Precipitation-Hardening Stainless Steel
The precipitation-hardening stainless steels are iron-nickel-chromium alloys containing one or more precipitation hardening elements such as aluminum, titanium, copper, niobium, and molybdenum. The precipitation hardening is achieved by a relatively simple aging treatment of the fabricated part.
The two main characteristics of all precipitation-hardening stainless steels are high strength and high corrosion resistance. High strength is, unfortunately, achieved at the expense of toughness. The corrosion resistance of precipitation-hardening stainless steels is comparable to that of the standard AISI 304 and AISI 316 austenitic alloys. The aging treatments are designed to optimize strength, corrosion resistance, and toughness. To improve toughness, the amount of carbon is kept low.
The first commercial precipitation-hardening stainless steel was developed by US Steel in 1946. The alloy was named Stainless W (AISI 635) and its nominal chemical composition (in wt. %) was Fe-0.05C-16.7Cr-6.3Ni-0.2Al-0.8Ti.
The precipitation hardening process involves the formation (precipitation) of very fine intermetallic phases such as Ni3Al, Ni3Ti, Ni3(Al,Ti), NiAl, Ni3Nb, Ni3Cu, carbides, and Laves (AB2) phases. Prolonged aging causes the coarsening of these intermetallic phases, which in turn causes the decline in strength, due to the fact that dislocations can bypass coarse intermetallic phases.
There are three types of precipitation-hardening stainless steels:
• Martensitic precipitation-hardening stainless steels, e.g., 17-4 PH (AISI 630), Stainless W, 15-5 PH, CROLOY 16-6 PH, CUSTOM 450, CUSTOM 455, PH 13-8 Mo, ALMAR 362, IN-736, etc.,
• Austenitic precipitation-hardening stainless steels, e.g., A-286 (AISI 600), 17-10 P, HNM, etc., and
• Semiaustenitic precipitation-hardening stainless steels, e.g., 17-7 PH (AISI 631), PH 15-7 Mo, AM-350, AM-355, PH 14-8 Mo, etc.
The type is determined by the martensite start and the martensite finish temperature (Ms and Mf) as well as the as-quenched microstructure.
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).
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