Microstructure of Ultrafine Acicular Bainite and Mechanical Properties of 3Cr2MnNiMo Mold Steel during Austempering

Herein, the effect of austempering treatment on martensite/bainite multiphase microstructure evolution and mechanical properties of 3Cr2MnNiMo mold steel are investigated, and the mechanical properties are compared with the conventional quenching and tempering (QT) treatment. It indicated that the morphology of martensite/bainite can be effectively tailored by changing the austempering temperature. With the increase of temperature from 280 to 360 degrees C, the microstructure changes from tempered martensite - bainite - blocky retained austenite (RA) to martensite - coarse bainite plate - blocky RA and irregular martensite/austenite (M/A) island. When the temperature reaches 420 degrees C, the martensite content reaches the maximum. The bainite morphology is surprisingly transformed from coarse bainite plate to ultrafine acicular shape owing to the reduction of the bainitic driving force and the sharp increase of carbon diffusion coefficient. Moreover, blocky RA and M/A islands disappear and are replaced by nanoscale filmy RA. The alternating distribution of variants has a refinement effect on the microstructure and the effective grain size reaches a minimum value. Compared with the conventional QT treatment, the tensile strength of austempering at 420 degrees C increased by 702 M Pa, but the toughness and ductility levels are almost the same.

Automated summary

In this study, the effect of austempering treatment on the microstructure and mechanical properties of 3Cr2MnNiMo mold steel was investigated. The results showed that by changing the austempering temperature, the morphology of martensite and bainite can be effectively controlled, resulting in improved strength of the material.