New 2Cr-Mo-Ni-W-V Steel for High-Pressure Rotors

In recent years, extensive research has been undertaken to improve the service properties of 1Cr-Mo-V steels with the aim of manufacturing high-pressure (HP) and medium-pressure (MP) turbine rotors having higher creep-rupture and toughness values. However, notched impact strength, fracture appearance transition temperature (FATT), and fracture toughness depend greatly on heat treatment. Oil quenching after austenitizing, which is common practice in Europe, gives higher values than air quenching, which is specified in the United States. A change in heat treatment, however, has little effect on the toughness in the core of turbine rotors. To improve this property, it is necessary to increase the alloy content of the steel. This has led to the development of a new 23Cr-Ni-Mo-W-V 8 8 steel for HP rotors. After testing this new steel at laboratory level, a 114-ton electroslag remelted (ESR) ingot was produced, from which a 42.5-ton turbine rotor with a 1370-mm maximum body diameter and a 1555-mm flange diameter was manufactured and extensively tested. Results obtained from transverse specimens showed an 0.2% yield strength of 650 N/mm², excellent elongation, reduction of area, and notched impact strength values, as well as a FATT of 273 K (0°C) at the surface, 293 K (20°C) at midradius, and 313 K (40°C) in the core. A modified production process will give a FATT of 293 K (20°C) in the core of the turbine rotor with excellent creep-rupture properties.