Phase composition and fine structure of the steel surface layer after plasma electrolytic treatment

The paper focuses on studying the phase composition and fine structure of 0.18-1Cr-3Ni-1Mo-Fe steel after the plasma electrolytic treatment provided by the surface saturation with carbon and nitrogen. Observations are performed using the transmission electron microscopy of thin foils. The steel samples are investigated before and after carbonitriding of their surface and the layer at a ~ 40 µm depth. It is shown that the steel structure in the initial state represents a mixture of perlite and ferrite grains. After carbonitriding the qualitative and quantitative changes are observed in the structure. Thus, a-phase in the surface layer is tempered lamellar martensite; at a ~ 40 µm depth it is both tempered lath martensite and high- and low-temperature lamellar martensite. There are also retained austenite layers in the surface layer, along the boundaries of lamellar martensite. And inside the lamellas the particles of alloyed cementite and M₂₃(C,N)₆, М₂С_0.61N_0.39, М_6.2С_3.5N_0.3, М(С,N)₂ and Cr₁₂Fe₃₂Mo₇Ni₇ carbonitrides are observed. At a depth of ~ 40 µm, along the boundaries of the martensite crystals there are retained austenite layers, whereas inside the lamellas and laths there are only particles of alloyed cementite and M₂₃(C,N)₆ carbonitride. The concentration of the interstitial atoms of carbon and nitrogen is the control parameter of the structure and phase composition. A change in this parameter results in the system deviation from thermodynamic equilibrium. The system transition to the equilibrium or quasi-equilibrium thermodynamic condition occurs through phase transformation of the crystal lattices which from the gradient structure and phase composition of the matrix, intermetallic metals and carbonitride compounds in a low-stability state.

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