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 fulfil the required mechanical properties as well as a sufficient resistance to corrosion. Two LP rotors with appr. 1 295 mm (51 in.) premachined diameter and ungashed weight of appr. 31 mt were manufactured. The steel was melted in a 125 mt electric are 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 (0.2K and 0.02%) of appr. 600 MPa (87 ksi) and appr. 550 MPa (80 ksi) respectively and tensile strength of appr. 780 MPa (113 ksi). This resulted in a FATT of appr. 16 degrees C (60 degrees F). Low hardness is important to susceptibility to stress corrosion. It was achieved to appr. 20 HRC. The results have met the assumption with respect to this steel for application as rotor material for geothermal power stations.