Residual Stress Control in Nozzle to Cylinder Butt Welds of Large-Scale Pressure Vessels Using Primary Plus Secondary Local PWHT Method

Abstract This study systematically investigated spot heating local postweld heat treatment (PWHT) for pressure vessel nozzle to cylinder butt welds through experimental and numerical analyses. A dual-stage local heat treatment methodology, i.e., primary plus secondary local PWHT (PS-PWHT), was developed specifically for nozzle to cylinder butt welds, with a systematic investigation of secondary heating parameters affecting residual stress reduction. The results demonstrate that the PS-PWHT method achieves 50–70% residual stress reduction at critical inner surface locations. Two optimized secondary heating configurations, i.e., elliptical saddle-shaped secondary heating (E-SH) and rectangular saddle-shaped secondary heating (R-SH), were established based on reverse bending moment analysis of axial/hoop heating bands. Geometric parameters were quantified with recommended secondary heating distances (WDC = 2Rt and WDS = 4Rt) and (2–3)t width range (where WDC and WDS are the primary and secondary heating distances for the hoop and axial heating bands, respectively, and t represents wall thickness). A heating temperature range of 300–450 °C was identified for secondary heating, balancing effective stress relief without material degradation and damage. The developed process control strategy demonstrates significant improvements in mitigating welding-induced residual stresses through the PS-PWHT method.