How To Write A Chemical Formula From A Diagram Precipitation-Hardening Stainless Steel

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Precipitation-Hardening Stainless Steel

Precipitation-hardening stainless steels are iron-nickel-chromium alloys with one or more precipitation-hardening elements such as aluminum, titanium, copper, niobium, and molybdenum. Rain-hardening is achieved through a relatively simple aging treatment of the fabricated part.

The two main characteristics of all rain-hardening stainless steels are high strength and high creep resistance. High power, unfortunately, is gained at the expense of durability. The corrosion resistance of stainless steels is comparable to AISI 304 and AISI 316 austenitic alloys. Anti-aging treatments are designed to increase strength, resistance to decay, and durability. To improve durability, 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 chemical composition (in wt.%) is 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. Long-term aging causes the hardening of these intermetallic phases, which causes a decrease in strength, because the dislocation can pass through the 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, for example, A-286 (AISI 600), 17-10 P, HNM, etc. 631), PH 15-7 Mo, AM-350, AM-355, PH 14-8 Mo, etc.

The type is identified by the onset of martensite and the temperature of the completion of martensite (Ms and Mf) and the quenched microstructure.

During the heat treatment of precipitation-hardness stainless steels, regardless of the type, austenitization in the single-phase austenite region is always the first step. Austenitization is followed by rapid cooling (quenching).

Martensitic Precipitation-Hardening Stainless Steel

During the heat treatment of precipitation-hardness stainless steels, regardless of the type, austenitization in the single-phase austenite region is always the first step. Austenitization is followed by rapid cooling (quenching).

Martensite finish temperature (Mf) of martensitic precipitation-hardening stainless steels – such as 17-4 PH (AISI 630), Stainless W, 15-5 PH, CROLOY 16-6 PH, i -CUSTOM 450, CUSTOM 455, PH 13- 8 Mo, ALMAR 362, and IN-736 – just above room temperature. So, in quenching heat treatment, they completely change to martensite. Precipitation hardening is achieved by a special aging treatment at 480 °C to 620 °C (896 °F to 1148 °F) for 1 to 4 hours.

The initial martensite temperature (Ms) of martensitic precipitation-hardening stainless steels is required to be above room temperature to ensure complete martensite-to-austenite transformation in quenching.

One of the most commonly used empirical equations to estimate the initial 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 of martensitic steels is achieved by reheating to temperatures where the finer intermetallic phases – such as Ni3Al, Ni3Ti, Ni3 (Al, Ti), NiAl, Ni3Nb, i – Ni3Cu, carbides, and Laves phase – precipitate.

The 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 commonly used of the three types of precipitation-hardening stainless steels. From a metallurgical point of view, they can be considered as the predecessors of nickel-based and cobalt-based superalloys. An example would be work on the Fe-10Cr-35Ni-1.5Ti-1.5Al austenitic precipitation-hardening alloy, which was carried out before the Second World War.

The first martensite 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 austenitic precipitation-hardening stainless steels is high enough to fully stabilize austenite at room temperature.

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

Hardening is achieved by the precipitation of a very fine, coherent, intermetallic Ni3Ti phase, when the austenite is reheated to high temperatures. Precipitation in austenitic precipitation-hardening stainless steels is much slower than in martensitic or semiaustenitic precipitation-hardening stainless steels. For example, to achieve near-maximum hardness in A-286 (AISI 600), 16 hours at 718 °C (1325 °F) is required.

As with all precipitation hardening steels, the strength of A-286 (AISI 600) can be further increased by cold working before aging.

Austenitic precipitation-hardening stainless steels do not have magnetic phases and, in general, have higher corrosion resistance than martensitic or semiaustenitic precipitation-hardening stainless steels.

Semiaustenitic Precipitation-Hardening Stainless Steel

Semiaustenitic precipitation-hardening steels are provided in a metastable austenitic state. They may contain up to 20 % delta ferrite in proportion to austenite at the solution temperature. The metastable nature of the austenitic matrix depends on the amount of austenite stabilizing and ferrite stabilizing elements.

Martensite finish temperature (Mf) of 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 – it’s good. below room temperature. As a result, their microstructure is mostly austenitic (and highly ductile) at quenching temperature treatment.

After forging, the austenite-to-martensite transformation is achieved by a conditioning treatment at about 750 °C (1382 °F), the main purpose of which is to increase the temperature of Mf to the temperature point by precipitation of alloy carbides (mainly chromium -rich M23C6 carbides). This, in turn, reduces the carbon and chromium content of austenite (see the above formula for Ms temperature which shows that if the amount of dissolved carbon and chromium in austenite is reduced, Ms temperature is raised significantly). The transformation to martensite is complete after cooling.

Cryogenic (subzero) treatment is required if higher temperatures are used, typically 930 °C to 955 °C (1706 °F to 1751 °F). At such a temperature, the number of alloy carbides that pass through is small, giving a temperature of Mf well below the temperature. The strength of martensite formed in this way (high-temperature conditioning + cryogenic treatment) is higher than that formed by transformation at low temperatures, due to the high carbon content of the former.

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