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

摘要

The microstructures of a 2.25/100 Cr-1/100 Mo-0.15 wt/100 C steel, and a steel with identical alloy- ing additions but a higher carbon level,2.25/100 Cr-1/100 Mo-0.4/100 C, have been examined in the marten- sitic condition for a variety of tempering heat treatments by transmission electron microscopy and atom probe field on 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 deg.c, in- dividual molecular ions of MoN~2+ were observed using atom probe selected area analysis.