MORPHOLOGY AND PHASE COMPOSITION OF DIFFUSION BORIDE LAYER BASED ON METALLOGRAPHIC ANALYSIS OF STEEL 45

Abstract

Chemical-thermal treatment in general and borating in particular are highly effective ways to increase the service life of working bodies and machine parts, as well as tools. At the same time, it is necessary to select possible grades of steels, which are preferably subjected to boriding, depending on the goal. To solve practical problems in order to control the quality of a boride diffusion coating, there is a need to determine the strength and morphological characteristics of the boride layer and its phase composition. Since the high- boride FeB phase has low performance characteristics (high brittleness, creates tensile stresses on the surface, leading to cracking and spalling in the boride layer), in most cases the content of the FeB phase in the coating is limited to an upper limit of no more than 10–15 % of the thickness of the boride layer. Determination of the phase composition of boride layers is preferably carried out by electron microscopy methods, however, under production conditions, this is a laborious task. One of the options for analyzing the structure and morphology of the results obtained is the visualization of the phase composition of boride diffusion layers using "color etching'' with the possibility of assessing the structural-phase state using optical microscopy. It is shown that the electrolytic etching method proposed in this work in a saturated aqueous solution of alkaline sodium picrate makes it possible to obtain high-contrast images of the microstructure of the boride layer, which can subsequently be automatically processed in modern software systems for metallographic analysis in order to qualitative and quantitative assessment of the structural-phase state. Compared to the chemical methods of "color etching", the electrochemical etching method does not require highly skilled personnel and can be used in a production laboratory.