Precipitation hardening stainless steels are iron-nickel-chromium alloys containing one or more precipitation hardening elements such as aluminum, titanium, copper, niobium, and molybdenum. Precipitation hardening is achieved by a relatively simple aging treatment of the manufactured part.
The two main characteristics of all precipitation hardening stainless steels are high strength and high corrosion resistance. Unfortunately, high strength comes at the expense of toughness. The corrosion resistance of precipitation-hardened stainless steels is comparable to standard austenitic alloys AISI 304 and AISI 316. 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-0.2 Al-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 coarsening of these intermetallic phases, which in turn causes a decrease in strength, due to the fact that thick intermetallic phases can be bypassed by dislocations.
There are three types of precipitation hardening stainless steels:
– Precipitation hardening martensitic stainless steels, for example, 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., – precipitation hardening austenitic stainless steels, for example, A-286 (AISI 600), 17-10 P, HNM, etc., and – precipitation hardening semi-austenitic stainless steels, for example, 17- PH 7 (AISI 631), PH 15-7 MB, AM-350, AM-355, PH 14-8 MB, etc.
The type is determined by the martensite start and martensite end temperatures (Ms and Mf), as well as the quenched microstructure.
During 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 relatively rapid cooling (annealing).
Precipitation Hardening Martensitic Stainless Steel
During 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 relatively rapid cooling (annealing).
The martensite finishing temperature (Mf) of precipitation hardening martensitic 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 MB, ALMAR 362 and IN-736: It is just above room temperature. Therefore, on cooling from the solution treatment temperature, they completely transform to 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 starting temperature (Ms) of precipitation-hardening martensitic stainless steels is required to be above room temperature to ensure complete transformation from martensite to austenite on cooling.
One of the empirical equations often used to predict the onset temperature of martensite (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 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 structure of martensite provides a large number of nucleation sites for the precipitation of intermetallic phases.
Precipitation Hardening Austenitic Stainless Steel
Austenitic grades are the least used of the three types of precipitation hardening stainless steels. From a metallurgical point of view, they can be considered precursors to nickel-based and cobalt-based superalloys. An example would be the work on the precipitation hardening austenitic alloy Fe-10Cr-35Ni-1.5Ti-1.5Al, which was done before World War II.
The martensite initiation temperature (Ms) of precipitation-hardenable austenitic 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 precipitation-hardening austenitic stainless steels is high enough to completely stabilize the austenite at room temperature.
The highly stable nature of the austenitic matrix eliminates all potential problems related to embrittlement, even at extremely low temperatures. Precipitation hardening austenitic stainless steels are therefore very attractive alloys when it comes to cryogenic applications.
Hardening is achieved by precipitation of a very fine, coherent, intermetallic Ni3Ti phase when the austenite is reheated to elevated temperatures. Precipitation in precipitation-hardening austenitic stainless steels is considerably slower compared to precipitation-hardening martensitic or semi-austenitic stainless steels. For example, to achieve near peak cure on A-286 (AISI 600), 16 hours at 718°C (1325°F) are required.
Like all precipitation hardening stainless steels, the strength of A-286 (AISI 600) can be further increased by cold working before aging.
Precipitation-hardening austenitic stainless steels do not contain magnetic phases and, in general, have higher corrosion resistance than precipitation-hardening martensitic or semi-austenitic stainless steels.
Semi-austenitic precipitation hardened stainless steel
Precipitation hardening semi-austenitic stainless steels are supplied in the metastable austenitic state. They may also contain up to 20% delta ferrite in equilibrium with austenite at solution temperature. The metastable nature of the austenitic matrix depends on the amounts of austenite stabilizing elements and ferrite stabilizing elements.
The martensite finishing temperature (Mf) of precipitation-hardening semi-austenitic stainless steels, such as PH 17-7 (AISI 631), PH 15-7 Mo, AM-350, AM-355 and PH 14-8 Mo, is well below room temperature. Consequently, its microstructure is efficiently austenitic (and highly ductile) on cooling from the solution treatment temperature.
After formation, the transformation from austenite to martensite is achieved by a conditioning treatment at approximately 750 °C (1382 °F), whose main objective is to raise the Mf temperature to room temperature close to room temperature through the precipitation of alloy carbides (mainly chrome-rich M23C6 carbides). This, in turn, reduces the carbon and chromium content of the austenite (see the formula above for the Ms temperature which shows that if the amount of dissolved carbon and chromium in the austenite is reduced, the Ms temperature increases significantly). The transformation to martensite is complete on cooling.
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, causing the Mf temperature to be well below room temperature. The strength of the martensite thus formed (conditioning at high temperature + cryogenic treatment) is higher than that formed by transformation at lower temperatures, due to the higher carbon content of the former.