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Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. [1] In plain-carbon steel , austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures.
Its primary crystalline structure is austenite (face-centered cubic). Such steels are not hardenable by heat treatment and are essentially non-magnetic. [2] This structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen.
As with conventional quench and tempering the material being heat treated must be cooled from the austenitizing temperature quickly enough to avoid the formation of pearlite. The specific cooling rate that is necessary to avoid the formation of pearlite is a product of the chemistry of the austenite phase and thus the alloy being processed.
Microstructures of four kinds of duplex stainless steel in each direction. Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N.
Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume. [4] Of considerably greater importance than the volume change is the shear strain , which has a magnitude of about 0.26 and which determines the shape of the plates of martensite.
2) Deformation in non-recrystallization regions. Austenite grains are elongated by rolling. Deformation bands might present within the band as well. Elongated grain boundaries and deformation bands are all nucleation sites for ferrite. 3) Deformation in austenite-ferrite two phase region. Ferrite nucleates and austenite are further work-hardened.
304 stainless steel is also used in the architectural field for exterior accents such as water and fire features. It is also a common coil material for vaporizers. Early SpaceX Starships used SAE 301 stainless steel in their construction, [8] before moving over to SAE 304L for the SN7 test tank [broken anchor] [9] and Starship SN8 in 2020. [10]
Subsequent investigations revealed that materials beyond ferrous alloys, such as non-ferrous alloys and ceramics, can also undergo diffusionless transformations. Consequently, the term "martensite" has evolved to encompass the resultant product arising from such transformations in a more inclusive manner.