Transitions in the strain hardening behaviour of tempered martensite

Martensite is a key constituent in advanced high strength steels (AHSS). The mechanical response of martensite has received particular attention in recent years due to its very high strain hardening rate. Previous studies have shown that the strain hardening behaviour of as-quenched and low temperature tempered martensite can be described by considering martensite as a composite, with constituents showing a spectrum of yield strengths or residual transformation stresses, or more likely, a combination of both effects. However, the composite models are unable to explain the flow behaviour of martensite tempered above 400°C when the composite effect starts to diminish and a transition in the strain hardening behaviour is observed. In this contribution, we conduct a systematic study on the flow behaviour of high temperature tempered martensite with three AHSS compositions. The evolution of the strain hardening behaviour is studied using monotonic and tension-compression tests while the evolution in microstructures is monitored using thermal analysis, interrupted X-ray diffraction and microscopy. A continuous loss of strain hardening capacity is observed in all steels during tempering at lower temperatures due to the diminishing composite effects. When the tempering temperature is above 500°C, all samples show strain hardening rates below the E/50 limit for dislocation storage and a gradual increase in the strain hardening capacity is observed during tempering. To rationalise the tension-compression flow behaviour of martensite tempered at 600°C, a combined isotropic and kinematic hardening model considering dislocation storage and back stresses generated at cementite particles is proposed. The need to combine the composite models that work well for as-quenched and low temperature tempered martensite, with the dislocation-based models that work well for high temperature tempered martensite, is discussed in the context of self-consistently describing the transitions in strain hardening behaviour as a function of tempering temperature.