Martensite can be divided into lath martensite and lamellar martensite. When the austenite carbon content is less than 0.20%, the quenched martensite is lath-like. When the austenite has a carbon content of more than 1.0%, it is in the form of a sheet.
Bainite has various histological forms and can be roughly classified into granular bainite, upper bainite and lower bainite according to its metallographic structure.
The first stage of tempering (≤250 ° C): decomposition of quenched martensite;
The second stage of tempering (200-300 ° C): the transformation of retained austenite;
The third stage of tempering (250-400 ° C): the transformation of carbide type;
The fourth stage of tempering (≥400 °C): spheroidization, coarsening of ferrite, recovery and recrystallization of ferrite.
According to the tempering temperature and the corresponding tissue change, the microstructure transformation process of the quenched steel during tempering can be divided into four stages:
1. Tempered martensite - The quenched steel is tempered at 150-250 ° C to form a tempered martensite structure.
2. Tempered tortite - After tempering at 350-450 °C, the quenched steel forms a tempered tortite structure.
3. Tempered Sorbite - After quenching at 500-650 °C, the quenched steel forms a tempered sorbite.
4. Tempered pearlite - quenched steel in the temperature range of 650 ° C - A1 after a long time tempering, that is, the formation of tempered pearlite structure.
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