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Steel Heat Treatment - Metallurgy and Technologies 2nd ed - G. Totten (CRC_ 2007) 3

Tham khảo tài liệu 'steel heat treatment - metallurgy and technologies 2nd ed - g. totten (crc_ 2007) 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | stage the recrystallization of martensite lath is more complete and there is a tendency for the formation of equiaxed grains and extensive grain growth. 2.4.2 Austenite Formation The early work on austenite formation by Robert and Mehl 35 focused on the nucleation of austenite from a ferrite-pearlite mixture. Various researchers have since shown the complexity of austenite formation from a two-phase mixture of ferrite and cementite and have made attempts at modeling austenite formation as a function of composition and microstructure 36-41 . On heating steel with a spheroidized ferrite cementite mixture the austenite phase nucleates at the ferrite-cementite boundary. With further heating the austenite phase consumes the cementite and then grows into ferrite through diffusion-controlled growth. In a pearlitic microstructure the austenite may nucleate in the cementite and grow into the colony by dissolving both ferrite and cementite. A typical micrograph of austenite growth into a pearlite colony is shown in Figure 2.12a. A schematic illustration of the growth is shown in Figure 2.12b. Recent work has shown that it is possible to model the above phenomenon with computational tools 42 . It is possible to construct TTT diagrams for the austenite growth for any steel to evaluate the effect of the initial microstructure. These diagrams do not show a C-curve behavior because both the driving force for austenite formation and the diffusivity increase with temperature. This results in monotonic acceleration of austenite formation as the temperature increases. The rate of austenite formation also increases with the presence of residual austenite in the microstructure as demonstrated by Yang and Bhadeshia who studied the austenite growth kinetics in a bainitic microstructure 43 44 . The microstructure contained residual austenite films and there was no requirement for nucleation of austenite and the austenite films grew with an increase in the temperature. After the completion