STRUCTURAL EVOLUTION, DIFFUSION AND HARDENING OF THE FE-CR-C-NBC DEPOSITS ON 65G STEEL DURING VACUUM ANNEALING

Abstract

A promising direction of increasing the durability of the working parts of soil processing equipment is the use of gasothermal coatings of the Fe-Cr-C system, alloyed with dispersed carbides, but the initial state of the coating-substrate border is characterized by insufficient adhesion. (Research purpose) The research purpose is establishing regularities of evolution of microstructure, phase composition, elemental distribution and microhardness of surface coating system Fe-Cr-C-NbC-TiC-W-Mo on steel 65G in the process of vacuum annealing at 1000 degrees Celsius in the interval of shutter speeds 60-300 minutes to substantiate technological modes of hardening. (Materials and methods) Samples measuring 20×20 mm of 65G steel were sprayed with iron-based composite powder. Conducted vacuum annealing in the furnace SSHE-1,2,5. Comprehensive characterization included scanning electron microscopy with EMF analysis, metallography and Vickers microhardness measurement. (Results and discussion) It was found that with increasing exposure time, the sharp heterophase border transforms into a gradient diffusion zone up to 40-50 micrometers thick. The original dendritic structure of the surfacing is replaced by an equiaxial polygonalized structure. Carbides NbC, TiC, W, Mo undergo diffusion-controlled coalescence, reaching a size of 8-12 micrometers. The microhardness of the substrate is monotonously reduced from 290 to 198 on the Vickers scale due to tempering and decarbonization, while the surface hardness after stabilization at 300 minutes exposure increases to 752 on the Vickers scale due to the combined contribution of dispersion hardening, the release of secondary chromium carbides and carbon diffusion saturation. (Conclusions) The vacuum annealing mode at 1000 degrees Celsius for 300 minutes is technologically justified for the formation of a highly adhesive coating-substrate composite. The optimal combination of the increased hardness of the surfacing and the developed diffusion zone provides a significant increase in the operational life of parts of soil processing equipment operating in conditions of intensive abrasive wear.