A Study on the Effect of Thermal Ageing on the Specific-Heat Characteristics of 9Cr–1Mo–0.1C (mass%) Steel

The effect of thermal ageing on the heat-capacity behavior of 9Cr–1Mo–0.1C (mass%) ferritic/martensitic steel has been studied using differential scanning calorimetry (DSC) in the temperature range from 473 K to 1,273 K. The DSC results in the case of slow cooled, normalized and tempered, and subsequently thermally aged samples (500 h to 5,000 h at 823 K (550 °C) and 923 K (650 °C), clearly marked the presence of both magnetic and α-ferrite + carbide → γ-austenite phase transformations that take place successively upon heating. Furthermore, for the case of fully martensitic microstructure, an additional exothermic transformation at about 920 K(647 °C), arising from carbide precipitation is noticed. This event is characterized by a sharp drop in CP. It is found that the α-ferrite + carbide → γ-austenite phase transformation temperature is only mildly sensitive to microstructural details, but the enthalpy change associated with this phase transformation, and especially the change in specific heat around the transformation regime, are found to be dependent on the starting microstructure generated by thermal ageing treatment. Prolonged ageing for about 500 h to 5,000 h in the temperature range from 823 K to 923 K (550 °C to 650 °C) contributed to a decrease in heat capacity, as compared to the normalized and tempered sample. This is due to the increase in carbide volume fraction. The martensitic microstructure is found to possess the lowest room-temperature CP among different microstructures.