Analysis of Creep and Stress Relaxation Data for Ultra-Supercritical Steam Turbine Materials

Development efforts are underway to qualify boiler and turbine materials for the ultra-supercritical (USC) steam conditions of 720/760°C and 31/35 MPa. An evaluation of the creep-rupture and stress relaxation data was performed for the critical turbine components of casing and bolting. The casing materials were Inconel 740, Udimet 500, and CCA617. Time-temperature parametric analysis of the rupture data was performed to determine the average rupture stress for a life of 100,000 hours at temperatures from 700 to 760°C. A multiple heat analysis was used for Inconel 740, and the average stress varied from 88 MPa to 109 MPa at 760°C. For Udimet 500, data for both cast and forged materials were used, and the average rupture stress was approximately the same, greater than 150 MPa at 760°C. The maximum useful temperature for CCA617 is about 730°C based on a 100 MPa/10⁵ hours criterion. The bolting materials were Nimonic 105 and Udimet 500. For Nimonic 105, the initial strain was 0.15% and the calculated relaxed stress at 760°C was 32 MPa (using the Larson Miller parameter). For Udimet 500, the initial strains were 0.2% and 0.3%. The calculated time to a relaxed stress of 47 MPa depended on the time-temperature parameter used, and ranged from 50,000 to 100,000 hours at 760°C for an initial strain of 0.3%.