Content Review: Plain Carbon Steel Alloy steels Influence of alloying elements Influence of hydrogen (unwanted alloy element) HSLA Steel specification Steel properties Appendix: Read for 4/17
1. Overview a) Exceeds minimum alloy contents b) Reasons for alloying elements c) Nomenclature (AISI designation system) d) Virtually always heat treated
2) Alloying Elements and their Effects in Steel Common classifications a) High Strength Low Alloy Steels b) Low Alloy Steels c) Chromium-Molybdenum Steels (Tool Steels)
Review : Carbon Steels Only 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).
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 particles Manganese increases the 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)
Alloy steels Alloy 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).
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
If a B shows up it means Boron containing. If H shows up means fits particular specs for hardenability Designation AISI
Alloy elements: Added to make steel better. Can be Higher yield stress Higher ductility Higher hardness Better machinability Higher service temperature Better corrosion resistance etc….. application specific
Change in lower yield stress point, low alloy steels Solid Solution Strengthener P is effective but when it gets into the GB it embrittles. See lecture 20. N is introduced by nitriding steel
Elements that promote austenite, I.e. a larger field Ni Mn
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.
Elements that lower M s Practically all ! If fully dissolved in phase M s = 561 - 474C - 33Mn - 17Ni - 17Cr - 21Mo
Carbide-forming elements Cr, W, Mo, V, Ti, Nb, Ta, Zr. Affinity for Carobon increases from left to right Some overlap with ferrite promoters (no accident think Fe 3 C ) Non Fe containing carbides Cr7C3 W2C, VC, Mo2C. Double carbides contain both Fe and Carbide former e.g Fe 4 W 2 C. 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 Fe 4 W 2 C or Fe 4 Mo 2 C; M23C6 represents Cr 23 C 6 etc. For how to stabilize these carbides at high T see Lect. 20
Carbide stabilizers The 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 are I.e. Mn will promote C to exist as Fe 3 C rather than being dissolved in the matrix. Thermo language would be that the activity of C is f(x i ). 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.
Nitrides Nitride former are similar C former TiN is a well known ultrahard compound. AlN precips distorts lattice and generates high dislocation density Nitrided surfaces are extremely hard
Nitrogen: Nitrogen can form a solid solution with ferrite at nitrogen contents up to about 6%. Above ~ 6% N, gamma prime with a composition of Fe 4 N. Above ~ 8%, the equilibrium product is compound, Fe 3 N. 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
Hydrogen An 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 situation a) 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 H 2 molecules generating interior pressure (do the thermo, pressure is tremendous) c) Hydrogen combining with C to form CH 4 again forming high pressure bubbles. d) hydrogen atmosphere around dislocation
Source of hydrogen cathodic protection, phosphating, pickling, and electroplating (notorious in high strength fasteners that also need to be corrosion resistant. welding with electrodes covered with coating containing moisture hydrogen used to cool (electric generators), reducing air friction (fly wheels), fuel (hydrogen economy) Low strength not susceptible High strength steel very susceptible.
Destination of hydrogen Solubility of hydrogen is enhanced in triaxial tensile fields
Diffusion is trap limited as first discovered by H. Johnson (look into the MS&E lounge)
The classic example is failure of retaining rings in hydrogen cooled electric generators
Why megawatt electric generators use hydrogen cooling The specific heat capacity of hydrogen in constant volume is 10.25 J/(kg/°C) 14 times that of nitrogen 32.5 times more than one of argon. Additional Benefit: Low friction loss
The problem is now much better understood, but led to some spectacular failures in the 70s Note that crack growth rate increases by an order of magnitude when hydrogen is present
S-N (Woehler) curves Note that fatigue limit falls to 1/5 of the value in air
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) + 1.75(Nb) + 1.5(Ti) + 0.75(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