Structural Transformations and Tribological Effects in the Surface Layer of Austenitic Chrome-Nickel Steel Initiated by Nanostructuring and Oxidation

Metallography, electron microscopy, and X-ray diffraction are used to study the effect of preliminary plastic deformation in the friction-contact zone on the structural transformations and wear resistance of 12Cr19N9T austenitic stainless steel subjected to subsequent oxidation in air at temperatures of 300-800 degrees C for 1 h. Severe deformation under dry sliding friction produces a two-phase (gamma + alpha) nanocrystalline structure in a similar to 10-mu m-thick surface layer of the steel. The microhardness is 5.2 GPa. Subsequent oxidation at 300-500 degrees C causes an additional increase in the microhardness of the deformed surface layer of steel to the value of 7.0 GPa. This is due to the active saturation of austenite and deformation-induced alpha'-martensite with oxygen atoms, which rapidly diffuse deep into the metal along the grain boundaries. The oxygen concentration in the surface layer and steel wear products reaches 8.5 wt %. The atoms of the dissolved oxygen efficiently pin dislocations in the gamma- and alpha' phases, increasing the strength and wear resistance of the surface of the 12Cr19N9T steel. Oxidation at 550-800 degrees C results in the formation of a large number of Fe3O4 (magnetite) nanoparticles, which increase the resistance of the steel to thermal softening and its wear resistance.