4 2) Alloying Elements and their Effects in Steel Common classificationsa) High Strength Low Alloy Steelsb) Low Alloy Steelsc) Chromium-Molybdenum Steels (Tool Steels)
5 Review : Carbon SteelsOnly element > 1% is Mn.P, S (except in electrical steels, Lect 20) are generally unwanted. Sulfur is sometimes added in controlled amounts to enhance machinability (embrittles) and P to increase hardening in particular during deep drawing (a cold working process where it can not move to GBs).
6 Cu increases the corrosion resistance and has some solid solution hardening Mn, in plain Carbon steels, works mostly to keep sulfur bound up so the it can not form FeS particlesManganese increases the g field (Hadfield steel is austenitic thanks to 13 % Mn) and is a potent carbide stabilizer. A stabilizer is an element that stabilizes the presence of an other compound.Role of Si: General impurity in Fe (from sand) used intentionally to deoxidize steel or to increase electrical resistivity (transformer steel)
8 Alloy steelsAlloy steels have compositions that exceed the limitations of C, Mn, Ni, Mo, Cr, Va, Si, and B set for carbon steels.Exception: steels containing more than 3.99% chromium are classified differently as stainless and tool steels.Alloy steels are always killed, but can use unique deoxidization or melting processes for specific applications.Alloy steels are generally more responsive to heat and mechanical treatments than carbon steels (you may recall that heat treating 1020 is generally not worth the cost).
9 Killed steels are steel that have been deoxidized by the addition of silicon or aluminium, before casting. (If you do not remember go back and revisit steel making, US and Europe, and continuing casting)In this case there is no (significant) evolution of gas during solidification.More generally, have a higher degree of chemical homogeneity and freedom from porosity
10 Designation AISIIf a “B” shows up it means Boron containing.If “H” shows up means fits particular specs for hardenability
11 Alloy elements:Added to make steel “better”. Can beHigher yield stressHigher ductilityHigher hardnessBetter machinabilityHigher service temperatureBetter corrosion resistanceetc….. application specific
12 Solid Solution Strengthener P is effective but when it gets into the GB it embrittles.See lecture 20.N is introduced by nitriding steelChange in lower yield stress point, low alloy steels
13 Elements that promote austenite, I.e. a larger g field Mn
14 Ferrite forming elements Cr, Si, Mo, W and Al.Fe-Cr alloys containing more than 13% Cr are ferritic at all temperatures up to incipient melting.
15 Elements that lower MsPractically all ! If fully dissolved in g phaseMs = C - 33Mn - 17Ni - 17Cr - 21Mo
16 Carbide-forming elements Cr, W, Mo, V, Ti, Nb, Ta, Zr.Affinity for Carobon increases from left to rightSome overlap with ferrite promoters (no accident think Fe3C )Non Fe containing carbidesCr7C3 W2C, VC, Mo2C.Double carbides contain both Fe and Carbide former e.g Fe4W2C.High-speed tool steels usually three types of carbides, which are usually designated M6C, M23C6 and MC. M represents sum of metal atoms. I.e M6C can represent Fe4W2C or Fe4Mo2C; M23C6 represents Cr23C6 etc. For how to stabilize these carbides at high T see Lect. 20
17 Carbide stabilizersThe stability of the carbides depends on the presence of other elements in the steel. Chemical equilibrium between different carbids is measure by K . K is weight ratio of C in cementite vs C contained in the matrix. K values areI.e. Mn will promote C to exist as Fe3C rather than being dissolved in the matrix. Thermo language would be that the activity of C is f(xi). Cr even more effective For that reason mallable cast iron (where you want the graphite to come out as globular C) can not contain any Cr.
18 NitridesNitride former are similar C formerTiN is a well known ultrahard compound. AlN precips distorts lattice and generates high dislocation densityNitrided surfaces are extremely hard
19 Nitrogen:Nitrogen can form a solid solution with ferrite at nitrogen contents up to about 6%.Above ~ 6% N, gamma prime g’ with a composition of Fe4N.Above ~ 8%, the equilibrium product is e compound, Fe3N.Nitrogen hardening is a “case hardening” process, producing a very hard thin layer at the surface. Ammonia, nitrogen plasma, or fluidized bed reactors are used
20 HydrogenAn unwanted element that generates, in high strength steels, hydrogen embrittlement. Hydrogen diffuses in metals, in trap limited diffusion process.The effect is due, depending on the situationa) weakening of the Fe-Fe bonds at the tip of a (growing) crack. The decohesion model can be explained with the effective d electron concentration (reviewed in prelim)b) reformation of H into H2 molecules generating interior pressure (do the thermo, pressure is tremendous)c) Hydrogen combining with C to form CH4 again forming high pressure bubbles.d) hydrogen atmosphere around dislocation
22 Source of hydrogencathodic protection,phosphating, pickling, and electroplating (notorious in high strength fasteners that also need to be corrosion resistant.welding with electrodes covered with coating containing moisturehydrogen used to cool (electric generators), reducing “air” friction (fly wheels), fuel (hydrogen economy)Low strength not susceptibleHigh strength steel very susceptible.
23 Solubility of hydrogen is enhanced in triaxial tensile fields Destination of hydrogen
24 Diffusion is trap limited as first discovered by H Diffusion is trap limited as first discovered by H. Johnson (look into the MS&E lounge)
26 The classic example is failure of retaining rings in hydrogen cooled electric generators
27 Why megawatt electric generators use hydrogen cooling The specific heat capacity of hydrogen in constant volume is J/(kg/°C)14 times that of nitrogen32.5 times more than one of argon.Additional Benefit: Low friction loss
28 The problem is now much better understood, but led to some spectacular failures in the 70’s Note that crack growth rate increases by an order of magnitude when hydrogen is present
29 S-N (Woehler) curvesNote that fatigue limit falls to 1/5 of the value in air
32 More than 3 elements.General approach is to lump all the austenite formers together as “effective Nickel” and all the ferrite formers as “effective Chromium”. Below is UK (Cambridge U)Ni equivalent = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)Cr equivalent = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) (Nb) + 1.5(Ti) (W)Different authors have different weighting factors The approach is popular in the welding industry to predict the phase in a weld. Schaeffler diagram.NOT an EQUILIBRIUM DIAGRAM. But neither is the Fe-C phase diagram… as we discussed