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Precipitation-Hardening Stainless Steel

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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 uncomplicated aging medicine of the fabricated part.

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How is Precipitation-Hardening Stainless Steel

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The two main characteristics of all precipitation-hardening stainless steels are high vigor and high corrosion resistance. High vigor is, unfortunately, achieved at the cost of toughness. The corrosion resistance of precipitation-hardening stainless steels is comparable to that of the accepted Aisi 304 and Aisi 316 austenitic alloys. The aging treatments are designed to optimize strength, corrosion resistance, and toughness. To enhance toughness, the number of carbon is kept low.

The first market precipitation-hardening stainless steel was advanced 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. Continued aging causes the coarsening of these intermetallic phases, which in turn causes the decline in strength, due to the fact that dislocations can bypass base 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, institution 450, institution 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 carefully by the martensite start and the martensite conclude climatic characteristic (Ms and Mf) as well as the as-quenched microstructure.

During the heat medicine 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 medicine 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 conclude climatic characteristic (Mf) of the martensitic precipitation-hardening stainless steels - such as 17-4 Ph (Aisi 630), Stainless W, 15-5 Ph, Croloy 16-6 Ph, institution 450, institution 455, Ph 13-8 Mo, Almar 362, and In-736 - is just above room temperature. Thus, upon quenching from the solution-treatment climatic characteristic they transform fully into martensite. Precipitation hardening is achieved by a particular aging medicine at 480 °C to 620 °C (896 °F to 1148 °F) for 1 to 4 hours.

The martensite start climatic characteristic (Ms) of the martensitic precipitation-hardening stainless steels is required to be above room climatic characteristic 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 climatic characteristic (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 fullness 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 carefully 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 climatic characteristic (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 article of the austenitic precipitation-hardening stainless steels is sufficiently high to fully stabilize austenite at room temperature.

The extremely stable nature of the austenitic matrix eliminates all the possible problems linked to embrittlement, even at extremely low temperatures. The austenitic precipitation-hardening stainless steels are therefore very animated 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 whether 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 vigor of A-286 (Aisi 600) can be additional increased by cold work prior to aging.

The austenitic precipitation-hardening stainless steels consist of 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 consist of up to 20 % of delta ferrite in equilibrium with the austenite at the clarification temperature. The metastable nature of the austenitic matrix depends on the amounts of austenite stabilizing and ferrite stabilizing elements.

The martensite conclude climatic characteristic (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 extremely ductile) upon quenching from the solution-treatment temperature.

After forming, the austenite-to-martensite transformation is achieved by a conditioning medicine at about 750 °C (1382 °F), whose main goal is to raise the Mf climatic characteristic to the vicinity of room climatic characteristic by the precipitation of alloy carbides (mainly chromium-rich M23C6 carbides). This, in turn, reduces the carbon and chromium article of the austenite (see the above given formula for Ms climatic characteristic which shows that if the number of dissolved carbon and chromium in austenite is reduced, the Ms climatic characteristic is significantly raised). The transformation to martensite is completed upon cooling.

A cryogenic (subzero) medicine is required if a high conditioning climatic characteristic is used, typically 930 °C to 955 °C (1706 °F to 1751 °F). At such high temperatures, the number of alloy carbides that precipitate is relatively small, rendering the Mf climatic characteristic well below room temperature. The vigor 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 article of the former.

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