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

Abstract

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.