OBRABOTKAMETALLOV TECHNOLOGY Vol. 24 No. 1 2022 Fig. 7. Change in the microhardness HV 0.025 in depth of the 03Cr16Ni15Mo3Ti1 steel surface layer (h is the distance from the surface) after dry diamond burnishing with a force Fb = 175 N In contrast to turning, dry diamond burnishing with a force of 175 N formed a pronounced surface layer with a thickness of 30...40 μm with a strongly deformed highly dispersed structure (the layer is marked with a dotted line in Fig. 8b). It can be seen that the deformation led not only to signifi cant austenitic structure dispersion, but also to occurrence of discontinuities in the form of micropores of various sizes – from fractions of a micrometer up to 5 μm (Fig. 8b). Similar micropores were formed in a thin surface layer of AISI 321 metastable austenitic steel as a result of friction treatment with a synthetic diamond indenter in an argon medium [27, 28]. It is known that micropores of deformation origin (submicropores) in plastic metallic materials are formed in the process of submicrocrack blunting that appear in deformable metal when moving dislocations are blocked by barriers such as inclusions, grain boundaries, sliding lines, etc. [29, 30]. Micropores occur as well due to the fact that during the passage of the sliding indenter and its individual microasperity, the metal moves from the zone of compressive stresses, in which deformation occurs in the shear conditions under pressure, into the zone of external tensile stresses [31, 32]. According to [33], the pores in metals under intense plastic deformation are formed precisely in the tension zones, while highly dispersed structures occur only in the zones of shear (compression). Tensile stresses also cause microcracking on the steel surface during burnishing (see Fig. 5b). It is important to note that the highly dispersed layer dotted in Fig. 8b is characterized by a maximum microhardness level of 400...420 HV 0.025 (see Fig. 7). Thus, during microdurometric measurements with loads of 0.245, 0.49 and 1.96 N on the Vickers indenter after dry burnishing with a natural diamond indenter with a force of 175 N, a microhardness level of 400...444 HV was registered on the surface of 03Cr16Ni15Mo3Ti1 austenitic steel and in a surface layer with a thickness of 40 μm. (see Fig. 6a; 7). a b Fig. 8. The structure of the 03Cr16Ni15Mo3Ti1 steel surface layer after fi nish turning (a) and dry diamond burnishing with the force of Fb = 175 N (b); cross-section, scanning electron microscopy, the dotted line indicates the layer boundary with dispersed structure
RkJQdWJsaXNoZXIy MTk0ODM1