Rationalization of Heat Treatment Parameters of the Surfaced High-Chromium Coatings Alloyed with a Complex of Boride Compounds

Introduction. Surface hardening of a large nomenclature of machine-building parts is performed by surfacing with iron-chromium-based powder wires, which ensures the production of metal coatings with high strength and corrosion resistance. At the same time, the resistance of coatings on an iron-chromium base is insufficient, when operating under abrasive wear, due to the small number of strengthening phases in the structure of the surfaced metal. The high operational properties of the surfaced metal can be obtained by combining solid-solution hardening and hardening by second-phase particles in an iron-based matrix. One of such effective method of hardening the metal is surfacing with a flux-cored wire alloyed with boron compounds. However, all the studies performed refer only to the metal coatings in the state after surfacing. The hardness of such coatings is high, which makes it difficult for machining. Purpose of the work: selection of rational parameters for thermal treatment of surfaced coatings based on chromium steel with carbide-boride-nitride alloying. The effect of heat treatment regimes on the microhardness, microstructure and phase composition of the coating metal surfaced by the high-chromium flux cored wire alloyed with complex boride compounds is studied. The composition was the following: 15% Cr vertical bar 0.5% B4C vertical bar 0.5% BN + 2.5% + TiB2 + 1.0% ZrB2. The methods of investigation are metallography; measurements of microhardness; X-ray phase analysis and transmission electron microscopy. Results and discussion. It is shown that tempering at 800 degrees C with a 2-hour equalizing ensures the hardness of the surfaced metal within the range of 32-37 HRC, which is acceptable for machining. The microstructure of the metal coatings after tempering is characterized by the structural components decay; the amount of boride eutectic and strengthening phases decreases and its size increases. It is found that to restore the high hardness of the metal after tempering with subsequent machining, it is advisable to conduct quenching from 1020 degrees C, providing a hardness within the range of 53-58 HRC. This heat treatment leads to the stabilization of the microhardness values at a high level, even higher than the level of the metal coatings microhardness after surfacing. It is shown that this is due to the formation of a composite structure with a martensitic matrix, an eutectic component based on chromium and iron borides Fe1,1Cr0,9B0,9, and dispersed inclusions of carbonitride, carbide and nitride particles for the most part Ti2CN and Cr7C3 and intermetallic compounds Cr4TiZr in the size from 0.4 to 6.5 mu m. The established rational parameters of heat treatment can be used in the technology of wear-resistant coatings surfaced with powdered wires alloyed with boride compounds.