Influence of oxide particles on the toughness of modified 9Cr-1Mo steel shielded metal arc weld metals with different Ni+Mn content

In the modified 9Cr-1Mo steel welds, Ni+Mn content of the filler metal and PWHT temperature range differ between U.S. and Japan. Hence, we investigated the influence of PWHT temperatures on the toughness of modified 9Cr-1Mo steel shielded metal arc weld metals with different Ni+Mn content, it was clarified that the oxide particle density and the hardness of matrix affect the toughness. The weld metal with a high Ni+Mn content of 2.6% partially transformed into austenite during holding on 1073 K which was more than Ac1. As a result, the freshly formed martensite of same microstructure as-welded state was formed, the hardness increased and the toughness decreased. As evidence, part of fractured surface after Charpy test is the same as that of as-welded state, spherical oxide particles (almost without any cracks) were observed at the center of the dimple. Therefore, it was suggested that the toughness decreases due to the partial precipitation of freshly formed martensite by carrying out PWHT, and oxide particles affect the decreasing of the toughness. © 2020 Japan Welding Society. All rights reserved.