Relationship between microstructure and tempering time and temperature in 55NiCrMoV7 steel

Microstructure of 55NiCrMoV7 steel during tempering was investigated using X-ray diffraction analysis technique. The results show that austenite in the steel decomposes gradually during tempering, and the carbides consist of the mass of M, C and a small quantity of VC. FWHM (Full-Width Half-Maximum) of M(211) of XRD patterns decreases linearly with tempering temperature, and exponentially with tempering time. FWHM of M(211) is considered as a representative parameter for indicating evolution of microstructure during tempering of the steel. One equation in form of Johnson-Mehl-A vrami relationship is proposed for describing the evolution of FWHM of M(211) in tempered steel with tempering temperature and tempering time. Parameters of the equation are identified in all tempering temperature range and in three temperature ranges, respectively. The precision of the latter is better than that of the former for predicting the evolution of FWHM of M(211).