Effect of Normalizing Followed by Deep Cryogenic Soaking on Mechanical Properties of P91 Martensitic Steel

Normalizing is a widely used heat treatment process for steel to induce desired mechanical and microstructural properties. In the present investigation, P91 martensitic steel was subjected to normalizing, followed by deep cryogenic soaking (DCS) using a muffle furnace and liquid nitrogen chamber. The specimens were reheated in the temperature range of 980–1150 °C and held isothermally for 10–135 min to homogenize the temperature. It is further subjected to air cooling at room temperature followed by deep cryogenic cooling at a temperature of − 196 °C for a time of 30 min in liquid nitrogen. The volumetric % of retained austenite (RA) envisages a logarithmic inclination with increasing austenitizing time for both “normalized” and “normalized followed by DCS” samples. It is ascribed to the steady reduction in thermal stability of the reversed austenite which was obtained at the elevated temperature during the austenitizing. However, the trend did not show either a constant (after reaching critical condition) or decreasing nature; therefore, it is suggested that the thermal equilibrium was not achieved. The variation in hardness also envisages a logarithmic inclination. The reduction in carbide stability at elevated austenitizing temperature (AT) and extensive recovery at the larger austenitizing time leads to reducing the hardness during the process.