Investigating the Influence of Post-Weld Heat Treatment on Carbon Steel Tube-to-Tubesheet Welds in Heat Exchanger

This study investigates the impact of post-weld heat treatment (PWHT) on carbon steel Tube-to-Tubesheet (TTS) welds, an essential consideration for the integrity and performance of shell and tube heat exchangers. The experimental work involved a 50-mm-thick carbon steel (CS) tubesheet SA 516 Gr. 70 N welded to CS tubes SA 179 with an outside diameter of 19.05 mm and a thickness of 2.11 mm. A total of twenty joints were welded using gas tungsten arc welding (GTAW), with ten subjected to PWHT and ten without PWHT. To assess the quality of the TTS joints as per ASME Section IX QW-193.1-QW-193.3, including visual testing (VT), penetrant testing (PT), and macro-examination for minimum leak path (MLP). Hardness testing was also conducted to validate the effectiveness of heat treatment and the integrity of welds. The findings revealed that the average MLP was greater in non-PWHT joints compared to PWHT joints primarily due to residual stresses. Hardness testing indicated that non-PWHT weld metal had a higher hardness (218 VHN) than the base metal (142 VHN) and the heat-affected zone (179 VHN), suggesting increased strength but also potential brittleness. In contrast, PWHT weld metal exhibited a hardness of 206 VHN, highlighting improved ductility and toughness, with all measurements remaining below the critical threshold of 250 VHN. This study highlights the importance of PWHT in enhancing the mechanical properties of welded structures, maintaining an optimal balance of strength and ductility critical for applications under stress and fatigue.