Effect of Postweld Heat Treatment Conditions on Mechanical Properties of 9Cr-1Mo-V Steel Welds for Pressure Vessel

2¼Cr-1Mo steel and 2¼Cr-1Mo-V steel which have high performance in high-temperature and high-hydrogen service are widely used in fabricating process equipment for the oil refining industry. With the demand for plants using low-grade crude oil expected to increase in the future, equipment will after to operate at higher temperatures and higher pressures to meet such demand. In addition, the demand for gasification process equipment for coal using high-temperature strength material, which is to be used as an alternative of oil, is expected to grow also. Although 2¼Cr-1Mo steel and 2¼Cr-1Mo-V steel are adopted for high-temperature and high-pressure equipment used at temperatures up to 450°C, 9Cr-1Mo-V steel is suited for operation at even higher temperatures and pressures. It is used as structural material for boilers of power plants and elsewhere. The application of 9Cr-1Mo-V steel is anticipated for oil refineries. However, it is necessary for the welds to consider the hardness and the low-temperature toughness in addition to the high-temperature strength. In this paper, the effect of postweld heat treatment (PWHT) conditions on mechanical properties of 9Cr-1Mo-V steel welds was investigated. The PWHT conditions are organized with reference to the tempering parameter (Larson-Miller Parameter), with the heat treatment conducted in an LMP range of 20.46 to 22.42 × 103. As LMP increased, the hardness and high-temperature strength decreased. In the hardness distribution of the welds subjected to PWHT, the hardness tended to maximize near the bond of the heat affected zone (HAZ); however, applying heat treatment to materials with an LMP equal to or higher than 21.15 × 103 allowed a hardness of 260HV or less to be obtained. In terms of the high-temperature strength, the welds met the specified value as the 0.2 proof stress of ASME SA387-91-2, the base metal, under the PWHT conditions of an LMP equal to 22.42 × 103; the tensile strength was 0.9 times or more of the specified value. The Charpy absorbed energy increased with the LMP, and, when an LMP of 21.99 × 103 or more was used as the PWHT conditions, an absorbed energy of 60 J or more could be obtained at 0°C, as well. From the above results, the effect of PWHT conditions on mechanical properties of the welds is clarified, and it is suggested that 9Cr-1Mo-V steel can be used for the equipment in oil refining plants.