Michael Patrick Manning

Martensitic 11% CrMoNiNb steel for turbine rotors in geothermal power stations

Turbine rotors in a geothermal power station were required in high alloyed 12% Cr steel. After some preliminary investigations it was found that a modified 11% CrMoNiNb steel will fulfill the required mechanical properties as well as a sufficient resistance to corrosion. Two LP rotors with approximately 1,295 mm (51 in.) premachined diameter and ungashed weight of approximately 31 mt were manufactured. The steel was melted in a 125 mt electric arc furnace and subsequently remelted into a 113 mt ESR ingot with 2,300 mm (90 in.) diameter. After forging and preliminary heat treatment the rotors were quality heat treated to a yield strength (0.2% and 0.02%) of approximately 600 MPa (87 ksi) and approximately 550 MPa (80 ksi) respectively and tensile strength of approximately 780 MPa (113 ksi). This resulted in a FATT of approximately 16 C (60 F). Low hardness is important to susceptibility to stress corrosion. It was achieved to approximately 20 HRC. The results have met the assumption with respect to this steel for application as rotor material for geothermal power stations.