Specific Features of the Formation of Diffusion Coatings Prepared by a Combined Chemical Thermal Treatment of Structural Steels

The main methods for enhancing durability of products prepared from structural steels are considered. The technologies of diffusion alloying from low-melting liquid-metal solutions (DALMMS) and a technology developed for the combined diffusion alloying of surface layers of the material of products that includes DALMMS and carburizing, namely, the combined chemical thermal treatment (CCTT), are described. The purpose of the work is to elucidate the influence of the steel compositions on the formation and elemental composition of diffusion-alloyed chromium-based surface layers (coatings) and to reveal differences and specific features in the formation of the diffusion-alloyed coatings after DALMMS and CCTT. Cylindrical specimens 20 mm in diameter and 30 mm long are subjected to DALMMS. The specimens are prepared from the carbon and alloyed steels: low-carbon steels St3 and 20Kh13, medium-carbon steels 40Kh and 40Kh13, and austenitic steel 12Kh18N10T. Some specimens are preliminarily subjected to vacuum carburizing. A lead–bismuth eutectic melt into which a specified amount of chromium is introduced is used as a technological medium (transport melt) for DALMMS. Metallographic studies are carried out on microsections prepared using a standard procedure. The thicknesses and structures of the coatings are determined on a Dura Scan Falcon 500 microhardness meter. The elemental compositions of the coatings are determined by electron probe microanalysis (EPMA) on a Tescan Lyra 3 scanning electron microscope equipped with an Oxford Ultim MAX system. The DALMMS and CCTT processes result in the formation of diffusion coatings. The thickness and elemental composition of the coatings depend on the trade mark of the steel and applied technology. After DALMMS, the percentage chromium content varies from 96.9 to 91.1%. The maximum concentration (96.9%) is observed on St3 steel. After CCTT, the Cr concentrations on the surfaces of all steels decrease compared to those on the coatings prepared via the DALMMS technology on the following steels: for St3, from 96.9 to 66.8%; for 40Kh, from 91.1 to 63.18%; for 20Kh13, from 93.18 to 62.54%; and for 12Kh18N10T, from 92.92 to 64.77%. The total thickness of the diffusion-alloyed coatings formed on all alloys under study ranges from 17 to 17.5 μm.