Numerical and experimental investigations on the temperature field in local post weld heat treatment of 9 % Cr heat resistant steel welded pipes

Abstract

Abstract Local post weld heat treatment (PWHT) is usually employed in the field fabrication of large-sized 9% Cr heat resistant steel welded components, such as power plant boilers. Temperature field in local PWHT plays critical roles in both achieving PWHT purposes and avoiding detrimental effects of local heating. The objective of this research is to quantify the effects of local PWHT parameters and pipe dimensions on the temperature field in local PWHT of 9% Cr heat resistant steel pipes and finding the optimal process window to engineer desired weld properties. Finite element (FE) models were developed to simulate the temperature field of local PWHT and carefully verified by eight experiments using four 9% Cr heat resistant steel pipes with typical dimensions employed in ultra-supercritical (USC) boilers. The results show that the through-thickness temperature gradient is severely aggravated with the increase of pipe diameter and wall thickness, which will lead to inadequate tempering in the weld metal close to the inside surface of the pipe. To enlarge heated band (HB) width is an efficient way to mitigate the through-thickness temperature gradient, and the axial temperature gradient can be effectively relieved by augmenting gradient control band (GCB) width. The through-thickness temperature gradient is the major factor in implementing successful local PWHT for 9% Cr heat resistant steel pipes. A much smaller process window is found in 9% Cr heat resistant steel pipes due to their higher PWHT temperatures and narrower range of permissible PWHT temperatures.