Surface layer of 40Kh steel after electromechanical treatment with dynamic force impact

The paper presents the results of complex studies of the structure, microhardness and depth of the hardened surface layer of 40Kh steel formed as a result of electromechanical treatment with dynamic application of a deforming force (EMT with impact). The research was carried out using optical microscopy, X-ray diffraction analysis, and microhardness methods. The method of electromechanical treatment with dynamic force impact consisted in simultaneous transmission of electric current pulses and deforming force through the contact zone of the tool with the part. As a result of shock-thermal effects with different current densities (j = 100 A/mm²; 300 A/mm²; 600 A/mm²), segments of the hardened layer of different sizes and structure composition are formed on the steel surface in cross-section. Analysis of structural and phase transformations in the surface layer of 40Kh steel subjected to dynamic electromechanical treatment indicates the formation of a specific structure of the white layer, the structure and properties close to the amorphous state of the metal with a maximum hardness HV = 8.0 – 8.5 GPa. As you move away from the surface, a transition zone is formed behind the segment of the white layer with a structure that does not have the characteristic needle structure of martensite. It was found that with an increase in the current density during shock electromechanical treatment, the depth of hardening increases by 4 – 5 times with a simultaneous increase in the heterogeneity of strength properties; the level of micro-stresses increases by 25 %. Experimental data on the structural state, microhardness and depth of the surface layer of 40Kh steel show that electromechanical treatment with dynamic (shock) application of the deforming force causes deeper transformations in the steel structure compared to traditional static EMT. The results obtained show that as a result of electro-mechanical processing with impact, the intensity of the temperature-force effect on the steel surface layer increases, which allows you to open the internal reserves of 40Kh steel and control the process of forming its structure and phase states.

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