The Kinetics of Mo₂C Precipitation in Tempered Martensite

Abstract

The rate of dissolution of Fe₃C and precipitation of Mo₂C during the tempering of a martensitic iron–carbon–molybdenum alloy at temperatures between 500 and 600° C (775 and 875 K) has been investigated by means of a magnetic balance. These kinetic observations have been correlated with the structural changes that accompany this precipitation reaction. The kinetics of the precipitation shows a discontinuity after ∼ 65% transformation. In the early stages precipitation occurs predominantly on dislocations. The discontinuity in the kinetics is associated with the recovery of the martensitic matrix and subsequent growth of precipitates in a relatively dislocation-free matrix. The activation energy for the precipitation process does not change with the change in reaction rate. The Mo₂C needles grow rapidly as recovery occurs and when growing in the recovered matrix the needle length increases parabolically with time.