Classification of Steels

C-Mn Steels:

Low-carbon steels contain up to 0.30% C.

Medium-carbon steels (up to 0.6%)

High-carbon steels (contain from 0.60 to 1.00% C )

Ultrahigh-carbon steels (1.25 to 2%)

High-Strength Low-Alloy Steels:High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed toprovide better mechanical properties and/or greater resistance to atmosphericcorrosion than conventional carbon steels in the normal sense because they aredesigned to meet specific mechanical properties rather than a chemicalcomposition.

The HSLA steels have low carbon contents (0.05-0.25% C) in order to produceadequate formability and weldability, and they have manganese contents up to2.0%.

Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium,niobium, titanium and zirconium are used in various combinations.

Classification•Control-rolled steels, hot rolled according to a predetermined rolling schedule,designed to develop a highly deformed austenite structure that will transform to avery fine equiaxed ferrite structure on cooling.

• Microalloyed steels, with very small additions of such elements as niobium,vanadium, and/or titanium for refinement of grain size and/or precipitationhardening.

•Dual-phase steels, processed to a micro-structure of ferrite containing smalluniformly distributed regions of high-carbon martensite, resulting in a productwith low yield strength and a high rate of work hardening, thus providing a high-strength steel of superior formability.

Low-alloy SteelsLow-alloy steels constitute a category of ferrous materials that exhibitmechanical properties superior to plain carbon steels as the result of additions ofalloying elements such as nickel, chromium, and molybdenum. Total alloycontent can range from 2 % up to levels just below that of stainless steels, whichcontain a minimum of 10% Cr.

As with steels in general, low-alloy steels can be classified according to:

•Chemical composition, such as nickel steels, nickel-chromium steels,molybdenum steels, chromium-molybdenum steels

Heat treatment, such as quenched and tempered, normalized and tempered,annealed.

(1) low-carbon quenched and tempered (Q&T) steels

(2) medium-carbon ultrahigh-strength steels

(3) bearing steels

(4) heat-resistant chromium-molybdenum steels (contain 0.5 to 9% Cr and 0.5to 1.0% Mo. The carbon content is usually below 0.2%)

Stainless Steels

Corrosion of carbon steels and low alloy steels is poor in severe environments.

Stainless steels contain sufficient amount of Cr which forms thin protective adherent layer of Cr2O3 film.

Used in Food, Chemical, Oil production and Power generation industries. (Utensils at home are made of stainless steel)

The stainless character occurs when the concentration of chromium exceeds about 12 wt%. However, even this is not adequate to resist corrosion in acids such as HCl or H2SO4; higher chromium concentrations and the judicious use of other solutes such as molybdenum, nickel and nitrogen is then needed to ensure a robust material.

Stainless steels are commonly divided into five groups:

• Ferritic stainless steels

• Austenitic stainless steels

• Martensitic stainless steels

• Duplex (ferritic-austenitic) stainless steels

• Precipitation-hardening stainless steels.

Effect of C on gamma loop

Role of alloying additions

Cr – Ferrite stabilizerForms (FeCr)2O3Corrosion protection in oxidizing environment~ 11% required to be stainlessHigher concentration required for more aggressiveenvironmentsStrong carbide / nitride former (Cr23C6) / Cr2NSolid solution strengtheningTends to form intermetallic compound at high Cr contentHigh Cr content promotes brittlenessC, N must be controlled to prevent brittleness

Ni – Austenite stabilizerImproves corrosion resistance in reducing environment (H2SO4)Lowers SCC resistance in Cl containing environmentDoes not form carbidesSolid solution strengtheningDoes not to form intermetallicsImproves toughness of ferrite / austeniteReduces DBTT

Mn – About 1-2% is presentUsually added to prevent hot shortness (a solidification cracking due to low melting eutectic)Austenite stabilizerAdded to increase solubility of N in special steelsSolid solution strengthening

Si – About 0.3 – 0.6% is present in most steels Added primarily as deoxidizer in steelImproves corrosion resistance when present 4-5%Improves fluidity (so added to weld filler metals)Forms intermetallic silicides (FeSi, Fe2Si etc.)Causes segregation during solidificaiton

Mo – Improves corrosion resistance particularly pitting / creviceAdded upto 6% or moreIn Austenitic steels increases elevated temperature strengthIn martenstitic steels added as carbide formerPromotes secondary hardening during temperingBeing ferrite stabilizer amount must be controlled

Carbide forming elements (Ti, Nb, V, W, Ta)

Ti, Nb are added for stabilization of carbon in austenitic stainless steelsTo prevent intergranular corrosion by preventing formation of Cr carbidesTa, W provide high temperature strength by forming fine carbidesPromote ferrite formation by tying up carbon

Precipitation formersTi, Al, Mo, Cu can cause precipitation Intermetallics like Ni3Al, Ni3Ti or pure Cu

C and N

Interstitial solutes and austenite stabilizersBut most of the time content should be < 0.1%Except in some martensitic gradesHigher carbon leads to formation of Cr carbides Deterioration of corrosion resistance

N is potent solid solution strengthenerIn recent higher N is being added upto 0.3% or so in austenitic steels

Other elements

S, Se and Pb are added to improve machiningThese elements reduce corrosion resistance and also unweldable

Ferritic stainless steels: typically contain more chromium and/or less carbon than the martensitic grades.

Ferritic stainless steels cannot be hardened by heat-treatment.

They exhibit lower strength but higher ductility/toughness.

Typical applications may include appliances, automotive andarchitectural trim (i.e., decorative purposes), as the cheapeststainless steels are found in this family (type 409).

Iron-chromium body-centred cubic solutions are such that there isa tendency under appropriate conditions for like atoms to cluster;at temperatures below a critical value, the solution tends toundergo spinodal decomposition into Cr rich and Fe-rich regions.

Austenite that forms at elevated temperature will transform to martensite during cooling to room temperature. Very slow cooling or isothermal holding required to avoid martensite

Presence of martensite could cause hydrogen embrittlement

If C content is low, presence of martensite could increase toughness / ductility

Presence of martensite resulted in loss of corrosion resistance

Interface of martensite / ferrite susceptible to IGC

Embrittlement Phenomena:

1) 475C embrittlement2) Sigma / Chi phase precipitation3) High temperature embrittlement

Austenitic stainless steels

Most widely used among all stainless steels.

Low yield strength.

Good corrosion resistance in different environments.

Good low temperature impact properties.

These steels are often in a metastable austenitic state at roomtemperature or below. Most grades have a Ms temperature wellbelow 0°C. However, plastic deformation can induce martensite attemperatures higher than MS.

The presence of Ni improves considerably the corrosion resistancewhen compared to the martensitic and ferritic grades.

Strength can be increased significantly by cold working

Good formability and weldability, Good high temperature corrosionresistance

Type 304 is the basic 18Cr 8Ni (18/8) austenitic stainless steel, sowidely used that it accounts for about 50% of all stainless steelproduction.

Other standard grades have different preferred applications; forexample, type 316 which contains up to 3 wt.% Mo, offers animproved general and pitting corrosion resistance, making it thematerial of choice marine applications and coastal environments.

Stabilized grades like 321 and 347 contain small additions of Tiand Nb to combine with C and reduce the tendency forintergranualar corrosion due to Cr carbide formation.

Higher SI and Al (and C) may be added to oxidation andcarburisation resistance and strength

Solidification mode can be austenitic or ferritic depending on composition

Sensitisation is one of the corrosion mechanisms which causes widespread problems in austenitic stainless steels.

In normal conditions, austenitic stainless steels are given a high-temperature heat-treatment, often called a solution-treatment, which gives a fully austenitic solid solution.

However, at temperatures below about 800°C, there is a tendency to precipitate chromium-rich carbides as the alloy enters the carbide plus austenite phase field.

Sensitisation

Preferential corrosion at grain boundaries

Reduction of carbon content.

Use of solutes (such as Nb, Ti, V or Ta) which have a greater affinity for carbon than chromium.

These are called stabilised stainless steels, for example, types 321 (Ti stabilised) and 347 (Nb stabilised) austenitic stainless steels.

Solution to problem of sensitisation

A variety of other factors impact on the problem, such as the austenite grain size and the crystallographic character of the grain boundaries.

Sensitisation can be avoided by grain boundary engineering by creating a crystallographic textures which favours low-energy boundaries which are less effective as heterogeneous nucleation sites for chromium carbides.

Grain boundary engineering is achieved through controlled thermomechanical processing

Martensitic Stainless Steels

The composition is such that the austenite in these steels is ableto transform into martensite. This allows a degree of control onthe mechanical properties by exploiting the phase change.

Typical heat-treatments consist of austenitisation at atemperature high enough to dissolve carbides followed byquenching to obtain martensite.

Given the high hardenability inherent in such alloys, the quenchrate required to achieve martensite is not high;

Oil and water quenching are used only when dealing with thicksections.

Typical compositions cover 12 to 18 Cr and 0.1 to 1.2 C wt%.As with other martensitic steels, a balance must be soughtbetween hardness and toughness.

Wide range of strengths possible 300 to 1900 Mpa

Applications where high strength and corrosion resistance required

Low carbon high Cr supermartensitic steels are used in oil and gas industries.

Maximum temperature is about 650C

Duplex stainless steels

Duplex stainless steels typically contain 50% austenite and 50% ferrite

They have higher strength and superior corrosion resistanceespecially to stress corrosion cracking and pitting

Higher thermal conductivity and lower thermal expansion

Not suitable for cryogenic applications due to higher DBTT

Embrittlement occurs at temperatures > 280C

Alloys are based on Fe-Cr-Ni-N

N increases strength and pitting corrosion resistance

W, Mo and Cu are added to improve corrosion resistance

During cooling, many embrittling phases could form

The two-phase mixture also leads to a marked refinement in the grain size of both the austenite and ferrite.

This, together with the presence of ferrite, makes the material about twice as strong as common austenitic steels.

Superior SCC resistance of duplex steels compared to austenitic steels

Precipitation Hardening Stainless Steels

Precipitation strengthening is the major strengthening mechanism

Matrix could be martensitic, semi-austenitic, austenitic

Ti and Al are added to form intermetallics Ni3Al, Ni3Ti

High strength (about 1500 MPa) with good ductility and toughnessand good corrosion resistance

Service temperatures in the range of 300 to 600C

Difficult to fabricate due to high strengths

Depending on the Ni content, the solidification route

or