Failure Transition Mechanism of Stress Rupture Performance of the Inconel 625/9 Pct Cr Steel Dissimilar Welded Joint

Based on a series of stress rupture tests at 620 °C under 110 to 170 MPa and at 650 °C under 80 to 110 MPa, the relationship between the stress and rupture time was obtained to evaluate the long-term performance of the welded joint (WJ). At 620 °C, the stress rupture occurred in the base metal of 9 pct Cr steel (9 pct Cr-BM), with the stress ranging from 130 to 170 MPa, yet the failure shifted to the heat-affected zone (HAZ) of 9 pct Cr steel (9 pct Cr-HAZ) with the stress ranging from 110 to 120 MPa. This failure behavior was observed at 650 °C with the turning point of 110 MPa. In particular, a ductile-to-brittle transition was determined when the rupture location shifted from 9 pct Cr-BM to 9 pct Cr-HAZ. Moreover, both the Laves phase adjacent to the M₂₃C₆ and the independent phases could be detected in the 9 pct Cr-HAZ after the stress rupture test, while only M₂₃C₆-type carbides could be found in the 9 pct Cr-BM. The appearance of the microhardness turning point and the formation of the Laves phase in the 9 pct Cr-HAZ are considered as the crucial factors resulting in the transition of the failure mode.