Boriding of transmission gear steel via CRTD-bor

Gears are used as a machine tool in power transmission and transferring systems. In automobiles, gears are made of steels due to their mechanical properties (i.e., their relative high strength and toughness values). However, in transmission systems where wear is the major risk, it is desired that the gears' tooth have extreme surface properties. In today's middle-class vehicles, the transmission gears are widely produced from AISI 8620 steel and their surface properties are improved via carburizing. There are many applications in which diffusion based surface modifications and / or coatings are performed to improve the surfaces of metallic materials. Along with the developing technologies, boriding is applied to steels in order to improve surface properties by forming hard layers on the surface of steels. In boriding process, boron is diffused to the surface of the materials and a layer with high wear and corrosion resistance is grown on the surface. In the case of boriding via the conventional pack/paste methods, the boride layers containing FeB and Fe2B phases are generally obtained on the applied steels. However, the difference between these two phases, especially the thermal expansion coefficient and hardness values, make it impossible to use in the applications where materials exposed to wear and surface fatigue. Despite the fact that there are researches in the literature conducted by conventional boriding techniques for the boriding of AISI 8620 transmission gear steel, boriding is not used in industrial applications due to the reasons of two phase boride layer formation. Electrochemical based KRTD-Bor (Cathodic Reduction Thermal Diffusion Boriding) technique, developed as an alternative to conventional boriding methods is an environmentally and low cost method. Plus these, this new boriding method makes it possible to form a homogeneous and very thick boride layer on the surface of iron and nonferrous materials in a short time. In this thesis, it is aimed to grow the industrially demanded single-phase hemiboride (Fe2B) layer on AISI 8620 transmission gear steel via the CRTD-Bor method in order to present an alternative surface modification way to carburizing and conventional boriding methods. With the scope of the thesis, the effects of current density (50-400mA / cm2), the electrolysis temperature (950-1100oC), and duration (15-120min) on the grown boride layers' chemistry, morphology and thickness were investigated. Moreover, the phase homogenization procedures were applied to obtain a single phase Fe2B. Experiments were carried out in a medium frequency induction furnace using the environmentally friendly electrolyte composition 90% Na2B4O7 + 10% Na2CO3, where the graphite crucible polarized as an anode and the steel substrate polarized as cathode. The morphology analyzes and thickness measurements were performed with an optical microscope and scanning electrone microscope (SEM), and the boride layer phases grown on the surface were determined by the thin film x-ray method (XRD). In the current density experiments, it was determined that 50 mA / cm2 could not provide enough cathodic protection where it was found that Fe2O3 and Fe3O4 phases were formed along with iron borides on the surface. At current densities of 100mA / cm2 and above, both Fe2B and FeB phases were grown on the substrate and the thickness of both layers were increased with increasing applied current densities. During all the boriding periods, the double phase (FeB + Fe2B) iron boride layers were uniformly grown on the steel substrate. In the boriding experiments carried out at 60 and 120 minutes, elemental boron was found on the surface beside the grown iron boride layers. After 15 minutes of CRTD-Bor, the thicknesses of total boride layer and FeB layers were measured as 50 μm and 30 μm, respectively. When the time was increased to 120 minutes, the thickness of total boride layer and the FeB layers extended to 160 μm and 115 μm respectively. In experiments where the effects of electrolysis temperature on the grown boride layers were examined, it was determined that the thickness of TBT was increased from 47 μm grown at 950 °C to 75 μm formed at 1100 °C. However, when the electrolysis temperature exceeds to 1000 oC, it is observed that the diffusion rate of boron atoms in the steel matrix became faster than the reduction of them, hence consequently the thickness values of FeB layers were decreased beyond 1000 oC. In order to grow a single-phase homogenous Fe2B layer on AISI 8620 transmission gear steel, phase homogenization (PH) process was adapted to CRTD-Bor treatment. The steel samples were borided at 950 oC with the current density of 200 mA / cm2 for 15 minutes and then anode-cathode polarization was cut off to initiate the PH stage where the top FeB phase convert into Fe2B layer. It was determined that the thickness of FeB phase decreased with increasing PH durations. A homogeneously 115 μm thick, single Fe2B phase boride layer was grown on the AISI 8620 steel substrate at 950 oC with applying 15 min of CRTD-Bor and additional 80 min of PH treatment. The Rockwell-C adhesion test revealed that the grown boride layer adhered to the steel substrate with the perfect (HF1) quality. In addition to the optimized condition, the electrolysis temperature was increased in order to reduce total CRTD-Bor process time which was 95 minute at 950 oC. Accordingly, CRTD-Bor experiments was planned to conduct at 1000 oC. Consequently, Single-phase Fe2B boride layer was homogenously grown after 15 min of CRTD-Bor and additional 45 minutes of PH, with the average thickness of 100 μm which exhibits the best adhesion quality (HF1) on the AISI 8620 transmission gear steel matrix. The grown boride layer on the steel substrate increased the hardness of the steel base material (300 HV) by 5 times, and the boride layer with the average hardness of 1600 HV was obtained. In order to examine the wear properties of the obtained hard boride layers, the pin on disk wear tests were conducted both in dry and oily medium. Consequently, it was determined that CRTD-Bor boriding improve the wear properties of the steel material. As a result of these investigation, homogenously thick, single-phase Fe2B boride layer was grown on the surface of AISI 8620 gear steel after 1 hour of CRTD-Bor treatment and it was found that the boride layer has a considerable contribution on the wear resistance of the steel.