Analysis of processes occurring in carbon steels during the isothermal transformation of austenite
The pearlitic transformation of austenite is a two-stage process: the first stage is associated with the formation of the phase that controls the growth of the pearlite column — cementite; the formation of ferrite and a thickening of the cementite plates occur in the second stage. Both stages are separate in time.
The formation of upper bainite consisting of a cementite skeleton and ferrite envelope, the latter forming by diffusionless means from decarbonized austenite, occurs within a narrow temperature interval (500–400°C).
Acicular (lower) bainite forms below 400°C via segregation of disperse cementite inclusions with their subsequent envelopment by martensite. The martensite is decarbonized by means of diffusion, transforming into ferrite.
At temperatures below 240°C, the occurrence of the diffusionless transformation of austenite of initial composition is possible in eutectoid steel. The martensite that forms during sufficient isothermal holding may decompose in accordance with two mechanisms: with the segregation of cementite (the formation of lower bainite), and with the segregation of ε-carbides inaccordance with the mechanism of spinoidal stratification (the formation of temper martensite).
All of the transformations that we have enumerated are reflected in diagrams of the isothermal decomposition of austenite, which have been constructed schematically for eutectoid, subeutectoid, and transeutectoid steels. A second region of pearlitic transformation, which expands with increasing carbon content in the steel, is located on these diagrams between the curves of the upper and lower bainite.
KeywordsFerrite Austenite Martensite Carbon Steel Cementite
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