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.
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