Characterisation of precipitation and coarsening of carbides during tempering in a low alloyed quenched and tempered steel

Low alloy quench and tempered (Q&T) steels plates up to 100mm thick are used in applications such as cranes and earth movers due to their combination of high strength and toughness. In order to ensure that appropriate tempering conditions are used to give optimum properties through thickness in Q&T steels it is desirable to be able to predict the effect of composition and tempering conditions (time and temperatures) on the microstructure and hence the hardness evolution. In this paper, the types and coarsening rates of carbides formed in a low alloyed Q&T steel have been investigated on tempering at 600 °C. It has been found that in the as-quenched condition auto-tempered martensite is present with needle-shaped epsilon and cementite particles, whilst after different tempering times (1 - 16 hours) cementite becomes the stable phase with an elliptical shape, which coarsens with time. Besides, the coarsening of inter-lath cementite with a faster rate is independent from that of intra-lath ones.