Influence of rolling temperature on the microstructure and mechanical properties of secondary hardening high Co–Ni steel bearing 0.28 wt% C

A secondary hardening high Co–Ni alloy steel with the composition of 13Co–8Ni–4Cr–1.5Mo–0.28C was heavily rolled in the temperature range of 600–1000 °C to study the influence of the rolling temperature. The effects of heavy rolling on the microstructure and mechanical properties were investigated in detail by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The severe deformation accelerated the aging kinetics and increased the degree of secondary hardening. These results were analyzed in terms of the dislocation-operated heterogeneous nucleation and the effective activation energy for precipitation during the early periods of aging. Based on the effective activation energy calculated from our DSC results, the M2C precipitation mode tended to change from continuous nucleation to site saturation with a decrease in rolling temperature. This phenomenon implies that the higher dislocation density within martensite due to the more severe deformation at lower temperatures plays a significant role as nucleation sites for M2C precipitation.

Research highlights▶ Precipitation behavior of M2C nucleated at dislocations was significantly influenced by rolling temperature. ▶ Severe deformation promoted the generation of dislocations, which can operate as heterogeneous nucleation sites for the M2C precipitation. ▶ Precipitation mode varied from continuous nucleation to site saturation due to higher density of dislocations.