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The steel phases
When steels with less than 0.8% carbon, known as a hypoeutectoid steel , are cooled from an austenitic phase the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, resulting in a cementite-ferrite mixture. Cementite is a hard and brittle intermetallic compound with the chemical formula of Fe3C. At the eutectoid, 0.8% carbon, the cooled structure takes the form of pearlite, named after its resemblance to mother of pearl. For steels that have more than 0.8% carbon the cooled structure takes the form of pearlite and cementite.
Perhaps the most important polymorphic form is martensite, a metastable phase which is significantly stronger than other steel phases. When the steel is in an austenitic phase and then quenched it forms into martensite, because the atoms "freeze" in place when the cell structure changes from FCC to BCC. Depending on the carbon content the martensitic phase takes different forms. Below approximately 0.2% carbon it takes an α ferrite BCC crystal form, but higher carbon contents take a body-centered tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.
Martensite has a lower density than austenite does, so that transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when water quenched, although they may not always be visible.
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Perhaps the most important polymorphic form is martensite, a metastable phase which is significantly stronger than other steel phases. When the steel is in an austenitic phase and then quenched it forms into martensite, because the atoms "freeze" in place when the cell structure changes from FCC to BCC. Depending on the carbon content the martensitic phase takes different forms. Below approximately 0.2% carbon it takes an α ferrite BCC crystal form, but higher carbon contents take a body-centered tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.
Martensite has a lower density than austenite does, so that transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when water quenched, although they may not always be visible.
www.china-sheetpiling.com