Effect of hardening friction treatment with hard-alloy indenter on microstructure, mechanical properties, and deformation and fracture features of constructional steel under static and cyclic tension

It has been studied how a hardening friction treatment with a sliding hard-alloy indenter influences the chemical composition and roughness of the surface, the structure, the distribution of the microhardness and the density of dislocations in depth of the surface layer, the mechanical properties upon static tension and the features of deformation and fracture upon cyclic tension of the annealed low-carbon (0.17 wt.% C) steel grade 20. It has been found that friction hardening of the steel (an increase in the microhardness up to 4.25 GPa) is due to a considerable dispersion of the ferritic base (with the formation of alpha-phase fragments no less than 100 nm in size) and pearlite colonies (crushing and partial dissolution of cementite plates) under the action of a severe friction deformation; hardening is not connected with the carryover of separate particles of the hard alloy to the steel surface. A finite-element model describing the process of the friction treatment with a sliding cylindrical indenter has been constructed. This model was used to determine how the number of the indenter strokes, the friction coefficient, and the shear component of the deformation affect the value of the accumulated deformation. It has been shown that the friction treatment improves the strength characteristics of the steel upon static tension. The hardened surface layer is susceptible to further considerable hardening (an increase in the microhardness up to 5.6-5.9 GPa) at the initial stages of the cyclic deformation and, therefore, has some margin of plasticity. The plastic flow in the surface-hardened steel under cyclic loading is due to the formation of numerous bands of localized deformation. It has been found that the friction-hardened surface layer is more prone to cracking under cyclic loading. The methods by which cracking of the hardened layer can be diminished have been discussed.