450-630 17 ≈ 150 -20 V 0.09 > 200-210 275 450-630 17 ≈ 150 -20 S 0.02- 0.04 CEV 0.45 max

C < 0.20 Hot-rolled bar 25-70 380 490-630 22 140-200 -20 Si < 0.55 > 70-90 350 490-630 20 140-200 -20 Mn < 1.60 > 90-180 350 490-630 20 140-200 0 V 0.09 S 0.13- 0.17

*Re: Upper yield stress (R

eH) or if discontinuous yield is absent 0.2 % proof stress, R

p0,2

** D, T or B for respectively round, square or flat bars, wall thickness for tubesBlue = Not stock standard

S3

55

J2

(SS

217

2)

Can

be c

ase

-h

ard

en

ed

or

nit

rid

ed

28

0

(S4

50

J0

) C

an

be c

ase

-h

ard

en

ed

or

nit

rid

ed

OV

AK

O 2

80

C

an

be c

ase

-h

ard

en

ed

, n

itri

ded

or

qu

en

ch

ed

-an

d-

tem

pere

d

E4

70

C

an

be c

ase

-h

ard

en

ed

or

nit

rid

ed

28

0D

5

20

M

(S3

55

J2)

Can

be c

ase

-h

ard

en

ed

or

nit

rid

ed

520

MW

+

Can

be c

ase

-h

ard

en

ed

or

nit

rid

ed

25

Grade Continued from previous page Mechanical properties



Impact test

SS-EN ISO 148-1


Dim- ension **

mm

Re*

N/mm2 min

Rm

N/mm2A5%

min HB

KVmin 27 J

at °C

C < 0.20 Cold-drawn bar 20-55 500 550-750 12 ≈ 200 +20 Si < 0.55 Mn < 1.60 V 0.09 S 0.02- 0.04

C < 0.20 Cold-drawn bar 20-55 500 550-750 ≈ 12 ≈ 200 +20 Si < 0.55 Mn < 1.60 V 0.09 S 0.13- 0.17

C < 0.26 Hot-rolled bar > 80-180 320 490-630 20 ≈ 200 0 Mn < 1.60 S 0.09- 0.15

C < 0.40 Hot-rolled bar - 100 580 850-1 000 14 250-300 Si < 0.50 > 100 580 850-1 000 14 250-300 Mn < 1.40 Cr < 0.25 Ni < 0.25 V < 0.16 S 0.020- 0.035



p0,2

** D, T or B for respectively round, square or flat bars, wall thickness for tubesBlue = Not stock standard

55

0M

S

35

5J2C

+C

Can

be c

ase

-h

ard

en

ed

or

nit

rid

ed

55

0M

W+

C

an

be c

ase

-h

ard

en

ed

or

nit

rid

ed

HY

DA

X 2

5

Can

be c

ase

-h

ard

en

ed

48

2

Su

itab

le f

or

ind

ucti

on

hard

en

ing

Heat treatment of constructional steelsConstructional steels are usually supplied and used in the hot-rolled condition. However, they sometimes require to be heat treated or processed as follows in order to improve certain properties.

Forging900 – 1 200°C Cooling freely in air.

Normalising 900 – 930°C Holding time 15-20 min. Cooling freely in air.

Quenching and temperingHardening 900 – 930°C. Holding time 15-30 min.

Quenching in water or polymer.Tempering 550 – 600°C. Air cooling.

Stress relieving550 – 600°C Holding time 1-2 h. Slow cooling.

Case hardeningCarburising 850 – 930°C. Hardening 780 – 830°C. Quenching in water, oil or salt bath.Tempering 150 – 200°C. Air cooling

26

CASE-HARDENING STEELSCase-hardening steels have a low content of carbon and are supplied in a soft, easy to machine condition. The component is first machined, then subjected to a surface hardening treatment (case hardening) and finally finished via grinding. Case-hardening steels are used in applications with requirements which may at first sight appear incompatible: wear resistance, toughness, ability to withstand impact and resistance to fatigue.

Case-hardening as a process involves heating the steel in a carburising medium, normally a gas mixture containing hydrocarbons. The carbon liberated from the gas mixture diffuses into the steel to an extent which is determined by temperature and time. The carburised components are then quenched to effect hardening which results in the combination of a hard wear-resistant surface (the case) and a tough core. The surface carbon content is typically between 0.8 and 1.0 % giving a surface hardness >60 HRC. The hardening depth can be anywhere between 0.2 and 1.5 mm depending on temperature, time and carbon activity, i.e. the medium used for carburising. It is possible to reduce the process time for carburising by selecting a steel with higher base carbon content, but there is then a risk that the core properties will be jeopardised.

Case-hardened components are characterised by high strength and excellent fatigue resistance. During hardening the outer layer with high carbon content would occupy a greater volume but for the fact that it is constrained by the softer core, with the result that high compressive residual stresses are developed in the case. This type of residual stress distribution is favourable for counter- acting the initiation and growth of fatigue cracks which require tensile stresses in order to develop.

Additional consequences of the volume increase when the carburised layer is hardened are dimensional and shape changes, which depend on the form of the component but which can be quite large. For parts with high requirements on dimensional tolerance, a finishing adjustment will be necessary usually by grinding. Dimensional changes will also depend on quench rate and are therefore less with a steel which can be hardened in oil, such

as 16NiCrS4 (SS 2511), than for one requiring hardening in water as S355J2 (SS 2172).

Carbonitriding is similar to case hardening but in addition to carbon even nitrogen is diffused into the steel. In respect of hardening, the effects of carbon and nitrogen are additive so that simpler, lesser-alloyed steels, which normally would need to be hardened in water, can be quenched in oil with benefits for dimensional and shape stability.

Nitriding is a surface hardening process which is carried out at far lower temperatures than case hardening and involves diffusion of nitrogen into the surface of a component. Unlike case hardening, no quenching operation is necessary and the hardening effect from nitrogen is attained directly. Nitriding can be effected in a variety of ways, via ammonia-containing gas, by immersion in special salts or through a plasma. With the gas method, it is possible to use a mixture of ammonia and hydrocarbons so that carbon as well as nitrogen is introduced into the steel. The process is then called nitrocarburising.

Nitrided layers are characterised by high hardness and good resistance to wear but also by low friction and some degree of corrosion protection. Furthermore, shape and dimensional changes are much smaller than with case hardening. Nitriding also improves fatigue resistance but not to the same degree as case hardening.

Ion nitriding is a process in which a plasma is created between the components to be treated and the wall of a chamber filled with nitrogen gas. The parts are bombarded with highly reactive nitrogen ions which diffuse into the steel in the same way as with other nitriding processes. Ion nitriding does not require so high temperatures, which means that dimensional changes are minimal at

the same time as the process permits a better control so that the toughness of the hardened layer and the corrosion protection afforded by it can better be optimised.

Nitriding is usually carried out as a final operation on finished components. Especially after ion nitriding, the surface finish is more or less the same as that prior to treatment. Parts nitrided via gas or salt bath may require a light polish if the application demands an extremely fine surface.

Generally speaking, the surface hardness achievable with nitriding increases with the alloy content in the steel. Some examples of typical levels attainable with ion nitriding are given below. Surface CoreGrade HV1 (*) HV10 (*)

C45E (SS 1672) 490 180S450J0/280 (SS2142) 650 20042CrMoS4 (SS 2244) 650 30016NiCrS4 (SS 2511) 730 17534CrNiMo6 (SS 2541) 650 290* See the section on hardness later in the guide.

It is noteworthy that nitriding of S450J0/280 results in the same hardness as considerably higher-alloyed grades. This is coupled to the micro-alloying with vanadium which is a very potent nitride former and gives high hardness even though the amount added is small. Manganese in this grade makes an additional contri-bution to the high hardness achieved.

The vanadium in S450J0 prevents microstructural coarsening at high temperatures which means that this grade is also very amenable to case hardening.

Hardenability is an important property for case-hardening steels since it determines the core properties after quenching and tempering. It is normally defined in terms of a Jominy diagram which shows hardness as a function of the distance for a sample

27




Impact test

SS-EN ISO 148-1


Dimension, D

mm

Re

N/mm2 min

Rm

N/mm2A5%

min HB

KVmin 27 J

at °C

16NiCrS4 C 0.15 Hot-rolled or forged >20-430 max 217(SS2511) Si <0.40 Cold drawn 10-20 500 625-750 10 200-240 Mn 0.90 Ni 1.00 Cr 0.80 S 0.02- 0.04

Other steel types that can be case-hardened are standard and micro-alloyed constructional steels and free-machining steels.

Hardenability from Jominy test (see SS-EN 10084)

Distance* (mm) 1.5 3 5 7 9 11 13 15 20 25 30 35 40

16NiCrS4 HRC Max 47 46 44 42 40 38 36 34 32 30 29 28 28(SS2511-08) HRC Min 39 36 33 29 27 25 23 22 20

*From the quenched end.

Heat-treatment of case-hardening steels

Forging900 – 1 200°C Rapid heating from 1000°C. Hold only until heated through. Free cooling in air.

Normalising860 – 890°C Free cooling in air. This heat treatment is carried out so as to refine grain size prior to case-hardening.

Annealing600 – 670°C Holding time 2h. Cooling in furnace or freely in air.

Case-hardeningCarburising 850 – 930°C. Temperature and time determined by

carburising medium and required hardening depth. Annealing 650 – 700°C. Will be necessary if the part is to be machined

after carburising. Hardening 780 – 830°C. Oil quench. (Direct hardening with quenching

immediately after carburising is sometimes practised). Tempering 150 – 200°C

that has been quenched by water at its one end in accord with a standard-ised procedure.

Depth of hardening is an essential parameter for all surface hardening

processes. This is most often defined as the distance in mm from the surface over which the hardness exceeds a specified level.

Process Hardness levelCase hardening: ≥550 HV1Carbonitriding: ≥550 HV1Nitriding: ≥400 HV1Induction hardening: ≥400 HV1

28

QUENCHED-AND- TEMPERED STEELSQuenched-and-tempered steels find application whenever a good combination of strength and toughness is required. Low-alloy quenched-and-tempered steels are supplied in a heat-treated execution. In this condition, these grades will, generally-speaking, withstand both static and dynamic loading better than carbon steels and weldable constructional steels. Hence, such materials offer an interesting alternative to lesser-alloyed steels if it is advantageous to reduce the weight of a component or construction that does not require welding.

The only low-alloy, quenched-and-tempered grade that is possible to weld using relatively simple procedures is 25CrMoS4 (SS 2225). Other grades with higher carbon content are more difficult to weld. If welding is absolutely necessary, attention should be given to the effects of changes of microstructure in the weld heat-affected zone. The material adjacent to the weld will be re-hardened and thereby embrittled. Further away from the weld is a region where the temperature has exceeded that used for tempering

and where the hardness and strength is lowered. Welds should therefore be placed where loading is least. If this is not feasible, it may be necessary to consider hardening and tempering once again of the finish-welded part.

Machining this type of steel is usually troublesome and tools wear quickly even if the cutting speed is reduced. However, M-treatment has a strong positive effect in relation to machining of quenched-and-tempered steels without significant negative effects as regards other properties.

This is discussed in more detail later on in the guide. The quenched-and-tempered grades in our stock at Tibnor are for the most part M-treated.

As already made clear, quenched-and-tempered steels are supplied in a heat-treated, ready-to-use execution and further heat treatment is usually not required. The medium-carbon steel C45E/R (SS 1672) is an exception and the stock standard is as-hot-rolled/ forged; any heat treatment that may be necessary will need to be carried out after machining of the component.

29




Impact test

SS-EN IS 148-1


Dim-ension**

mm

Re*

N/mm2 min

Rm

N/mm2

A5%

min HBMin Jat °C

C 0.47 Cold-drawn /C45E 8-16 320 590-740 9 165-220 Si 0.25 Hot-rolled /C45R >16-40 310 590-740 14 165-220 Mn 0.60 >40-63 300 590-740 14 165-220 S <0.035/ >63-300 280 590-740 14 165-220 C45E Forged/C45E+N >300-550 280 590-740 16 165-220 0.02- -03 Quenched-and-tempered -100 370 630-780 17 180-230 0.04/ -04 Quenched-and-tempered -40 430 650-800 16 190-235 C45R -05 Quenched-and-tempered -16 490 700-850 14 205-250

C 0.26 -03 Quenched-and-tempered -100 500 700-850 17 205-250 27@-20 Si 0.25 -05 Quenched-and-tempered -40 700 900-1050 13 270-325 27@-20 Mn 0.62 -06 Quenched-and-tempered >100-160 410 640-780 16 185-230 27@-20 Cr 1.05 Cold-drawn 15-20 700 900-1050 10 275-325 Mo 0.20 S 0.02- 0.04

C 0.26 Quenched-and-tempered -16 700 900-1100 12 45@+20 Si <0.40 >16-40 600 800-950 14 50@+20 Mn 0.75 >40-100 450 700-850 15 50@+20 Cr 1.05 >100-160 400 650-800 16 45@+20 Mo 0.23 S 0.02- 0.04

C 0.42 -05 Quenched-and-tempered -105 690 900-1050 12 270-310 27@-20 Si 0.25 -04 Quenched-and-tempered >105-160 600 800-950 14 235-285 27@-20 Mn 0.75 Cold-drawn 15-20 700 900-1050 10 275-320 Cr 1.05 Mo 0.20 S 0.02- 0.04

C 0.42 Quenched-and-tempered -16 900 1100-1300 10 30@+20 Si <0.40 >16-40 750 1000-1200 11 35@+20 Mn 0.75 >40-100 650 900-1100 12 35@+20 Cr 1.05 >100-160 550 800-950 13 35@+20 Mo 0.23 >160-250 500 750-900 14 35@+20 S 0.02- 0.04

C 0.36 -03 Quenched-and-tempered -275 700 900-1050 12 270-325 27@-20 Si 0.25 Cold-drawn 10-20 700 900-1100 10 275-335 Mn 0.70 Cr 1.40 Ni 1.40 Mo 0.23 S 0.02- 0.035

C 0.34 Hot-rolled. quenched -16 1000 1200-1400 9 35@+20 Si <0.40 -and-tempered >16-40 900 1100-1300 10 45@+20 Mn 0.65 >40-100 800 1000-1200 11 45@+20 Cr 1.50 >100-160 700 900-1100 12 45@+20 Ni 1.50 >160-250 600 800-950 13 45@+20 Mo 0.23 S 0.02- Forged, quenched-and-tempered 285-610 600 800-950 13 240-290 27@-40 0.035



p0,2

** D, T or B for respectively round, square or flat barsBlue = Not stock standard

C4

5E

/C4

5R

(S

S 16

72)

Su

itab

le f

or

ind

ucti

on

h

ard

en

ing

SS

2225

M

(25

CrM

oS

4)

Su

itab

le f

or

ind

ucti

on

h

ard

en

ing

25

CrM

oS

4

Su

itab

le f

or

ind

ucti

on

h

ard

en

ing

SS

224

4 M

(4

2C

rMo

S4

) S

uit

ab

le f

or

ind

ucti

on

h

ard

en

ing

42C

rMo

S4

S

uit

ab

le f

or

ind

ucti

on

h

ard

en

ing

SS

25

41 M

(3

4C

rNiM

o6

)S

uit

ab

le f

or

surf

ace

tr

eatm

en

t via

in

du

cti

on

hard

en

ing

o

r n

itri

din

g

34

CrN

iMo

6

Su

itab

le f

or

surf

ace

tr

eatm

en

t via

in

du

cti

on

hard

en

ing

o

r n

itri

din

g

Quenching and tempering of C45E/RHardening 820 – 860°C. Quenching in water, polymer or fast-quenching oil.Tempering 600 – 650°C. Free cooling in air.

30

YIELD AND TENSILE STRENGTH FOR QUENCHED- AND-TEMPERED STEELS Below you find a comparison between the old SS-standard and the current SS-EN-standard.

Yield strength minimum

RP0.2

min, N/mm2

Diameter in mm

RP0.2

min, N/mm2

Diameter in mm

RP0.2

min, N/mm2

Diameter in mm

1 200

1 000

SS-EN 10083-25CrMoS4

SS 2225-03

SS 2225-04

SS 2225-05800

600

400

200

020 40 60 80 100 120 140 160

1 200

1 000

800

600

400

200

020 40 60 80 100 120 140 160

SS-EN 10083-42CrMoS4

SS 2244-04SS 2244-05

1 200

1 000

SS-EN 10083-34CrNiMo6SS 2541-03SS 2541-08

SS 2541-05

SS 2541-04

800

600

400

200

020 40 60 80 100 120 140 160

31

The change to SS-EN means that requirements based upon the old SS-standard may need re-assessing in order to check conformance to the new standard. It could be necessary to change grade or to see whether or not the requirements can be modified.SS-EN is such that a given level of

yield or tensile strength applies within a certain range of dimensions while the old SS standard allowed for different strength levels for one and the same size. For example, diameter 80 mm for grade SS 2541 could be specified as 2541-03 with yield/tensile strength 700 N/mm2 and

900 N/mm2 respectively or as 2541-05 with respectively 800 N/mm2 and 1000 N/mm2. However, in SS-EN, this dimension is available only with the combination 800 and 1000 N/mm2, i.e. the same as 2541-05.

Tensile strength minimum

Rmmin, N/mm2

Diameter in mm

Rmmin, N/mm2

Diameter in mm

Rmmin, N/mm2

Diameter in mm

1 200

1 000

SS-EN 10083-25CrMoS4SS 2225-03

SS 2225-04

SS 2225-05

800

600

400

200

020 40 60 80 100 120 140 160

1 200

1 000

800

600

400

200

020 40 60 80 100 120 140 160

SS-EN 10083-42CrMoS4SS 2244-04

SS 2244-05

1 200

1 000

800

600

400

200

020 40 60 80 100 120 140 160

SS-EN 10083-34CrNiMo6SS 2541-03SS 2541-08

SS 2541-04

SS 2541-05

32

SPRING STEELSSpring steels contain about 0.5 % carbon which means that high levels of yield and tensile strength plus excellent resistance to fatigue can be achieved via quenching and tempering. As the name implies, the principal area of application is springs but the high strength means that these steels also function well in tools, wear parts and for some machine components.

For spring applications, spring steels are tempered in the range 350-500°C to attain a hardness of 44-50 HRC, corresponding to tensile strengths 1 400-1 650 N/mm2. The associated high yield strength is concomitant with good spring properties since large amounts of elastic energy can be stored and released repeatedly. However, the toughness is less good at these high strength levels.

Both silicon-chrome and chrome-vanadium spring-steel grades are included in Tibnor’s stock programme. At a given hardness level, these steels are more or less equivalent but the Cr-V steel has better hardenability and can be used for heavier sections. Remember that the product stocked is either as-hot-rolled or annealed and that finished parts must be heat treated in order to realise high strength.

For smaller dimensions, results similar to those achieved by hardening and tempering can be attained by cold working followed by heat treatment at low-temperature.

The service temperature of components made from spring steels should not exceed 200°C (Si-Cr) or 225°C (Cr-V); higher temperatures will result in loss of strength.

33




Impact test

SS-EN ISO 148-1


Dimensionmm

Re

N/mm2 min

Rm

N/mm2A5%

min HB

KVmin 27 J

at °C

C 0.56 -00 As hot-rolled All <248 Si 1.7 flat bars Mn 0.85 Cr 0.3 S 0.035

C 0.52 -02 Annealed All <240 Si 0.28 round bars Mn 0.85 Cr 1.15 V 0.15 S 0.035

SS

20

90

(5

6S

i7)

SS

223

05

1CrV

S4

Heat treatment

Forging SS 2090 SS 2230Cooling freely in air. 800 – 1 025°C 800 – 1 050°C

Hot formingCooling freely in air. 830 – 900°C 800 – 900°C

AnnealingHolding time 0.5 h after attaining 680 – 720°C 730 – 750°C full temperature. Furnace cool ca 20°C /h to 650°C, followed by cooling free in air.

Stress relievingHolding time ≈ 2 h after 550 – 650°C 550 – 650°C attaining full temperature. Furnace cool to 500°C, followed by cooling free in air.

Hardening in oil 850 – 910°C 840 – 870°C

TemperingShould be carried out immediately the material can be touched by hand. Temperature 350-600°C. See tempering graph for SS2230 below.

Tempering temperature (C°)100 300 500 700

600

200

400

Hardness (HV30)

SS2230 oil quenched from 860°, holding time 1 h.

34

BEARING STEELSBearing steels were developed specifically for service as ball and roller bearings. This application requires good resistance to wear at the same time as the repeated nature of the loading on bearings demands a high level of fatigue strength. Hence, bearing steels can be and are used in other applications with the same basic requirements, i.e. combined fatigue and wear resistance. In terms of their chemical analysis, bearing steels are very similar to simpler tool-steel grades.

Since bearing steels contain about 1 % carbon, they can be considered as an alternative for parts that usually are case hardened. In such an instance, the gain is that case hardening, which is a time-consuming and therefore expensive heat treatment, can be re- placed by a straightforward hard ening and tempering operation. The properties obtained thereby are more or less equivalent to what is attainable via case hardening. If a hard surface in combination with a tough core is required, then bearing steels can even be induction hardened.

Let’s look at an example:

Component Requirements(See photo on left below) D40 X 300 mm Surface hardness

≥60 HRC; good straightness; close tolerances.

One can envisage several alternatives to manufacture this part; two are described below.

S355JR case hardened to a depth of about 0.8 mm with the final tolerance being achieved by finish grinding after the surface treatment. In order to attain the required hardness, this grade would for the relevant dimension need to be hardened by quenching in water which will result in significant changes in

dimension and shape along with a considerable risk for hardening cracks. Hence, this choice of steel and heat-treatment method will necessitate an appreciable finish grinding allowance in order that the requirements on tolerance and straightness can be met.

100Cr6 (Ovako 803) induction hardened. The base steel is admittedly more expensive but the part will be straight after hardening and a minor final adjustment by grinding is all that will be required in order to achieve the dimensional tolerances.

This constitutes a good example of how choosing a more expensive material can reduce the overall cost to manufacture a part; induction hardening costs far less than case hardening and a time-consuming and therefore expensive finishing operation is avoided.

35

Heat treatment of bearing steels

Full annealing800 – 820°C Holding time 2-5 h after attaining full temperature. Furnace

cool 15–20°C/h to 650°C followed by free cooling in air.

Stress relieving550 – 650°C Holding time 2 h after attaining full temperature.

Furnace cool to 500°C followed by free cooling in air.

Hardening in oil830 – 875°C For large and/or complicated parts the quench should

be interrupted at 100-150°C with subsequent double tempering.

Surface hardening treatment This type of steel can be induction hardened and tempered

at 150-200°C to attain a surface hardness of 60-65 HRC.

Tempering100 – 500°C Tempering in the interval 250-350°C causes embrittlement

and should be avoided. Tools for blanking are normally tempered at 150-200°C. If better toughness is needed then a tempering temperature >350°C should be chosen.




Impact test

SS-EN ISO 148-1


Dimensionmm

Re

N/mm2 min

Rm

N/mm2A5%

min HB

KVmin 27 J

at °C

C 1.00 -02 Annealed bar All 340 ≈640 30 ≈190 Si 0.25 -06 Cold-finished bar 225-260 Mn 0.35 Cold-finished tube ≈8801) ≈9801) ≈101) ≈300 Cr 1.50

C 0.97 Annealed tube All 370 ≈670 27 ≈200 Si 0.30 Mn 0.20 Cr 1.80 Mo 0.35

C 0.97 Annealed bar All 390 ≈690 25 ≈210 Si 0.30 Mn 0.30 Cr 1.80 Mo 0.35

1)Approximate values depending on degree of reduction.

100

Cr6

(O

VA

KO

80

3)

100

CrM

o7

(OV

AK

O 8

24

)

100

CrM

o7-3

(O

VA

KO

825

)

36

HARD-CHROME BARSHard-chrome-plated bars do not represent a steel type but rather a special execution. The principal application is for piston rods in hydraulic and pneumatic cylinders. Induction-hardened hard-chrome bars characterised by a hardened zone beneath the chrome layer, which is resistant to both wear and impact, have proved usable even for service as pivot pins.

As standard, Tibnor stocks hard chrome-plated bar in grade 280X, which is a micro-alloyed, low-carbon construc-tional steel with an analysis corres - ponding to EN-SS S450J0 (SS 2142). However, the steel is optimised such that the yield and tensile strengths are about 20 % higher than is normal for EN-SS S450J0, and this improvement is achieved without compromise in regard to weldability or machinability. This higher strength provides an opportunity to downsize piston rods thereby enabling savings in weight and cost to be achieved.

Hard-chrome bar can also be supplied in an induction-hardened execution, in which case the steel base is Ovako 482, a micro-alloyed steel with medium carbon level. The surface hardness of the induction-hardened layer is minimum 55 HRC and the depth of hardening about 2 mm. This product is used for hydraulic applications where the piston rods run the risk of being damaged by impact or similar external factors.

Chromium metal has excellent corrosion properties, but nevertheless the corrosion resistance afforded by hard-chrome plating of steel is only

moderate. This is because the chromium layer is hard and has considerable internal stress which results in a network of fine micro-cracks. However, by good control of the plating process and suitable finishing, it is possible to achieve sufficient corrosion resistance for normal applications involving exposure to damp air or oxygenated water. If your application requires long-term exposure to a salty/marine or acidic environment, then it is a good idea to seek the advice of Tibnor for alternative products with better corrosion resistance.




Impact test

SS-EN ISO 148-1

Typical analysis %

Execution and tolerance

Dim- ension, D

mm

Re*

N/mm2 min

Rm

N/mm2A5%

min. HB

KVmin 27 J

at °C

C 0.18 Hard-chrome plated 10–18 520 650–800 12 200-240 Si 0.35 f7 20-90 520 600–800 19 200-240 -20 Mn 1.55 > 90-140 440 600–750 19 180-230 V 0.10 S 0.025

C 0.39 Hard-chrome plated 12-125 580 850-1 000 14 250-300 Si 0.40 induction-hardened f7 Mn 1.20 V 0.13 S 0.020


eH) for 280X, R

p0.2 for 482

Additional characteristics - Surface finish: Ra≤0.2μm, Rt≤2μm. Straightness: ≤0.1 mm/0.5 m for D<30 mm, ≤0.1 mm/m for larger diameters. Chrome layer: thickness minimum 20μm with minimum hardness 850 HV0.1.

Blue = Not stock standard

28

0X

(S

45

0J0

)

48

2

(38

Mn

VS

5)

38

FREE-MACHINING STEELSFree-machining steels as the name suggests are designed to be easy to machine. Very high cutting speeds are possible and the chips are short and easy to transport, which is a considerable advantage when working unmanned with CNC-equipment. The excellent machinability derives from additions of sulphur either on its own or in combination with lead.

Most of the dimensions in Tibnor’s stock programme of free-machining steels are in a cold-drawn execution with close tolerances (h9-h11) and smooth surfaces. Furthermore, the cold working contributes further to the excellent machinability. The good dimensional tolerances constitute an additional advantage when processing in automatic machines.

It should be remembered that surface defects from hot-rolling such as cracks, scratches and impressions

are not eliminated by cold drawing even though the depth of such will be decreased. The cold-finished surface should therefore not be left unworked if the application involves load variations and concomitant risk for fatigue. The standardised maximum allowable crack depth on cold-drawn rounds is 2 % of the diameter per side (see SS-EN 10277-1, class 1).

Another issue, which can arise if un-machined as-drawn material is to be surface treated, is so-called

”orange-peel surface”. If appearance is important, the surface should first be improved by fine machining or grinding prior to any surface-treatment operation.

Free-machining steels are optimised for machinability and this is achieved at the expense of other properties, especially ductility and toughness. This type of material should therefore only be used for components or constructions that in service are subjected to low loads.

39




Impact test

SS-EN ISO 148-1


Dimension*mm

Re

N/mm2 min

Rm

N/mm2A5%

min HB

KVmin 27 J

at °C

C <0.14 -04 Cold-finished 5-10 440 510-810 6 150-250 Si 0.05 drawn >10-16 410 490-760 7 150-220 Mn 1.10 >16-40 370 460-710 8 140-240 S 0.30 >40-63 300 400-650 9 130-230 Pb 0.25 >63-100 245 360-630 9 120-220 Peeled 80-140 240 360-520 10 <170

C 0.43 Cold-finished - 16 510 710-860 6 >210 Si 0.10 drawn >16-40 460 650-800 7 >185 Mn 1.40 >40-90 390 620-780 8 >180 S 0.26 Pb 0.25

C 0.36 Cold-finished 5-10 500 660-960 6 210-270 Si 0.20 drawn >10-16 440 620-920 6 200-260 Mn 0.90 S 0.20 Pb 0.25

C <0.20 Cold-finished 20-55 500 550-750 12 ≈ 200 +20 Si <0.55 drawn Mn <1.60 V 0.09 S 0.13- 0.17

C <0.20 Hot-rolled 25-70 380 490-630 22 150-200 -20 Si <0.55 >70-90 350 490-630 20 150-200 -20 Mn <1.60 >90-180 350 490-630 20 150-200 0 V 0.09 S 0.13- 0.17

11S

Mn

Pb

30

+C

(S

S 19

14)

Can

be c

ase

-h

ard

en

ed

MA

CH

50

Can

be

qu

en

ch

ed

-an

d-

tem

pere

d

or

ind

ucti

on

h

ard

en

ed

36

SM

nP

b14

+C

(SS

1957+

Pb

-04

)

55

0M

W+

520

MW

+

* D, T, B or N for rounds, squares, flats and hexagons respectively.

40

M-STEELS FOR MACHINABILITYMachining represents a large portion of the total cost of manufacturing a component, often up to 50 %. As a complement to the programme of free-achining steels, Tibnor has therefore made the decision to stock all grades of quenched-and-tempered steels and case-hardening steels in M-execution. The so-called M-treatment confers improved machinability without significant negative effect for other properties. For a given degree of tool wear, M-treated steels can be machined at 20-30 % greater cutting speed; conversely, machining with the same data as for equivalent conventional steels results in up to four times longer tool life.

The improved machinability derives from the special nature and shape of non-metallic inclusions in the steel. Instead of hard aluminium oxides, which cause excessive tool wear, the characteristic inclusions in M-steels consist of calcium aluminates surrounded by a skull of calcium sulphide. This type of inclusion is relatively soft and does not wear

tooling to the same degree while at the same time a lubricating film containing calcium and sulphur is established between the tool and the chips.

The manufacture of M-steels necessitates careful control if the improvement of machinability is to be achieved consistently from heat to heat. Calcium boils at a relatively low

temperature and will be lost from the melt unless the addition is made in a proper way. If too little calcium ends up in the steel, the inclusions that are formed do not have the correct character and the positive effect for machinability is impaired or is maybe even absent completely.

41

The lubrication effect from M-steels:M-treatment eliminates hard, abrasive inclusions at the same time as a lubricating film is created on the cutting edge at high machining speeds.

High-speed machining of steel which has not undergone M-treatment results in significant flank wear after a short time. For the same steel which has been M-treated, the point at which rapid wear occurs is shifted to longer times. Crater wear is close to zero thanks to the lubricating film which develops at the cutting edge.

The positive effect of M-treatment is greatest when the cutting speed, and thereby the cutting-edge temperature is high. The M-effect is therefore very pronounced when machining with coated carbide tooling but far less for cutting at the lower speeds with non-coated high-speed steel (HSS) tools. The improvement in machinability is also significant for cutting with cermets and certain types of ceramic inserts. The table summarises the effect of M-treatment for machining with various types of cutting tool.

Standard steel

Flank wear

Normal crater wear

Chip Chip

Lubricating film

Flank wear

M-steel

Flank wear, mm

Crater wear, mm

Turning parameters: depth of cut = 2.5 mm, feed = 0.4 mm/rev., cutting speed = 450 m/minInsert: GC 415 P15

0

0.2

0.4

0.6

5 10 15 20 25 30Contact time (min)

Wear criterion VB=0.3 mm

Standard steel

0

M-steel

Compatibility between various types of cutting tools and M-steels

Tool material Conditions Effect of M-treatment

HSS uncoated High speeds Quite goodHSS uncoated Low speeds Less goodHSS TiN-coated High speeds Quite goodHSS TiN-coated Low speeds GoodCarbide P10 uncoated High stability ExcellentCarbide P20 uncoated Normal GoodCarbide P30 uncoated Normal Less goodCarbide-coated TiC-Al

2O

3-TiN Normal Excellent

Carbide-coated Al2O

3 Normal Good

Cermets (all) Fine machining ExcellentMixed ceramic Al

2O

3+TiC High stability Excellent

Ceramic Al2O

3 High stability Less good

05 10 15 20 25 30

Contact time (min)0

0.2

0.4

Wear criterion KT=0.18 mm

Standard steel

M-steel

Photo: Struers

42

HARDNESSThe term hardness defines the resistance offered by the steel (or any other material) to indentation by an external force. Hence, we measure hardness by pressing a ball or tip with a predetermined load into the surface of the steel. From the size of the resulting impression in terms of its area or depth, the hardness can be assessed; a soft material gives a large/deep impression and a hard material a small/shallow one.

In what follows, you find a description of the three most common methods to measure the hardness of steel.

Brinell (HB)The indenter is a ball of hardened steel or cemented carbide (D= 10 mm) and the force used to make the impression is usually 3 000 kgf (3 tons). This method is mostly used for soft/medium-hard steels.

Rockwell C (HRC)This test, in which the indenter is a conical-shaped diamond, is most often used for hard steels. Its main advantage is speed since the hardness is read off directly from a scale on the hardness tester. Rockwell C hardness measurement requires quite careful sample preparation by grinding or polishing.

Vickers (HV)The Vickers indenter is a pyramid-shaped diamond and the test can be used over the entire hardness spectrum. The load can be adjusted between 0.1 and 30 kgf. Hence, when reporting Vickers hardness, the load should also be specified, for example HV1 or HV10. When determining hardness profiles on surface-hardened parts treated by case hardening, nitriding etc., it is best if the hardness impressions are not too large and 1 kgf is a suitable load for such measurements.

All three methods have their specific advantages and limitations. Since by alloying and heat treatment, the hardness of steel can vary over a very wide range (from soft and formable to hard and wear resistant), one test or the other is used depending on steel grade, heat-treat condition and test-piece geometry.

HB- or HV-values are calculated by dividing the applied load by the area of the resulting impression and the

hardness in these instances is reported as kgf/mm2. This means that for a given steel, the numerical values for HB and HV are quite similar (the deviation is about 5 %). On the other hand, HRC is based on the depth of the impression and is very approx-imately a tenth of the values for HB and HV. Furthermore, since Rockwell impressions are rather deep, the test is not really suitable for thin parts.

For unalloyed and low-alloy steels, there exists quite a close correlation between hardness and tensile strength. If the HB-value for a certain steel is divided by 3 and the result multiplied by 10, the answer is surprisingly close

to the tensile strength in N/mm2. This relationship is useful if one requires an estimate for tensile strength (after heat treatment, for example) but only has access to a hardness tester. The correlation between tensile strength and hardness works somewhat better for HB than for HV.

In the table below, a comparison is given between hardness values obtained using the different methods and tensile strength. An exact conversion is not possible and the values should be regarded as approximate only. The table is identical to the one given in SS-EN ISO 18265.

43

Correlation between various hardness values and tensile strength

HV10 HRC HB Rm, N/mm2 HV10 HRC HB R

m, N/mm2

150 143 480 290 28.5 276 920 160 152 510 300 29.8 285 950 170 162 540 310 31.0 295 990 180 171 570 320 32.2 304 1020 190 181 600 330 33.3 314 1050 200 190 635 340 34.4 323 1080 210 200 670 350 35.5 333 1115 220 209 695 370 37.7 352 1175 230 219 725 400 40.8 380 1275 240 228 755 420 42.7 399 1345 250 22.2 238 785 450 45.3 423 1440 260 24.0 247 825 470 46.9 442 1500 270 25.6 257 855 500 49.1 466 1610 280 27.1 266 880 550 52.3 509 1805

44

WELDINGThe higher the carbon and/or alloy content of a steel, the less suitable it is to be welded. In other words, steels with higher strength and hardness (wear resistance) are more difficult to weld. It is preferable that the carbon level is below 0.25 % and that the sulphur content is also low if a component requires welding as a step in its manufacture.

A simple way of quantifying weldability for constructional steels, carbon steels and low-alloy steels is through the carbon equivalent value (CEV):

CEV = C+Mn+(Cr+Mo+V)+(Cu+Ni) 6 5 15

(symbols relate to content in weight %)

A steel with low CEV is easy to weld and vice versa.

An element, which is very negative for weldability but which does not appear in the carbon-equivalent formula, is sulphur, for which does not appear in the weld metal can give rise to hot cracking. The negative influence of sulphur is limited when also manganese is present since sulphur is then bound as manganese sulphide (MnS). For this reason, the ratio Mn:S should be at least 10:1 in steels that are to be welded. Free- machining steels containing sulphur are not at all suitable for welding, even grades where the content of carbon and other alloying elements is low.

Weldability can be improved to some degree by prior pre-heating. This has the effect of lowering the cooling rate after welding which counteracts the formation of brittle microstructures in the heat-affected zone of the weld. The slower cooling also contributes to elimination of any eventual hydrogen in the weld, thereby reducing the risk for cold cracking as a result of hydrogen embrittlement. Steels with CEV

> 0.55 % should always be pre-heated prior to welding. Furthermore and irrespective of CEV, it is always a good idea to pre-heat whenever parts with larger cross-sections are to be welded.

Standard constructional steels and micro-alloyed steels with CEV <0.55 % are most suitable for constructions or components where welding is required. These grades can in most cases be welded without pre-heating and do not normally require any post-weld treatment so long as the section is not too large. If higher strength is needed, then EN-SS 25CrMo4 (SS 2225), which has quite good weldability especially in smaller dimensions, can be considered.

Even higher-strength steels with more carbon and appreciable alloy content can be welded successfully as long as correct procedures are followed and suitable consumables selected. Pre-heating is an absolute requirement and the heat-input should be limited by building up the joint with a large number of smaller beads, all to lessen the risk for occurrence of brittle regions in the weld metal and heat-affected zone. If quenched-and-tempered steels with higher carbon content need to be welded, then a second full hardening and tempering heat treatment of the finish-welded part is to be recommended, at least for critical cases. A second heat treatment carries the additional benefit that residual stresses are reduced and that

any hydrogen which has been introduced is expelled. The latter is important since higher-strength steels are more sensitive for hydrogen embrittlement.

With the exception of free-machining steels, grades which are stocked in a cold-drawn execution can be welded without problem. However, one should be aware that the strength level and hardness can decrease somewhat in the heat-affected zone.

MAG welding with shielding gas and wire consumables gives better control and less risk for contamination by hydrogen. For MMA/SMA-welding, basic electrodes are to be preferred and it is important that these are properly dried so that ingress of hydrogen is mitigated to as great a degree as possible. If you are uncertain as to details concerning welding procedure, it is advisable to seek the advice of either your Tibnor represen-tative or your supplier of welding equipment and consumables. This applies particularly whenever higher-strength steels with elevated contents of carbon and alloying elements are to be welded.

The following table lists suitable consumables (ESAB-designations) and gives some general recommend-ations for welding of some of the eng-ineering steel grades in Tibnor’s stock programme.

45

MAG welding MMA/SMA welding

Grade Wire/shielding gas* Electrode Pre-heat? Other remarks

S235 12.51/M21 OK48.00 Not normally Femax 33.XX Femax 33.XX required are rutile electrodes (hydrogen uptake can give problems)

S355/280/520M/550M 12.51/M21 OK 48.00, OK 55.00 > 150°C for larger See above 12.64/M21 Femax 38.65 dimensions

C45E 12.64/M21 OK 74.78 > 200°C

25CrMoS4 (SS 2225) 13.12/M21 OK 74.78, OK 78.16 > 150°C In critical cases, 13.29/M21 unless parts are hardening and very small tempering after welding may be necessary

16NiCrS4 (SS 2511) 13.12/M21 OK 74.78 > 150°C Compatible with 13.29/M21 unless parts are base steel only very small

42CrMoS4 (SS 2244) 13.12/M21

OK 75.75, OK 76.18 > 300°C In critical cases, 13.29/M21 hardening and tempering after welding may be necessary *M21 = 80 % Ar, 20 % CO

2

46

COLD FORMINGThe cold formability of a material relates to the degree to which it can be worked without cracking in a cold-forming operation, like bending for example. The cold formability of steel is coupled to its ductility. Hence, high-strength steels are generally more difficult to cold form since ductility is reduced as carbon content and strength level are raised (see diagram below). The ductility of steels can be increased by reducing the carbon content to very low levels (< 0.01 %) but this kind of material, usually in the form of thin strip, is used for press forming and deep drawing. For the grades and executions which are the focus of our attention in this guide, relevant cold-forming operations are bending, cold heading or cold forging.

Among the steel types that are discussed in this guide, standard constructional and micro-alloyed steels show best cold formability. Generally speaking, the ductility of this type of material is improved by normalising, which might be worth considering if the manufacture of a part necessitates extensive cold forming.

Non-metallic inclusions have a strong negative influence on ductility since they act as initiation points for cracks. Hence, a clean steel has better cold formability than one containing large amounts of inclusions. Basically all types of inclusions are deleterious for ductility, even those that are deliberately present as in free-machining steels and M-steels.

Non-metallic inclusions bear an influence on cold formability in a second way. During hot-rolling, the inclusions are elongated in the

longitudinal direction of a bar (or tube) and give rise to so-called fibre. One consequence of this is that the ductility is considerably less in a direction transverse to the bar axis than parallel to it. This difference is of significance in bending where tensile stresses are generated on the outside of the bend. Hence, for flat bars, the bendability is quite a lot better if the bend axis is at right angles to the length direction of the bar rather than parallel to it.

The typical inclusions in M-steels are not elongated during hot-rolling to the same degree as other inclusion types, such as manganese sulphides and silicates. Hence, the difference in ductility between longitudinal and transverse directions is less pronounced for this type of steel treated with silicon-calcium than for other steels containing large amounts of inclusions for the purpose of

improving machinability. Even so, cold formability both parallel to and transverse the rolling direction is always best if the steel is clean and the number of inclusions is reduced to a minimum.

Cold heading and cold forging are forming methods which for the most part involve loading in compression. Under such circumstances, cold formability is normally much better than in processes where tensile stresses are generated, such as bending. However, even in cold heading and cold forging, tensile stresses can arise at free surfaces as a result of frictional forces between the work material and the tool. Such stresses are generated at right angles to the axis of compression and can give rise to cracking if they act in a direction corresponding to the transverse direction of the original rolled bar.

The dependence of ductility (fracture elongation, A5

, in longitudinal direction) on tensile strength for a number of different steel types.

0 2 0001 5001 000

Hot-rolled

Normalised

Constructional and micro-alloyed steels

Quenched-and-tempered steels

Very low carbon steels

Spring steels

500

60

30

0

Fracture elongation, %

Tensile strength, N/mm2

47

FATIGUE AND HOW THE RISK FOR FATIGUE FAILURE CAN BE LESSENEDFatigue is a damage process occurring under conditions of variable loading and which is characterised by the initiation and growth of cracks at stress levels considerably less than the tensile strength of a material. A rough estimate is that 80-90 % of all failures of machine parts and constructions can be put down to fatigue. In many instances, it is possible to avoid fatigue failure by giving some thought to suitable shape and/or surface finish of a component.

A part that has failed due to fatigue will have a fracture surface with a very characteristic appearance. It is normally quite easy to see where the failure has started, most often but not always at an external surface. The crack-growth phase is characterised by convex so-called striations concentric with the starting point; this part of the fracture surface is rather flat. When the fatigue crack has grown to a critical length, the final failure of the part takes place and the fracture surface is then considerably more irregular.

The fatigue properties for steel are normally presented as a so-called Wöhler diagram (see the example below) in which the stress amplitude (half the difference between the greatest and least stress in a load cycle) is plotted against the number of cycles to failure. The number of cycles that the material can resist before it breaks decreases as the stress amplitude increases.

Steel is rather unique among common metallic materials in that it exhibits a fatigue limit, i.e. fatigue failure does not occur if the stress amplitude remains below a certain level. For the data shown in the Wöhler diagram below, the fatigue limit is about 380 N/mm2. Hence, when one refers to the fatigue strength of a certain steel, it is normally the level of fatigue limit that is implied.

The fatigue strength of a steel component depends upon a number of factors. The most important ones are listed below. – Fatigue strength increases with

increasing hardness and tensile strength.

– Steel cleanliness. Non-metallic inclusions, and especially very hard inclusions, have a negative effect on fatigue resistance.

– Surface finish. A polished surface shows much better fatigue strength than, for example, one with a rough, hot-rolled finish.

- Stress concentration effects associated with sharp corners, sudden changes in section, fillets etc. are very negative in relation to fatigue resistance.

– Residual stresses affect fatigue resistance, negatively if such are tensile (in the vicinity of welds for example) and in a positive sense if the stresses are compressive in nature. Favourable compressive residual stresses can be generated either by cold working (shot peening or roller burnishing) or through heat treatment (case hardening, induction hardening, nitriding etc.).

In addition, fatigue strength is influenced by the character of the loading that a part might be subjected to, for example bending, pushing/pulling or twisting. The type of loading is defined by the mean stress which is the average of the highest and lowest values in a load cycle. As an example, consider a rotating axle subjected to a constant

Wöhler diagram for steel Ovako 482 (38MnV5 – medium-carbon, micro-alloyed steel) as determined by rotating-bend testing

1 00010010 10 000 100 000

900

500

100

Not broken

Stress amplitude, N/mm2

No. of cycles to failure, thousands (logarithmic scale)

48

load (rotating bending). A given point on the surface will experience alternating tensile and compressive stresses with the same magnitude but opposite signs; the mean stress is therefore zero. The influence of mean stress on fatigue resistance can be summarised as follows:- Fatigue strength is lowered from the

level for mean stress zero if the load cycle is dominated by tensile stresses (mean stress positive).

- The fatigue strength will be increased from the level for mean stress zero if the load cycle is predominantly compressive in nature (mean stress negative). In fact, fatigue does not occur at all if a component or construction is subjected to only compressive stresses.

Load cycles consisting only of tensile stresses, so-called pulsating tension, are the most dangerous ones.

Corrosion and corrosive environ-ments are extremely deleterious in relation to fatigue. Furthermore, the characteristic fatigue limit for steel no longer exists under corrosive conditions and failure can occur even at very low levels of loading. Unfortunately, certain surface treatment methods aimed to protect against corrosion, plating with chrome or nickel for example, also impair fatigue resistance.

Welds are particularly dangerous from a fatigue standpoint. There are two reasons for this; on the one hand, typical weld defects such as large inclusions, cracks, pores, lack of penetration etc. can function as starting points for failure, and secondly, welds are inevitably associated with unfavourable tensile residual stresses.

The negative effect on fatigue strength derived from stress raisers, surface finish and corrosive environ-ment become progressively more pronounced as the tensile strength increases. In other words, high-strength steels are more sensitive to the said effects. The diagram below shows the degree to which the fatigue strength is reduced as a function of tensile strength.

The fatigue strength (fatigue limit) obtained in bending of polished samples of low-carbon constructional steels and quenched-and-tempered steels with R

m < 1200 N/mm2 can be

roughly approximated to half the tensile strength. For loading in pulsating tension with positive mean stress and minimum stress zero, the fatigue limit is lowered to 35-40 % of the tensile strength.

Measures whereby the risk for fatigue failure of a part can be reduced if not completely eliminated are:

1. Select a material with better fatigue strength.

2. Change the design of the component so that stress raisers are avoided or at least their effects lessened.

3. Improve the surface finish.4. Treat so as to generate compressive

residual stresses in the surface.5. If welding is necessary, give

consideration to the location of welds.

Difficulties in defining the load variations in a given application combined with lack of material data for fatigue strength means that it can sometimes be problematic to design and dimension a component so that the risk for fatigue failure is minimised. For safety-critical parts, the only alternative is to measure service load variations and to test finished components subjected to realistic load spectra. Of course, this type of testing is expensive. For less critical applications, the risk for failure can be lessened by adhering to some or all of the measures listed above and which now will be discussed in more detail.

1. Select a material with better fatigue strength

The fatigue resistance of steels increases with increasing tensile strength (hardness). The diagram on the next page shows the approximate relationship between tensile strength and fatigue limit for bend loading.

For tensile strengths < 1200 N/mm2, the fatigue limit is about 50 % of the tensile strength. However, this percentage decreases successively as the tensile strength is increased.

As has already been pointed out, non-metallic inclusions are negative in relation to fatigue resistance and steels used for components which are subjected to high levels of fluctuating

Illustrating the reduction in fatigue strength derived from different surface finishes, a stress raiser (notch) and corrosive environment.

(Source: K B Lundqvist ”Strength of Materials”, Albert Bonniers Förlag (1959), p.44 – in Swedish).

% decrease of fatigue strength in relation to polished

1 0005000 1 500 2 000

Salt-water environment

Fresh-water environment

Hot rolled

Ring-shaped V-notch

Rough machined

Fine machined

Ground

Polished100%

50

75

25

0

Tensile strength (Rm) N/mm2

49

load should be clean, i.e. contain only a small amount of non-metallic inclusions. The reason is that inclusions can function as starting points for fatigue and as the tensile strength increases, the critical inclusion size to initiate fatigue is reduced. Steel cleanliness is therefore of particular importance for high-strength steels which are usually manufactured using special refining procedures coupled with vacuum treatment. The oxygen content of the material is by so doing reduced along with the amounts of hard (and consequently dangerous) oxide inclusions.

In some steel types, such as free-machining steels and M-steels, additions such as sulphur and calcium are made in order to deliberately promote certain types of inclusion with the aim of improving machinability. Such inclusions can affect fatigue resistance negatively and the use of free-machining steels for parts which are subject to appreciable load variations is not to be recommended. As regards M-steels, the characteristic inclusions (calcium aluminates with a skull of calcium sulphide) exert only a minor negative influence on fatigue resistance so long as the hardness is below 350HB and the inclusions remain relatively small. At higher hardness levels, M-treatment reduces fatigue strength.

2. Limit as far as possible stress raisers and notch effects

Stress concentrations due to changes in section, holes and sharp corners give rise to a considerable decrease in fatigue strength. Hence, for best fatigue resistance, it is essential that:

- The transition from one section to another is gradual.

- Corners have generous as opposed to very sharp radii.

- Fillets are eliminated as far as possible.

Notch effects are sometimes unavoid-able, in components with threads for example. There is then no choice other than to select a steel with suitable strength, assess the notch effect of the thread (can be found in standard tables) and dimension accordingly depending on the loading to which the part is subjected. It is worth noting that rolled threads exhibit far better fatigue strength than ones which have been machined.

3. Improve surface finishComponents where there is a risk for fatigue failure must be manufactured with care. The finer the surface finish, the better is the fatigue resistance, and the sensitivity for sub-standard surface finish increases with the level of tensile strength. It is not by chance that balls, rings and rollers in bearings, with a hardness of 60-62 HRC, are fine ground or polished.

4. Introduce favourable residual stresses

Fatigue cracks start and grow only during the tensile part of a load cycle. This means that compressive residual stresses are favourable for fatigue strength since they counteract tensile stresses felt as a result of the service loading. Surface compressive stresses can be created by cold working, for example shot peening or roller

burnishing, or through suitable heat treatment. All surface heat-treatment methods, case hardening, nitriding, induction hardening etc, give rise to an increase of volume in a surface layer which is counteracted by the underlying material. In this way, compressive stresses are introduced which are very positive from the point of view of fatigue.

The improvement of fatigue strength resulting from the generation of surface compressive stresses is greatest for loading modes in which the highest stress is attained at the surface, e.g. bending or twisting. In axial push-pull loading, when the stress is relatively constant over the cross section of a component, surface compressive stresses have far less effect or even no effect at all; this is irrespective whether the stresses are derived from cold working or heat treatment.

5. Avoid welds or at least consider where they are placed

As already pointed out, welds are very negative in respect of fatigue resistance and it is preferable that welding is avoided in machine applications where there is risk for fatigue failure. If this is not possible, then one should bear in mind the following:- Welds should be located where

loads (stresses) are lowest.- Fatigue strength can and should be

improved via careful weld finishing. Suitable methods are grinding of the weld bead or TIG-remelting of the weld metal and the transition zone with the base material. In both instances, the aim is to reduce the number of welding defects which potentially can initiate fatigue cracks. However, these finishing procedures can be time-consuming and therefore costly. A cheaper finishing method, which nevertheless gives some improvement of fatigue life, is cold hammering of the weld bead.

- Machine components with welding as a manufacturing step should be stress relieved in order to reduce the level of tensile residual stress.

Variation of fatigue limit in rotating bending as a function of tensile strength for various types of steel.

0 2 5002 0001 5001 000

Carbon steelsConstructional steels

Quenched-and-tempered steels

Spring steels

Bearing steels

500

900

500

700

100

300

Fatigue limit, N/mm2

Tensile strength, N/mm2

50

REDUCING WEIGHT OF COMPONENTS AND CONSTRUCTIONS Engineers and designers have strived towards lighter constructions for many years but the driving force for weight reduction has intensified in recent times, especially in the motor-vehicle industry. It has to some degree proved possible to replace steel with lighter materials like aluminium or plastics but for parts which are heavily loaded, there is really no economical alternative to steel. The strength range which is achievable with steel is so broad that it is in principle always possible to swap ”ordinary” steels, such as S355JR, for a higher-strength material allowing the dimensions of the part to be reduced, thereby saving weight. Unfortunately, it is seldom so simple and what is feasible depends on which properties have been used as the basis for dimensioning of the part at the design stage.

In many instances, the principal requirement for a component or construction is that it is elastically stiff; in other words, bending, twisting, axial elongation or axial compression can only be tolerated to a limited degree.

All steels have more or less the same elastic modulus, so it is not possible to reduce dimensions without a greater elastic shape change, i.e. stiffness is reduced. But sometimes, it is possible to decrease weight without compromising stiffness. Consider for example, a simply supported tube with dimensions 50/30 mm, length 1 m which is subjected to a load of 1.5 tons at its middle. The maximum deflection is about 5 mm independent of steel grade so S355JR (SS 2172) is a sound economic choice. However, if instead we change to a tube with dimensions 70/65 mm, with everything else unaltered, the deflection is still about 5 mm but it is no longer possible to use S355JR since the stress at the outside of the tube would then

exceed the yield strength of this material. On the other hand, the yield strength of the micro-alloyed grade E470 (Ovako 280) is greater than the largest bending stress so this material constitutes a feasible alternative. The difference now is that the tube weighs only 4.2 kg compared with 9.9 kg for the 50/30-tube, a weight saving of almost 60 %!

If instead the service requirement is that no permanent shape changes can be allowed, i.e. plastic deformation is not tolerated, then a weight saving will always be possible by changing to a steel with higher strength. As an example, let us look at twisting (torsion) of a solid bar (see sketch). An axle in S355JR (SS2172) with diameter 50 mm and length 1 m can be subjected to a twisting moment of up to approx-imately 9kN.m before the outside of the axle starts to deform plastically (9 kN.m corresponds to transmission of a power of 100 kW at 100 rpm). However, the same power can be transmitted by a 40 mm axle if the

steel is changed to the quenched-and-tempered grade 42CrMoS4 (SS 2244) giving a weight saving of more than 35 %. However, it is noteworthy for this particular example, that the angle of twist when plastic deformation starts is only 10° for S355JR but it is 26° for the steel with higher yield strength. This greater elastic twisting must be tolerated in order that the full weight saving can be enjoyed.

For components or constructions subjected in service to variable loading and thereby risk for fatigue failure, weight savings can almost always be achieved by switching to a steel with higher strength/hardness. After all fatigue resistance increases with tensile strength. But, as we have already seen in the section dealing with fatigue, such a change requires some prior consideration. The influence of surface finish, stress raisers (e.g. changes in section etc.) and welds is far more prevalent when the strength level of the base steel is raised. Hence, if the aim is to save

51

weight by reducing the dimensions of a component susceptible to fatigue, then one is perhaps forced to improve surface finish, optimise the design in relation to stress raisers and in some instances, eliminate or re-locate welds.

In loading modes where the greatest stress occurs at the surface of a part, for example bending or twisting, surface treatments such as case hardening, induction hardening, nitriding etc., which result in an increase of strength/hardness of the outer layer, will allow dimensions to be reduced. One must, however, take into account that the hardened surface is

more brittle than the base steel and therefore cannot deform plastically without cracking. It is less well documented that buckling resistance when long, thin parts are loaded axially is improved considerably by surface hardening and especially induction hardening. Since the buckling load is often well below that corres-ponding to the yield strength of the material, an appreciable dimensional reduction will be achievable if the buckling resistance can be increased. This possibility is of special interest for high-strength, micro-alloyed steels since the induction-hardened layer

then has sufficient toughness that the part will bend rather than fracture when the buckling load is exceeded.

The examples given above are certainly not the only ones, but serve as an indication of what is possible by applying a little thought. Higher-strength steels are admittedly more expensive but allow a weight reduction which in certain applications may well motivate the extra cost. Furthermore, the higher price per kilo is often compensated by the fact that the purchased weight per component is reduced.

Bending of tube with free ends

Twisting (torsion) of round bar A––>A’

A

A’

52

CONSTRUCTIONAL STEELS

SS SS-EN 10025-2:2004 TIBNOR DIN W.Nr AISI/SAE AFNOR

1312 S235JR 1312/S235JR St 37-2 1.0116 A570 Gr.36 E 24-2 2172 S355JR S355J2 S355JR 1.0045 1518 A 50-2

MICRO-ALLOYED STEELS

SS SS-EN 10025-2:2004 TIBNOR DIN W.Nr AISI/SAE AFNOR

2142 S450J0 280/280X 20MnVS6 1.5217 20MV6 2144-01 S355J2 520M/520MW+ St 52-3 1.0045 2144 S355J2+C 550MW+ St 52-3 1.0045

CASE-HARDENING STEELS

SS SS-EN 10084 TIBNOR DIN W.Nr AISI/SAE AFNOR

2127 16MnCrS5 16MnCr5 1.7131 2506 21NiCrMo2 21NiCrMo2 1.6523 8620 15NCD2 2511 16NiCrS4 2511M/16NiCrS4 15CrNi6 1.5919 3115 16NC5 2523 17NiCrMoS6-4 17CrNiMo6 1.6587 18NCD6

FREE-MACHINING STEELS

SS SS-EN 10277-3 TIBNOR DIN W.Nr AISI/SAE AFNOR

2144-01 S355J2 520MW+ 2144 S355J2+C 550MW+ 1914 11SMnPb30 1914 95MnPb28 1.0718 12L14

1957+Pb-04 36SMnPb14 1957 1.0765

BEARING STEELS

SS SS-EN ISO 683-17 TIBNOR DIN W.Nr AISI/SAE AFNOR

2258 100Cr6 803/2258 100Cr6 1.3505 52100 100C6 100CrMo7 824 100CrMo7 1.3537 A4853 100CD7 100CrMo7-3 825 100CrMo7-3 1.3536

HOT-FINISHED SEAMLESS TUBES


2142 E470 E470 20MnVS6 1.8905 20MV6 2142 E470 280 20MnVS6 1.8905

SPRING STEELS


2090 56Si7 2090 55Si7 1.5026 9255 55S7 2230 51CrVS4 2230 51CrV4 1.8159 6150 50CV4

QUENCHED-AND-TEMPERED STEELS


1672 C45E 1672/C45E 2C45 1.1191 1045 XC45 1672M C45R 1672M/C45R 2225 25CrMo4 2225/25CrMo4 25CrMo4 1.7218 4130 25CD4 2225M 25CrMoS4 2225M/25CrMoS4 2234 34CrMo4 34CrMo4 1.7220 4135 35CD4 2244 42CrMo4 2244/42CrMo4 42CrMo4 1.7225 4140 42CD4 2244M 42CrMoS4 2244M/42CrMoS4 42CrMoS4 2541 34CrNiMo6 2541M/34CrNiMo6 34CrNiMo6 1.6582 4340 35NCD6

STANDARDS FOR STEEL GRADESBelow you find a table which compares SS- (discontinued Swedish standards) and the new SS-EN standards with

Tibnor’s internal designations and other international standards. There is seldom exact correspondence between

different standards so the comparisons given should rather be regarded as “the closest equivalent standard”.

53

SS 2244BROWN

16NiCrS4GREYISH BLUE

OVAKO803,824,825

RED

SS 2541MAROON

S355J2BLACK

520 MGREENISH YELLOW

OVAKO 280ORANGE

C45GREEN

550MW+PINK

E470DARK BLUE

MACH 50SILVER

SS 1914BLUE

Free-machining steels

520MW+BLACK/

GREENISH YELLOW

The colour coding for an SS-standard is the same as for the equivalent in SS-EN.

SS 2225OLIVE GREEN

SS 2230BROWN/YELLOW

COMP. AXLEWHITE

Constructional steels, case-hardening steels, quenched-and-tempered steels and bearing steels

COLOUR CODING

SS 2090RED/GREEN

54

CERTIFICATIONSS-EN 10204 is the standard which specifies certification of metallic materials. In this section, we give a short description of the contents and limitations of the various types of inspection documents which are defined in this standard.

Declaration of compliance with order 2.1In this, the manufacturer certifies that the goods supplied are in accordance with the requirements of the order but no specific test results are given.

Test report 2.2This is a declaration that the goods supplied are in accordance with the order but which also includes non-specific test results from similar material.

Inspection certificate 3.1This is a document issued by the manufacturer which not only declares that the goods supplied are in accordance with the order but also includes specific test results on the material actually delivered. A 3.1 certificate must be validated by a manufacturer’s inspection representative that is independent of any production department.

Inspection certificate 3.2This type of certificate is issued jointly by the manufacturer’s inspection representative together with an inspection representative authorised by the purchaser. These two instances make a mutual declaration that the goods supplied are in accordance with the order and also validate specific test results on the material actually delivered. Such so-called third-party certification normally involves an inspector from a specified company or authority either being present during testing or actually performing the testing stipulated by the purchaser.

What information can be found in these different certificate types?

Test report 2.2This type of report will normally give the typical analysis along with minimum guaranteed mechanical properties.

Inspection certificate 3.1Quenched-and-tempered steelsChemical analysis, yield stress, tensile stress, elongation to fracture, area reduction at fracture and impact toughness.

Case-hardening steelsChemical analysis, hardness and Jominy-values.

Constructional steelsChemical analysis, yield stress, tensile stress, elongation to fracture, area reduction at fracture (optional) and impact toughness.

Inspection certificate 3.2The test data to be reported in this type of certificate are regulated by the purchase order or agreement. In other words, the purchaser shall define the testing to be done and the manufacturer must confirm that such

testing can indeed be carried out. The purchaser shall also specify the company or authority that is to constitute the third party.

Unless agreed otherwise in advance, mechanical testing data reported on 3.1 or 3.2 certificates relate to samples taken at a standardised location in, for example, a bar.

Quenched-and-tempered steelsFor dimensions D > 25 mm, the test sample is taken such that its centre is at least 12.5 mm from the bar surface. For D ≤ 25 mm, the centre of the test sample should coincide with the centre of the bar.

Constructional steelsFor dimensions D, B or T > 25 mm, the tensile-test sample is taken such that its centre is at least one third of the distance between the surface and centre of the bar. If D, B or T ≤ 25 mm, the centre of the tensile-test sample should coincide with the centre of the bar. As regards the sampling location for testing of impact toughness, the one side of the sample should be at least 2 mm from the surface of the bar.

Important to note!From the above, it will be clear that the sampling location is in many instances close to the bar surface. For heavy-section bars, this means that the mechanical properties at the centre can differ quite considerably from the values certified. For the quenched and tempered grades SS-EN 25CrMo4 and SS-EN 42CrMo4, the maximum dimension defined in the appropriate standard (SS-EN 10083) is 160 mm. This is because, for these steels with relatively low alloy content, the mechanical properties at the bar centre would for larger diameters deviate excessively from those certified. The grade SS-EN 34CrNiMo6, which is more highly alloyed, is standardised up to larger dimensions, D=250 mm.

56

CONTACT US

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Tibnor is the steel and metal supplier to the Nordic region. We assure the flow of materials to industrial users in the Nordic and also the Baltic countries.

We work strategically and long-term together with both customers and suppliers in order to develop the best material, logistic, production and administrative solutions. We are therefore an essential link for

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