The influence of carbonitriding conditions on microstructure and mechanical properties of 25CrMo4 low-alloy steel

Abstract

Carbonitriding is an important industrial process applied for the improvement of the mechanical characteristics of the component of many steels employed in several machines parts like cam shafts, crank shafts and gears to enhance fatigue strength and wear resistance. In this study, the influence of the gaseous carbonitriding on the enhancement of surface characteristics and mechanical properties was investigated using different parameters for low alloy steel. The analysis and characterization of the treated material were carried out employing optical microscopy (OM) and scanning electron microscopy (SEM) and X-ray diffraction methods (retained austenite and residual stresses). Microhardness and tensile tests were performed and the fracture mechanisms of the various materials were ultimately examined and discussed. The main contribution of this research work is to show and optimize the selection of the appropriate parameters for this kind of steel in industrial field. Diffusion mechanism and microstructure proved that the process of high temperature gas carbonitriding allowed enhancing the mechanical properties of material. This process resulted in a rise of the yield strengths and a loss in a pronounced ductility associated with the brittle intergranular fracture surface caused by the nitride precipitation in the grain boundaries.