Carbide precipitation in martensite during the early stages of tempering Cr- andMo-containing low alloy steels

Abstract

The microstructures of a 2.25% Cr–1% Mo–0.15 wt% C steel, and a steel with identical alloying additions but a higher carbon level, 2.25% Cr–1% Mo–0.4 wt% C, have been examined in the martensitic condition for a variety of tempering heat treatments by transmission electron microscopy and atom probe field ion microscopy. Ultra-thin films which were carbon enriched have been observed at some of the martensite lath boundaries. Brightly imaging clusters of Mo and C atoms, which represent the earliest stages of the precipitation of Mo 2C carbide were observed. In the very early stages of ageing at 350°C, individual molecular ions of MoN 2+ were observed using atom probe selected area analysis. No partitioning of Cr, Mo and Mn, was observed between cementite and martensite after tempering at 350°C for times up to 40 h. The enrichment behaviour and interface concentrations of Cr, Mn and Mo in the cementite after prolonged ageing at 450°C for 187 h were found to be the same in both the low and high carbon alloys, despite the fact that the predicted equilibrium concentrations of the various alloying elements in the cementite were very different. These results demonstrate that cementite, in Cr and Mo containing alloy steels, precipitates from supersaturated ferrite via a paraequilibrium transformation mechanism.