10 - New martensitic steels

Abstract

Boundary and sub-boundary hardening is shown to be the most important strengthening mechanism for creep of martensitic 9Cr steel and is enhanced by fine dispersions of precipitates along boundaries. Based on the boundary and sub-boundary hardening, new martensitic 9Cr steel has been alloy-designed for application to thick-section boiler components, such as main steam pipes and headers, with steam temperature of 650°C maximum in coal-fired power plants. Boron stabilizes fine distributions of M23C6 carbides along prior austenite grain boundaries in a 9Cr-3W-3Co-0.2V-0.05Nb steel for up to long times during exposure at 650°C, which increases the creep strength at low stresses and long times. Boron also suppresses the degradation in creep strength due to brittle Type IV fracture in the heat-affected zone of welded joints at 650°C. The addition of nitrogen causes the precipitation of nanometer-size MX nitrides, which significantly increases the creep strength. The addition of boron and nitrogen without any formation of boron nitrides during normalizing heat treatment exhibits not only much higher creep rupture strength of base metal than conventional martensitic steels Gr.91, Gr.92, and Gr.122 but also substantially no Type IV fracture in welded joints at 650°C. This is a new martensitic 9Cr steel MARBN (martensitic 9Cr steel strengthened by boron and MX nitrides), which contains 120–150ppm boron and 60–90ppm nitrogen. MARBN also exhibits enough creep rupture ductility, larger than or the same as Gr.91, because of low nitrogen concentration. The formation of protective Cr2O3-rich oxide scale is achieved on the surface of MARBN by pre-oxidation treatment in argon gas, which significantly improves the oxidation resistance in steam at 650°C.