Alloy steel elements

1. 1. Overview
a) Exceeds minimum alloy contents
b) Reasons for alloying elements
c) Nomenclature (AISI designation system)
d) Virtually always heat treated

2. 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)

3. 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).

4. 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 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)

5. Review: Classification

6. 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).

7. 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

8. If a “B”
shows up it
means Boron
containing.
If “H” shows
up means fits
particular
specs for
hardenability
Designation AISI

9. 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

10. 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

11. Elements that promote austenite, I.e. a larger g field
• Ni
• Mn

12. 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.

13. Elements that lower Ms
Practically all ! If fully dissolved in g phase
Ms = 561 - 474C - 33Mn - 17Ni - 17Cr - 21Mo

14. 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 Fe3C )
• Non Fe containing carbides
Cr7C3 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

15. 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 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.

16. 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

17. 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

18. 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 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

20. 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.

21. Destination of hydrogen
Solubility of
hydrogen is
enhanced in
triaxial tensile
fields

22. Diffusion is trap
limited as first
discovered by H.
Johnson (look into
the MS&E lounge)