Introduction. Low-temperature plasma carburization is an effective way to increase the hardness of thermally unhardened austenitic chromium-nickel steels. Usage of low-energy (up to 1 Kev) electron beams for plasma surface modification allows not only to generate plasma efficiently, but also to heat objects placed in the plasma to a high temperature without using additional external heating. However, in the literature there is no information about the carburization of austenitic stainless steels using plasma generated by an electron beam. The carburization temperature has a significant impact on the level of characteristics and the phase composition of austenitic steels. It is also important to take into account that the usage of ion-plasma chemical-thermal treatments can lead to a change in the roughness of the treated surface. The aim of the work is to study the effect of the temperature carburization in the plasma of a low-energy electron beam in the range TC=350-500 °C on the phase composition, roughness, depth and hardening of the carburized layer of austenitic steel AISI 321. The research methods are microhardness measurement, X-ray phase analysis, scanning electron microscopy and optical profilometry. Results and discussion. Carburization in the plasma generated by the electron beam at TC=350-500 °C provides an increase in the microhardness of the surface of austenitic steel by 5.5 times (up to ~ 1100 HV 0.025). It is found that the depth of the hardened layer strongly depends on the temperature of carburization and is 25 µm at TC=350 °C, and with further increase in the temperature of carburization increases up to 200 µm at TC=500 °C. The effective hardening of the surface layer of stainless steel is associated with the formation of carbon-supersaturated austenite γC and carbides Cr23C6 at TC=350-500 °C, as well as carbides Cr7C3 at TC=500 °C. It is shown that the carburization of the electropolished steel surface at temperatures of 400-500 °C is accompanied by an increase in the roughness parameter Ra to 0.73-1.06 µm. The decrease in the carburization temperature to TC=350 °C leads to the formation of a surface with a significantly lower roughness parameter Ra=0.15 µm. Thus, carburization in the plasma of a low-energy electron beam at temperatures TC=350-500 °C is an effective way to increase the hardness of thermally unhardened austenitic Cr-Ni steel, and carbon modification at a minimum temperature TC=350 °C also provides a low roughness of the carburised surface. This allows us to consider that this technology is a promising finish hardening treatment of precision products made of austenitic stainless steel.
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Funding:
The reported study was funded by RFBR according to the research project №18-38-00561_mol_a. It was done within the state assignment for IES UB RAS, theme No. AAAA-A18-118020790148-1, and the assignment for IMP UB RAS, theme “Structure” No. AAAA-A18-118020190116-6. The studies were performed at the Centers of Collaborative Access “Plastometry” of the Institute of Engineering Science, Ural Branch, Russian Academy of Sciences.
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