HRSG-Header Welds Residual-Stress Evaluation and Creep-Assessment Through the Application of Italian Code, American Standard

Joint design, preheating, post-weld-heat treatment and operator ability are key factors for the occurrence of critical residual stresses in the welds. In particular, they may affect creep life results of the analysis: to evaluate this, their characterization requires modeling welding process, such as that carried out in the present work for ASTM A 335-Grade P22 weld. It is a weld of the two analyzed in previous work on the high-temperature-section (superheater/reheater) lower headers of the bottom-supported heat-recovery steam generator (HRSG). Present study includes modeling the only weld-lay, gas tungsten arc, manual, for the finned-tube angle joint to the cylinder. It is the same as the first of the three weld-lays for the other previously analyzed, end-plug circumferential “V”-groove butt joint. The material considered for base metal is 2¼Cr 1Mo forged, normalized, tempered with creep strain rates higher than the weld’s as it appeared from analysis of previous work. Presently, the study first compares max tangential stress evaluated by thermal analysis on the circumferential weld with the average applied normally on the joint, sustained case. Roughly, the latter one has nearly same importance as axial membrane stress on the cylinder wall for the end-plug butt joint, pressure case; it is smaller for the finned-tube angle joint. Then, study compares previous creep results obtained considering SRF = 0.9 for the finned-tube joint weld with those counting the residual-stress increase in stress analysis (different creep law’s coefficients). Finally, study compares creep results for the finned-tube joint weld obtained with and without residual-stresses (same creep power-law). The objective is to comprehend residual-stress influence over creep-redistribution and creep relaxation.