MARTENSITIC STEEL CPJ7 FOR IMPROVED HIGH-TEMPERATURE CREEP CAPABILITIES IN POWER PLANTS

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650 degrees C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing similar to 7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson. Miller Parameter curve for CPJ7 at 650 degrees C almost coincides with that of COST E at 620 degrees C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M23C6 precipitates at 650 degrees C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.