Effect of HAZ Microstructure on HIC Failure Mechanism in SA-508 Steel Welds

According to ASME Case N-888-1, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW and Machine GTAW Temper Bead Technique, a 48-hour waiting period before conducting the final nondestructive examination (NDE) is required when ferritic filler weld metal is used. The purpose of the 48-hour hold is to confirm the absence of hydrogen-induced cracking in the temper bead heat-affected zone. In previous research, the effect of post-weld heat treatment (PWHT) and temper bead welding (TBW) on the hydrogen-induced cracking (HIC) susceptibility in the coarse-grained heat-affected zone (CGHAZ) in welds of SA-508, P-No. 3 Group 3, pressure vessel steel was investigated using the Delayed Hydrogen Cracking Test (DHCT). In this previous study, the Gleeble™ thermomechanical simulator was used to generate six CGHAZ microstructural conditions: as-welded (AW), PWHT, and AW with a single TBW reheat at 675°C, 700°C, 725°C, and 735°C. Hydrogen is introduced to the specimen through cathodic charging under in-situ constant tensile stress. The HIC susceptibility for these microstructures was ranked by the DHCT at a diffusible hydrogen level significantly exceeding typical GTAW and SMAW processes, as well as at a low hydrogen level through varying current density. The work described in this paper investigates the susceptibility to HIC of these same CGHAZ microstructures by fracture surface examination of failed tests, and cross-section microstructural analysis of non-failed tests under a scanning electron microscope (SEM). Future steps include evaluating hydrogen content using gas chromatography for each condition. The results from this study will be used to consider the potential elimination of the NDE hold time requirement in Case N-888-1 when ferritic weld metal is used.