Study On Boronizing Process Of FeB+KBF₄ And The Corrosion Resistance Of Boride Layer At High Temperature

In this thesis, two-way method was used to study boronizing process, structure and performance of FeB-KBF4 system, the single factor method was adopted to optimize boronizing temperature and holding time. According to the investigation of boronizing kinetics, the boronizing mechanism of FeB-KBF4 system was discussed, and the boronizing heat treatments were also carried out to observed the changes of microstructure and property. A process of boronizing followed by eutecticum treating by high-frequency induction heating and the resulting case microstructure were investigated. At the end of this study, the high-temperature corrosion behavior of boron coating on 35 steel was studied under 5%O2 and 5%O2+500ppm HCl atmosphere. The main conclusions are as follows:The content of FeB and KBF4 effected signifieantly to case depth, which increased gradually with the increase of FeB and KBF4. The optimum content of boronizing agent included 15%~25%wt FeB and 10%wt KBF4, and the optimum soaking time at treating temperature of 900~950℃ was among 2~4h. Activation energies(Q) of borided 35 steel was determined as 185kJ/mol. The structures of boride layers consisted of simple Fe2 B phase or Fe2B+FeB diphase and the micro hardness was among 1200~2000HV0.1. Diffusion heat treatment and quenching with high temperature tempering treatment followed by boronizing reduced the surface concentration of B element, improved the compactness of boride. The optimum process of boronizing followed by eutecticum treating by high-frequency induction heating was that soaking time at treating temperature of 1100~1200℃ was among 5~10s. The eutecticum area structure included Fe2B+α-Fe or Fe2B+α-Fe+little Fe3 C hypoeutectic and Fe2B+ Fe3(C,B)+ α-Fe eutectic structure, and the hardness was among 500~700HV0.1。The high-temperature corrosion tests showed that the corrosion kinetics rule was parabolic and the corrosion resistance of boronized specimens were much better than unboronized steel. The existence of HCl can crossover surface oxidation which lead to serious corrosion and the weight gained 2~4 times compered to atmosphere without HCl. The X-Ray indicated that the corrosion products consisted of iron oxides, boride oxides and iron-boride oxides. Amorphous B2O3 formed as a glassy protective layer on the coating surface and controled the diffusion of oxygen through the glassy layer which resulted in the slow reaction rate at 400 ℃. However, At 600℃, B2O3 evaporated, and the formation of iron borates was preliminary which also diminished the formation of iron oxides.