RESOURCE-SAVING APPROACH TO THE TECHNOLOGY OF LASER SURFACE ALLOYING OF METAL WORKPIECES AND ITS THERMODYNAMIC SUBSTANTIATION: A CASE STUDY OF ALLOYING WITH CHROMIUM

Abstract

This article is devoted to the investigation of the prospects for resource-saving development of laser surface alloying technology for machine-building blanks by replacing the expensive pure metal powders used for this purpose with their simple oxides, while ensuring complete slag-free reduction of the latter using a strongly and rapidly oxidizing gas such as methane. To justify the effectiveness of the proposed approach, a specific case study was conducted involving the laser surface alloying of AISI 1045 structural steel with chromium recovered from its trivalent oxide Cr₂O₃ of chemical purity. To this aim, a complete thermodynamic analysis of the interaction of Cr₂O₃ - CH₄ components was performed in the temperature range of 400 - 2500 K, which is relevant for laser surface doping, with consideration of the decomposition of part of the methane that was directly exposed to the focused laser beam. It has been established that the chromium reduction process proceeds in two stages: solid phase (1150 - 1350 K) and liquid phase (1550 - 2000 K). In the first case, the process proceeds with the formation of chromium carbides Cr₃C₂ and Cr₇C₃, and in the second case, with the decomposition of the latter into elemental (condensed) chromium and free (amorphous) carbon, which contributes to the further complete carbothermal reduction of the residual locations of the initial oxide layer. It is recommended that the second stage of recovery be carried out under dynamic vacuum conditions, ensuring the removal of CO and H₂ reaction gases. The generalization and implementation of the proposed technological solution can lead to significant savings in expensive alloying element powders, as well as reduce their irretrievable losses in the form of burn-off and blow-off caused by laser evaporation. It is noted that the proposed resource-saving approach to surface laser alloying technology may serve as a significant driver for further progress in this important scientific and technical field.