Effects of post-weld heat treatments on dissimilar joints of rigid pipes made of API X65 steel and Inconel 625 for offshore applications

This paper evaluated the effects of different post-weld heat treatment (PWHT) routes on the microstructural, mechanical, and electrical behavior of dissimilar welded joints be-tween API X65 steel and Inconel 625, widely used in offshore systems such as risers and mechanically lined pipes. Welded specimens were analyzed in four conditions: as-welded (AW), stress-relieved (SR), quenched (Q), and quenched followed by tempering (QT). Microstructural analysis, Rockwell C hardness testing, and electrical resistivity measurements using a four-point probe method were applied to assess joint performance across regions such as the base metal, weld metal, and heat-affected zones (HAZ 1, HAZ 2, HAZ 3). In the AW condition, hardness varied significantly (e.g., 11 HRC in base metal and 35 HRC in HAZ 2), and resistivity ranged from 17.80 to 120.50 μΩ·m, indicating localized hardening and residual stress. After PWHT, hardness became more uniform (17–25 HRC), and resistivity increased but stabilized (52.80–63.00 Ω·m), reflecting improved homogeneity. The treatments reduced internal stress, refined microstructures, and promoted more predictable electrical behavior. These findings support the effectiveness of combined PWHT (SR + Q + QT) in enhancing the integrity of dissimilar joints, contributing to safer and more durable offshore pipeline applications.