The kinetics of softening and microstructure evolution of martensite in Fe-C-Mn steel during tempering at 300 °C.

A fundamental understanding of the properties of tempered martensitic steel during the production and service of fasteners helps the development of cost-effective, micro-alloyed, medium-carbon steel for future automotive fasteners, which have to fulfill more stringent requirements due to the on-going trend of down-sizing the car engine. We have studied the relation between the softening kinetics of martensite and the kinetics of the evolution of the microstructure in high-purity Fe-C-Mn steel during tempering at 300 °C by means of nano-indentation, SEM, and EBSD. The as-quenched specimen consist of martensite blocks that are auto-tempered and non-tempered. We find that the nano-hardness that is measured directly at the martensite boundaries is significantly higher than the nano-hardness that is measured in the martensite matrix. The boundary regions soften with increasing tempering time, whereas the nano-hardness of the tempered matrix remains approximately constant with increasing tempering time. The kinetics of martensite softening can be described by three stages that are related to the evolution of the microstructure. However, most of the softening takes place during the first stage of tempering: The macroscopic softening of martensite is mainly related to: (i) the nano-scale softening of boundary regions, (ii) the reduction in area fraction of the boundaries regions, and (iii) the reduction in area fraction of the non-tempered matrix regions.