Page 216 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

Duplex stainless steels feature the “best of both worlds” since they contain both
ferritic and austenitic phases. The ferritic phase helps to circumvent the problems
associated with stress-corrosion cracking in chloride environments, while the austenitic
component helps to improve the generally low strength and ductility of purely ferritic
steels. This biphasic steel is generated through careful heating/cooling of the Fe–Cr–Ni
alloy. When these materials initially solidify, they are 100% ferritic in nature; subsequent
cooling causes the precipitation of austenite within individual ferrite crystals.
Strengthening of stainless steels is carried out through cold-working processes –
rolling into sheets or drawing into wires/rods at temperatures around 25 C. This

generates a strong material through formation of a distorted bcc lattice that is
roughly analogous to martensite. Although we indicated that austenite may be
stabilized at room temperature through alloying, this is only a metastable phase. In
fact, if the steel is cooled to subzero temperatures or cold worked, the ferrite phase
will be generated. Since the austenite would have contained at least a small amount
of carbon, the resulting ferrite phase will also contain carbonaceous suspensions,
i.e., resulting in a martensite-like structure that will possess high hardness.
From a re-examination of the gamma loop in Figure 3.24, one can see that heating
stainless steel with typical Cr concentrations >11 wt.% will not yield austenite upon
heating, as this region is outside the loop. Hence, it is not possible to quench harden
these materials through transformation to the martensite phase. For example, aus-
tenitic, ferritic, and duplex stainless steels may not be hardened through heat
treatment due to their high Cr/C ratios. Interestingly, increasing the carbon concen-
tration will extend the gamma loop (Figure 3.29), allowing one to austenize the
stainless steel and harden through fast quenching to martensite.

Figure 3.29. The effect of carbon on the gamma loop. Reproduced with permission from Stainless Steels,
Lacombe, P.; Baroux, B.; Berauger, G. eds. Copyright 1993 EDP Sciences, Les Ulis, France.