Effects of Chemical Composition and Heat Treatment on Creep Properties of ASME Grade 91 Type Steel

Effects of the chemical composition and heat treatment on the creep properties of ASME Grade 91 type steel were experimentally investigated using materials whose chemical compositions and heat treatment conditions in the steel making process were completely controlled. Regarding chemical composition, only the Al, Cr and Ni contents were systematically varied while keeping the contents of the other elements and heat treatment conditions constant. Regarding heat treatment, the normalizing and tempering temperatures were varied while keeping the contents of chemical components constant. The creep tests of materials were performed for approximately up to 50,000 h at 650°C. The creep strength of Grade 91 type steel decreased with increasing Al content under test conditions of short-term to long-term range. On the other hand, the effect of Cr on the creep life of Grade 91 type steel depended on the stress or time range, and the creep strength of the steel decreased with high Cr content under test conditions of only long-term range. No effect of Ni on the creep life of the materials was observed in the test data obtained in this study. As creep tests are currently being conducted at 625°C and 60 MPa, which are conditions closer to the actual service conditions of main steam piping at ultra-super critical fossil power plants, the creep deformation data at present indicate that the above trends hold true for long-term range. As for the effect of heat treatment, the creep life of the materials tended to increase with increasing normalizing temperature or decreasing tempering temperature. Regarding the results obtained in this work, it seems that within the scope of the material standards for Grade 91 type steel, the effect of chemical composition on creep life is greater than that of heat treatment.