Microstructure evolution and mechanical properties of simulated HAZ in a Ni-17Mo-7Cr superalloy: effects of the welding thermal cycles

Abstract

Welding thermal cycles with different peak temperatures were reproduced by a Gleeble testing machine to simulate the heat-affected zone of a Ni-17Mo-7Cr superalloy-welded joint. The effects of the thermal cycle times and peak temperatures on the microstructure and tensile properties of the thermal simulation specimens were systematically studied. The results show that no eutectic carbides are observed below 1300 °C, even when the number of thermal cycles is increased to 5. However, the eutectic reaction occurs above 1300 °C and causes a significant change in the carbide morphology because it transforms from a granular to lamellar structure. Moreover, the carbide composition transforms from (Ni, Cr)3Mo3C for the primary carbides to Ni3(Mo, Cr)3C for the lamellar carbides after the eutectic reaction. Tensile tests show that the thermal cycle has a minor effect on the tensile properties, except when the specimen underwent 5 thermal cycles with a peak temperature of 1340 °C. This is mainly due to the severe liquation reaction caused by elevated temperatures from the welding heat source, which induces cracking along the grain boundaries and results in a degradation of the tensile properties.