Martensite microstructure of 9–12%Cr steels weld metals

Abstract

Several new grades of steel have been developed in recent years, which have been based on the high-strength 9–12%Cr steels. These steels were developed to meet the proposed demand by power generation companies to increase efficiency to operating at higher temperatures by and thus burning less fossil fuel, principally coal, and thereby reducing costs and also meeting the increasingly stringent environmental requirements. The 9–12Cr steels used for high temperature components in power plants are generally required to possess good mechanical properties, corrosion resistance, creep strength and fabricability. Although such steels normally have a fully martensite microstructure, they are also susceptible to the formation of delta ferrite, mainly during the welding process. Delta ferrite has several detrimental effects on such properties as creep, ductility and toughness. Thus, it is important to avoid its formation. In this study the formation of delta ferrite in the weld metal of high-strength 9–12%Cr steels has been analysed for several samples with variations of key alloying elements. The results indicate that the most effective way to avoid delta ferrite in the weld metal is to reduce the chromium equivalent value to as low a value as possible. A fully martensite microstructure is obtained when both the Schneider chromium equivalent is lower than 13.5 and the difference between the chromium and nickel equivalent is lower than 8.