Prediction of Residual Stress Distributions in External Attachment Welding Applications to Evaluate the Need of PWHT

摘要

There has been clear uncertainty whether post-weld heat treatment (PWHT) should be required for external attachment (non-pressure retaining) welds in petrochemical industry applications where stress corrosion cracking (SCC) is a possibility. An industrial criterion established by NACE SP0472-2010, paragraph 3.5.1, indicates that PWHT is not required if no tensile residual stresses extend through the entire thickness. To investigate this problem, commercial FEA software, SYSWELD, was utilized to predict the extent and level of residual stresses of such welds through the thickness of the pressure vessel shell. The study used the GMAW process to deposit single bead-on-plate welds with ER70S-6 wire on ASTM-516 grade 70 PVQ steel plates of 6.3, 12.7 and 19.0mm thicknesses. Microstructural analysis through the use of metallography and micro-hardness tracing of weldments were performed as a mean to qualify the modeling phase quantification results. Following the through-thickness direction, the peak of tensile residual stresses was invariably observed in the Sub- and Inter-critical HAZ regions. In addition, through-thickness longitudinal tensile stresses existed in 6.3 and 12.7 mm thick plates whereas both tensile and compressive longitudinal stresses were observed in 19.0 mm thick plates. Maximum longitudinal tensile stresses at 85-104% of the base metal yield strength existed in the bottom surface of the 6.3 mm thick plates whereas maximum longitudinal tensile stresses reached only 8-12% of the yield strength in the 19.0 mm thick plates. The conclusion indicates that 6.3 mm thickness applications demand PWHT whereas welding 12.7 and 19.0mm thick plates are possibly safe depending on the amount of SCC threshold stress and loading conditions.