OBRABOTKAMETALLOV Vol. 24 No. 1 2022 TECHNOLOGY A similar level of deformation hardening (up to 4.1...4.4 GPa) was observed as a result of intensive plastic chromium-nickel austenitic steel deformation by ultrasonic impact treatment with strikers [34], ultrasonic peening in vacuum [35] and equal-channel angular pressing (extrusion) [36]. When processing a workpiece made of AISI 304 metastable austenitic steel by fi nishing turning and diamond burnishing with lubricating fl uid on a turning and milling center, hardening on the surface and surface layer with a thickness of 75 μm to 380...450 HV 0.025 was achieved [23]. Nanostructuring surface machining SMAT of 316L (02Cr17Ni12Mo2Mn2) austenitic steel, similar in composition to 03Cr16Ni15Mo3Ti1 steel studied in this work, led to surface hardening up to 4.5 GPa [37, 38] and surface layer nanostructuring with a thickness of 40 μm with formation of 15 % nanocrystalline strain-induced martensite. In our earlier study [13], on the surface of 03Cr16Ni14Mo3Ti1 austenitic steel under the conditions of friction treatment with a sliding indenter made of synthetic diamond in a nonoxidizing argon medium, an increase in microhardness up to 720 HV 0.025 was observed with a total depth of a gradient-hardened layer of 300 μm. A high coeffi cient of friction (f = 0.47) in the process of friction treatment with a synthetic diamond indenter [13] contributed to more intensive steel hardening than in this work, while when burnishing with a natural diamond indenter, even without lubricating fl uid, the coeffi cient of friction does not exceed 0.1 [39]. In contrast to the works [37, 38], the study [13] observed an almost complete absence of deformation γ→α transformation: in a surface layer ~7 μm thick, no more than 1.5% (vol.) of strain-induced α’-martensite was formed during friction treatment of 03Cr16Ni14Mo3Ti1 steel. The noted result is due to the increased content of nickel (a strong austenite stabilizer) in 03Cr16Ni14Mo3Ti1 steel compared with its amount in 316L (02Cr17Ni12Mo2Mn2) steel [37, 38]. At the same time, in [13], as a result of friction treatment, nano- and submicrocrystalline austenitic structures were formed on the surface of 03Cr16Ni14Mo3Ti1 steel, the occurrence of which was preceded by generation of dislocation cell and band structures. The formation in stable-to-deformation and metastable austenitic steels under frictional impact of highly disordered crystals of nano- and submicrometer dimensions [13-15, 40] occurs at the fi nal stage of structure transformation due to cell turns and its decrement as a result of the development of a rotational deformation mechanism due to friction [41]. Thus, the established increase in microhardness of stable-to-deformation 03Cr16Ni15Mo3Ti1 austenitic steel to 400…444 HV as a result of dry burnishing with an indenter made of natural diamond can be explained by formation of highly dispersed austenite in the surface layer and by corresponding activation of grain boundary and dislocation mechanisms of hardening. Conclusions As a result of the experimental study of the normal force magnitude effect during dry burnishing by a spherical indenter with a radius of 2 mm made of natural diamond at a sliding speed of 10 m/min and a feed value of 0.025 mm/rev on the surface roughness formation and surface layer hardening of stable-todeformation 03Cr16Ni15Mo3Ti1 austenitic steel, it is established: 1) in the studied variation range of the normal burnishing force of 100...200 N, the smoothing coeffi cient of the steel surface initial microprofi le after fi nish turning is 79...90 %, the greatest smoothing with a decrease in the average roughness parameter Ra from 1.0 to 0.1 μm is achieved at a force of 150 N; 2) with diamond burnishing, the initial (after turning) surface is strengthened by 15...43 % (up to 382...444 HV), as the burnishing force increases from 100 to 175 N, a non-monotonic increase in the average microhardness occurs from 409 to 444 HV 0.05 3) burnishing with a load of 175 N forms a gradient-hardened layer with a thickness of 300...350 μm with generated individual micro-fractures on the surface in the form of buildups and microcracks, the maximum hardening of the steel surface is due to formation of a highly dispersed surface layer with a thickness of 30...40 μm with a structure of highly dispersed austenite and corresponding activation of grain boundary and dislocation mechanisms of hardening.
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