The effects of internal hydrogen on tensile properties of solution-treated and precipitation-hardened iron-based super alloys SUH660 (A286) were investigated. Tensile specimens were exposed to 10 MPa and 100 MPa hydrogen gas at 270 °C for 300 hours to obtain uniform distribution of hydrogen content, and then the tensile tests were conducted at room temperature in air. Reduction of area of the solution-treated SUH660 slightly decreased with increase in the internal hydrogen content, whereas that of the precipitation-hardened SUH660 largely decreased. SEM observation showed that large-size and small-size dimples were formed on tensile fracture surfaces of the solution-treated SUH660, and that the area of the small-size dimples increased with increase in the internal hydrogen content. The increase in the area of small dimples resulted in the slight decrease in reduction of area. Three dimensional analysis of the two stereo SEM images showed that the area of small-size dimples was void sheet. Therefore, it was inferred that hydrogen enhanced the formation of void sheet. In the case of the precipitation-hardened SUH660, large-size and small-size dimples as well as facets were formed on tensile fracture surfaces. The area fraction of facets increased with increase in the internal hydrogen content, resulted in the large decrease in the reduction of area. Therefore, it was inferred that hydrogen also enhanced the formation of facets. Based on the results, the index of hydrogen embrittlement in tensile fracture of hydrogen-charged specimens was proposed.
