Evaluation on Reheat Cracking of Multi-pass Weld Metal of YP 690 QT Steel

Abstract

This study evaluated the relationship between reheat cracking that occurs in the reheated weld zone formed by subsequent multi-pass welding and the preheat/interpass temperature. The multi-pass weldment was YP 690 MPa high-strength steel, which is used in mobile offshore platform units. The welding process was flux-cored arc welding (FCAW), which coupons were constrained by welding to simulate an actual constraint condition. Three preheat/interpass temperature ranges were employed. The characteristics of the weldment subjected to preheat/interpass temperatures higher than 80°C satisfied the relevant specifications. In the bending testing, a weldment subjected to a preheat/interpass temperature range of 50-60 °C was fractured. The results demonstrated reheat cracking in the reheated weld zone in multi-pass weldments. Reheating cracking (RHC) was initiated at carbides precipitated along the grain boundaries, induced by multipass heating; some cracks were arrested in the reheated zone and the others propagated to the columnar weld metal. The hardness of the cracked specimens was higher than that of the specimen subjected to a higher preheat/interpass temperature, confirming that a susceptible microstructure had formed due to the faster cooling rates. In the analysis of weld residual stress, the specimen with the lower preheating/interpass temperature range showed higher tensile residual stress than that of high preheating/interpass temperature range. Therefore RHC in the reheated zone of the weldment was mainly affected by grain boundary carbides, a higher hardness, and residual stresses. Based on the above discussion, the mechanism of RHC and assessment of structural integrity is presented. Key words: flux cored arc welding, reheating cracking(RHC), preheat/interpass temperature, quenched-tempered(QT), residual stress, structural integrity