| Boronizing technique, especially solid-boronizing technique, is widely used in the industrial production due to the superior performance of the boride layer. There are some problems in these processes, however, that have hindered the widespread application of high-temperature boronizing technique. Such as the large deformation of the work piece, the brittleness of the boride layer and easy spall ing form the substrate. Solid multi-element boronizing at low temperature was proposed to solve those problems with the shortcoming that the boride layer is thin. Experiments indicated that the depth of the boride layer can be increased by fabricating artificial defects in the surface of substrates. It has important practical significance to study the effect of artificial defects on the growth of the boride layer for the solid multi-element boronizing technique at low temperature.The main material for the experiment is low-medium carbon. Fast multiple rotation rolling (FMRR) and Surface high frequency induction quenching were utilized to perform pretreatment in the top surface of substrate, and then samples were Cr-Rare earth-boronized under low-temperature by using ferroboron type boronizing agents. The mechanism of surface pretreatment was studied. The Cr-Rare earth-boronizing process was performed under 600℃×6 h and 650℃×6 h. High resolution transmission electron microscopy (HRTEM), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), X-ray diffraction (XRD), Electron micro hardness tester (EMHT), and acoustic emission instrument (AE) were utilized to characterize artificial defects and the microstructure and properties of the boride layer.The boronizing process on steel 45 was conducted under 650℃ for 6h, and the Cr-Rare earth-boronizing agents were optimized by uniform design. The components of the optimized Cr-Rare earth-boronizing agents were as follows:LaCl3,4%, KBF4, 5%, HC FeCr,5%, other components were zoomed according to the primary proportion. Experimental results indicated that continuous, uniform, and dense boride layer was obtained by the optimized Cr-Rare earth-boronizing agents.Thereocial model was established by taking the grain size refinement as main lines, and the stress for movement of dislocation and the extension of cracks as the assistant lines to characterize the formation of the microstructure in the top surface of pretreated samples. Experimental results of surface nanocrystallization showed that a deformation layer with the depth of approximately 60μm was obtained in the top surface of substrate by Surface nanocrystallization. High-density dislocations, twins, and amorphous phase were observed in the top surface layer of Surface nanocrystalliz specimen. Grains were refined to nanometer order of magnitude by Surface nanocrystallization. The average grain sizes in the top surface of steel 20 surface nanocrystallized for different time were approximately 90nm-100nm,72nm-80nm and 24nm-30nm. The average grain sizes in the top surface of steel 45 surface nanocrystallized for different time were approximately 113nm-120nm,94nm-100nm and 31nm-40nm.The surface nanocrystallization followed by boronizing at low temperature indicated that the boronizing process was improved significantly by surface nanocrystallization. Results indicated that the boride layer showed columnar growth, which was perpendicular to the surface of substrates. The morphology of the boride layer formed on surface nanocrystallized samples was saw-toothed. And the boride layer was dense, straight, continuous and uniform, and adhered well to the metallic substrate. The depth of boride layers Cr-Rare earth-boronized at low temperature was obviously increased by surface nanocrystallization. The depth of steel 20 boronized at 600℃ was increased to 1.9,3.3 and 4.9 times when surface nanocrystallized for different time. The depth of steel 20 boronized at 650℃ was increased to 1.4,1.8 and 2.8 times when surface nanocrystallized for different time. The depth of steel 45 boronized at 600℃ was increased to 1.4,1.7 and 2.5 times when surface nanocrystallized for different duration time. The depth of steel 45 boronized at 650℃ was increased to 1.4,1.6 and 2.6 times when surface nanocrystallized for different duration time.The surface quenching treatment followed by boronizing at low temperature indicated that the boronizing process was improved slightly by surface quenching treatment, and the effect is inferior to the surface nanocrystallization. Results indicated that the boride layer has no columnar characteristics, gaps exist between substrate and boride layer, and cavities exist in the surface layer. However, the boride layer was continuous, uniform and dense, and had no exfoliation phenomenon. The depth of the boride layer was increased also. It was obtained that the depth of steel 20 boronized at 600℃ was increased to 1.4,1.6 and 2.9 times when surface quenched for different time. The depth of steel 20 boronized at 650℃ was increased to 1.3,1.6 and 1.9 times when surface quenched for different time. The depth of steel 45 boronized at 600℃ was increased to 1.3,1.7 and 2.0 times when surface quenched for different time. The depth of steel 45 boronized at 650℃ was increased to 1.2,1.3 and 1.5 times when surface quenched for different time.Rare earth participated in the whole boronizing process, and played an important role as improving the penetrating speed. Chrome element changed the space bond structure of the boride layer, and decreased the brittleness. The results of TFESEM and EDS indicated that nanograins in the top layer of surface nanocrystallized samples participated in Cr-Rare earth-boronizing process, and the chrome element distributed in boride layers and was non-uniform. The La element was mainly distributed in the olumnar crystal apex, grain boundaries, and holes (the La element was mainly distributed in the boride layer and holes for quenched samples), and the La element was enriched in holes. XRD results showed that the boride layer obtained at 650℃ on steel 45 surface nanocrystallized for 60min (quenched for 14s) consisted of single phase Fe2B. Results of brittleness of the boride layer obtained by two methods indicated that the brittleness of the boride layer on surface treated samples decreased. Acoustic emission signal peak is almost non-existent, and there was a linear relationship between En and P basically. The calculation of K value showed that the brittleness of the boride layer Cr-Rare earth-boronized at low temperature decreased by 40% and 37%, compared to that obtained by single-element boronizing process. Results of microhardness measurement indicated that the microhardness of the boride layer increased first and then decreased along the direction perpendicular to the surface of boride layer from surface to the matrix with smaller gradient.The mechanism of surface pretreatment mainly displayed in the absorption and diffusion of active atoms. The stretching of grains, sever deformation, obvious refinement, a lot of grain boundaries, dislocations and high stress region in deformation layer caused by FMRR treatment supply more diffusion channel and auxiliary energy for the diffusion of boron atoms in subsequent Cr-RE-boronizing process, and improve the penetrating speed of boron atoms. The refinement of microsturcture (compared with the substrate), the great increase of hardness and obvious increase of the internal stress in surface hardening layer caused by high-frequency induction surface quenching supply more diffusion channel and auxiliary energy for the diffusion of boron atoms in subsequent Cr-RE-boronizing process, and improve the penetrating speed of boron atoms.The calculation of diffusion coefficient and diffusion activation energy of boron atoms before and after surface pretreatment showed that the diffusion coefficient constant of boron atoms increased obviously by surface pretreatment. Diffusion coefficient constant of boron atoms was 3.9×10-13m2/s,11.5×10-13m2/s and 27.9×10-13m2/s boronized at 600℃, and 7.3×10-13m2/s,12.9×10-13m2/s and 33.1×10-13m2/s boronized at 650℃. The average diffusion activation energy of boron atoms decreased obviously by surface pretreatment.The average diffusion activation energy of boron atoms was decreased by 8986.12Jmol-1,17268.29Jmol-1 and 22942.20Jmol-1 boronized at 600℃, and 4994.47.Jmol-1,8756.52Jmol-1 and 15632.64Jmol"1 boronized at 650℃. Analysis of oncentration profile assumed for the growth model of the borided layer was utilized to explain the change of the adsorption concentration of boron atoms in interface of Fe2B/a-Fe. The mechanism of the effect of surface pretreatment on the Cr-Rare earth-boronizing process was also studied. |
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