Grain Boundary Feature and Its Effect on Mechanical Property of Ni 690 Alloy Layer Produced by GTAW

Ni690 alloy surfacing layers were fabricated by gas tungsten arc welding (GTAW) with two different heat inputs, namely, large heat input (LHI) and small heat input (SHI). The high temperature performance of the surfacing layer was evaluated by employing Gleeble 3500 thermal/mechanical simulator. It is found that the ultimate tensile strength (UTS) of the LHI samples was higher than that of the SHI samples after reheat thermal cycles, regardless of the reheating temperature. The EBSD result shows that the proportion of high angle grain boundaries (GBs, >15 degrees) in the LHI sample was obviously higher than that in the SHI sample. And more M23C6 particles were found to precipitate at the high angle GBs. The relations among UTS, GB angle distribution and M23C6 precipitations were analyzed. Moreover, the fracture modes were characterized by optical microscope (OM) and scanning electron microscope (SEM). The fracture mode was ductile fracture with deep dimples at 700 degrees C. While it changed to brittle intergranular fracture at 900 degrees C. As the temperature was enhanced to 1050 degrees C, the fracture returned to transgranular mode, with shallow dimples.