In Situ Formation of Molybdenum Borides at Hardfacing by Arc Welding with Flux-Cored Wires Containing a Reaction Mixture of B4C/Mo

Abstract

This paper reports a study into the formation of the phase composition, structure, and properties of arc welding coatings by the flux-cored electrode materials from the Fe-Mo-B-C system. The welding alloys were applied using the flux-cored arc welding (FCAW) electrodes, which consisted of a shell made from the low-carbon steel filled with a reaction powder mixture that contained boron carbide and molybdenum in a ratio of 1:1. The calculation of the phase composition of alloys that correspond to the surfaced layers by a CALPHAD method using the Thermo-Calc OpenCalphad software shows that under the equilibrium conditions the boride phases of molybdenum and ferrite cannot co-exist. The main phase of such alloys is a FeMo 2 B 2 compound, which forms the eutectics with austenite. Given that the eutectic structures with borides are characterized by high brittleness, the introduction of components was conducted in the form of a reaction mixture in order to obtain the in situ formed boride phases in the form of separate structural components. Analysis of the results of studying the microstructure and phase composition of coatings reveals that they consist of three main structural components: the eutectic (FeMo 2 B 2 +ferrite) and the grains of molybdenum tetraboride MoB 4 . Thus, under the conditions of arc welding using the reaction mixture, an irregular structure is formed, which is favorable in terms of ensuring wear resistance due to the high microhardness of MoB 4 >27 GPa. The hardness of the coatings obtained is at the level of 63–65 HRC, and the wear resistance is higher compared to standard high-chromium alloys (grades Т620 and Т590) by 2‒2.5 times. This makes it possible to recommend the coating of a given system for hardfacing the working surfaces of equipment in the coal, processing, woodworking industries, etc., where abrasive wear is the dominant type of surface wear