The effects of chemical composition of Nimonic 80A-alloy on the microstructure and properties of gas-borided layer

Abstract

Ni-based alloys are characterized by great resistance to corrosion and high temperature oxidation. Diffusion boriding is well-known as a process which can improve the wear protection of these alloys. In this paper, continuous gas boronizing in N2–H2–BCl3 atmosphere is proposed for the production of the boride layer on Nimonic®80A-alloy. Microstructural characterization of this layer is studied with the use of an optical microscope, scanning electron microscope, energy-dispersive X-ray microanalysis and X-ray diffraction. The diffusion zone mainly consists of a mixture of nickel and chromium borides, occurring in the compact boride zone. Beneath this zone, there are also some areas in which borides appeared at grain boundaries. However, it is an exceptional situation. In most cases this zone is invisible. The relatively high chromium content in this alloy results in diminished depth of the boride layer in comparison with Inconel®600-alloy containing less chromium. The increase in concentration of chromium causes that grain-boundary diffusion of boron more difficult. Hence, during boriding of Nimonic®80A-alloy volume diffusion plays the more important role, which requires the high activation energy. As a consequence, the zone with borides at grain boundaries can obtain a limited depth or cannot occur. Chromium content also influences some mechanical properties of the layer. Its higher content causes an increase in hardness because of the higher percentage of harder chromium borides in the compact boride zone. The limited iron concentration in this alloy improves the quality of the layer. The microstructure is free of any porosity. It improves the tribological properties of the layer in comparison with Inconel®600-alloy.