Effect of normalization temperature on the creep strength of modified 9Cr-1Mo steel

The effect of normalization temperature from 850 °C to 1050 °C on the structure and creep-rupture properties of modified 9Cr-1Mo steel was studied. Normalization at temperatures below 925 °C resulted in structures containing significant polygonized, recovered ferrite. The ferrite structures had poor creep-rupture strength: roughly two orders of magnitude increase in minimum creep rate or decrease in rupture life for 850 °C compared to 1050 °C normalization at test conditions of 600 °C and 145 MPa. Room-temperature strength and hardness were also reduced. The microstructure after normalization at the standard 1050 °C temperature consisted of tempered martensite with fine M₂₃C₆ carbide along prior austenite and lath boundaries and fine MX carbonitride precipitates within the laths. Normalization at temperatures between 925 °C and 1000 °C also resulted in reduced creep strength in comparison with 1050 °C normalization, even though tempered martensite microstructures were formed and little change in room-temperature strength was observed; the reduction was attributed to subtle differences in the MX precipitates. The effect of reduced normalization temperature was more pronounced for higher-temperature, lower-stress creep-rupture conditions.