OBRABOTKAMETALLOV Vol. 24 No. 1 2022 TECHNOLOGY material (Fig. 6). When measuring with a load of 0.49 N, as the burnishing force increases from 100 to 175 N, a non-monotonic increase in microhardness occurs from 409±17 HV 0.05 to 444±7 HV 0.05 (see Fig. 6a). The established maximum level of steel surface microhardness after burnishing with a load of 175 N is observed despite individual micro-destructions in the form of buildups and microcracks as a result of metal redeformation under burnishing effect (Fig. 5b). a b Fig. 6. Dependences of microhardness HV (a) and the hardening coeffi cient δHV (b) of the 03Cr16Ni15Mo3Ti1 steel surface on the burnishing force Fb: microhardness measurements at loads on the Vickers indenter 0.49 N (curves 1) and 1.96 N (curves 2) With a further increase in the burnishing force to 200 N, a decrease in the microhardness of the deformed surface to 422 ± 3 HV 0.05 is observed (Fig. 6a). This can be explained by an occurrence, at the maximum burnishing force, of an over-peening effect, which leads to accumulation of surface damage and local destruction of the thin steel surface layer. This statement is supported by occurrence of noticeable irregularities on the 3D profi logram of the smoothed surface (see Fig. 2d) and a corresponding abrupt increase in roughness after a burnishing force increase from 175 to 200 N (see Fig. 3a). From the data of Fig. 6a it also follows that at measurements using a higher load on the Vickers indenter (of 1.96 N) with increasing the burnishing force, the microhardness of the work surface increases monotonically from 382±4 HV 0.2 after burnishing with a force of 100 N and achieves maximum 421±4 HV 0.2 after burnishing with a force of 200 N. Consequently, the microhardness reduction HV 0.05, associated with the re-peening when increasing burnishing force from 175 to 200 N, affects only a very thin nearsurface layer. Figure 6b shows the effect of the burnishing force on the hardening coeffi cient δHV calculated by formula (2) when burnishing with respect to the microhardness of the initial (after turning) surface of the steel under study. A lower level of initial microhardness (310 ± 10 HV 0.05), found when measuring a thinner layer with a load of 0.49 N than when using a load of 1.96 N (330±9 HV 0.2), indicates damage accumulation directly on the steel surface during the fi nish turning process, causing some material softening. According to Fig. 6b diamond burnishing provided 31...43 % hardening in a thin near-surface layer with an extremum at a burnishing force of 175 N and 15...27 % hardening in a thicker surface layer with a maximum microhardness at a burnishing force of 200 N. Fig. 7 shows microhardness distribution in depth of the gradient-hardened steel surface layer after burnishing at a load of 175 N, which provided a maximum microhardness of 444±7 HV 0.05 of the burnished surface. While moving away from the burnishing surface, the microhardness measured at a load of 0.245 N decreases from 400...420 HV 0.025 to 220...250 HV 0.025 at a depth of 300...350 μm. The study of cross-sections with a scanning electron microscope showed that after fi nishing turning, the large austenitic grain structure is preserved in the sample surface layer (Fig. 8a). Since the technological turning (lathe machining) is designed for sizing cut, accelerated removal of the material cuttings does not create favorable conditions to accumulate large degrees of plastic deformation and structure dispersion accompanying this process in the workpiece surface layer.
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