MICROSTRUCTURE AND PROPERTIES OF NITROCEMENTED LAYER OF CAST NEEDLE SPROCKETS MADE OF CARBON STEEL

This article presents the results of scientific and experimental investigations of cast rotary needle sprockets of earth-moving machines made of carbon steel. Chemical composition, mechanical properties and macro- and microstructure parameters of the same steels are studied. The surface layer of steel was calculated after chemical-thermal preparation - nitrocementation with a coating thickness of between 0.6 and 1 mm. On an MBS-9 metallographic macroscope, the layer thickness was measured at various magnifications. The optimal nitrocementation modes have been implemented with concurrent hardening and low tempering. The surface layer macrostructures of specially prepared samples with different carbon concentrations, consisting of a fine-grained structure, are given. The structure of the nitro-cemented layer should consist of fine crystalline martensite, small quantities of fine evenly scattered carbonitrides, and a smaller quantity of residual austenite under optimal saturation conditions. In all carbonaceous steels investigated with equivalent carbon and nitrogen content, the simple residual austenite content in the surface layer is found to be the same. Subsequent increases in nitrogen concentration in the layer lead to an increase in residual austenite content and a decrease in the hardness of the surface coating of samples and machine parts. With higher carbon and nitrogen concentrations, the structure of the steel sublayer also changes. The surface layer hardness of the samples after quenching and low tempering is also analyzed at 56-58 HRC, and the microhardness is 570-690 HB. The surface of the samples consists of a 0.6-0.8 mm thick coating, and further along the depth of the layer the structure of martensite. With the same structure of the surface zone of carbonaceous steels due to a sharp decrease in carbon concentration along the depth of the layer, the structure of martensite with austenite at a shallower depth transitions to martensitic-troostitic and then to troostitic-sorbitic structure of steel. All reference grain sizes are characterized by scores. Fine-grained steels have high hardness and wear resistance of steel parts. Further the process occurs at a lower temperature of 840-860⁰ C instead of 920-950⁰ C; layer thickness is less; less deformation and warping of parts is obtained; hardness and resistance to wear and corrosion are increased. It is established that the hardness and wear resistance of the sprocket after optimal heat treatment increases by 2-3 times.