Hardening of steels modifying their surfaces with ion-plasma nitriding in glow discharge

Steel hardening technology is considered, which implies modification of the steel surface with the method of ion-plasma nitriding in glow discharge. Ion-plasma nitriding is a multi-factor process, which requires the study of the influence of nitriding process conditions on the structure of modified layers, which, in its turn, determines their mechanical properties. The subjects of research included: austenitic steel 12X18Н10T, carbon steel Ст3 and structural steel 45. There were two conditions of plasma creation during the research: free location of samples on the surface of the cathode (configuration I) and inside the hollow cathode (configuration II). Optimal parameters of the ion-plasma nitriding process have been determined, which provide stability of the process and create conditions for intensive diffusion of nitrogen into the steel surface. Hydrogen was added to the argon-nitrogen gaseous medium to intensify the nitriding process. Working pressure in the chamber was maintained within the range of 250-300 Pa, the duration of the process was 120 minutes. Comparative characteristics of the structure and microhardness of the modified surfaces of the steels under study for two ion-plasma nitriding technologies are presented. Metallographic examination of the structure of the surface modified layers in the cross section showed the presence of the laminated nitrided layer, which consists of different phases and has different depths, depending on the material of the sample and treatment mode. Nitrided layer of 12Х18Н10Т steel consisted of four sublayers: upper "white" nitride layer, double diffuse layer and lower transition layer. The total depth of the nitrided layer after the specified treatment time reached 23 μm, use of hollow cathode increased it by 26% to 29 μm. The nitrided layers of steel Ст3 and steel 45 consisted of two sublayers – thick "white" nitride layer and general diffuse layer with a thickness of about 18 μm. The microhardness of the nitrided layer of steel Ст3 was 480 HV, increasing by 2,5 times, and for steel 45 was 440 HV, increasing by 1,7 times. The use of hollow cathode for these steels reduces the depth of the nitrided layer, but at the same time the microhardness increases due to the formation of a thicker and denser nitride layer on the surface. The results of the conducted research can be used to strengthen the surfaces of the steel parts in rocket and space technology, applying high-strength coatings.