Creep Resistant Martensitic Steels for Operation at High-Temperatures in Power Generation Applications

Increasing the temperature capabilities of ferritic/martensitic 9-12% Cr steels can help in increasing the operating temperature of land-based turbines and minimize the use of expensive high-temperature alloys in the hot section. A creep resistant martensitic steel, CPJ-7, was developed with an operating temperature approaching 650°C. Subsequently, another creep resistant martensitic steel, JMP, was designed with the potential to operate at or above 650°C. The design of the alloys originated from computational modelling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately forty heats of CPJ, each weighing approximately 7 kg, were vacuum induction melted and varied by compositions. Variations in the C and N contents were used to tailor microstructures with desirable carbides and carbonitrides. Variations in Cr, Mo, Co, Ta and Cu were investigated. Overall, CPJ-7 presented superior creep properties when compared to COST alloys and P91/92. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M23C6 precipitates at 650°C. Following the work on CPJ-7, the JMP steels were designed with higher Co for increased solid solution strengthening, Si for oxidation resistance and different W and Mo concentrations for matrix strength and stability. The JMP steels showed increases in creep life compared to CPJ-7 between 118 to 150% at 650°C for testing at various stresses between 138 MPa and 207 MPa. On a Larson-Miller plot, the performance of the JMP steels surpasses that of state-of-the-art MARBN steel. Approximately 17 years of cumulative creep data is reported for CPJ-7 and 21 years for the JMP steels which encompasses various compositions. The relationships between composition-microstructure-creep properties will be discussed including characterization of microstructures after >20,000 hours in creep.