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–hardeninaxg 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).

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.

Austenitic Precipitation–Hardening Stainless Steel

The austenitic grades are the least widely used of the three types of precipitation–hardening stainless steels. From a metallurgical point of view, they can be considered to be the precursors of the nickel–based and cobalt–based superalloys. An example would be the work on Fe–10Cr–35Ni–1.5Ti–1.5Al austenitic precipitation–hardening alloy, which was conducted before the Second World War.

The martensite start temperature (Ms) of the austenitic precipitation–hardening stainless steels – such as A–286 (AISI 600), 17–10 P, and HNM – is so low that they cannot be transformed into martensite. The nickel content of the austenitic precipitation–hardening stainless steels is sufficiently high to fully stabilize austenite at room temperature.

The highly stable nature of the austenitic matrix eliminates all the potential problems related to embrittlement, even at extremely low temperatures. The austenitic precipitation–hardening stainless steels are therefore very attractive alloys when it comes to cryogenic applications.

Strengthening is achieved by the precipitation of very fine, coherent, intermetallic Ni3Ti phase, when the austenite is reheated to elevated temperatures. Precipitation in austenitic precipitation–hardening stainless steels is considerably more sluggish compared to either martensitic or semiaustenitic precipitation–hardening stainless steels. For example, in order to achieve near–maximum hardening in A–286 (AISI 600), 16 hours at 718 °C (1325 °F) is required.

Like all precipitation–hardening stainless steels, the strength of A–286 (AISI 600) can be further increased by cold work prior to aging.

The austenitic precipitation–hardening stainless steels contain no magnetic phases and, in general, have higher corrosion resistance than the martensitic or semiaustenitic precipitation–hardening stainless steels.

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 (see the above given formula for Ms temperature which shows that if the amount of dissolved carbon and chromium in austenite is reduced, the Ms temperature is significantly raised). 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.