Microstructure characterization and HCF fracture mode transition for modified 9Cr-1Mo dissimilarly welded joint at different elevated temperatures

The high cycle fatigue (HCF) tests of modified 9Cr-1Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 108 cycles based on S-N curve can be estimated as a half of weld joint's yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones (IC-HAZs) of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2 (BM2) is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion-extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin (SNDFCO) from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E (BM1) due to its higher susceptibility of temperature. Besides, it is found that the δ-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.