Investigation of SAW heat input on modified 9Cr-1Mo steel: microstructure, mechanical properties, and residual stress

This study investigates the impact of welding heat input on weldments of modified 9Cr-1Mo (P91) steel, a high-strength material that requires high-energy welding processes like submerged arc welding. In the as-welded condition, P91 steel welds primarily consist of untempered martensite, which transforms into tempered martensite during post-weld heat treatment (PWHT). Electron spectroscopy analysis reveals the presence of M₂₃C₆ and MX carbonitride precipitates at grain boundaries. Increasing the heat input leads to greater quantities of precipitates in the prior austenite grain boundaries, which can affect material properties. Weldment hardness profiles exhibit modest improvements, while ultimate tensile strength and toughness decrease with higher welding heat input, potentially due to the formation of a ferritic phase. Residual stress distributions are noticeably influenced by the welding heat input level.