Martensite transformation, microsegregation, and creep strength of

The 9Cr-1Mo-V (P91) steel is a high strength alloy that normally transforms completely to martensite during air cooling. This means that issues related to the handling of weldments during welding operations, and between the completion of welding and post weld heat treatments are important concerns. Measurements of specimen dilations during controlled thermal cycles were used to follow the martensite transformation in a P91 base metal and a weld deposit. For identical thermal cycles, the base metal transformed completely to martensite after normalizing, but the weld deposit did not. The effect of this difference in transformation behavior was that austenite was retained in the weld deposit after tempering. The retained austenite transformed to martensite upon cooling from the tempering temperature, resulting in small amounts of untempered martensite in fully heat treated weld deposit. An analysis of microsegregation by computational thermodynamics suggested that partitioning of alloying elements increased the range between the martensite transformation start and finish temperatures in the weld deposit, increasing the likelihood of retained austenite in it. Creep testing at 600°C and stresses of 120 MPa and 186 MPa showed that weld deposit specimens with untempered martensite had low creep rates and relatively long rupture lives