Effect of heating rate on temperature field of local post weld heat treatment of P91 steel pipe and parameter optimization

Heating rate is the key parameter of local post weld heat treatment (PWHT) of P91 steel pipes, which determines the efficiency and quality of the heat treatment. This article takes the P91 steel pipe of Φ575 mm × 35 mm as the object, carries out the local PWHT tests at 5 heating rates between 79- 479 ℃/h, and analyzes the influence of the heating rate on the actual heating time (refers to sum of the nominal heating time for the outer wall of the pipe to reach the steady state of the temperature and the lag time for the temperature of the inner wall of the pipe reach the steady state), as well as the influence of the axial and radial temperature gradients of the pipe. Afterwards, the finite element calculation model for PWHT of the P91 steel pipe is established to optimize the heating rate parameters. The results show that increasing the heating rate results in the decrease of the actual heating time of the entire pipe joint, which can improve the efficiency of the heat treatment. When the heating rate is lower than 179 ℃/h, increasing the heating rate significantly shortens the actual heating time; when the heating rate exceeds 179 ℃/h, the effect of increasing the heating rate on shortening the actual heating time is reduced. Furthermore, the change of the heating rate has a relatively small effect on the radial temperature gradient, yet has a greater effect on the axial temperature gradient. Based on the axial temperature gradient control criteria and the optimized calculation results, the optimized calculation formula is obtained for the maximum heating rate of local PWHT of P91 steel pipe with different wall thickness. Finally, an application plan for quickly selecting the maximum heating rate in engineering is presented.