The absence of work hardening in steel quenched and tempered at around 623K

摘要

The Precision tubular specimens of a pure vacuum smelted quench and temper steel were prepared. For some of them a very small and for others a very large austenitic grain size were brought about. Then the specimens were quenched and tempered at 623 K and tested at room temperature using torsion tests. The tests showed that the inherent austenitic grain size had effects on the shape of the stress - strain curves. In the case of the large inherent austenitic grain size yield stress was about 200 MPa lower than in the case of the small one and there was a narrow work hardening zone after yielding. The rate of work hardening at the strain of 0.2 was nil after yielding in both cases. Some cylindrical specimens were also prepared and heat treated as above and compressed 60%. The microstructure of tested specimens was studied by means of a high resolution TEM and SEM. After tempering at 623 K, there were plate-like precipitates in the microstructure. The plate sizes and the overall view of microstructures did not depend on inherent austenitic grain size. During deformation the habit planes of the plates were turned towards the slip planes. It is proposed that the nil work hardening rate results from the reorientation of martensite laths and plate precipitates and fragmentation of the latter during deformation.