Residual stress quantification of external attachment welding applications to evaluate the need of post-weld heat treatment

Abstract

Investigated since the 1950s, welding-induced tensile residual stresses have inherently imposed challenges in almost all fusion welding processes. Such residual stresses can be as high as the local yield strength in a weld metal and surrounding HAZ and are well identified to promote several service failures such as brittle fracture, fatigue and stress corrosion cracking (SCC). Petrochemical industry has almost always utilized post-weld heat treatment (PWHT) to relieve welding residual stresses in weldments that are exposed to processing environments that can potentially induce SCC to maintain safe operations during equipment lifetime. However, there has been clear uncertainty whether PWHT should be required for external attachment welds in petrochemical equipment that are not pressure retaining. 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 wall thickness. To investigate this problem, an FEA software, Sysweld, was utilized to analyze the extent and level of residual stresses of such welds through the thickness of the pressure vessel shell. For validation, the study also used the GMAW process to deposit single bead-on-plate welds with ER70S-6 wire on ASTM-516 grade 70 PVQ steel plates of 6.35 mm (1/4 in.), 12.7 mm (1/2 in.), and 19 mm (3/4 in.) thicknesses. An experimental welding matrix of 106-150A, 20-22V and 6.5 mm/s travel speed was followed. Microstructural analysis and distortion and temperature measurement of weldments were performed as means to qualify the FEA results. In addition, hole-drilling strain gauge method according to ASTM E837 was used for residual stress measurement on the bottom surfaces of the welds intended for SCC environment exposure. Both FEA and the hole drilling method show that residual stresses varied as a function of heat input and base metal thickness with a maximum deviation of 9% of the base metal yield strength. Following the through-thickness direction, the peak of tensile residual stresses was invariably observed in the Sub- and Inter-critical HAZ regions. In the bottom surface, maximum tensile stresses at 86-104% of the yield strength were reached in the 6.35 mm (1/4 in.) thick plate application whereas the maximum stresses reached only 26-35% of the yield strength in the 19 mm (3/4 in.) thickness. Following the experimental procedures, the conclusion indicates that welding applications on 6.35 mm (1/4 in.) thick plates demands PWHT whereas welding on 12.7 mm (1/2 in.) thick plate is deemed safe depending on the amount of SCC threshold stress. Due to the low amount of residual stresses observed in the 19 mm (3/4 in.) thick plate, the study concluded that for pressure vessel plates of 19 mm thickness or greater, PWHT can be waived or optional.