Classification of Steels C-Mn Steels: High-Strength Low-Alloy Steels:

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1 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 to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition.

2 The HSLA steels have low carbon contents ( % C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. 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 a very 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 precipitation hardening. Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, thus providing a highstrength steel of superior formability.

3 Low-alloy Steels Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2 % up to levels just below that of stainless steels, which contain 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 0.5 to 1.0% Mo. The carbon content is usually below 0.2%)

4 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 Cr 2 O 3 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 H 2 SO 4 ; higher chromium concentrations and the judicious use of other solutes such as molybdenum, nickel and nitrogen is then needed to ensure a robust material.

5 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.

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7 Effect of C on gamma loop

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12 Role of alloying additions Cr Ferrite stabilizer Forms (FeCr) 2 O 3 Corrosion protection in oxidizing environment ~ 11% required to be stainless Higher concentration required for more aggressive environments Strong carbide / nitride former (Cr 23 C 6 ) / Cr 2 N Solid solution strengthening Tends to form intermetallic compound at high Cr content High Cr content promotes brittleness C, N must be controlled to prevent brittleness

13 Ni Mn Austenite stabilizer Improves corrosion resistance in reducing environment (H 2 SO 4 ) Lowers SCC resistance in Cl containing environment Does not form carbides Solid solution strengthening Does not to form intermetallics Improves toughness of ferrite / austenite Reduces DBTT About 1-2% is present Usually added to prevent hot shortness (a solidification cracking due to low melting eutectic) Austenite stabilizer Added to increase solubility of N in special steels Solid solution strengthening

14 Si About % is present in most steels Added primarily as deoxidizer in steel Improves corrosion resistance when present 4-5% Improves fluidity (so added to weld filler metals) Forms intermetallic silicides (FeSi, Fe 2 Si etc.) Causes segregation during solidificaiton Mo Improves corrosion resistance particularly pitting / crevice Added upto 6% or more In Austenitic steels increases elevated temperature strength In martenstitic steels added as carbide former Promotes secondary hardening during tempering Being ferrite stabilizer amount must be controlled

15 Carbide forming elements (Ti, Nb, V, W, Ta) Ti, Nb are added for stabilization of carbon in austenitic stainless steels To prevent intergranular corrosion by preventing formation of Cr carbides Ta, W provide high temperature strength by forming fine carbides Promote ferrite formation by tying up carbon Precipitation formers Ti, Al, Mo, Cu can cause precipitation Intermetallics like Ni 3 Al, Ni 3 Ti or pure Cu

16 C and N Interstitial solutes and austenite stabilizers But most of the time content should be < 0.1% Except in some martensitic grades Higher carbon leads to formation of Cr carbides Deterioration of corrosion resistance N is potent solid solution strengthener In recent higher N is being added upto 0.3% or so in austenitic steels Other elements S, Se and Pb are added to improve machining These elements reduce corrosion resistance and also unweldable

17 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 and architectural trim (i.e., decorative purposes), as the cheapest stainless steels are found in this family (type 409). Iron-chromium body-centred cubic solutions are such that there is a tendency under appropriate conditions for like atoms to cluster; at temperatures below a critical value, the solution tends to undergo spinodal decomposition into Cr rich and Fe-rich regions.

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23 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

24 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) 475 C embrittlement 2) Sigma / Chi phase precipitation 3) High temperature embrittlement

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27 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 room temperature or below. Most grades have a Ms temperature well below 0 C. However, plastic deformation can induce martensite at temperatures higher than M S. The presence of Ni improves considerably the corrosion resistance when compared to the martensitic and ferritic grades. Strength can be increased significantly by cold working Good formability and weldability, Good high temperature corrosion resistance

28 Type 304 is the basic 18Cr 8Ni (18/8) austenitic stainless steel, so widely used that it accounts for about 50% of all stainless steel production. Other standard grades have different preferred applications; for example, type 316 which contains up to 3 wt.% Mo, offers an improved general and pitting corrosion resistance, making it the material of choice marine applications and coastal environments. Stabilized grades like 321 and 347 contain small additions of Ti and Nb to combine with C and reduce the tendency for intergranualar corrosion due to Cr carbide formation. Higher SI and Al (and C) may be added to oxidation and carburisation resistance and strength

29 Solidification mode can be austenitic or ferritic depending on composition

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31 Sensitisation Sensitisation is one of the corrosion mechanisms which causes widespread problems in austenitic stainless steels. In normal conditions, austenitic stainless steels are given a hightemperature 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.

32 Preferential corrosion at grain boundaries

33 Reduction of carbon content. Solution to problem of sensitisation 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.

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

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37 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

38 Martensitic Stainless Steels The composition is such that the austenite in these steels is able to transform into martensite. This allows a degree of control on the mechanical properties by exploiting the phase change. Typical heat-treatments consist of austenitisation at a temperature high enough to dissolve carbides followed by quenching to obtain martensite. Given the high hardenability inherent in such alloys, the quench rate required to achieve martensite is not high; Oil and water quenching are used only when dealing with thick sections. Typical compositions cover 12 to 18 Cr and 0.1 to 1.2 C wt%. As with other martensitic steels, a balance must be sought between hardness and toughness.

39 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 650 C

40 Duplex stainless steels Duplex stainless steels typically contain 50% austenite and 50% ferrite They have higher strength and superior corrosion resistance especially 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 > 280 C Alloys are based on Fe-Cr-Ni-N N increases strength and pitting corrosion resistance

41 W, Mo and Cu are added to improve corrosion resistance During cooling, many embrittling phases could form

42 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.

43 Superior SCC resistance of duplex steels compared to austenitic steels

44 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 Ni 3 Al, Ni 3 Ti High strength (about 1500 MPa) with good ductility and toughness and good corrosion resistance Service temperatures in the range of 300 to 600 C Difficult to fabricate due to high strengths

45 Depending on the Ni content, the solidification route or

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