This paper presents results obtained from examining the effect of heat treatment and production method on the hydrogen induced stress cracking (HISC) susceptibility of UNS N06625 (Alloy 625). Five different versions of Alloy 625 were examined: Hot forged material heat treated at 920 °C, 1010 °C and 1100 °C respectively, material produced by hot isostatic pressing (HIP) and material that has been solution annealed and strain hardened (cold worked) at a temperature below secondary phase formation. The different versions of Alloy 625 were initially characterized by identifying the critical pitting temperature (CPT) by use of a modified ASTM G48 Method C test. Resistance to Hydrogen Embrittlement (HE) has been examined by use of stepwise loading of tensile specimens in a hydrogen charging environment in CortestTM proof rings, followed by fractography characterization and microstructural examination by use of Optical Light Microscope (OLM) and Scanning Electron Microscope (SEM). The hot forged and the HIP’ed versions of Alloy 625 that subsequently were heat treated, were found to be susceptible to HISC. The evaluation was based on a decrease in strength and ductility, as well as brittle fracture surfaces and secondary cracking on the samples’ outer surface. Among the hot forged and heat-treated versions, the samples heat treated at 1010°C were found to be the most susceptible to HISC, while (solution) heat treatment at 1100°C resulted in the lowest HISC susceptibility amongst the hot forged and heat-treated qualities. The solution annealed (1150 °C) and cold worked version showed a higher resistance to HISC, as no reduction in fracture strength, nor any secondary cracking were observed. The amount of grain boundary precipitations was found to be the main factor influencing the HISC susceptibility, where an increased amount of grain boundary precipitations led to higher susceptibility. The effect of grain size on the HISC susceptibility of samples charged with hydrogen were also considered, and the results indicate that larger grains have a beneficial effect on the ductility.

Several offshore field failures in recent years have been attributed to Hydrogen Induced Stress Cracking (HISC) of high strength, highly corrosion resistant Precipitation Hardened Nickel Alloys (PHNA’s) such as UNS N07716, UNS N07718 and UNS N07725.

Hence, HISC is a constant concern regarding subsea components subjected to high tensile stress, and the industry is searching for solutions to their technical needs: High strength corrosion resistant alloys (CRA’s) resistant to seawater (high Pitting Resistance Equivalent number (PREN)) but also resistant to HISC.

For PHNA’s, improved processing (chemical composition limits and processing temperatures) and improved quality control methods as well as refined acceptance criteria are all under consideration.