Effect of Heat Input on Fracture Behavior of X80 Girth Filler Weld Under Service Loads

We focused on studying the fracture behavior of filler welds with varying heat inputs under coupling tension‐bending loads. A set of in situ devices was designed and combined with scanning electron microscopy (SEM) to compare the morphology evolution and fracture behaviors of filler welds with varying heat inputs under coupling tension‐bending loads. The results indicated that the fracture mechanism did not change when the low heat input filler welds transitioned from uniaxial tension loads to coupling tension‐bending loads. For high heat input filler welds, the change from ductile to brittle fracture. Low heat input formed a mixed phase structure of fine bainite and ferrite. The low density of martensite‐austenite (M‐A) constituents led to lower debonding ratios, resulting in a lower local strain coefficient and ductile fracture. Clustered M‐A constituents were formed within the coarse bainite under high heat input. The coupling tension‐bending loads increased the equivalent plastic strain and stress triaxiality of the multilayer M‐A constituents. As a result, a significant amount of debonding occurred at the interface between the M‐A constituents and the coarse bainite matrix, and sharp microcracks emerged, ultimately leading to brittle fracture.