Effect of partitioning of Mn and Si on the growth kinetics of cementite in tempered Fe–0.6 mass% C martensite

The effects of Mn and Si addition on the growth rate of cementite in Fe–0.6 mass% C martensite have been studied by means of scanning electron microscopy, transmission electron microscopy and a three-dimensional atom probe. The growth rate of the cementite during tempering at 723 K decreases substantially with the addition of Si due to the redistribution of Si between the cementite and ferrite matrix. Mn retards cementite coarsening more effectively than Si at 923 K. In tempering at both 723 and 923 K, the Si concentration in the cementite starts to decrease from an early stage of precipitation, whereas the cementite develops initially without the redistribution of Mn, before the Mn gradually enriches into the cementite during tempering. Calculations of phase boundaries for stable equilibrium (partition local equilibrium) and metastable equilibria (para and negligible-partition local equilibriums) have revealed that there is a sufficient driving force for the formation of paracementite in the Mn-added alloys. On the other hand, paracementite is difficult to form in the Si-containing alloy because the cementite becomes unstable due to the dissolution of Si.