Investigation On Preparation Of High Nitrogen Stainless Steels And High Stainless Steel Coatings By Mechanical Alloying

The low-cost high nitrogen stainless steels that replace nickel with nitrogen are attracting more and more attention due to its excellent mechanical properties, corrosion resistance and biocompatibility. The study and development of high nitrogen stainless steels are highly valued by researchers in Home and Abroad. However, the solubility of nitrogen in Fe at room temperature is only 0.045%. In order to obtain high nitrogen contents, it is necessary to select appropriate technology for high nitrogen stainless steel production. Based on these, this work carried out the study about preparation of high nitrogen stainless steels and high nitrogen stainless steel coatings by mechanical alloying method.Firstly, nitrogen gas was used as nitrogen resource. The pure metal powder mixtures were milled with multiple cycles of nitrogen aeration and nitriding of powder was achieved through solid-gas reactions. The influence of milling time and cycles of nitrogen aeration on the microstructure, nitrogen contents, phase transformation of powders and sintered samples, as well as the sintered samples properties were investigated. Besides, the formation mechanism of high nitrogen stainless steels powders manufactured by mechanical alloying in nitrogen was discussed. The results showed that increasing the milling time and cycles of nitrogen aeration resulted in the increase of nitrogen contents, the transformation of ?-Fe to ?-Fe, and the refinement of crystalline size. Meanwhile, the sintered samples with fully austenitic structure, higher density and microhardenss, and better corrosion resistance were obtained by increasing the milling time and cycles of nitrogen aeration.Secondly, chromium nitrides and nitrogen gas were used as nitrogen resource. The mixtures of pure metal powder and chromium nitrides were milled in nitrogen atmosphere. Nitriding of powders was achieved through both the solid-solid and solid-gas reactions. The effects of milling time on the microstructure, nitrogen contents, and phase transformation of powders and sintered samples were investigated. And the properties of sintered samples with different milling time were studied. The variation behavior of nitrogen element during the mechanical alloying process was properly analyzed according to results. It indicated that the increase of the milling time in favor of the dissociation of chromium nitrides and the increase of nitrogen content in substrate powders. After milling for 30 h, the powders with fully austenitic structure were obtained. There was iron nitrides precipitated during the sintering process. The microhardness of the sintered sample reached to 549HV0.1. The corrosion resistance of the sintered samples was improved by increasing the milling time.Finally, 430 ferritic stainless steels were used as substrate and as-milled high nitrogen stainless steel powders with high nitrogen content were used as raw materials. The high nitrogen stainless steel coatings with certain thickness were prepared on the surface of the 430 ferritic stainless steels by mechanical alloying method. The effects of milling time on phase, the microstructure of the coatings, the adhesive strength between coatings and substrates, and the microhardness, as well as wear and corrosion resistance of coatings were analyzed. The results suggested that the density, adhesive strength between coatings and substrates, the microhardness, as well as wear and corrosion resistance of coatings increased by increasing the milling time. High nitrogen stainless steel coating with 60?m of thickness was prepared on the surface of substrate. The microhardness of coating surface was 478HV0.1, showing a twofold improvement upon the substrates. And the corrosion resistance of the substrate was improved by forming the coating.