Distribution of carbon in martensite during quenching and tempering of dual phase steels and consequences for damage properties

The microstructural evolution of martensite in as-quenched and quenched and tempered Fe-0.15C-0.215Si-1.9Mn-0.195Cr wt.% dual phase (DP) steels processed to give four different errite/martensite ratios was studied. It was found that partial thermodynamic equilibrium was obtained after intercritical annealing for 130 s. The local carbon distribution in as-quenched martensite was heterogeneous for all quenching temperatures. Significant carbon enrichment was observed at the ferrite/martensite interface at annealing temperatures of 790°C, whereas carbon depletion occurred when the annealing temperature was reduced to 755°C. A possible explanation for the carbon profile in terms of the effect of Mn partitioning on the austenite phase transformation kinetics is given. The kinetics of carbide formation during tempering is strongly influenced by these carbon gradients. A simple analysis shows that the interface carbon depletion observed at lower intercritical annealing temperatures could induce a beneficial increase in the void nucleation strain ! n, due to a reduction in the backstress at the ferrite/martensite interface which decreases the local stress triaxiality. We estimate that the upper limit for the improvement in the asquenched microstructure is ~8%, so the effect could provide a moderate delay in the onset of damage. Further, we propose that the improvement in damage resistance during tempering is mainly due to dispersed void formation at tempered carbides and that this mechanism will be compromised if those carbides are localised at ferrite/martensite interfaces. This argument mitigates for the carbon-depleted interface structure obtained at lower intercritical temperatures