Identification of carbonitride precipitates in a 1wt.%Cr0.5wt.%Mo steel using analytical electron microscopy

Abstract

The initial stages of carbonitride precipitation, in a normalized 1wt.%Cr0.5wt.%Mo steel subjected to isothermal heat treatment at 600 and 650°C, have been studied using electron microdiffraction, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. In the asnormalized steel, fine-scale (100 nm) equiaxed particles of f.c.c. CrN (a = 0.414 nm) have been identified at ferrite grain boundaries. With isothermal aging, an apparently continuous change in composition is observed in these grain boundary particles, forming firstly cubic close-packed carbonitrides (Cr, Mo) (N, C), followed by a transition to h.c.p. (Mo, Cr) 2 (C, N) and eventual formation of Mo 2C. Within the ferrite grains, short elliptical precipitate rods are detectable in extraction replicas from specimens aged for 25 h at 650 °C. These particles have been identified as h.c.p. Mo 2C. No significant changes in composition or structure were observed to accompany the growth of these intraferritic particles during isothermal aging.