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 (73µm in particle size). 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 Bore and Silicon in the 
Nickel 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.