Neutron diffraction study of the low-temperature ageing of martensite in dual-phase steel

Neutron diffraction measurements were used to obtain a better understanding of the ageing behaviour in dual-phase ferrite + martensite steels. Therefore, the influence of different ageing treatments on the tempering of low-alloyed 0.4 and 0.7% C FeMnCrMo-martensite and 0.1% C FeMnCrMo dual-phase steels was investigated. On tempering at 170 and 350 °C, the sequential precipitation of g- and h-carbides, as well as the loss of tetragonality was observed in the martensite. The decrease in austenite lattice parameter after tempering is due to stress relaxation in the material. These observations offer an acceptable explanation for the large yield stress increase after ageing at 170 °C.