Diffusion coatings formation features, obtained by complex chemical-thermal treatment on the structural steels

Abstract

Introduction. The main methods of increasing the efficiency of products made from structural steels are considered. A description of diffusion saturation from liquid metal media solutions (DSLMMS) is given. Also, complex diffusion saturation technology (CDS), including DSLMMS and carburization is shown. The purpose of the work is to reveal the effect of steel composition on the process of formation and elemental composition of diffusion-saturated surface layers (coatings) based on chromium, as well as to establish differences and regularities in the processes of formation of diffusion-saturated coatings after DSLMMS and CDS. The methods of investigation. Cylindrical specimens 20 mm in diameter and 30 mm long were subjected to DSLMMS. The specimens were made of carbon and alloyed steels: St3, 20-Cr13, 40-Cr, 40-Cr13, 12-Cr18-Ni10-Ti. At the same time, some of the specimens were previously subjected to vacuum cementation. An eutectic Pb-Bi with the specified content of Cr was used as a transport medium when executing DSLMMS. Metallographic studies were carried out on microsection prepared according to the standard method. Studies to determine the thickness of coatings and its structure were carried out on the Dura Scan Falcon 500 Microhardness Tester. The elemental composition of the coatings was determined by the method of electron microprobe analysis on a Tescan Lyra 3 scanning electron microscope with the Oxford Ultim MAX PCMA system. Results and discussion. As a result of the research, it was revealed that the formation of saturated coatings occurs with DSLMMS and CDS. At the same time, the thickness of the coatings and its elemental composition depend on the steel grade and the technology used. After DSLMMS concentration of Cr varies from 96.9% to 91.1%. At the same time, the maximum concentration of 96.9% is observed on steel St3. After CDS, on the surfaces of all steel samples, the concentration of Cr decreases in comparison with the coatings obtained by the DSLMMS technology on steels: St3 from 96.9% to 66.8%; 40-Cr from 91.1% to 63.18%; 20-Cr13 from 93.18% to 62.54%; 12-Cr18-Ni10-Ti from 92.92% to 64.77%. The total thickness of diffusion-saturated coatings formed on all the alloys studied ranges from 17 to 17.5 µm.