Effect of Heat Treatment on the Creep Rupture Strength of a Cr-Mo-V Steel for Steam Turbine Shafts: Studies on the high temperature strength of a Cr-Mo-V steel for steam turbine shafts-I

To investigate the cause for dispersion of the creep rupture strength of the actual largesized steam turbine shaft forgings, creep rupture tests were conducted with specimens heattreated at different austenitizing temperatures, different cooling rates from austenitizing temperature and different tempering temperatures. The types of the specimens used were both plain bar specimens and notched bar specimens, and testing temperatures were 510°C, 538°C, 565°C and 593°C. Creep rupture data were treated with Larson-Millers' parameter and the results were illustrated by master rupture curves. It was found that the creep rupture strength of the plain bar specimens was increased remarkably by elevating the austenitizing temperature from 950°C to 1000°C. It is supposed that this difference of the austenitizing temperature will be the most important factor for the dispersion of the creep rupture strength among the actual steam turbine shaft forgings. Such cooling rate from austenitizing temperature as may produce bainitic structures has little effect on the creep rupture strength, while the tempering temperature exercises a relatively large effect when a tempering treatment is carried out at the temperatures between 675°C and 700°C. The following was found about the effect of austenitizing temperature on the high temperature notch sensitivity of rotor materials. According to the conditions of the tests, specimens austenitized at 950°C showed notch strengthening, while notch weakening was observed with specimens austenitized at 1000°C and 1050°C. While specimens austenitized at 950°C and tempered at various temperatures showed notch strengthening, oil quenched specimens from austenitizing temperature exhibited notch strengthening at first and then notch weakening as the Larson-Millers' parameter increased. This tendency in notch sensitivity of the materials is approximately corresponding to the tendency of hardness change measured in ruptured specimens, and it was ascertained that high temperature notch sensitivity of a material was influenced by its structural changes.