Development and characterization of alternative boriding composition by different boron derivatives

Boriding is a diffusion-based thermochemical surface hardening process that can be applied to a wide variety of materials such as ferrous, non-ferrous and cermet. As a result of the boriding process, very hard boride layers are formed on the surface of the materials by diffusion of boron into materials. These very hard layers which are formed on the material surface significantly increase the wear resistance of the material. The surfaces of the borided machine components maintain their tribological properties at high temperatures and also exhibit good corrosion resistance against some acids. Imported and costly boriding products (mixtures) are generally used in powder-pack (solid) boriding process, which is applied to increase the wear and corrosion resistance of machinery parts and molds in industrial area. Within the scope of the thesis, various studies have been done to prepare and characterize boriding products which may be alternative to imported boriding products for using in powder-pack boriding process. In powder-pack boriding process, the boriding medium constitutes from various boriding components such as boron source, activator and diluent. Boric acid, borax pentahydrate, borax decahydrate, disodium octaborate tetrahydrate, anhydrous borax and boron oxide were provided from ETİMADEN Works General Directorate and used as boron source in alternative boriding products. A large number of mixtures have been prepared to obtain alternative boriding products by mixing the boron compounds and the other chemicals (activator: potassium tetrafluoroborate and diluent: silicon carbide) at a certain ratio. SAE 1020 steel samples are borided by powder-pack boriding method using alternative boriding mixtures and imported boriding product (EKABOR® II) at the determined temperatures and times. Microstructural (Optical Microscope, SEM), chemical (XRD, EDS, WDS), mechanical (Microhardness, Rockwell C adhesion test, Residual Stress Measurement, Wear Test, Surface Roughness Measurements), thermal (TGA, DTA) characterization tests and diffusion kinetics studies were performed on samples which are borided with alternative boriding mixture and commercial (imported) boriding product. Also the results were compared with each other. As a result of microstructural examinations (OM, SEM) carried out after the boriding process, it was determined that all samples had saw-tooth morphology. The XRD analysis showed that the boride layers formed on the surface of the steel specimens were single phase (Fe2B). As a result of EDS and WDS analyzes performed in the cross-sectional area of steel samples borided with K1-D6 and K4-D6 mixtures, the amount of boron element in the layer was determined and the obtained results support that the layer has single-phase structure. It was determined that the surface hardness of the samples reached high hardness values in the 1200-2000 HV range by the boriding process. The Rockwell C adhesion test showed that the adhesion strength of the boride layers was sufficient. The residual stress values for the selected specimens were calculated experimentally and theoretically and it has been revealed that residual stress were found to be compressive for single phase boride layer. As a result of the wear test carried out by the ball on disk method, it was found that the wear rate of the borided samples was at least 46,7 times and at most 96,8 times lower than wear rate of the unborided samples. Surface roughness measurement results were obtained and surface morphologies of borided samples by different mixtures were evaluated. It was observed that the surface roughness values of samples increased with increasing boriding temperature and time. Diffusion kinetics of samples which is borided by K1-D6, K4-D6 and EKABOR® II boriding compounds were calculated and parameters of growth kinetics were obtained. The empirical equations related to these parameters were obtained and the predictability of boride layers thickness depending on different temperatures and times was investigated. In addition, a regression model for these processes was also constituted and alternative equations were obtained for estimating the thickness of the boride layer. The performed boriding processes by alternative boriding mixtures (K1-D6 and K4-D6) and the chemicals and powders that form alternative boriding mixtures were analyzed separately by thermal characterization methods (DTA and TGA) and possible chemical reactions that may occur during the boriding process were revealed.