WIDE-GAP JOINTS IN INCONEL 738 SUPER ALLOY BY VACUUM BRAZING: MICROSTRUCTURE AND MICROHARDNESS

Abstract

In this investigation, the effect of the parameters during vacuum brazing of wide-gaps (0.6 to 1.8 mm) in a Nickel-based super alloy Inconel 738, were studied. The temperatures used were 1120 °C and 1150 °C with times of 120 and 240 min. The filler metal was a mix of Ni-11Cr-3.5Si-2.25B-3.5Fe paste with Inconel 738 powder with a particle size of 73µm. From observations with Scanning Electron Microscopy, three microstructural zones were identified: Athermally Solidified Zone (ASZ), Isothermally Solidified Zone (ISZ) and Diffusion-Affected Zone (DAZ). The eutectic compounds identified by X-Ray Energy Dispersive Spectroscopy in the ASZ were of the Fe-Cr-Si type. When the brazing time was increased to 240 min and the temperature to 1150 °C with a gap of 0.6 mm and the application of a mixture ratio of 80/20 (Ni paste and Inconel powder, respectively), the volumetric fraction of eutectics decreased in the ASZ favoring isothermal solidification, diffusion of B and Si in the Ni matrix; hardness reached 260 HV, while the width of the DAZ decreased by 39% on average. The latter indicates that an adequate selection of brazing parameters allows to obtain better joints, with a reduced process time and a decrease in intermetallics, which are the main reason of cracks and fractures.