Hydrogen Induced Cracking Susceptibility in the Heat Affected Zone of SA-508 Pressure Vessel Steel

Abstract

ASME Case N-888, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW and Machine GTAW Temper Bead Technique, requires a 48-hour hold time before nondestructive examination (NDE) can be performed to ensure hydrogen-induced cracking (HIC) did not occur. The aim of this work is to assess and characterize the HIC susceptibility of SA-508 pressure vessel steel. The results from this study will be used to consider potential elimination of the NDE hold time requirement in Case N-888. The Gleeble™ thermo-mechanical simulator was used to recreate CGHAZ for five weld conditions; as-welded, post-weld heat treated (PWHT), and four single-reheat temperatures of 675 °C, 700 °C, 725 °C, and 735 °C to simulate temper bead welding (TBW). Metallurgical characterization revealed a tempered martensitic microstructure for all TBW and the PWHT conditions. The 735 °C TBW sample developed ferrite along the prior austenite grain boundaries. The single reheats to 675 °C, 700 °C and 735 °C reduced the as-welded CGHAZ hardness from 425HV0.5 respectively to 313, 298, and 278HV0.5. A fourth TBW condition was added at 725 °C to eliminate ferrite formation seen at 735 °C. The TBW at 725 °C produced a microstructure of tempered martensite with a hardness of 298HV0.5. The HIC susceptibility is being evaluated using the Delayed Hydrogen Crack Test (DHCT) developed at the Ohio State University. Samples of SA-508 steel with the five CGHAZ microstructural conditions are loaded at 90% of the base metal yield strength and simultaneously electrolytically charged with hydrogen. The HIC susceptibility is ranked by the time to failure (full specimen separation) and sustained mechanical energy.