Identification of the nitrides formed during the annealing of a low-carbon low-aluminium steel

The precipitation of a low carbon low aluminium steel with a sub-stoichiometric [Al]/[N] atomic ratio was investigated during an annealing at 600°C by Transmission Electron Microscopy (TEM) and chemical analyses by X-ray Energy Dispersive Spectroscopy (EDX) on thin foils and carbon extraction replicas. These studies showed that in this steel, the precipitates which form have a platelet-like morphology associated with a cubic structure (with a = 0.412 ± 0.005 nm) and the following orientation relationship with iron: (001)p // (001)Fe and [110]p // [010]Fe. The EDX microanalyses of these precipitates revealed that they contain, in addition to aluminium and nitrogen, chromium and/or manganese. It was suggested that in the early stages of annealing, the initial precipitates are of CrN or (Cr,Mn)N type. During a prolonged annealing, they serve as nucleation sites for the final precipitates of (Al,Cr)N and (Al,Cr,Mn)N type.