Tempering of Martensitic Steel Studied by Electron Microscopy and Diffraction

Tempering of martensitic steel can be devided into four stages : initial, first, second and third. Structural changes have been studied in detail, and the following results have been obtained. At the initial stage, (1) the formation of interstitial carbon atom cluster, (2) the formation of modulated structure, and (3) the formation of long period ordered phase with respect to the interstitial carbon atoms or vacancies take place. At the first stage, the precipitation of η-Fe_2C and the formation of low carbon martensite take place. At the second stage, the decomposition of retained austenite produces θ-Fe_3C and α-iron. At the third stage, micro-syntactic intergrowth of X-Fe_5C_2 and θ-Fe_3C and a gradual transformation of X to θ take place