Feasibility of MARBN Steel for Application to Thick Section Boiler Components in USC Power Plant at 650 °C

A 9Cr-3W-3Co-VNbBN steel, designated MARBN (MARtensitic 9Cr steel strengthened by Boron and Nitrides), has been alloy-designed and subjected to long-term creep and oxidation tests for application to thick section boiler components in USC power plant at 650^oC. The stabilization of lath martensitic microstructure in the vicinity of prior austenite grain boundaries (PAGBs) is essential for the improvement of long-term creep strength. This can be achieved by the combined addition of 140ppm boron and 80ppm nitrogen without any formation of boron nitrides during normalizing at high temperature. The addition of small amount of boron reduces the rate of Ostwald ripening of M₂₃C₆ carbides in the vicinity of PAGBs during creep, resulting in stabilization of martensitic microstructure. The stabilization of martensitic microstructure retards the onset of acceleration creep, resulting in a decrease in minimum creep rate and an increase in creep life. The addition of small amount of nitrogen causes the precipitation of fine MX, which further decreases the creep rates in the transient region. The addition of boron also suppresses the Type IV creep-fracture in welded joints by suppressing grain refinement in heat affected zone. The formation of protective Cr₂O₃ scale is achieved on the surface of 9Cr steel by several methods, such as pre-oxidation treatment in Ar gas, Cr shot-peening and coating of thin layer of Ni-Cr alloy, which significantly improves the oxidation resistance of 9Cr steel in steam at 650^oC. Production of a large diameter and thick section pipe and also fabrication of welds of the pipe have successfully been performed from a 3 ton ingot of MARBN.