Because Inconel 718 has been used extensively for aerospace applications, detailed heat treatment studies were conducted by Walter and Chandler (ref. 16, 23) for rocket engine applications in a high pressure hydrogen environment. Table 17.8 presents the results of HEE Index for Inconel 718 in combinations of five different heat treatments and product forms, which also included weld and heat-affected zone (HAZ) materials. In all heat treatment cases, Inconel 718 in plate-form appears to be less influenced by hydrogen than in rolled bar and forging forms. Microstructure analysis shows that heat treatment A produced a fine-grained structure with discontinuous Ni3Nb intermetallic for the plate product, which was the least embrittled by hydrogen. However, this same heat treatment A also resulted in a rolled bar and forged products that contained large grains and continuous intergranular Ni3Nb particles that formed a preferential fracture path, resulting in a structure that was the most embrittled in hydrogen environment.