Microstructural evolution and creep damage accumulation in Grade 92 steel weld for steam pipe applications

Grade 92 steel is a commonly used material for steam pipe and tube applications in the power generation industry. The advantages of Grade 92 steel include excellent creep resistance and sufficient corrosion resistance, which provide a long service lifetime expectancy for components. Grade 92 steel is typically heat treated by a normalisation process performed at approximately 1100°C followed by a tempering process at approximately 750°C. The resulting microstructure of Grade 92 steel is composed of a tempered martensitic matrix with secondary precipitates distributed both on lath and grain boundaries and within laths and grain interiors. Welds in thick-section steam pipes made from Grade 92 steel are typically fabricated by a gas tungsten arc root process followed by a multi-pass submerged arc fill process. Post weld heat treatment (PWHT) is then performed on as-fabricated welds, helping to relieve residual stress and stabilise the martensitic microstructure. The microstructures in the HAZs of these complicated welds have not to date been fully understood. There is a lack of systematic microstructural investigations to define the different regions of the microstructure across the HAZ as a function of the welding process.