atlas technical handbook stainless steel

The

Atlas Steels

Technical Handbook

of

Stainless Steels

Copyright Atlas SteelsRevised : July 2010

Produced byAtlas Steels

Technical Department

ATLAS STEELS Technical Handbook of Stainless Steels

Page 2www.atlassteels.com.au

FOREWORDThis Technical Handbook has been produced as an aid to all personnel of Atlas Steels, their customers and theengineering community generally. It is intended to be both background reading for technical seminars conductedby Atlas Steels Technical Department, and also as a source of ongoing reference data. Any suggestions forimprovements, additions or corrections would be very welcome; these should be directed to:

Technical Manager, Atlas SteelsTelephone +61 3 9272 9999, E-mail [email protected]

Copies of this handbook can be downloaded from the Atlas Steels web site at www.atlassteels.com.au.

Details of specific products are given in the Atlas Steels Product Reference Manual, the series of Atlas GradeData Sheets and in Atlas Technotes, as listed below; copies of all these can be viewed or downloaded from theAtlas Steels Website.

Atlas Steels Technotes1. Qualitative Sorting Tests for Steels2. Pitting & Crevice Corrosion of Stainless Steels3. Stainless Steels - Properties & Equivalent Grades4. Machining of Stainless Steels5. Cleaning, Care & Maintenance of Stainless Steels6. Life Cycle Costing7. Galvanic Corrosion8. "L", "H" and Standard Grades of Stainless Steels9. Stainless Steel Tube for the Food Industry10. Restrictions of Hazardous Substances (RoHS)11. Magnetic Response of Stainless Steels12. Pipe Dimensions

Atlas Grade DatasheetsConcise datasheets, covering all the common stainless steels, include chemical composition, mechanical andphysical properties, fabrication and application of each grade. Again these are available from the AtlasSteels Website.

Information from any Atlas publications can be freely copied, but it is requested that the source beacknowledged.

Atlas Steels Technical DepartmentAtlas Steels Technical Department comprises experienced metallurgists, based at our Melbourne Head Office.We offer a free information service, including: Metal grade selection Fabrication information Special steels applications Specification assistance (such as equivalents of foreign specifications and trade names) Metallurgical properties of metals Supply of technical literature published by Atlas Steels and other metals institutions and bodies.We also have our own metallurgical laboratory providing facilities to assist in investigation of products andapplications.

This assistance is provided as a free service to Atlas Steels' valued customers, and to all members of theAustralian and New Zealand engineering community.

Freecall 1800 818 599 (from within Australia) or +61 3 9272 9999E-mail [email protected]

Limitation of LiabilityThe information contained in this Technical Handbook is not an exhaustive statement of all relevant information. Itis a general guide for customers to the products and services available from Atlas Steels and no representation ismade or warranty given in relation to this information or the products or processes it describes.

Published by Atlas Steels Technical DepartmentCopyright Atlas Steels



TABLE OF CONTENTSFOREWORD 2

Atlas Steels Technotes 2Atlas Grade Datasheets 2Atlas Steels Technical Department 2Limitation of Liability 2

TABLE OF CONTENTS 3

THE FAMILY OF MATERIALS 4

Steel Grade Designations 4

STAINLESS STEELS - INTRODUCTION TOTHE GRADES AND FAMILIES 6

The Families of Stainless Steels 6Austenitic Stainless Steels 6Ferritic Stainless Steels 7Martensitic Stainless Steels 7Duplex Stainless Steels 7Precipitation Hardening Stainless Steels 7

Characteristics of Stainless Steels 7Standard Classifications 7

Austenitic and Duplex Stainless Steels 8Ferritic and Martensitic Stainless Steels 9Comparative Properties of the Stainless SteelAlloy Families 10

CORROSION RESISTANCE 11

General Corrosion 11Pitting corrosion 11Crevice corrosion 12Stress corrosion cracking (SCC) 12Sulphide Stress Corrosion Cracking (SSC) 13Intergranular corrosion 13Galvanic corrosion 14Contact corrosion 14

HIGH TEMPERATURE RESISTANCE 16

Scaling Resistance 16Creep Strength 16Structural Stability 17Environmental Factors 17Thermal Expansion 17

CRYOGENIC PROPERTIES 19

MAGNETIC PROPERTIES 20

Magnetically Soft Stainless Steels 20

MECHANICAL PROPERTIES 21

Mechanical Properties of Wire 22Mechanical Properties of Bar 22

FABRICATION 23

Forming Operations 23Machining 25Welding 26Soft Soldering 27Brazing ("Silver Soldering") 27

HEAT TREATMENT 29

Annealing 29

Hardening 30Stress Relieving 30Surface Hardening 31

SURFACE FINISHING 32

Passivation 32Pickling 32Degreasing 33Electropolishing 33Grinding and Polishing 33Mechanical Cleaning 34Blackening 34

SURFACE CONTAMINATION INFABRICATION 35

Contamination by Carbon Steel 35Contamination by Chlorides 35Contamination by Carbon 35

DESIGN CONSIDERATIONS INFABRICATION OF STAINLESS STEELS 36

Grade Selection for Fabrication 36Design to Avoid Corrosion 36Specific Design Points - To Retain CorrosionResistance 37

GUIDELINES FOR STAINLESS STEEL GRADESELECTION 39

APPENDICES 41

Grade Summary - Stainless & Heat ResistingSteels 41Comparison of Grade Specifications ofStainless Steels 43Typical Physical Properties - AnnealedCondition 44Hardness Conversions 45Unit Conversions 46Dimensional Tolerances for Bar 47Further Information 48Internet Sites of Interest 49



THE FAMILY OF MATERIALSMaterials can be divided into metals and non-metals; the history of civilisation has largely beencategorised by the ability to work metals - hence"bronze age" and "iron age" - but until quiterecently most large-scale construction was still innon-metals, mostly stone or masonry and wood.

Today a vast number of materials compete fortheir share of the market, with more newmaterials being added every year. Someparticularly exciting developments are nowoccurring in the fields of ceramics, plastics andglasses and composites of these materials. Theday of the ceramic car engine is probably not allthat far off - already there are some hightemperature components made from the newgeneration of tougher ceramics, and the modernmotor vehicle also offers many examples of theuse of engineering plastics. Recent developmentsin metals have re-asserted their competitiveposition in auto engineering, in particular the useof aluminium and magnesium alloys. A majorrevolution under way at present is thereplacement of much copper telecommunicationscabling with glass optical fibre. For metals tocompete they must be able to demonstratesuperior properties to their competitors.

In a similar fashion each of the metals has tocompete for its market share, based ondemonstrated superiority of properties oreconomics. It is therefore worth identifying thevarious metals available and indicating just whattheir most important features are. A basicdifferentiation is to divide metals into "ferrous"and "non-ferrous", ie those iron-based and all theothers.

Amongst the non-ferrous metals the mostimportant for engineering applications are thefamilies of aluminium alloys (with very lowdensities, high electrical and thermal conductivity,good formability and good corrosion resistancethese find applications in aircraft, high tensionelectricity conductors, yacht masts etc) and ofcopper alloys (with very high electrical andthermal conductivities and ready formability thesefind their principal applications in electricalwiring). Other important non-ferrous alloys (analloy is simply a mixture of two or more metals)are the brasses and bronzes.

The family of ferrous metals incorporates a vastnumber of alloys. Those alloys containing a veryhigh proportion of carbon (over about 2%) arecalled cast irons. Virtually all of the remainder aretermed steels and these can be found in eithercast form (produced by pouring molten metal intoa mould of the shape of the finished part) orwrought form (cast as ingots or continuous castbillets or slabs, but then hot rolled or forged toproduce bars, plates or complex shapes such asrail sections and beams). They can also be formedto finished shape by sintering powdered metal athigh temperature. Steels are categorised by theirmajor alloying elements (carbon, manganese,chromium, nickel and molybdenum) and by thepresence or absence of minor elements (silicon,sulphur, phosphorus, nitrogen and titanium), asshown in the table in Figure 1.

"Micro" additions of alloys are also present insome grades.

Atlas Steels distributes product from all fourcategories (plain carbon, low alloy, stainless andtool steels).

Steel Grade DesignationsDesignation systems for metals vary widely. In thepast every producer had their own name for eachgrade they produced - some examples were"Duraflex" (BHP's name for 1045) and "Sixix"(Atlas Steels Canada's name for M2 high speedsteel).

Thankfully this practice is now reducing, withbenefits to all users. In some instances there isjustification for the use of a specific trade name,for instance where a manufacturer has made a

Type TypicalGrade

AlloyContent

TypicalUses

plaincarbonsteels

1020 0.2% C bridges,buildingframes,machineryshafts

lowalloysteels

4140 0.4% C1.0% Cr0.2% Mo

highlystressedshafts,forgedmachinecomponents

stainless& highalloysteels

304 0.05% C18% Cr9% Ni

corrosionresistanttanks, bolts,springs

toolsteels

H13 0.4% C1.05% Si5.2% Cr1.3% Mo1.0% V

tools forcasting andhot forging

Figure 1 Typical grades in each steel group



grade significantly different from other similarproducts. This is particularly appropriate in newproduct areas such as duplex stainless steels,where national standards lag behind commercialalloy development, and where grades are stillevolving. Some producers, however, cling to theuse of trade names for quite standard grades inthe hope of generating sales on the basis ofperceived rather than actual product superiority.

Apart from trade designations a variety of namingsystems exist, supported by one or otherstandards body. In Australia metals designationstend to more or less follow those of the USA -principally the American Iron and Steel Institute(AISI) and American Society for Testing andMaterials (ASTM). These bodies many years agodeveloped three-digit designations for stainlesssteels, four-digit designations for carbon and lowalloy steels and one letter plus one or two digitdesignations for tool steels. All three systemshave proven inadequate for coping with an on-going series of new alloy developments, so a newUnified Numbering System (UNS) has beenimplemented by ASTM and the Society ofAutomotive Engineers (SAE). The UNSdesignations have been allocated to all metals incommercial production, throughout the world; asingle letter indicates the alloy family (N = nickelbase alloys, S = stainless steels, etc) and fivedigits denote the grade. This system is nowincorporated in all ASTM standards, and also somestandards from other countries such as Australia.

Japanese grade designations are based on theAISI/ASTM designations as far as stainless steelsare concerned, with some modifications andexceptions. They follow their own system for othersteels.

Until recent years British Standards had adesignation system for steel grades based uponthe AISI/ASTM system, but with extra digits tospecify slight variants of grades, eg 316S31 was aparticular variant of Grade 316 stainless steel.

Other European national standards were quitedifferent, and also differed among themselves.The most commonly encountered system was theGerman Werkstoff (Workshop) Number giving allsteels a single digit plus four digit designation, eg"1.4301" for Grade 304. A second identifierassociated with each grade is the DIN designation,eg "X 5 CrNi 18 9" for Grade 304.

In addition to these national specifications thereare International Standards (ISO) which tend tofollow various European systems, and a newlydeveloping set of "Euronorm" (EN) Europeanspecifications from the European Union. TheseEuronorms have now largely replaced nationalspecifications from Britain, Germany, France andother member nations. For stainless steels themost common grade designation system inAustralia and New Zealand is that of the AmericanASTM. There is however increased awareness ofthe Euronorm system.

An interesting development in 2006 was the minorrevision of composition limits for grade 304 flatrolled product in ASTM A240 with the specific aimof harmonizing it with the European equivalentgrade 1.4301. There is a long way to go, butperhaps this is the beginning of a truly uniformworld-wide agreement on specifications.

Numerous cross references between grades arepublished; a very comprehensive publication is theGerman "Stahlschlssel" (Key to Steel). This isparticularly good for German specifications butdoes cover all significant steel specifyingcountries, including Australia, and lists tradenames in addition to national specifications.Several web sites also give specificationcomparisons.

A summary of these grade equivalents (or nearalternatives) is shown in Appendix 2.



STAINLESS STEELS - INTRODUCTION TO THE GRADES AND FAMILIES

The group of alloys which today make up thefamily of stainless steels had their beginning in theyears 1900 to 1915. Over this period researchwork was published by metallurgists in France andGermany on ferritic, martensitic and austeniticstainless steels. Commercialisation began in theyears 1910 to 1915, with the Americans Dansitzenand Beckets work on ferritic steels and Maurerand Strauss developing austenitic grades inGermany. The often-related development ofmartensitic steels was in 1913 in Sheffield,England; Harry Brearley was trying a number ofalloys as possible gun barrel steels, and noticedthat samples cut from one of these trial Heats didnot rust and were in fact difficult to etch. When heinvestigated this curious material - it containedabout 13% chromium - it lead to the developmentof the stainless cutlery steels for which Sheffieldbecame famous.

Although the consumption of stainless steels isgrowing very rapidly around the world (average of5.8% per annum in the Western world over theperiod 1950 to 2001) average per capitaconsumption in Australia is very low bycomparison with other developed, and manydeveloping countries. In 1999 Australians eachconsumed about 5kg, compared with about 8kgper head in France, 13kg in Japan, 16kg inGermany, 26kg in Singapore and 38kg in Taiwan.On average each Chinese consumed about 1.3kg,but this figure is rapidly rising.

The Families of Stainless SteelsStainless steels are iron based alloys containing aminimum of about 10.5% chromium; this forms aprotective self-healing oxide film, which is thereason why this group of steels have theircharacteristic "stainlessness" or corrosionresistance. The ability of the oxide layer to healitself means that the steel is corrosion resistant,no matter how much of the surface is removed;this is not the case when carbon or low alloy steelsare protected from corrosion by metallic coatingssuch as zinc or cadmium or by organic coatingssuch as paint.

Although all stainless steels depend on thepresence of chromium, other alloying elementsare often added to enhance their properties. Thecategorisation of stainless steels is unusualamongst metals in that it is based upon the natureof their metallurgical structure - the terms useddenote the arrangement of the atoms which makeup the grains of the steel, and which can beobserved when a polished section through a pieceof the material is viewed at high magnificationthrough a microscope. Depending upon the exactchemical composition of the steel themicrostructure may be made up of the stablephases austenite or ferrite, a "duplex" mix of

these two, the phase martensite created whensome steels are rapidly quenched from a hightemperature, or a structure hardened byprecipitated micro-constituents.

The relationship between the different families isas shown in Figure 2. A broad brush comparison ofthe properties of the different families is given inFigure 5.

Austenitic Stainless SteelsThis group contain at least 16% chromium and6% nickel (the basic grade 304 is sometimesreferred to as 18/8) and range through to the highalloy or "super austenitics" such as 904L and 6%molybdenum grades.

Additional elements can be added such asmolybdenum, titanium or copper, to modify orimprove their properties, making them suitable formany critical applications involving hightemperature as well as corrosion resistance. Thisgroup of steels is also suitable for cryogenicapplications because the effect of the nickelcontent in making the steel austenitic avoids theproblems of brittleness at low temperatures, whichis a characteristic of other types of steel.

The relationship between the various austeniticgrades is shown in Figures 3.

Figure 2 Families of stainless steels



Ferritic Stainless SteelsThese are plain chromium (10 to 29%) gradessuch as Grades 430 and 409. Their moderatecorrosion resistance and poor fabricationproperties are improved in the higher alloyed andstabilised grades such as 439 and 444 and in theproprietary grade AtlasCR12.

The relationship between the various ferriticgrades is shown in Figure 4.

Martensitic Stainless SteelsMartensitic stainless steels are also based on theaddition of chromium as the major alloyingelement but with a higher carbon and generallylower chromium content (eg 12% in Grades 410and 416) than the ferritic types; Grade 431 has achromium content of about 16%, but themicrostructure is still martensite despite this highchromium level because this grade also contains2% nickel.

The relationship between the various martensiticgrades is shown in Figure 4.

Duplex Stainless SteelsDuplex stainless steels such as 2205 and 2507(these designations indicate compositions of 22%chromium, 5% nickel and 25% chromium, 7%nickel but both grades contain further minoralloying additions) have microstructurescomprising a mixture of austenite and ferrite.Duplex ferritic - austenitic steels combine some ofthe features of each class: they are resistant tostress corrosion cracking, albeit not quite asresistant as the ferritic steels; their toughness issuperior to that of the ferritic steels but inferior tothat of the austenitic steels, and their strength isgreater than that of the (annealed) austeniticsteels, by a factor of about two. In addition theduplex steels have general corrosion resistancesequal to or better than 304 and 316, and ingeneral their pitting corrosion resistances aresuperior to 316. They suffer reduced toughnessbelow about -50oC and after exposure above300oC, so are only used between thesetemperatures.

The relationship between the various duplexgrades is shown in Figures 3.

Precipitation Hardening Stainless SteelsThese are chromium and nickel containing steelswhich can develop very high tensile strengths. Themost common grade in this group is "17-4 PH";also known as Grade 630, with the composition of17% chromium, 4% nickel, 4% copper and 0.3%niobium. The great advantage of these steels isthat they can be supplied in the "solution treated"condition; in this condition the steel is justmachinable. Following machining, forming etc. thesteel can be hardened by a single, fairly lowtemperature "ageing" heat treatment which

causes no distortion of the component.

Characteristics of StainlessSteelsThe characteristics of the broad group of stainlesssteels can be viewed as compared to the morefamiliar plain carbon "mild" steels. As ageneralisation the stainless steels have:- Higher work hardening rate Higher ductility Higher strength and hardness Higher hot strength Higher corrosion resistance Higher cryogenic toughness Lower magnetic response (austenitic only)

These properties apply particularly to theaustenitic family and to varying degrees to othergrades and families.

These properties have implications for the likelyfields of application for stainless steels, but alsoinfluence the choice of fabrication methods andequipment.

Standard ClassificationsThere are many different varieties of stainlesssteel and the American Iron and Steel Institute(AISI) in the past designated some as standardcompositions, resulting in the commonly usedthree digit numbering system. This role has nowbeen taken over by the SAE and ASTM, whoallocate 1-letter + 5-digit UNS numbers to newgrades. The full range of these standard stainlesssteels is contained in the Iron and Steel Society(ISS) "Steel Products Manual for Stainless Steels",and in the SAE/ASTM handbook of UnifiedNumbering System. Certain other grades do nothave standard numbers, but are instead coveredby other national or international specifications, orby specifications for specialised products such asstandards for welding wire. The following diagramsshow most of the grades of stainless steelsdistributed by Atlas Steels and also some otherimportant grades, identified by their gradenumbers or common designations, illustratingsome of the important properties of the variousfamilies of grades.



Austenitic and Duplex Stainless Steels

Austenitic Stainless Steel

304Basic Grade

310

316 317

316L

304L

308L

303

302HQ

253MAS30815

904L 6MoS31254

321

347

Free Machining grade

Low work hardening rate for cold heading

Welding consumable grades

Weld stabilized grades

Weld stabilized grades

Increasing corrosion resistance >>>>

Duplex Stainless Steels

2304S32304

2250S31803

super duplexgrades

Increasing high temperature resistance >>>

Figure 3 The families of Austenitic and Duplex Stainless Steels



Ferritic and Martensitic Stainless Steels

Ferritic Stainless Steels

430basic grade

444

409 3CR12

430F

420

431

440A

Free machining grade

Utility grades with increasingtoughness >>>

410basic grade

Higher corrosion resisting weldable grade.

440B 440C

Martensitic Stainless Steels

Higher hardness grade

Higher corrosion resistanceand higher toughness grade

Increasing hardness after heat treatment>>>

416 Free machining grade

Figure 4 The families of Ferritic and Martensitic Stainless Steels



Comparative Properties of the Stainless Steel Alloy Families

Alloy Group MagneticResponse(note 1)

WorkHardening

Rate

CorrosionResistance

(note2)

Hardenable Ductility HighTemperatureResistance

LowTemperatureResistance

(note 3)

Weldability

Austenitic Generally No Very High High By Cold Work Very High Very High Very High Very High

Duplex Yes Medium Very High No Medium Low Medium High

Ferritic Yes Medium Medium No Medium High Low Low to High

Martensitic Yes Medium Medium Quench &Temper

Low Low Low Low

PrecipitationHardening

Yes Medium Medium Age Hardening Medium Low Low High

Notes1. Attraction of the steel to a magnet. Note some austenitic grades can be attracted to a magnet if cold worked, cast or welded.2. Varies significantly between grades within each group. e.g. free machining grades have lower corrosion resistances, those grades higher in chromium,

molybdenum and nitrogen have higher resistances. Corrosion resistance is not primary related to the alloy group (structure) but more by the composition.3. Measured by toughness or ductility at sub-zero temperatures. Austenitic grades retain ductility to cryogenic temperatures.

Figure 5 Comparative properties



CORROSION RESISTANCE

Although the main reasons why stainless steelsare used is corrosion resistance, they do in factsuffer from certain types of corrosion in someenvironments and care must be taken to select agrade which will be suitable for the application.Corrosion can cause a variety of problems,depending on the applications:

Perforation such as of tanks and pipes, whichallows leakage of fluids or gases,

Loss of strength where the cross section ofstructural members is reduced by corrosion,leading to a loss of strength of the structureand subsequent failure,

Degradation of appearance, where corrosionproducts or pitting can detract from adecorative surface finish,

Finally, corrosion can produce scale or rustwhich can contaminate the material beinghandled; this particularly applies in the case offood processing equipment.

Corrosion of stainless steels can be categorisedas:-General CorrosionPitting CorrosionCrevice CorrosionStress Corrosion CrackingSulphide Stress Corrosion CrackingIntergranular CorrosionGalvanic CorrosionContact Corrosion

General CorrosionCorrosion whereby there is a general uniformremoval of material, by dissolution, eg whenstainless steel is used in chemical plant forcontaining strong acids. Design in this instance isbased on published data to predict the life of thecomponent.

Published data list the removal of metal over ayear - a typical example is shown in Figure 6.

Tables of resistance to various chemicals arepublished by various organisations and a verylarge collection of charts, lists, recommendationsand technical papers is available through the AtlasSteels Technical Services Department.

Pitting corrosionUnder certain conditions, particularly involvinghigh concentrations of chlorides (such as sodiumchloride in sea water), moderately hightemperatures and exacerbated by low pH (ie acidicconditions), very localised corrosion can occurleading to perforation of pipes and fittings etc.This is not related to published corrosion data as itis an extremely localised and severe corrosionwhich can penetrate right through the crosssection of the component. Grades high inchromium, and particularly molybdenum andnitrogen, are more resistant to pitting corrosion.

The Pitting Resistance Equivalent number (PRE)has been found to give a good indication of thepitting resistance of stainless steels. The PRE canbe calculated as:

PRE = %Cr + 3.3 x %Mo + 16 x %NOne reason why pitting corrosion is so serious isthat once a pit is initiated there is a strongtendency for it to continue to grow, even althoughthe majority of the surrounding steel is stilluntouched.

The tendency for a particular steel to be attackedby pitting corrosion can be evaluated in thelaboratory. A number of standard tests have beendevised, the most common of which is that givenin ASTM G48. A graph can be drawn giving thetemperature at which pitting corrosion is likely tooccur, as shown in Figure 7.

Figure 7 Critical pitting temperaturesfor different alloys, rated by ASTMG48A test

The graph is based on a standard ferric chloridelaboratory test, but does predict outcomes in

Figure 6 Iso-corrosion curves forvarious stainless steels in sulphuricacid



many service conditions.

A very common corrosive environment in whichstainless steels are used is marine, generally up toa few hundred metres from quiet (eg: bay) water,or up to a few kilometres from a shore withbreaking waves. Corrosion in this environment issometimes called tea staining a term used byASSDA (Australian Stainless Steel DevelopmentAssociation) to describe light surface rusting,usually visible as a fairly wide-spread brown-reddiscolouration. A very full description of thecauses and prevention of tea staining is given inthe ASSDA Technical Bulletin on the topic,available at the ASSDA website atwww.assda.asn.au

For many years 316 has been regarded as themarine grade of stainless steel. It must berecognised however, that in the more aggressivemarine environments 316 will not fully resistpitting corrosion or tea staining. It has beenfound that finer surface finishes generally withan Ra value of no coarser than 0.5m will assist inrestricting tea staining. It is also important thatthe surface is free of any contaminants.

Crevice corrosionThe corrosion resistance of a stainless steel isdependent on the presence of a protective oxidelayer on its surface, but it is possible under certainconditions for this oxide layer to break down, forexample in reducing acids, or in some types ofcombustion where the atmosphere is reducing.Areas where the oxide layer can break down canalso sometimes be the result of the waycomponents are designed, for example undergaskets, in sharp re-entrant corners or associatedwith incomplete weld penetration or overlappingsurfaces. These can all form crevices which canpromote corrosion.

To function as a corrosion site, a crevice has to beof sufficient width to permit entry of thecorrodent, but sufficiently narrow to ensure thatthe corrodent remains stagnant. Accordinglycrevice corrosion usually occurs in gaps a fewmicrometres wide, and is not found in grooves orslots in which circulation of the corrodent ispossible. This problem can often be overcome bypaying attention to the design of the component,in particular to avoiding formation of crevices or atleast keeping them as open as possible.

Crevice corrosion is a very similar mechanism topitting corrosion; alloys resistant to one aregenerally resistant to both. Crevice corrosion canbe viewed as a more severe form of pittingcorrosion as it will occur at significantly lowertemperatures than does pitting. Further details ofpitting and crevice corrosion are given in AtlasTechnote 2.

Stress corrosion cracking (SCC)Under the combined effects of stress and certaincorrosive environments stainless steels can besubject to this very rapid and severe form ofcorrosion. The stresses must be tensile and canresult from loads applied in service, or stresses setup by the type of assembly e.g. interference fits ofpins in holes, or from residual stresses resultingfrom the method of fabrication such as coldworking. The most damaging environment is asolution of chlorides in water such as sea water,particularly at elevated temperatures. As aconsequence many stainless steels are limited intheir application for holding hot waters (aboveabout 50C) containing even trace amounts ofchlorides (more than a few parts per million). Thisform of corrosion is only applicable to theaustenitic group of steels and is related to thenickel content. Grade 316 is not significantly moreresistant to SCC than is 304. The duplex stainlesssteels are much more resistant to SCC than arethe austenitic grades, with grade 2205 beingvirtually immune at temperatures up to about150C, and the super duplex grades are moreresistant again. The ferritic grades do notgenerally suffer from this problem at all.

In some instances it has been found possible toimprove resistance to SCC by applying acompressive stress to the component at risk; thiscan be done by shot peening the surface forinstance. Another alternative is to ensure theproduct is free of tensile stresses by annealing asa final operation. These solutions to the problemhave been successful in some cases, but need tobe very carefully evaluated, as it may be verydifficult to guarantee the absence of residual orapplied tensile stresses.

From a practical standpoint, Grade 304 may beadequate under certain conditions. For instance,Grade 304 is being used in water containing 100 -300 parts per million (ppm) chlorides at moderatetemperatures. Trying to establish limits can berisky because wet/dry conditions can concentratechlorides and increase the probability of stresscorrosion cracking. The chloride content ofseawater is about 2% (20,000 ppm). Seawaterabove 50oC is encountered in applications such asheat exchangers for coastal power stations.

Recently there has been a small number ofinstances of chloride stress corrosion failures atlower temperatures than previously thoughtpossible. These have occurred in the warm, moistatmosphere above indoor chlorinated swimmingpools where stainless steel (generally Grade 316)fixtures are often used to suspend items such asventilation ducting. Temperatures as low as 30 to40C have been involved. There have also beenfailures due to stress corrosion at highertemperatures with chloride levels as low as 10ppm. This very serious problem is not yet fullyunderstood.



Sulphide Stress CorrosionCracking (SSC)Of greatest importance to many users in the oiland gas industry is the material's resistance tosulphide stress corrosion cracking. The mechanismof SSC has not been defined unambiguously butinvolves the conjoint action of chloride andhydrogen sulphide, requires the presence of atensile stress and has a non-linear relationshipwith temperature.

The three main factors are:a) Stress level

A threshold stress can sometimes beidentified for each material - environmentcombination. Some published data show acontinuous fall of threshold stress withincreasing H2S levels. To guard againstSSC NACE specification MR0175 forsulphide environments limits the commonaustenitic grades to 22HRC maximumhardness.

b) EnvironmentThe principal agents being chloride,hydrogen sulphide and pH. There issynergism between these effects, with anapparently inhibiting effect of sulphide athigh H2S levels.

c) TemperatureWith increasing temperature, thecontribution of chloride increases but theeffect of hydrogen decreases due to itsincreased mobility in the ferrite matrix.The net result is a maximum susceptibilityin the region 60-100C.

A number of secondary factors have also beenidentified, including amount of ferrite, surfacecondition, presence of cold work and heat tint atwelds.

Intergranular corrosionIntergranular corrosion is a form of relatively rapidand localised corrosion associated with a defectivemicrostructure known as carbide precipitation.When austenitic steels have been exposed for aperiod of time in the range of approximately 425to 850C, or when the steel has been heated tohigher temperatures and allowed to cool throughthat temperature range at a relatively slow rate(such as occurs after welding or air cooling afterannealing), the chromium and carbon in the steelcombine to form chromium carbide particles alongthe grain boundaries throughout the steel.Formation of these carbide particles in the grainboundaries depletes the surrounding metal ofchromium and reduces its corrosion resistance,allowing the steel to corrode preferentially alongthe grain boundaries. Steel in this condition issaid to be "sensitised".

It should be noted that carbide precipitationdepends upon carbon content, temperature and

time at temperature, as shown in Figure 8. Themost critical temperature range is around 700C,at which 0.06% carbon steels will precipitatecarbides in about 2 minutes, whereas 0.02%carbon steels are effectively immune from thisproblem.

It is possible to reclaim steel which suffers fromcarbide precipitation by heating it above1000C, followed by water quenching to retainthe carbon and chromium in solution and soprevent the formation of carbides. Moststructures which are welded or heated cannot begiven this heat treatment and therefore specialgrades of steel have been designed to avoid thisproblem. These are the stabilised grades 321(stabilised with titanium) and 347 (stabilisedwith niobium). Titanium and niobium each havemuch higher affinities for carbon than chromiumand therefore titanium carbides, niobiumcarbides and tantalum carbides form instead ofchromium carbides, leaving the chromium insolution and ensuring full corrosion resistance.

Figure 8 Susceptibility to intergranularcorrosion of grade 304 with variouscarbon contents (ref AWRA Technote 16)

Another method used to overcome intergranularcorrosion is to use the extra low carbon gradessuch as Grades 316L and 304L; these haveextremely low carbon levels (generally less than0.03%) and are therefore considerably moreresistant to the precipitation of carbide.

Many environments do not cause intergranularcorrosion in sensitised austenitic stainless steels,for example, glacial acetic acid at roomtemperature, alkaline salt solution such as sodiumcarbonate, potable water and most inland bodiesof fresh water. For such environments, it wouldnot be necessary to be concerned aboutsensitisation. There is also generally no problem inlight gauge steel since it usually cools very quicklyfollowing welding or other exposure to hightemperatures. For this reason thin gauge sheet isoften only available in standard carbon content,but heavy plate is often only available in lowcarbon L grades. More information on L, H andstandard grades is given in Atlas Technote 8.

It is also the case that the presence of grain



boundary carbides is not harmful to the hightemperature strength of stainless steels. Gradeswhich are specifically intended for theseapplications often intentionally have high carboncontents as this increases their high temperaturestrength and creep resistance. These are the "H"variants such as grades 304H, 316H, 321H and347H, and also 310. All of these have carboncontents deliberately in the range in whichprecipitation will occur.

Sensitised steels have also been found to sufferfrom intergranular stress corrosion cracking(IGSCC). This problem is rare, but can affectunstabilised ferritic and austenitic grades if theyare treated or held in appropriate temperatureranges.

Galvanic corrosionBecause corrosion is an electrochemical processinvolving the flow of electric current, corrosion canbe generated by a galvanic effect which arisesfrom the contact of dissimilar metals in anelectrolyte (an electrolyte is an electricallyconductive liquid). In fact three conditions arerequired for galvanic corrosion to proceed ... thetwo metals must be widely separated on thegalvanic series (see Figure 9), they must be inelectrical contact, and their surfaces must bebridged by an electrically conducting fluid.Removal of any of these three conditions willprevent galvanic corrosion.

The obvious means of prevention is therefore toavoid mixed metal fabrications or only use thoseclose together in the galvanic series; copper alloysand stainless steels can generally be mixedwithout problem for instance. Frequently this isnot practical, but prevention can also be byremoving the electrical contact - this can beachieved by the use of plastic or rubber washersor sleeves, or by ensuring the absence of theelectrolyte such as by improvement to draining orby the use of protective hoods. This effect is alsodependent upon the relative areas of the dissimilarmetals. If the area of the less noble material (theanodic material, further towards the right in Figure9) is large compared to that of the more noble(cathodic) the corrosive effect is greatly reduced,and may in fact become negligible. Conversely alarge area of noble metal in contact with a smallarea of less noble will accelerate the galvaniccorrosion rate. For example it is common practiceto fasten aluminium sheets with stainless steelscrews, but aluminium screws in a large area ofstainless steel are likely to rapidly corrode.

The most common instance of galvanic corrosionis probably the use of zinc plated carbon steelfasteners in stainless steel sheet. Further details ofgalvanic corrosion are given in Atlas Technote 7.

Contact corrosionThis combines elements of pitting, crevice and

galvanic corrosion, and occurs where smallparticles of foreign matter, in particular carbonsteel, are left on a stainless steel surface. Theattack starts as a galvanic cell - the particle offoreign matter is anodic and hence likely to bequickly corroded away, but in severe cases a pitmay also form in the stainless steel, and pittingcorrosion can continue from this point. The mostprevalent cause is debris from nearby grinding ofcarbon steel, or use of tools contaminated withcarbon steel. For this reason some fabricatorshave dedicated stainless steel workshops wherecontact with carbon steel is totally avoided.

Figure 9 Galvanic series for metals inflowing sea water. More negativevalues for stainless steels are for activeconditions, such as in crevices

All workshops and warehouses handling or storingstainless steels must also be aware of thispotential problem, and take precautions to preventit. Protective plastic, wood or carpet strips can beused to prevent contact between stainless steelproducts and carbon steel storage racks. Otherhandling equipment to be protected includes forklift tynes and crane lifting fixtures. Clean fabricslings have often been found to be a usefulalternative.

If stainless steel does become contaminated bycarbon steel debris this can be removed bypassivation with dilute nitric acid or pickling with amix of hydrofluoric and nitric acids. See the latersection on pickling and passivation for furtherdetails.

Contamination by carbon steel (also referred to as



free iron) can be detected by:

A ferroxyl test is very sensitive, but requiresa freshly made up test solution. Refer toASTM A380.

Copper sulphate will plate out copper on freeiron. Again details in ASTM A380.

The simplest test is to wet the surfaceintermittently for about 24 hours; anycontamination will be revealed as rust spots.



HIGH TEMPERATURE RESISTANCE

The second most common reason stainless steelsare used is for their high temperature properties;stainless steels can be found in applications wherehigh temperature oxidation resistance isnecessary, and in other applications where hightemperature strength is required. The highchromium content which is so beneficial to the wetcorrosion resistance of stainless steels is alsohighly beneficial to their high temperaturestrength and resistance to scaling at elevatedtemperatures, as shown in the graph of Figure 10.

Scaling ResistanceResistance to oxidation, or scaling, is dependenton the chromium content in the same way as thecorrosion resistance is, as shown in the graph ofFigure 10. Most austenitic steels, with chromiumcontents of at least 18%, can be used attemperatures up to 870C and Grades 309, 310and S30815 (253MA) even higher. Mostmartensitic and ferritic steels have lowerresistance to oxidation and hence lower usefuloperating temperatures. An exception to this isthe ferritic grade 446 - this has approximately24% chromium, and can be used to resist scalingat temperatures up to 1100C.

The table in Figure 11 shows the approximatemaximum service temperatures at which thevarious grades of stainless steels can be used toresist oxidation in dry air. Note that thesetemperatures depend very much on the actual

environmental conditions, and in some instancessubstantially lower temperatures will result in

destructive scaling.

Creep StrengthThe high temperature strength of materials isgenerally expressed in terms of their "creepstrength" - the ability of the material to resistdistortion over a long term exposure to a hightemperature. In this regard the austenitic stainlesssteels are particularly good. Design codes such asAustralian Standard AS1210 "Pressure Vessels"and AS4041 "Pressure Piping" (and correspondingcodes from ASME and other bodies) also stipulateallowable working stresses of each grade at arange of temperatures. The low carbon versions ofthe standard austenitic grades (Grades 304L and316L) have reduced strength at high temperatureso are not generally used for structuralapplications at elevated temperatures. "H"versions of each grade (eg 304H) have highercarbon contents for these applications, whichresults in significantly higher creep strengths. "H"grades are specified for some elevatedtemperature applications. A more completedescription of the application of L, H and standardaustenitic grades is given in Atlas Technote 8.

The scaling resistances of the ferritic stainlesssteels are generally as suggested by the graph ofFigure 10. So 11% chromium grades (409 orAtlasCR12) have moderate sealing resistances,17% grades (430) have good sealing resistanceand 25% Cr grades (446) have excellent sealingresistance. The ferritic structure however, doesnot have the high creep strength of the austeniticgrades, so the use of ferritics at very hightemperatures is often limited to low stressapplication. Addition of niobium to ferritic gradesdoes improve the creep strength substantially, andsome niobium-stabilised grades find application inhigh temperature auto exhaust components forinstance.

Figure 10 Effect of chromium content onscaling resistance

Grade Intermittent(C)

Continuous(C)

304309310316321410416420430

S30815

87098010358708708157607358701150

925109511509259257056756208151150

Figure 11 Maximum service temperaturesin dry air, based on scaling resistance (ref:ASM Metals Handbook)



Although the duplex stainless steels have goodoxidation resistance due to their high chromiumcontents, they suffer from embrittlement ifexposed to temperatures above about 350C, sothey are restricted to applications below this.

Both martensitic and precipitation hardeningfamilies of stainless steels have high strengthsachieved by thermal treatments; exposure ofthese grades at temperatures exceeding their heattreatment temperatures will result in permanentsoftening, so again these grades are seldom usedat elevated temperatures.

Structural StabilityThe problem of grain boundary carbideprecipitation was discussed under intergranularcorrosion. This same phenomenon occurs whensome stainless steels are exposed in service totemperatures of 425 to 815C, resulting in areduction of corrosion resistance which may besignificant. If this problem is to be avoided the useof stabilised grades such as Grade 321 or lowcarbon "L" grades should be considered. It mustbe understood that a high carbon content, as inthe H grades, such as 304H, is beneficial toelevated temperature strength. Such steels donot have good aqueous corrosion resistance, butthis is often not a problem. Refer to AtlasTechnote 8 for further details.

A further problem that some stainless steels havein high temperature applications is the formationof sigma phase. The formation of sigma phase inaustenitic steels is dependent on both time andtemperature and is different for each type of steel.In general Grade 304 stainless steel is practicallyimmune to sigma phase formation, but not sothose grades with higher chromium contents(Grade 310) with molybdenum (Grades 316 and317) or with higher silicon contents (Grade 314).These grades are all prone to sigma phaseformation if exposed for long periods to atemperature of about 590 to 870C. Sigma phaseembrittlement refers to the formation of aprecipitate in the steel microstructure over a longperiod of time within this particular temperaturerange. The effect of the formation of this phase isto make the steel extremely brittle and failure canoccur because of brittle fracture. Once the steelhas become embrittled with sigma it is possible toreclaim it by heating the steel to a temperatureabove the sigma formation temperature range,however this is not always practical. Becausesigma phase embrittlement is a serious problemwith the high silicon grade 314, this is nowunpopular and largely replaced by high nickelalloys or by stainless steels resistant to sigmaphase embrittlement, particularly S30815(253MA). Grade 310 is also fairly susceptible tosigma phase formation in the temperature range590 to 870C, so this "heat resistant" grade may

not be suitable for exposure at this comparativelylow temperature range and Grade 321 is often abetter choice.

Environmental FactorsOther factors which can be important in the use ofsteels for high temperature applications arecarburisation and sulphidation resistance. Sulphurbearing gases under reducing conditions greatlyaccelerate the attack on stainless alloys with highnickel contents. In some instances Grade 310 hasgiven reasonable service, in others grade S30815,with a lower nickel content is better, but in othersa totally nickel-free alloy is superior. If sulphurbearing gases are present under reducingconditions it is suggested that pilot test specimensbe first run under similar conditions to determinethe best alloy.

Thermal ExpansionA further property that can be relevant in hightemperature applications is the thermal expansionof the particular material. The coefficient ofthermal expansion is expressed in units ofproportional change of length for each degreeincrease in temperature, usually x10-6/C,m/m/C, or x10-6cm/cm/C, all of which areidentical units. The increase in length (ordiameter, thickness, etc) can be readily calculatedby multiplying the original dimension by thetemperature change by the coefficient of thermalexpansion. For example, if a three metre longGrade 304 bar (coefficient of expansion 17.2m/m/C) is heated from 20C to 200C, thelength increases by:

3.00 x 180 x 17.2 = 9288 m = 9.3 mm

The coefficient of thermal expansion of theaustenitic stainless steels is higher than for mostother grades of steel, as shown in the followingtable.

This expansion coefficient not only varies betweensteel grades, it also increases slightly withtemperature. Grade 304 has a coefficient of 17.2 x10-6/C over the temperature range 0 to 100C,but increases above this temperature; details ofthese values are given in the table of physical

Coefficient ofThermal Expansion

(x10-6/C)

Carbon SteelsAustenitic SteelsDuplex SteelsFerritic SteelsMartensitic Steels

1017141111

* or micrometres/metre/C

Figure 12 Coefficient of thermal expansion- average values over the range 0-100C



properties in Appendix 3 of this Handbook, and inthe individual Atlas Grade Data Sheets.

The effect of thermal expansion is most noticeablewhere components are restrained, as theexpansion results in buckling and bending. Aproblem can also arise if two dissimilar metals arefabricated together and then heated; dissimilarcoefficients will again result in buckling orbending. In general the quite high thermalexpansion rates of the austenitic stainless steelsmean that fabrications in these alloys may havemore dimensional problems than similarfabrications in carbon or low alloy steels, inferritic, martensitic or duplex stainless steels.

The non-austenitic stainless steels also havesomewhat higher thermal conductivities than theaustenitic grades, which may be an advantage incertain applications. Thermal conductivities ofstainless steels are again listed in the table ofphysical properties in Appendix 3 of thisHandbook, and in the individual Atlas Grade DataSheets.

Localised stresses from expansion during heatingand cooling can contribute to stress corrosioncracking in an environment which would notnormally attack the metal. These applicationsrequire design to minimise the adverse effects oftemperature differentials such as the use ofexpansion joints to permit movement withoutdistortion and the avoidance of notches and abruptchanges of section.



CRYOGENIC PROPERTIES

The austenitic stainless steels possess a uniquecombination of properties which makes themuseful at cryogenic (very low) temperatures, suchas are encountered in plants handling liquefiedgases. These steels at cryogenic temperatureshave tensile strengths substantially higher than atambient temperatures while their toughness isonly slightly degraded. Typical impact strengthsare as shown in Figure 13.

Note: There is substantial variation in cryogenicimpact properties, depending on testmethods and steel condition.

Considerable austenitic stainless steel hastherefore been used for handling liquefied naturalgas at a temperature of -161C, and also in plantsfor production of liquefied gases. Liquid oxygenhas a boiling temperature of -183C and that ofliquid nitrogen is -196C.

The ferritic, martensitic and precipitationhardening steels are not recommended for use atsub-zero temperatures as they exhibit a significantdrop in toughness, even at only moderately lowtemperatures, in some cases not much belowroom temperature.

The duplex stainless steels have a better lowtemperature ductility than the ferritic andmartensitic grades; they are generally quiteuseable down to at least -50C but have reducedductility below this temperature. This thereforeusually places a lower temperature limit on theirusefulness. The "ductile to brittle transition" fromwhich these grades suffer is also a commonfeature of carbon and low alloy steels, some ofwhich have Ductile to Brittle TransitionTemperatures (DBTT) close to 0C.

These limitations are incorporated into designcodes, such as for instance the material designminimum temperature (MDMT) quoted in theAustralian pressure vessel code AS1210.

The ductile to brittle transformation is not apermanent change; a steel that is brittle at lowtemperatures returns to its ductile condition whenit is brought back to higher temperatures.

Figure 13 Typical impact energies of stainlesssteels down to cryogenic temperatures.



MAGNETIC PROPERTIES

Magnetic Permeability is the ability of a material tocarry magnetism, indicated by the degree to whichit is attracted to a magnet. All stainless steels,with the exception of the austenitic group, arestrongly attracted to a magnet. All austeniticgrades have very low magnetic permeabilities andhence show almost no response to a magnet whenin the annealed condition; the situation is,however, far less clear when these steels havebeen cold worked by wire drawing, rolling or evencentreless grinding, shot blasting or heavypolishing. After substantial cold working Grade304 may exhibit quite strong response to amagnet, whereas Grades 310 and 316 will in mostinstances still be almost totally non-responsive, asshown in Figure 14.

The change in magnetic response is due to atomiclattice straining and formation of martensite. Ingeneral, the higher the nickel to chromium ratiothe more stable is the austenitic structure and theless magnetic response that will be induced bycold work. Magnetic response can in some casesbe used as a method for sorting grades ofstainless steel, but considerable caution needs tobe exercised.

Any austenitic (300 series) stainless steel whichhas developed magnetic response due to coldwork can be returned to a non-magnetic conditionby stress relieving. In general this can be readilyachieved by briefly heating to approximately 700 800C (this can be conveniently carried out bycareful use of an oxy-acetylene torch). Note,however, comments elsewhere in this publicationabout sensitisation (carbide precipitation) unlessthe steel is a stabilised grade. Full solutiontreatment at 1000 1150C will remove allmagnetic response without danger of reducedcorrosion resistance due to carbides.

Many cold drawn and/or polished bars have anoticeable amount of magnetism as a result of theprevious cold work. This is particularly the casewith grades 304 and 303, and much less so for thehigher nickel grades such as 310 and 316, asshown in the graph of Figure 14. Even within thechemical limitations of a single standard analysisrange there can be a pronounced variation in therate of inducement of magnetic response fromcold work.

Austenitic stainless steel castings and welds(which could be viewed very small costings) areusually deliberately designed to have a minorproportion of ferrite. Approximately 5-12% offerrite assists in preventing hot cracking. Thismicrostructure responds slightly to a magnet, andin fact ferrite meters based on measuring thisresponse can be used to quantify the proportion offerrite.

If magnetic permeability is a factor of design or isincorporated into a specification, this should beclearly indicated when purchasing the stainlesssteel from a supplier.

Magnetically Soft StainlessSteelsIn some applications there is a requirement for asteel to be "magnetically soft". This is oftenrequired for solenoid shafts, where it is necessaryfor the plunger to respond efficiently to themagnetic field from the surrounding coil when thecurrent is switched on, but when the current isswitched off the magnetic field induced in the steelmust quickly collapse, allowing the plunger toreturn to its original position. Steels which behavein this way are said to be magnetically soft. Forcorrosion resisting applications there are ferriticstainless steels which are magnetically soft,usually variants of a grade "18/2" (18% chromiumand 2% molybdenum) but with very tightlycontrolled additions of silicon and often withsulphur added to make them free machining.Special mill processing guarantees the magneticproperties of the steels.

Figure 14 Magnetic response of austeniticstainless steels after cold work.



MECHANICAL PROPERTIESThe mechanical properties of stainless steels arealmost always requirements of the productspecifications used to purchase the productsdistributed by Atlas Steels.

For flat rolled products the properties usuallyspecified are tensile strength, yield stress (almostalways measured as 0.2% proof stress),elongation and sometimes Brinell or Rockwellhardness. Much less frequently there arerequirements for impact resistance, either Charpyor Izod. Bar, tube, pipe, fittings etc. also usuallyrequire at least tensile strength and yield stress.Hardness is commonly measured for annealedtube. These properties give a guarantee that thematerial in question has been correctly produced,and are also used by engineers to calculate theworking loads or pressures that the product cansafely carry in service.

Typical mechanical properties of annealedmaterials are as in the graph of Figure 15. Notethat the high cold work hardening rate of theaustenitic grades in particular results in actualproperties of some commercial products beingsignificantly higher than these values. The yieldstress (usually measured as the 0.2% proofstress) is particularly increased by even quiteminor amounts of cold work. More details of thework hardening of stainless steels are given in thesection of this handbook on fabrication.

An unusual feature of annealed austenitic stainlesssteels is that the yield strength is a very lowproportion of the tensile strength, typically only40-45%. The comparable figure for a mild steel isabout 65-70%. As indicated above a small amountof cold work greatly increases the yield (muchmore so than the tensile strength), so the yieldalso increases to a higher proportion of tensile.Only a few % of cold work will increase the yieldby 200 or 300MPa, and in severely cold workedmaterial like spring temper wire or strip, the yield

is usually about 80-95% of the tensile strength.

As engineering design calculations are frequentlymade on yield criteria the low yield strength ofaustenitic stainless steels may well mean thattheir design load cannot be higher than that ofmild steel, despite the tensile strength beingsubstantially higher. Design stresses for variousgrades and temperatures are given in AustralianStandards AS1210 "Pressure Vessels" and AS4041Pressure Piping.

The other mechanical property of note is theductility, usually measured by % elongation duringa tensile test. This shows the amount ofdeformation a piece of metal will withstand beforeit fractures. Austenitic stainless steels haveexceptionally high elongations, usually about 60-70% for annealed products, as shown in Figure16. It is the combination of high work hardeningrate and high elongation that permits the severefabrication operations which are routinely carriedout, such as deep drawing of kitchen sinks andlaundry troughs.

Hardness (usually measured by Brinell, Rockwellor Vickers machines) is another value for thestrength of a material. Hardness is usually definedas resistance to penetration, so these testmachines measure the depth to which a very hardindenter is forced into a material under the actionof a known force. Each machine has a differentshaped indenter and a different force applicationsystem, so conversion between hardness scalesmay not be very accurate. Standard tables havebeen produced, and a summary of these forstainless steels, is given in Appendix 4 of thishandbook. These conversions are onlyapproximate, and should not be used to determineconformance to standards. Conversions for carbonand low alloy steels are more reliable than thosefor austenitic stainless steels.

0

100

200

300

400

500

600

700

800

Auste

nitic

Duple

x

Ferri

tic

Mild

Steel

Alum

inium

Brass

Ten

sile

and

Proo

fStr

ess(

MPa

)

Tensile Strength Proof Strength

Figure 15 Typical tensile properties of annealedmaterials

0

10

20

30

40

50

60

70

Auste

nitic

Duple

x

Ferri

tic

Mild

Steel

Alum

inium

Brass

Elon

gatio

n(%

)

Figure 16 Typical elongations of annealedmaterials



It is also sometimes convenient to do a hardnesstest and then convert the result to tensilestrength. Although the conversions for carbon andlow alloy steels are fairly reliable, those foraustenitic stainless steels are much less so. Nostandard conversion tables for hardness to tensilestrength are published for austenitic alloys, andany conversions should be regarded withsuspicion.

Mechanical Properties of WireThe mechanical properties of the majority of thestainless steel wire and bar products stocked byAtlas Steels are generally sufficiently described bythe tensile strength. Each of the wire productsrequire mechanical properties which are carefullychosen to enable the product to be fabricated intothe finished component and also to withstand theloads which will be applied during service. Springwire has the highest tensile strength; it must besuitable for coiling into tension or compressionsprings without breaking during forming. However,such high tensile strengths would be completelyunsuitable for forming or weaving applicationsbecause the wire would break on forming.

Weaving wires are supplied in a variety of tensilestrengths carefully chosen so that the finishedwoven screen will have adequate strength towithstand the service loads, and yet soft enoughto be crimped and to be formed into the screensatisfactorily.

Mechanical properties of cold Heading wire forfasteners are another example where a carefulbalance in mechanical properties is required. Inthis type of product the wire must be ductileenough to form a quite complex head but the wiremust be hard enough so that the threads will notdeform when the screw or bolt is assembled intothe component. Good examples are roofing bolts,wood screws and self-tapping screws; to achievethe mechanical properties required for suchcomponents requires careful consideration of thecomposition of the steel so that the workhardening rate will be sufficiently high to formhard threads on thread rolling and yet not so highas to prevent the head from being formed.

Mechanical Properties of BarFor bar products a compromise must also bemade; a large proportion of bar will be machined,so it is important that the hardness be not toohigh, but better load carrying capacity is achievedif the strength is high, and for drawn bar a goodbright finish is achieved only by a reduction whichsignificantly increases strength levels.

In practice round austenitic stainless steel barfrom about 5mm up to about 26mm in diameter ismost commonly manufactured by cold drawing toits final size, typically on a Schumag or similar

machine. The resulting finish is called BrightDrawn or BD. The approximately 10% reductionin area of this draw gives a significant increase intensile strength and much more so in proof stress.By contrast, bars with diameters of about 26mmand over are most commonly manufactured bytaking hot rolled annealed bar and simply cuttingoff the rough and uneven outer surface to achievefinal size. Such a finish is called Smooth Turned orST. Final straightening does slightly increase thestrength (and more so the surface hardness) butthese large sized bars have properties quite closeto annealed.

Typical mechanical properties for these productsare shown in Figure 17.

Finish Sizeapprox(mm)

TensileStrength

(MPa)

YieldStrength

(MPa)

Elongation(%)

Drawn 5-26 700 600 35Turned >26 600 300 55

There are other implications of these twomanufacturing routes -

1. Large diameter ST bars will have bettermachinabilities than the same grade in smallerBD sizes, because a lower hardness, lowerstrength steel is easier to cut.

2. Large diameter ST bars will be free of surfacelaps or seams because the hot rolled surface hasbeen cut off in turning. The smaller BD bars mayhave minor surface imperfections because nometal has been removed; the original hot rolledsurface has merely been burnished smooth bydrawing, thus covering minor blemishes.

3. All drawn wire and bar has a higher surfacehardness because that is where most of the coldwork is concentrated. This makes little differencein the small diameter wires and bars, but for barapproaching 25mm in diameter the surface willbe significantly higher hardness than its centre.Even ST bar has a higher surface hardnessbecause of cold working imparted by finalmachining and bar straightening.

Figure 17 Typical mechanicalproperties of 304/316 round bars



FABRICATIONForming OperationsOne of the major advantages of the stainlesssteels, and the austenitic grades in particular, istheir ability to be fabricated by all the standardfabrication techniques, in some cases even moreseverely than the more well-known carbon steels.The common austenitic grades can be folded,bent, cold and hot forged, deep drawn, spun androll formed. Because of the materials' highstrength and very high work hardening rate all ofthese operations require more force than forcarbon steels, so a heavier machine may beneeded, and more allowance may need to bemade for spring-back.

Austenitic stainless steels also have very highductility, so are in fact capable of being veryheavily cold formed, despite their high strengthsand high work hardening rates, into items such asdeep drawn kitchen sinks and laundry troughs.Few other metals are capable of achieving thisdegree of deformation without splitting.

All metals work harden when cold worked and theextent of work hardening depends upon the gradeselected. Austenitic stainless steels work hardenvery rapidly, but the ferritic grades work hardenonly a little higher than that of the plain carbonsteels. The rapid cold working characteristics ofaustenitic stainless alloys makes them particularlyuseful where the combination of high strength andcorrosion resistance are required, such as formanufacture of springs for corrosiveenvironments. The relationship between theamount of cold work (expressed as "% reductionof area") and the resulting mechanical propertiesis shown in the chart in Figure 18.

Figure 18 Effect of Cold work on tensilestrength of stainless steels. Values aretypical for cold drawn wire.

It is important to realise that work hardening isthe only way in which austenitic stainless steels

can be hardened. By contrast the martensiticstainless steels (eg. 410, 416, 420 and 431) canbe hardened by a quench-and-temper thermaltreatment in the same way as carbon and lowalloy steels. Ferritic stainless steels (such asGrades 430, 439 and 444) are similar to austeniticgrades in that they can only be hardened by coldworking, but their work hardening rates are low,and a substantial lift in strength cannot beachieved.

In cold working such as cold drawing, tensileproperties over 2000 MPa may be obtained withGrades 301, 302 and 304. However, these veryhigh tensile properties are limited to thin sectionsand to fine wire sizes.

As the size increases, the amount of cold worknecessary to produce the higher tensile propertiescannot be practically applied. This is due to thefact that the surface of larger sections rapidlywork hardens to the extent that further work isnot practical, while the centre of the section is stillcomparatively soft. To illustrate this point, Grade304 6mm round, cold drawn with 15 per centreduction in area will show an ultimate strength ofabout 800 MPa. A 60mm round, drawn with thesame reduction will have about the same tensile infull section. However, if sections are machinedfrom the centre of each bar, the 60mm round willshow much lower tensile properties, whereas the6mm round will test about the same as in fullsection. Also the 6mm round can be cold workedto much higher tensile properties, whereas the60mm round must be annealed for further coldwork, because of excessive skin hardness.

In general, the austenitic grades showing thegreatest work hardening rate will also have thehighest magnetic permeability for a given amountof cold work.

Ferritic and martensitic alloys work harden atrates similar to low carbon steel and are magneticin all conditions at room temperatures. Wire sizesmay be cold worked to tensile properties as highas approximately 1000 MPa. However, bar sizesare seldom cold worked higher than 850 MPa.Although the ferritic grades (e.g. 430, 409 andAtlasCR12) cannot be heat treated, themartensitic grades (e.g. 410, 416 and 431) areusually heat treated by hardening and temperingto develop mechanical properties and maximumcorrosion resistance. Cold working, therefore, ismore of a sizing operation than a method ofproducing mechanical properties with thesegrades.

The rate of work hardening, while relativelyconsistent for a single analysis, will show amarked decrease in the rate as the temperatureincreases. This difference is noticeable at



temperatures as low as 80. At slightly increasedtemperature there is also a reduction in strengthand increase in ductility. Advantage is taken ofthis in some deep drawing applications as well asin "warm" heading of some difficult fasteners.

Another feature of cold forming of stainless steelsis that more severe deformation is possible atslower forming speeds - this is quite different fromcarbon steels which have formabilities virtually thesame no matter what the forming rate. So theadvice given to those attempting difficult coldheading (or other high speed forming operations)is to slow down; stainless steel is almost alwaysheaded slower than is carbon steel.



MachiningAustenitic stainless steels are generally regardedas being difficult to machine, and this has led tothe development of the free-machining Grade 303.There are also free-machining versions of thestandard ferritic (Grade 430) and martensitic(Grade 410) grades - Grades 430F and 416respectively - these grades have improvedmachinability because of the inclusions ofManganese Sulphide (formed from the sulphuradded to the steel) which act as chip breakers.

The free-machining grades have significantly lowercorrosion resistances than their non-freemachining equivalents because of the presence ofthese non-metallic inclusions; these grades areparticularly prone to pitting corrosion attack andmust not be used in aggressive environments suchas for marine exposure. The free-machininggrades containing high sulphur levels also havereduced ductility, so cannot be bent around a tightradius nor cold forged. Because of the sulphuradditions these grades are very difficult to weld,so again would not be chosen for weldedfabrication.

Ugima Improved Machinability GradesRecently a number of manufacturers have offered"Improved Machinability" versions of the standardaustenitic Grades 304 and 316. These steels areproduced by proprietary steel melting techniqueswhich provide enough of a chip-breaking effect tosignificantly improve the machinability, but theystill remain within the standard grade compositionspecifications and still retain mechanicalproperties, weldability, formability and corrosionresistance of their standard grade equivalents.These materials are marketed under trade namessuch as "Ugima". For "Ugima" the improvement inachievable machining speed is about 20% overthe equivalent standard grades; in addition it iscommonly experienced that greatly enhanced toollife is obtained, which considerably reduces thecost of machining. In many instances this is ofeven greater benefit than is the potentialimprovement in cutting speed.

A "Ugima 303" is available as a "super-machineable" grade; like other 303 stainlesssteels weldability, formability and corrosionresistance are compromised in order to achievemaximum machinability. Relative machinabilitiesof various stainless steels, expressed ascomparison of achievable cutting speeds, areshown in the graph of Figure 19. Moreinformation on machining of stainless steels is inAtlas Technote 4.

Rules for Machining Stainless SteelsSome general rules apply to most machining ofaustenitic stainless steels:

1. The machine tool must be sturdy, havesufficient power and be free fromvibration.

2. The cutting edge must be kept sharp at alltimes by re-sharpening or replacement.Dull tools cause glazing and workhardening of the surface. Sharpeningmust be carried out as soon as the qualityof the cut deteriorates. Sharpeningshould be by machine grinding usingsuitable fixtures, as free-hand sharpeningdoes not give consistent and long-lastingedges. Grinding wheels must be dressedand not contaminated.

3. Light cuts should be taken, but the depthof the cut should be substantial enough toprevent the tool from riding the surface ofthe work - a condition which promoteswork hardening.

4. All clearances should be sufficient toprevent the tool from rubbing on thework.

5. Tools should be as large as possible tohelp to dissipate the heat.

6. Chip breakers or chip curlers prevent thechips from being directed into the work.

7. Constant feeds are most important toprevent the tool from riding on the work.

8. Proper coolants and lubricants areessential. The low thermal conductivity ofaustenitic stainless alloys causes a largepercentage of the generated heat to beconcentrated at the cutting edges of thetools. Fluids must be used in sufficientquantities and directed so as to flood boththe tool and the work.

0 20 40 60 80 100Relative Machinability (%)

303Ugima 303

304Ugima 304

316Ugima 316

410416430

430F431

2205

Figure 19 Relative machinabilities ofstainless steels



WeldingThe weldabilities of the various grades of stainlesssteels vary considerably. Nearly all can be welded,and the austenitic grades are some of the mostreadily welded of all metals. In general thestainless steels have weldabilities which dependupon the family to which they belong.Recommendations for welding the common gradesare given in Figure 20, and in the individual Atlasgrade data sheets available on the Atlas Steelswebsite. Australian Standard AS 1554.6 coversstructural welding of stainless steels, and gives anumber of pre-qualified conditions for welding.Pre-qualified welding consumables for welding ofsame-metal and mixed-metal welding are alsogiven in AS 1554.6. This excellent standard(available from Standards Australia) also enablesspecification of welding procedures appropriate toeach particular application. It is also highlyrelevant to non-structural welding.

Austenitic Stainless SteelsThe austenitic grades are all very readily welded(with the exception of the free-machining grade303 noted elsewhere). All the usual electricwelding processes can be used - Manual Metal Arc(MMAW or "stick"), Gas Tungsten Arc (GTAW orTIG), Gas Metal Arc (GMAW or MIG), Flux Cored(FCAW), Submerged Arc (SAW) and laser. A fullrange of welding consumables is readily availableand standard equipment can be used.

The use of low carbon content grades (304L and316L) or stabilised grades (321 or 347) needs tobe considered for heavy section product which isto be welded. This overcomes the problem of"sensitisation" discussed in the previous section onintergranular corrosion. As the sensitisationproblem is time/temperature dependent, so thinmaterials, which are welded quickly, are notusually a problem. It should be noted that if afabrication has become sensitised during weldingthe effect can be reversed and the materialrestored to full corrosion resistance by a fullsolution heat treatment.

The free-machining grade 303 is notrecommended for welded applications as it issubject to hot cracking; the Ugima improvedmachinability grades, Ugima 304 and Ugima 316,offer a much better combination of reasonablemachinability with excellent weldability.

Duplex Stainless SteelsDuplex stainless steels also have good weldability,albeit not quite as good as that of the austenitics.Again all the usual processes can be used, and arange of consumables is available. For the mostcommon duplex grade 2205 the standardconsumable is a 2209 - the higher nickel contentensures the correct 50/50 ferrite/austenitestructure in the weld deposit, thus maintainingstrength, ductility and corrosion resistance. One of

the advantages of duplex stainless steels overaustenitics is their comparatively low coefficient ofthermal expansion. This is only a little higher thanthat of carbon steels, as shown in the table in thesection of this handbook on high temperatureproperties of stainless steels.

Ferritic Stainless SteelsThe ferritic grades are weldable but not as readilyas the austenitic grades. The three majorproblems encountered are sensitisation, excessivegrain growth and lack of ductility, particularly atlow temperatures. Sensitisation can be avoided byuse of stabilised grades. Filler metal can be of asimilar composition or more commonly anaustenitic grade (e.g. Grades 308L, 309, 316L or310) which is helpful in improving weld toughness.The excessive grain growth problem is difficult toovercome, so most grades are only welded in thingauges, usually up to 2 or 3mm. Heat input mustbe kept low for the same reason. Stabilised ferriticgrades include 409 and 430Ti and the moremodern 444 (Durinox F18MS), 439 (Durinox F18S)and Durinox F20S. These possess considerablybetter weldability compared to the unstabilisedalternatives such as 430. Such grades can bewelded, even in structural or pressure vesselapplications, but only in thin gauges.

AtlasCR12 and AtlasCR12Ti are proprietary ferriticgrades which have a very low carbon content andthe remaining composition and the mill processingroute balanced to enable welding. The CR12 typesare the only ferritic grades weldable in heavysection plate. As for other ferritic grades it isnormal to use austenitic stainless steel fillers.

Martensitic Stainless SteelsMartensitic stainless steels can if necessary bewelded (again with the high sulphur freemachining grade 416 being not recommended) butcaution needs to be exercised as they will producea very hard and brittle zone adjacent to the weld.Cracking in this zone can occur unless much careis taken with pre-heating and with post weld heattreatment. These steels are often welded withaustenitic filler rods to increase the ductility of thedeposit.

Welding Dissimilar MetalsWelding together of different metals, such as ofGrade 316 to Grade 444 or a stainless steel to amild steel, can be carried out, although someextra precautions need to be taken. It is usuallyrecommended that over-alloyed austenitic weldingrods, such as Grade 309, be used to minimisedilution effects on the stainless steel component.The composition of the weld deposit resulting fromdissimilar grade welding is shown in the Schaefflerdiagram or its successors by De Long and morerecently the WRC. Australian Standard AS 1554.6contains a table giving the pre-qualifiedconsumables for each combination of dissimilarmetal welds.



Further Information on WeldingSpecific recommendations are given in Figure 19.Further details on welding of stainless steels aregiven in the booklet "Guidelines for the WeldedFabrication of Nickel-containing Stainless Steelsfor Corrosion Resistant Services" (Nickel InstituteReference Book, No. 11 007) available from theNickel Institute. Specific details on applicationscan be provided by the welding electrode, gas andequipment manufacturers. Excellent information isalso included in WTIA Technote 16 on "WeldingStainless Steels ". This is available from theWelding Technology Institute of Australia.

Soft SolderingAll grades of stainless steel can be soldered withlead-tin soft solder. Leaded solders should not beused when the product being soldered is used forfood processing, serving or transport. Solderedjoints are relatively weak compared to thestrength of the steel, so this method should not beused where the mechanical strength is dependentupon the soldered joint. Strength can be added ifthe edges are first lock-seamed, spot welded orriveted. In general welding is preferable tosoldering.

Recommended procedure for soldering:-

1. The steel surfaces must be clean and freeof oxidation.

2. A rough surface improves adherence ofthe solder, so roughening with grindingwheel, file or coarse abrasive paper isrecommended.

3. Use a phosphoric acid based flux.Hydrochloric acid based fluxes requireneutralising after soldering as anyremnant traces will be highly corrosive tothe steel. Hydrochloric acid based fluxesare not recommended for soldering ofstainless steels.

4. Flux should be applied with a brush, toonly the area being soldered.

5. A large, hot iron is recommended. Use thesame temperature as for carbon steel, buta longer time will be required because ofstainless steel's low thermal conductivity.

6. Any type of solder can be used, but atleast 50% tin is recommended. Solderwith 60-70% tin and 30-40% lead has abetter colour match and greater strength.

Brazing ("Silver Soldering")When welding is impractical and a stronger jointthan soft soldering is required, brazing may beemployed. This method is particularly useful forjoining copper, bronze, nickel and other non-ferrous metals to stainless steel. The corrosionresistance of the joint will be somewhat lower thanthat of the stainless steel, but in normal

atmospheric and mildly corrosive conditionsbrazed joints are satisfactory. Because mostbrazing operations involve temperatures at whichcarbide precipitation (sensitisation) can occur inthe austenitic grades, low carbon or stabilisedgrades (304L, 316L or 321) should be used.Ferritic grades such as 430 can be quenched fromthe brazing temperatures, but hardenablemartensitic grades (410, 420, 431) should not beheated above 760C when brazing. The freemachining grades 303, 416 and 430F shouldgenerally not be used as a dark scum forms on thesurface when fluxing and heating, which adverselyaffects the appearance of the steel.

Recommended procedure for brazing:-

1. Use silver brazing alloys with meltingpoints from 590-870C. Select the alloyfor best colour match.

2. Remove dirt and oxides from the steelsurfaces and apply flux immediately.

3. A slightly reducing flame should be playedacross the joint to heat uniformly.

4. For high production work use inductionheating or controlled atmosphere furnaces(argon, helium, vacuum or dissociatedammonia with dew point of about -50C).

5. After brazing remove all remaining fluxwith high pressure steam or hot water.

6. When brazing grade 430 use a silversolder with 3% nickel. This alloy alsohelps to minimise crevice corrosion whenused with austenitic grades.



Grade Pre-heat Post Weld Heat Treatment Filler

304 (a) Cool rapidly from 1010-1090C only if welding over

about 3 5mm thickness.

308L

304L (a) Not required 308L

309 (a) Usually unnecessary as this grade is generally used at

high temperatures (b).

309

310 (a) As for 309 310

316 (a) Cool rapidly from 1060-1150C only if welding over

about 3 5mm thickness.

316L

316L (a) Not required 316L

321 (a) Not required 347

253MA (a) Not required S30815(h)

410 (c) Air cool from 650-760C 410 (d)

430 (c) Air cool from 650-760C 430 (d)

444 (a) Not required 316L

AtlasCR12 (g) Not required 309 (e)

2205 (f) Not generally required 2209

This table gives broad over-view recommendations. Further details are available from welding consumablesuppliers. For critical application, welding procedures should be qualified in accordance with AS1554.6 orother applicable standards.

Notes:

a. Unnecessary when the steel is above 15C.b. Where corrosion is a factor, 309S and 310S (0.08% Carbon maximum) are used, with a post

weld heat treatment of cooling rapidly from 1120-1180C.c. Pre-heat at 200-320C; light gauge sheet is frequently welded without pre-heat.d. May be welded with 308L, 309 or 310 electrodes without pre-heat if the steel is above 15C.e. May be welded with 309, 309L, 309Mo, 309MoL, 316L or 308L, depending on the application.f. If temperature is below 10C then a 50C pre-heat is recommended.g. In case of critical structural welding of AtlasCR12 destined for corrosive environments, please

refer to Atlas Steels Technical Department.h. 309 Consumables can be used if a reduced creep strength and oxidation resistance can be

tolerated.

Figure 20 Recommendations for welding of stainless steels.



HEAT TREATMENT

Stainless steels are often heat treated; the natureof this treatment depends on the type of stainlesssteel and the reason for the treatment. Thesetreatments, which include annealing, hardeningand stress relieving, restore desirable propertiessuch as corrosion resistance and ductility to metalaltered by prior fabrication operations or producehard structures able to withstand high stresses orabrasion in service. Heat treatment is oftenperformed in controlled atmospheres to preventsurface scaling, or less commonly to preventcarburisation or decarburisation.

AnnealingAustenitic Stainless SteelsThe austenitic stainless steels cannot be hardenedby thermal treatments (but they do harden rapidlyby cold work). Annealing (often referred to assolution treatment) not only recrystallises thework hardened grains but also takes chromiumcarbides (precipitated at grain boundaries insensitised steels) back into solution in theaustenite. The treatment also homogenisesdendritic weld metal structu

atlas technical handbook stainless steel

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