CARBIDE PRECIPITATION IN MARTENSITE DURING THE EARLY STAGES OF TEMPERING CR-CONTAINING AND MO-CONTAINING LOW-ALLOY STEELS

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 microsc opy and atom probe field ion microscopy. Ultra-thin films which were c arbon enriched have been observed at some of the martensite lath bound aries. Brightly imaging clusters of Mo and C atoms, which represent th e earliest stages of the precipitation of Mo2C carbide were observed. In the very early stages of ageing at 350 degrees C, individual molecu lar ions of MoN2+ were observed using atom probe selected area analysi s.No partitioning of Cr, Mo and Mn, was observed between cementite and martensite after tempering at 350 degrees 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 degrees C for 187 h were found to be the same in both the low and high carbon alloys, despite t he fact that the predicted equilibrium concentrations of the various a lloying elements in the cementite were very different. These results d emonstrate that cementite, in Cr and Mo containing alloy steels, preci pitates from supersaturated ferrite via a paraequilibrium transformati on mechanism. (C) 1998 Acta Metallurgica Inc.