Heat treatment and product forms

Heat treatment and product forms can have a profound effect on the degree of hydrogen embrittlement for certain types of superalloys with complex microstructures. In general, wrought superalloys, including the PM process, have been found to have a slightly less influence by high pressure hydrogen environment than cast superalloys with similar compositions and heat treatment (ref. 20). For instance, Table 17.7 shows the influence of heat treatment and product forms on the HEE index, based on NTS ratio, for five different types of superalloys tested at room temperature with pressure at 34.5 MPa (5 ksi). For Astroloy in standard heat treatment, a moderate improvement for the HEE index can be achieved when this material is in wrought (PM) product form instead of cast. For Incoloy 100 and Waspaloy, both in PM product form, different heat treatment conditions give rise to a small change in HEE index. However, the effect of heat treatment becomes very profound for Inconel 718 in wrought form. Choosing between (955 and 1037 °C) (1750 and 1900 °F) as solution annealing temperature, Inconel 718 can become extremely embrittled (HEE index = 0.53) or slightly embrittled (HEE index = 0.92) by high pressure hydrogen gas, respectively.