Influence of Oxide Particles and Hardness on the Toughness of Modified 9Cr-1Mo Steel Weld Metals

The main welding processes used for modified 9Cr-1Mo steel for the main steam pipes in ultra-supercritical thermal power plants are shielded metal arc welding (SMAW), submerged arc welding (SAW), and gas tungsten arc welding (GTAW). However, welding is rarely performed with the understanding that the toughness of the weld metals differs depending on the welding process. We investigated the dominant factors that determine the toughness of weld metals for these welding processes with different post-weld heat treatment (PWHT) temperatures. Both SMAW and SAW weld metals with high oxygen content possessed low toughness in their as-welded state, with many oxide particles (MnSiO₃) observed on fracture surfaces. In contrast, the GTAW metal did not have any oxide particles and exhibited high toughness. Therefore, the presence of oxide particles facilitated crack formation at the interface between the oxide particles and the matrix, leading to low toughness. The toughness of SMAW and SAW metals was improved by PWHT, which was accompanied by a reduction in the oxide particle density on the fracture surface. The hardness is decreased by a decrease in the carbon content in the matrix with a decrease in the solid solution strengthening of carbon by precipitation of carbides. This decrease was reflected by the lower impact force measured during the Charpy test on the interface between the oxide particles and the matrix. Therefore, the dominant factors determining the toughness are the density of the oxide particles and the hardness of the matrix. Carbides precipitated by PWHT did not affect toughness. From the above, the toughness properties of modified 9Cr-1Mo steel weld metals were clarified from a metallographic point of view.