Al And N Express Your Answer As A Chemical Formula Precipitation-Hardening Stainless Steel

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Precipitation-Hardening Stainless Steel

Precipitation-hardenable stainless steels are iron-nickel-chromium alloys that contain one or more precipitation-hardening elements such as aluminum, titanium, copper, niobium, and molybdenum. Precipitation hardening is achieved by relatively simple aging 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 hardening stainless steels is comparable to that of standard AISI 304 and AISI 316 austenitic alloys. Aging procedures are designed to optimize strength, corrosion resistance and toughness. The amount of carbon is kept low to improve toughness.

The first commercial precipitation hardening stainless steel was developed in 1946 by US Steel. The alloy was named Stainless W (AISI 635) and had a nominal chemical composition (wt%) of 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 coarsening of these intermetallic phases, which in turn causes a decrease in strength, as dislocations can bypass the coarse intermetallic phases.

There are three types of precipitation hardening stainless steel:

– Martensitic precipitation-resistant 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-resistant stainless steels, e.g. A-286 (AISI 600), 17-10 P, HNM, etc., and – semi-austenitic precipitation-resistant 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 start and finish temperature of martensite (Ms and Mf), as well as the quenched microstructure.

In the heat treatment of precipitation-hardening stainless steels, regardless of their type, austenitizing in the single-phase austenite region is always the first step. Austenitization is then followed by relatively rapid cooling (quenching).

Martensite precipitation hardening stainless steel

In the heat treatment of precipitation-hardening stainless steels, regardless of their type, austenitizing in the single-phase austenite region is always the first step. Austenitization is then followed by relatively rapid cooling (quenching).

Martensitic finish temperature (Mf) for martensitic precipitation-resistant 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 and IN -736 – is slightly above room temperature. Thus, after quenching from the solution treatment temperature, they completely transform into martensite. Precipitation curing is achieved with a single aging at 480°C to 620°C (896°F to 1148°F) for 1 to 4 hours.

Martensitic precipitation hardening stainless steels must have a martensite initiation temperature (Ms) above room temperature to ensure complete martensite-to-austenite transformation during quenching.

One of the empirical equations often used to predict the martensite onset temperature (°F) is:

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 heating to a temperature at which very fine intermetallic phases such as Ni3Al, Ni3Ti, Ni3(Al,Ti), NiAl, Ni3Nb, Ni3Cu, carbides and Laves phase are precipitated.

The lath martensite structure provides numerous nucleation sites for the precipitation of intermetallic phases.

Austenitic precipitation-hardening stainless steel

Of the three precipitation-resistant stainless steels, the austenitic grades are the least used. From a metallurgical point of view, they can be considered the precursors of nickel- and cobalt-based superalloys. An example would be the work done before World War II with Fe-10Cr-35Ni-1.5Ti-1.5Al austenitic precipitation-hardening alloy.

Austenitic precipitation-hardening stainless steels—such as A-286 (AISI 600), 17-10 P, and HNM—have a martensite onset temperature (Ms) so low that they cannot be transformed into martensite. The nickel content in austenitic precipitation-resistant stainless steels is high enough to fully stabilize austenite at room temperature.

The very stable nature of the austenite matrix eliminates any potential spalling problems even at very low temperatures. Therefore, austenitic precipitation-resistant stainless steels are very attractive alloys for cryogenic applications.

Strengthening is achieved by the precipitation of a very fine, coherent intermetallic Ni3Ti phase when the austenite is reheated to an elevated temperature. Precipitation in austenitic precipitation-hardened stainless steels is considerably slower compared to either martensitic or semi-austenitic precipitation-hardened stainless steels. For example, A-286 (AISI 600) requires 16 hours at 718°C (1325°F) to reach near maximum cure.

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

Austenitic precipitation-hardened stainless steels do not contain magnetic phases and generally have higher corrosion resistance than martensitic or semi-austenitic precipitation-hardened stainless steels.

Semi-austenitic precipitation-hardening stainless steel

Semi-austenitic precipitation-resistant stainless steels are supplied in a metastable austenitic state. They may also contain up to 20% delta ferrite, which is in equilibrium with austenite at the solution temperature. The metastability of the austenite matrix depends on the amount of austenite-stabilizing and ferrite-stabilizing elements.

The martensite finishing temperature (Mf) of semi-austenitic precipitation hardening stainless steels – eg 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) after quenching from the solution processing temperature.

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

When high conditioning temperatures are used, typically 930°C to 955°C (1706°F to 1751°F), cryogenic (subzero) treatment is required. At such high temperatures, the amount of alloy carbides that precipitate is relatively small, making the Mf temperature well below room temperature. The strength of martensite formed in this way (high temperature conditioning + cryogenic treatment) is higher than the strength of martensite formed by transformation at lower temperatures, due to the higher carbon content of the former.

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