Strengthening mechanism and thermal stability of severely deformed ferritic/martensitic steel

The strengthening mechanism of the ultrafine-grained ferritic/martensitic steel (T91) prepared by equal-channel angular pressing (ECAP) is studied by microhardness measurement, transmission electron microscopy and internal friction. The results show that dislocation strengthening rather than grain boundary strengthening plays a dominant role in the increase in microhardness. Upon annealing at different temperatures, it is found that there is an evident decrease in microhardness between 650 and 700 °C for the ECAPed specimens processed by 1 pass and 4 passes. An internal friction peak is also observed in the vicinity of 670 °C for the ECAPed specimens during the first heating but disappears during the second heating. The TEM observations indicate that there is no evident change in the grain size for the ECAPed specimens before and after the annealing at 700 °C for 1 h. It is concluded that the softening around 670 °C stems mainly from the dislocation annihilation.