UNS N07725: Hydrogen Induced Stress Cracking Resistance Determination and Optimization

Abstract UNS N07725 fully conforming to API 6ACRA has experienced past failures due to Hydrogen Induced Stress Cracking (HISC), which could be attributed to the presence of undesired grain boundary phases. The authors have published improvements to the HISC resistance, but these investigations have been limited to smaller dimensions or required chemical restrictions of the original alloy composition. This work focuses on further improvements of the HISC resistance of a fully API 6ACRA-conforming alloy within the chemical composition boundaries of N07725, for bar diameters ranging from 1 to 7.5 inches. Internal testing capabilities of NACE TM0198-Appendix C have been implemented. High-resolution scanning electron microscopy with electron dispersive X-ray spectroscopy and electron backscatter diffraction were used to analyze the HISC failure behavior and the microstructure. An API 6ACRA-conforming modification of the heat treatment parameters makes it possible to achieve the desired plastic elongation of 8% in NACE TM0198-Appendix C testing over all tested bar diameters. Results of an underlying heat treatment study highlight the fundamental mechanism for decreasing HISC resistance with increasing bar diameter.