Research on the Dependence of the Microhardness of Modified Surfaces of Titanium Alloys on the Depth of Nitrogen Saturation During Vacuum Ion Plasma Nitriding in the Thermocyclic Mode
Authors:   Karpenko Physico-Mechanical Institute of NAS of Ukraine, Student M.M. , Markovych Sergiy ,   Central Ukrainian National Technical University, Hvozdetskii V.M. ,   Karpenko Physico-Mechanical Institute of NAS of Ukraine, Zadopozna Kh.P. ,   Karpenko Physico-Mechanical Institute of NAS of Ukraine
Published: December 2023

Electric arc spraying of coatings is common in many branches of industrial production, in particular to restore the geometry of machine parts worn in operational conditions, to increase their protection against abrasive and gas-abrasive wear (at the same time, both at climatic and at technologically determined elevated temperatures). Coatings sprayed using powdered wires are characterized by high chemical heterogeneity, which significantly distinguishes them from electric arc coatings made of solid wires. This is due to the different chemical composition of the droplets formed from the molten powder wires and carried by the air jet to the surface of the substrate, forming a coating on it. The charge with alloying elements in its composition (including difficult-to-melt ones such as FH, B4C, FHB) does not have time to fully melt and mix with the melt of the steel shell. It is clear that because of this, the melt droplets of flux-cored wires dispersed by an air jet will have a different chemical composition and , as a result, the coatings formed from these droplets on the surface of the substrate will be characterized by high heterogeneity and significant chemical heterogeneity, which will affect their physical and mechanical properties at different operating temperatures and especially when exposed to corrosive environments. To ensure high corrosion resistance of electric arc coatings in aggressive environments, it was necessary to reduce their chemical microheterogeneity and achieve a sufficiently high chromium content (it should be more than 12 wt.%) in each of its lamellae. To achieve this goal, ingredients were added to the composition of the charge, which enabled the formation of eutectics with a low melting temperature, with the dissolution of such refractory components of the powder wire charge as carbides, borides, refractory metals and alloys. Microhardness measurements showed that the highest hardness was achieved by electric arc coatings made of powder-coated wires №. 2 (20X16Р3Н2ГС) and powder-coated wires №. 5 (Х17Р3С). This happened due to the presence in the charge of 3 wt.% of boron, which entered their charge as part of the FHB-2 ferrochromiumboron powder. Phase analysis of these EDPs revealed the segregation of finely dispersed FeCrB and FeCr2B borides in their ferrite structure. However, the cohesive strength of these coatings did not exceed 100 MPa. This was explained by the fact that during filing, residual first-order tensile stresses could occur in their structure, which, as a rule, contribute to the cracking of the coatings during their subsequent mechanical processing. Therefore, before applying coatings from such powdered wires, the base for spraying should be heated to 150...200ºС. Conclusions. 1. To ensure complete fusion of the components of the powder-coated wire charge with each other and with its steel sheath, it is proposed to add Fe-Mn, Fe-Si ferroalloy powders to the powder-coated wire charge, which are able to interact with the refractory components of the charge with the formation of low-temperature eutectics. The legality of such a component composition of the charge of flux-cored wires as an effective method of reducing the melting temperature of the components has been experimentally substantiated. 2. The addition of ferrosilicon, ferromanganese powders and self-fluxing alloy PН-10Н-01 to the charge of flux-cored wires based on ferrochromium and ferrochromium ensured high hardness of electric arc coatings, low heterogeneity of the chromium content in the lamellae, and, as a result, high corrosion resistance, which comparable to that of stainless steel