Tribological properties of the low-carbon steels with different micro-structure processed by heat treatment and severe plastic deformation

This paper presents the results of tribological investigations conducted on steel 20 with the carbon content of up to 0.2%. The steel was studied in the three conditions: initial (hot-rolled), after heat-treatment (quenching + tempering) and after heat treatment with subsequent severe plastic deformation (SPD) performed by equal channel angular pressing technique (ECAP). It was stated that after various treatments the material acquires various structural conditions and possesses various strength properties and has a considerable difference in oxygen content in the surface layer. This influences the tribological properties during the contact with tool steel. The lowest values of adhesive bond shear strength, friction coefficient and wear rate are demonstrated in the material after martempering with subsequent SPD by ECAP technique. As a result of complex investigations, it has been revealed that severe plastic deformation by equal channel angular pressing technique enhances efficiently the strength of the low-carbon steel due to grain structure refinement observed by the sample of the low-carbon steel. This considerably influences the decrease of the friction coefficient and its adhesive component. The increased oxygen content on the material surface after SPD compared to both initial condition and after heat-treatment contributes to stronger passivation of the surface due to formation of oxide films, which together with strain hardening contributes to decrease of the friction coefficient in the specified temperature range, and increase of wear resistance as well. The surface of the investigated material after SPD treatment by the ECAP technique possesses a highest bearing capacity and requires more time for wearing-in in friction assemblies. Oxygen content increase in the form of metal oxides on the surface of low-carbon steels is accompanied by a decrease of the adhesive component of friction coefficient.