| In recent years, the research in the field of high nitrogen steel (HNS) has come into a new upsurge. The type of HNS are not limited to the special kinds of steel such as austenitic stainless steel and high-speed steel. It extends to the general kinds of steel. Now there are more and more methods of the preparation of HNS, which are coming to maturity. Nevertheless, owing to the limitation of the processing technology and the needed investment, so far HNS has not yet been used widely and the potential of HNS has not been recognized completely. To probe into the possibility of production of HNS in large scale in our country, the 35CrMoV steel with high nitrogen content were prepared by using either the technology of filling the tube with the steel powder-sintering-forging or the technology of powder explosive compaction and then sintering. The thermodynamics and kinetics of nitrogen dissolution in iron and/or ferroalloy were studied and the problems appearing in the preparation course of HNS were discussed in this paper. The microstructure and the mechanical properties of the HNS were also measured and analyzed. Finally the statistical thermodynamic properties of the aFe-N system were investigated.The high nitrogen 35CrMoV steel, whose nitrogen content was 0.1 wt%, was prepared by using the technology of filling the tube with the steel powder-sintering-forging. The density of the samples was found to be about 7.8g/cm3 and the mechanical properties of them were discovered as much better than those of the low nitrogen35CrMoV steel. The nitrogen content of the high nitrogen 35CrMoV steel samples produced through explosive compaction is 0.15wt% and the density of the sintering samples is 6.9 g/cm3. Since this was just the first experiment, much crack was found in the center of the cross-section of the sample. So the forging procedure was not carried out. However, in the majority of the rim region of the samples, the microstructure was very tight. Therefore, it is expected to be possible and successful to prepare HNS through explosive compaction.After annealing the HNS samples prepared through the technology of filling the tube with the steel powder-sintering-forging, the room tensile elongation would be above 13%. As for the tempered samples, the room tensile strength of them was even increased by 30% compared with the standard strengths of the low nitrogen steel (LNS). Meanwhile, the elongation of the sample was found to keep on the level of the standard value of LNS. The temperature of the resistance to softening on tempering has been increased. As compared with the standard high temperature mechanical properties of LNS, the properties of HNS at 600C were equivalent to those of LNS at 500 C. With the increase of temperature, the tensile strength of HNS decreased while the tensile elongation increased.The reasons of the nitrogen improving the obdurability of the steel can be given as follows. First, since the chromium and vanadium elements in 35CrMoV steel are strong nitride formers, they interact with nitrogen and some finer disperse carbonitrides appear. Because the melt points of these carbonitrides are very high, they are too hard to dissolve into the matrix at 900 C. They blockthe growth of the austenitic grains during heating and result in the finer products of the austenite. During deformation process, they could also pin down the dislocations. Meanwhile, nitrogen improves the stability of the austenite. The analysis of the microstructures of the forging, annealing, quenching and tempering samples showed that the microstructure of HNS is obviously finer than that of LNS. Besides a large amount of lath martensite, there are some bainite, M-A microstructure and twin martensite in the quenched HNS samples. As compared with the microstructure of LNS, the precipitated carbides in the HNS samples after 650~670 掳C tempered are very fine and their distribution in the matrix is in a random way. Furthermore, some much finer precipitates are also observed in the HNS samples.There are a few characterist...
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