Weld metal microstructure analysis of 9–12% Cr steels

Abstract

High-strength 9–12% Cr steels have been developed to meet the demand by power generation companies to increase efficiency by operating at higher temperatures. The 9–12% Cr 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 martensitic microstructure, they are also susceptible to the formation of delta ferrite, mainly during the welding process. Delta ferrite has several detrimental effects on properties such as creep, ductility and toughness. Thus, it is important to avoid its formation. This paper analyses the microstructure in the weld metal of high-strength 9–12% Cr steels 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 to obtain a fully martensitic microstructure is to reduce the ferrite former elements as low as possible. The partial substitution of Mo for W favours austenite stability and would be expected to improve the mechanical properties at high temperature.