Wide Gap Brazing of Inconel 738LC Nickel-Based Superalloy: Metallurgical and Mechanical Characteristics

Abstract

This paper addresses the microstructure properties relationship of wide gap brazed Inconel 738LC. Amdry 718 and a Ni-Cr-Fe-Si-B alloy were used as high melting particles (HMPs) and low melting particles (LMPs), respectively. The effect of the amount of LMPs, 30, 40, and 50 pct, on the microstructure and the shear strength of the joint, was investigated. The microstructure in the brazing zone consists of Ni-based solid solution and eutectic-type microconstituents that is nickel-rich and chromium-rich borides. Nickel-rich borides only observed in the presence of the high amount of LMPs, because of the low Cr-B ratio in the filler alloy. At the brazing/base metal interface, high diffusion of melting point depressant (MPD) elements to the base metal led to the formation of small blocky Cr-rich borides in Ni-rich solid solution matrix. The nickel-rich and chromium-rich borides have a hardness value of about 1200 and 830 HV, respectively, which is remarkably higher than the other regions. By increasing the amount of LMPs from 30 to 50 pct, the joint shear strength decreases from 422 to 373 MPa due to the higher volume fraction of the brittle boride phase in the joint. For all brazed joints, the cleavage fracture mode was observed, which can be ascribed to the interlinked network of the brittle eutectic-type microconstituents. The X-ray diffraction pattern confirms that the preferred location for crack propagation is eutectic boride phases.