Effect of plasma nitriding process on the microstructure and mechanical properties of AISI 4140 low alloy steel

In this study, the effect of plasma nitriding on the structure and mechanical properties of cold drawn low alloy AISI 4140 steel has been investigated. The steel was subjected to traditional quenching process used for hardening of slide of pistol in the light weapon industry and plasma (ion) nitriding which is a thermochemical process used as an alternative method to harden the same parts in the European weapon industry. The structural and mechanical properties especially the fatigue behaviour of the raw, quenched and plasma nitrided AISI 4140 steel were examined. As a result of structural investigations, it was seen that the microstructure of raw material consisted of ferrite and perlite phases and the grains were oriented by the effect of deformation. It was also determined that microstructure was largely converted into martensite and residual austenite phases after traditional hardening and tempering treatments, It was determined that chemical composition of the surface of plasma nitrided steel was changed and a white layer including iron nitrides and a diffusion layer including alloy nitrides are formed on the surface. The effective hardness depth of the steel surface due to the nitrogen diffusion was measured as 0.25-0.30 mm. The highest hardness, tensile strength and impact toughness values were obtained from the plasma nitrided steel. Besides, the steel also exhibited the best fatigue performance in plasma nitriding condition. It was also found that the Basquin's equation could be used to express the fatigue behaviour of AISI 4140 steel in all states. Examining the fractured surfaces showed that, the fatigue cracks of raw material and traditional hardened specimens initiated from the surface but, the fisheye cracks formed in a region between diffusion layer and core in plasma nitrided specimens.