Semiaustenitic Precipitation-Hardening Stainless Steel
The semiaustenitic precipitation-hardening stainless steels are supplied in the metastable austenitic condition. They may also contain up to 20 % of delta ferrite in equilibrium with the austenite at the solution temperature. The metastable nature of the austenitic matrix depends on the amounts of austenite stabilizing and ferrite stabilizing elements.
The martensite finish temperature (Mf) of the semiaustenitic precipitation-hardening stainless steels — such as 17-7 PH (AISI 631), PH 15-7 Mo, AM-350, AM-355, and PH 14-8 Mo — is well below room temperature. Consequently, their microstructure is predominantly austenitic (and highly ductile) upon quenching from the solution-treatment temperature.
After forming, the austenite-to-martensite transformation is achieved by a conditioning treatment at about 750 °C (1382 °F), whose main goal is to raise the Mf temperature to the vicinity of room temperature by the precipitation of alloy carbides (mainly chromium-rich M23C6 carbides). This, in turn, reduces the carbon and chromium content of the austenite. As the below given empirical formula for Ms shows, if the amount of dissolved carbon and chromium in austenite is reduced, the Ms temperature is significantly raised:
Ms = 2160 – 66·(% Cr) – 102·(% Ni) – 2620·(% C + % N)
where Cr = 10-18 %, Ni = 5-12.5 %, and C + N = 0.035-0.17 %.
The transformation to martensite is completed upon cooling.
A cryogenic (subzero) treatment is required if a high conditioning temperature is used, typically 930 °C to 955 °C (1706 °F to 1751 °F). At such high temperatures, the amount of alloy carbides that precipitate is relatively small, rendering the Mf temperature well below room temperature. The strength of the martensite that is formed in this way (high-temperature conditioning + cryogenic treatment) is higher than that formed by transformation at lower temperatures, due to a higher carbon content of the former.
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