OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. 5 No. 3 2023 Fig. 10. Experimental results for parts made of U10A steel: a – optical microscopy; b – the distribution of microhardness and residual stresses in the surface layer ( – residual stresses obtained by X-ray determination); c – microstructure of base metal and transition zone; d – microstructure of the hardened layer a b c d Studying the graph of the microhardness distribution of the surface layer (see fig. 10a, b), three characteristic regions can be distinguished. The first region, designated as zone I, is characterized by a stable average microhardness value. The second region, or zone II, is the transition zone. Finally, the third region, or zone III, does not undergo structural and phase changes. The thickness of the hardened layer is defined as the distance from the surface to the area containing 50 % martensite. The transition layer is a region between the surface layer of a hardened metal with a constant average value of microhardness and a zone of material that has not undergone structural-phase transformations. The base metal is presented by a lamellar pearlite (fig. 10c). In addition, globular cementite with sizes from 1 to 5 µm is observed in the base metal. The transition zone, with a thickness of 0.172 mm under these processing modes (fig. 10a, b), consists of martensite (light), perlite (dark) and globular cementite (fig. 10c). The presence of perlite and cementite globules indicates that the heating temperatures of this section did not exceed the Ac3 temperature and the soaking time at this temperature was insignificant. Martensite with differently etched plates and retained austenite are observed in the hardened layer (fig. 10d). With distance from the base metal, the amount of globular cementite decreases. The hardened layer of the studied steel grade, obtained by HEH HFC at a hardening depth of 0.52 mm, has a microhardness of 910 HV. In addition, the maximum value of residual compressive stresses on the working surface of the plug is approximately sc max ≈ -700 MPa.
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