Features of the formation of Ni-Cr coatings obtained by diffusion alloying from low-melting liquid metal solutions

Abstract

Introduction. The main ways to increase steel parts properties are considered. The rationale for choosing Ni and Cr as the main components of the coating is given. The technology of diffusion alloying from low-melting liquid metal solutions (DALMMS) is given. The purpose of this work is to identify the features of the coatings formation with simultaneous diffusion saturation of nickel and chromium structural steels using the DALMMS technology. Methodology. Cylindrical specimens with a diameter of 20 mm and a length of 30 mm were subjected to DALMMS. The specimens were manufactured of the following structural steels: carbon steel St3, alloyed carbon steels 40Cr, 40Cr13, and extrafine steel 30CrMnSiNi2. As a technological medium with DALLMS (transport melt), a Pb-Li eutectic melt with the specified content of Ni and Cr was used. Metallographic studies were carried out on microsections prepared according to the standard methodology. 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 X-ray microanalysis on a Tescan Lyra 3 scanning electron microscope with the Oxford Ultim MAX PCMA system. Results and discuss. It is revealed that the formation of diffusive Ni-Cr coatings occurs with DALMMS. With DALMMS of structural steels contained carbon in cementite form two-layers coatings are formed: surface carbide layer and transition solid-soluble one. At the same time, the chromium content in the surface layers reaches 80 % with a nickel content of 1.5 %. The maximum Ni concentration is observed in the transition layer and amounts to 21 % at a depth of 5 µm on steel 30CrMnSiNi2 and 13 % at a depth of 4.5 µm for steel 40Cr. Carrying out the DALMMS on steels containing carbon in the form of chromium carbides, or containing carbon in small amounts, leads to the formation of single-layer coatings based on solid solutions. The Ni content in the coating reaches 40 %, the chromium content for steel St3 is 14.5 %; for steel 40Cr13 it was 9 %.