Page 207 - Materials Chemistry, Second Edition

194 3 Metals

concentrations, martensite may be formed at temperatures well below 0 C. Hence,
high-C steels must be quenched in low-temperature media (e.g., dry ice/acetone, liquid
nitrogen) to ensure full conversion of austenite to martensite.
The effect of alloying elements may be understood by examining the austenite/
ferrite regions of the Fe–C phase diagram with respect to the alloy concentrations
(Figure 3.24). Ferrite-forming elements such as Al, Si, W, Cr, and Mo result in a
contraction of the austenite region, forming a gamma loop. By contrast, austenite-
stabilizing elements such as C, N, Mn, Ni, and Cu cause an expansion of the
austenitic phase boundary. Austenite stabilizers inhibit the nucleation and growth
of ferrite and pearlite/bainite phases, assisting in the formation of pure martensite
upon quenching. In general, since bcc ferrite contains more voidspace than fcc
austenite, larger interstitial dopants may be incorporated into these lattices. Hence,
ferrite stabilizers tend to be larger in contrast to the smaller size of austenite
stabilizers. Most importantly, in accord with the “like dissolves like” principle,
bcc and fcc dopants will tend to stabilize ferrite and austenite, respectively.
A slow cooling rate will give the greatest opportunity for controlled atomic
migration within the lattice, and growth of large ordered crystallites. However, in
the rapid non-equilibrium conditions used to form martensite, there is no time for
carbon diffusion to occur; this yields a supersaturated solution, with >2 wt.% C
present as an interstitial impurity. During this phase transition, the fcc lattice of
austenite is transformed to a distorted bcc lattice, commonly referred to as body-
centered tetragonal bct (Figure 3.25). The degree of distortion from a perfect bcc
lattice (cubic lattice axes ratio, c/a ¼ 1) is amplified with increasing carbon con-
centration (Eq. 18):
c
ð18Þ ¼ 1 þ 0.045(wt% C)
a
The fcc–bct conversion, known as the Bain transformation, is a diffusionless
process. That is, unlike the previous high-temperature conversions we saw earlier
(e.g., austenite to ferrite), martensite can form at temperatures significantly below
room temperature, within 1 10 7 s. Such a fast growth rate precludes the decipher-
ing of the exact mechanism for the nucleation and growth of martensite. However,
leading theories suggest that the growth initiates from dislocations in the solid. [11]

Tempering
Even though martensite exhibits a high hardness, the as-quenched material is much too
brittle and highly stressed for structural applications. The ductility and toughness of
martensite is greatly improved through post-annealing, a process known as tempering.
This process relieves stresses in the solid through conversion of bct martensite into bcc
ferrite, with precipitation of iron carbide particulates. It is important to note that the
annealed structure is not simply pearlite/ferrite, but is best referred to as tempered
martensite. During the annealing process for martensite, a number of key transforma-
tions occur:
(i) 50–250 C: Interstitial carbon atoms in martensite begin to diffuse within the

bct lattice. This results in precipitation hardening from the formation of