Effects of the treatment parameters on the formation and the properties of diffusion layers formed by electrothermochemical and thermochemical boronizing on steel surfaces

Diffusion coating treatments are applied to many types of steels in order to improve their surface properties. In this work, diffusion processes which result in the formation of multi-layered diffusion layers were studied. Diffusion layers which were generated on the surface of three different types of steel samples (plain carbon steel, interstitial-free steel, alloyed steel) were formed by the thermal diffusion of boron and titanium. During the diffusion of boron (boronizing), additional electrical energy was applied to the system. Some of the steel samples, having been cut into convenient dimensions, were boronized, some of them were titanium diffused, some of them were first boronized and then titanium diffused and some of them were first titanium diffused and then boronized. The chemical reactions that occur during the diffusion of titanium were determined by thermodynamic calculation. As mechanical evaluation, hardness and wear tests were applied. Additionally, fracture toughness values of the interstitial–free steel which were first titanium diffused and then boronized were calculated. The formation of the boride layer by boronizing was studied as a kinetic model. As a result of the optical microscopy and the scanning electron microscopy work, it was understood that boride layers and titanium based diffusion layers were generated by boronizing and titanium diffusion respectively. While the presence of diffusion layers which were composed of compounds of boron and titanium was detected on the surface of the steel samples which were first boronized and then titanium diffused, on the surface of the steel samples which were first titanium diffused and then boronized, diffusion layers which were formed only by titanium compounds were observed and no presence of the boron compounds was detected except small discontinuous regions to which boron had been segregated. However, it was understood that boron diffusion could be achieved besides the titanium diffusion if the applied current density and the duration were increased during boronizing which was carried out after titanium diffusion. The formation of the boride layers by this way was observed. A no crack containing diffusion layer having a thickness of almost 200 µm was obtained by first boronizing and then titanium diffusion.