Martensitic Precipitation-Hardening Stainless Steel
During the heat treatment of precipitation-hardening stainless steels, regardless of their type, austenitization in the single-phase austenite region is always the first step. Austenitization is then followed by a relatively rapid cooling (quenching).
The martensite finish temperature (Mf) of the martensitic precipitation-hardening stainless steels — such as 17-4 PH (AISI 630), Stainless W, 15-5 PH, CROLOY 16-6 PH, CUSTOM 450, CUSTOM 455, PH 13-8 Mo, ALMAR 362, and IN-736 — is just above room temperature. Thus, upon quenching from the solution-treatment temperature they transform completely into martensite. Precipitation hardening is achieved by a single aging treatment at 480 °C to 620 °C (896 °F to 1148 °F) for 1 to 4 hours.
The martensite start temperature (Ms) of the martensitic precipitation-hardening stainless steels is required to be above room temperature in order to ensure a full martensite-to-austenite transformation upon quenching.
One of the empirical equations that is often used to predict the martensite start temperature (in °F) is as follows:
Ms = 2160 – 66·(% Cr) – 102·(% Ni) – 2620·(% C + % N)
where Cr = 10-18 %, Ni = 5-12.5 %, and C + N = 0.035-0.17 %.
Precipitation hardening in the martensitic steels is achieved by reheating to temperatures at which very fine intermetallic phases — such as Ni3Al, Ni3Ti, Ni3(Al,Ti), NiAl, Ni3Nb, Ni3Cu, carbides, and Laves phase — precipitate.
A lath martensite structure provides an abundance of nucleation sites for the precipitation of intermetallic phases.
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