Al-Containing Nano-Bainite Steels Using For Crossing And First-Principles Calculation For Hydrogen Embrittlement Characteristics

The aim of the present dissertation is to investigate the microstructure, mechanical properties and the wear resistance of nano-bainite in low-alloyed Al-containing steels using for crossing, and to calculate hydrogen embrittlement in Al-containing bainitic steels with the first-principles, in order to provide basic data in designing the crossing of new kinds of bainitic steels, and to research hydrogen embrittlement in microcosmos.In this paper, according to chemical constituents of crossing used in high-speed railways and with meeting demand of performance, we have designed the bainitic steels Al-contained used for crossing. With the test of the conventional mechanical properties, we find the optimal craft of normalizing and tempering; By the analysis of microstructure and mechanical properties, the craft of austempering can be gained. Through the nanoindentation and nanoscratch, we can study microcosmic properties of Al-containing bainitic steels. The phase composition and microstructural morphology were studied by X-ray diffractometer, optical microscope and transmission electron microscope. At last, the wear-resistance under the condition of dry sliding friction and wear is also measured.The results show that,the optimal craft of normalizing and tempering is austenitized at 900℃for 0.5 h, then tempered at 350℃for 2 h;The craft of austempering is austenitized at 920℃for 1h, then isothermally quenched at 350℃for 10 h;After normalized and tempered, the rockwell hardness of specimen is 63.2 HRC, the impact toughness is 46.9 J/cm2; After austempered, the rockwell hardness of samples is 47 HRC, the impact toughness is 85.4 J/cm2; The mechanical properties of specimen austempered can meet the demand of the crossing in the railway. The results of the nanoindentation and nanoscratch have shown that friction coefficient and micro-hardness of specimen austempered are less than that normalized and tempered, and the elastic modulus of samples austempered is bigger than that normalized and tempered. With the load of 200 N or 300 N and the spin rates of 200 r/min, the mass loss of samples austempered is less than standard samples of high speed steel, which indicates that the wear-resistant of samples austempered is better.According to first principles, we can calculate the formation energy of FCC structure of Fe with different content of Al or Si. The results show that Al by which replacing Si in steels can accelerate bainitic transformation; When H atom enters into the FCC structure of Fe with Al atom as replaceable atom, the relative formation energy of Fe-Al system is bigger than that of Fe-Si system, which indicates that it is more stabile that H atom enters into Fe-Si system. Consequently, the hydrogen embrittlement of bainitic steel containing Al is lower than that of bainitic steel containing Si.