The effect of alloying elements on cementite coarsening during martensite tempering

Abstract

The effects of anti-segregating elements (Si and Al) and segregating elements (Cr and Mo) on cementite precipitation during tempering of Fe-C-Mn martensites have been quantitatively investigated in terms of both cementite chemistry and particle size evolution. Si and Al additions both accelerate the Mn partitioning kinetics and decelerate the particle growth kinetics. They are trapped in cementite during the initial stage of precipitation but are rejected from cementite after prolonged tempering. By comparison, small Cr and Mo additions do not strongly affect the Mn partitioning kinetics or particle growth kinetics, even though both Cr and Mo partition to cementite. The experimental measurements of cementite coarsening with time-dependent chemistry evolution are compared with a cementite coarsening model recently proposed by Wu et al. that has been generalised to multicomponent steels. The model can describe well the partitioning kinetics of segregating elements Mn/Cr/Mo and cementite particle growth kinetics simultaneously in a range of higher order systems. At 600 °C, the model works very well under local equilibrium (LE) and volume diffusion assumptions. The retardation by Si/Al on cementite coarsening and resulting accelerated Mn partitioning can be understood by the kinetic barrier from the migration of Si/Al spikes in front of growing cementite. At lower temperatures of 500 °C and 400 °C, the model can also reasonably well describe the Mn/Cr/Mo partitioning kinetics (and particle growth kinetics). However, to better describe the particle growth kinetics, the interfacial condition has to be treated with a deviation from LE due to Si/Al solute trapping. The Cr and Mo mobilities in cementite are enhanced by the same factor as for Mn mobility in cementite previously tuned by Wu et al., and the simulated Mn/Cr/Mo partitioning kinetics indicate these substitutional diffusion data are reasonably well estimated, which is further demonstrated in the simulation of Al rejection kinetics.