| Through the use of XRD and SEM analysis technology, the microstructure evolutions of 55NiCrMoV7 hot die steel, tempered from 100℃ to 700℃ and tested in low cyclic fatigue (LCF) from 20 to 600℃,were investigated. The quantitative metallographic method and corresponding preparation techniques were also developed. The results were concluded as follows:(1) With the increase of tempering temperature and time,the microstructures of 55NiCrMoV7 steel tempered are in turn lath-martensite, sorbite, troostite and pearlite. The results of image processing showed that the roundness of carbides declines and cementites changes from strip to global, as time as carbides sizes become smaller. The coalescence of carbides was observed when the steel tempered above 600癈.(2) The FWHM (X-ray diffraction half-width) of M(211) in the steel decreases with tempering temperature and time, and linearly increases with tempering hardness. Therefore, the FWHM of M(211) is a characteristic parameter indicating the microstructure change of the steel during tempering.(3) The kinetic equations were established at the first time among the FWHM of M(211), tempering temperature and tempering time for the tempered steel. The kinetic equations not only predicted well the FWHM of M(211) of the tempered steel, but also lay the foundation for the development of complex mechanical behavior models combined with macro-, meso- and microscopic dimensions.(4) During LCF test, the FWHM of M(211) decreases, the drop of FWHM of M(211) is linearly related with temperature of fatigue and original hardness of the steel.(5) When the fatigue temperature is lower than the tempering temperature of the steel, the drop of FWHM of M(211) is mostly resulted from mechanical loading, and minority from thermal effect. However, once the fatigue temperature is above the tempering temperature, the change of FWHM of M(211) induced by mechanical loading drops sharply, with a quickly increased aging which is a dominant factor of the steel softening.
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