Cementite coarsening during the tempering of Fe-C-Mn martensite

Abstract

Cementite precipitation during martensite tempering has become an important processing step in some advanced high strength steels (AHSS). This tempering can lead to the formation of Mn-partitioned cementite which has a strong influence on subsequent austenite reversion during intercritical annealing. In this contribution, both the cementite Mn content and particle size evolution have been quantitatively monitored during the tempering of Fe-C-Mn martensite at temperatures between 400 °C and 600 °C. Mn progressively partitions to cementite during the coarsening reaction from the earliest tempering times. This coarsening of the cementite occurs in a matrix that contains strong Mn concentration gradients that drive partitioning of the Mn to the cementite. A model is developed to simultaneously describe the time, temperature and bulk alloy dependence of the mean cementite size and Mn composition evolution. In the model, both the Mn flux, driven by the composition gradient, and C flux, driven by capillarity, have been taken into account to determine the operative tie-lines at the cementite and martensite/ferrite interface. The effect of the cementite Mn content on the dissolution of smaller cementite particles that supply C to the larger particle growth is considered by coupling the growth and dissolution of cementite under conditions of multi-component diffusion.