Reasons for the absence of work hardening in quenched and tempered steel after tempering at around 623 K

A medium carbon Si steel was quenched and tempered at around 638 K and tested using torsion tests with tubular specimens at and below room temperature. The mechanical tests showed that the test temperature had no effect on the rate of work hardening. At low temperature, the stress states increased but the rate of work hardening was nil as it was at room temperature. It seems that the diffusion of carbon has no effect on the rate of work hardening. The microstructure of the tested specimens was studied by means of carbon replicas using transmission electron microscopy. After tempering at 638 K, there were plate-like precipitates in the microstructure. During deformation, these plates deformed and turned their habit planes towards the slip plane. No microcracks were seen in the precipitates. It seems that the precipitates are coherent with the matrix. During deformation the dislocations pass through the precipitates, which may be the reason why precipitates turn their direction.