Structure of High-Speed Alloy after Plasma Surfacing in Nitrogen Medium and Heat Treatment

Abstract: We investigate the influence of the plasma surfacing and subsequent high-temperature tempering on the surface morphology and the elemental composition of the deposited coatings made of the R18-type high-hardness chromium-tungsten steel additionally alloyed with aluminum and nitrogen (C 0.86%; Cr 4.84%; W 17.0%; Mo 5.40%; V 0.50%; Al 0.65%; N 0.06%). We engaged the 30HGSA steel with high mechanical properties as the base metal. The usage of low-temperature preliminary and accomplished heating, as well as using a thermal surfacing cycle, was a feature of the surfacing. The surfacing cycle consisted of three main stages: the first stage limited the heating duration and provided the high cooling rate within the high temperature domain, prevented the grain growth and the austenite decomposition and thus formation of the equilibrium low-strength structures; the second stage consisted in austenitic state of the deposited metal; in the third stage, we obtained the deposited metal with low tendency to the crack formation. We investigated the surface morphology and the elemental composition of the coatings by means of scanning electron microscopy and X-ray spectral microanalysis in two states: directly after surfacing and after surfacing and high-temperature tempering. In the first case, we discover that the main part of the material surface represents the pearlite grains. The complex composition cementite and the variable composition (Fe4W2N, FeWN2, and Fe4W2C) iron-, tungsten-, and molybdenum-based compounds fill the joints and the grain boundaries. There are also aluminum-based solid solutions and, possibly, the AlN phase. The high-temperature tempering results in a hardness increase, a change in the grain shape and size, a quantitative change in the elemental composition, and a uniform distribution of alloying elements over the material volume. We show that the proposed method of the plasma surfacing with the high-temperature tempering meets all basic requirements to the surface of the cold-rolled work rolls – this fact is confirmed by the test results of the surfaced roll batch. © 2020, Allerton Press, Inc.