The Influence of Heterogeneous Microstructure on Low-Cycle Fatigue Behavior in NiCrMoV Welded Joint

The heterogeneous microstructure of NiCrMoV welded joint plays a crucial role in influencing the mechanical properties. However, its effects on the low-cycle fatigue (LCF) life have not been widely researched. In order to reveal the effect of the strain amplitude and subzone proportion of the welded joint on the LCF performance, the digital image correlation (DIC) technique was applied to investigate the strain distribution and evolution of the welded joint during the strain-controlled LCF tests. The results indicated that the strain localization shifted from fine grain heat-affected zone (FGHAZ) to the weld metal of fine grain zone (WM-FGZ) when the LCF strain amplitude was 0.3%, while the final strain concentration and fracture of the whole welded joint were all located in WM-FGZ regardless of various strain amplitudes. Therefore, the WM-FGZ was considered as the weak position due to its lower yield strength. However, the LCF life with high proportion of WM-FGZ tested by the 12-mm extensometer was much higher than the one tested by the 25-mm extensometer because the ability of WM-FGZ bearing the extra deformation required from other subzones could be improved. This research could provide the theoretical guidance and reference for improving the LCF properties of the welded NiCrMoV steel.