Austenitic phases, carbides

The best-known eutectoid reaction is that which occurs in steel where the austenite phase, stable at high temperatures, transforms into (he eutectoid structure known as pcarlitc In this transformation, the austenite phase, containing 0.8% carbon in solid solution, transforms to a mixture of ferrite (nearly pure body-centered cubic irom anti iron-carbide (Fe-.Ct. Al atmospheric pressure, the equilibrium temperature for this reaction is 723 C. This temperature is the eutectoid temperature... 

The temperature-composition section at 6.3 mass% W was calculated by [1998Hac] during examination of a ternary C-Fe-W steel with the purpose of characterizing the carbide morphologies that arise during the diffusional decomposition of austenite. Phase stability at the melting point of a low-alloyed steel (1527°C) was estimated by [2002Bab]. 

A precipitation of various carbides or intermetallic phases in the austenite phase ... 

When a steel is cooled sufficiendy rapidly from the austenite region to a low (eg, 25°C) temperature, the austenite decomposes into a nonequilihrium phase not shown on the phase diagram. This phase, called martensite, is body-centered tetragonal. It is the hardest form of steel, and its formation is critical in hardening. To form martensite, the austenite must be cooled sufficiently rapidly to prevent the austenite from first decomposing to the softer stmeture of a mixture of ferrite and carbide. Martensite begins to form upon reaching a temperature called the martensite start, Af, and is completed at a lower temperature, the martensite finish, Mj, These temperatures depend on the carbon and alloy content of the particular steel. 

F, Ferrite, C, Carbides, A, Austenite M, Martensite TS, as tempered IP, Intermetallic phases PH, Precipation hardened H T, Heat treated and tempered, H, Heat-treated. 

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