THE CREEP BEHAVIOUR OF THREE ALLOY STEELS FOR STEAM TURBINE APPLICATIONS
Griffin, Shane H.
Understanding the elevated temperature behaviour of materials is extremely important for the efficient selection of materials for use in steam turbines, which require high operating temperatures to achieve maximum efficiency. In this study, an attempt to characterize the creep behaviour of three ASTM grade high alloy steels to determine their suitability for use in various steam turbine applications was made. The steels of interest were ferritic in nature and include a 12%CrMoV stainless steel, a 1 %CrMoV steel, and a steel suitable for nitriding. The testing consisted mainly of creep tensile tests to determine the creep behaviour of these materials at temperatures near the service temperature, which is approximately 538°C. Other tests such as tensile and hardness tests were also performed to determine or verify other material properties. Scanning and transmission electron microscopy and Xray analysis work were performed on as-received materials and ruptured creep, specimens. The results of the creep testing showed that the creep strength and oxidation resistance of the 12%CrMoV steel was significantly greater than that of the two low alloy steels, although the rupture ductility was significantly lower. The shape of the creep curve for each steel was also well established. The rupture data was used to create a master curve for each steel using time-temperature parameters. The Goldhoff-Sherby parameter was used for the two low alloy steels, while the Larson-Miller parameter proved to fit the stainless steel best. Many particles were observable in the microstructure of these steels, which are most likely carbide and nitride particles. The largest of these particles were examined using EDX analysis and large amounts of chromium were found to be present, suggesting that they are likely chromium carbides. Many smaller particles were also observable which are likely vanadium nitrides, although they were too small for reliable EDX analysis. The creep behaviour of these steels under service conditions is not easily characterized using data obtained from short term creep tests performed at much higher stresses than would be seen in service.