Specific Features of the Formation of Ni–Cr Coatings Prepared by Diffusion Alloying from Low-Melting Liquid-Metal Solutions

Abstract

The main methods for increasing durability of products prepared from structural steels are considered. The choice of Ni and Cr as the main components of the coating is substantiated. The technology of diffusion alloying from low-melting liquid-metal solutions (DALMMS) is described. The purpose of the work is to reveal specific features of coating formation with the simultaneous diffusion saturation of structural steels with nickel and chromium according to the DALMMS technology. Cylindrical specimens with a diameter of 20 mm and a length of 30 mm are subjected to DALMMS. The specimens are prepared from the following structural steels: St3, 30KhGSN2A, 40Kh, and 40Kh13. A lead–lithium eutectic melt into which specified amounts of nickel and chromium are introduced serves as a technological medium (transport melt) of DALMMS. The DALMMS process is carried out at 1050°C for 300 min. Metallographic studies are conducted 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 for EPMA measurements. The DALMMS process results in the formation of diffusion Ni–Cr coatings. The DALMMS process on the structural carbon and low-alloy steels affords two-layer coatings: the surface carbide layer and transition solid solution layer are observed. The chromium content in the surface layers can reach 80% at a nickel content of 1.5%. The maximum nickel concentration is observed in the transition layer and amounts to 21% at a depth of 5 μm for 30KhGSN2A steel and 13% at a depth of 4.5 μm for 40Kh steel. The DALMMS process on the steels containing significant amounts of carbide-forming elements or containing low quantities of carbon leads to the formation of single-layer coatings based on solid solutions. The nickel content in the coating reaches 40%, the chromium content for St3 steel is 14.5%, and the chromium content for 40Kh13 steel is 9%.