| Stainless surgical steel for clinical use is almost 316L or 317L stainless steel. The corrosive resistances of these stainless steels are not so satisfactory. With the rapid development of nitrogen-containing steels, the beneficial impact of nitrogen was well recognized. As a result of the beneficial impact of nitrogen, the high nitrogen stainless steel is superior to regular stainless steel in its mechanical properties and corrosive resistance. Pressure metallurgy technology level has increased rapidly in recent years, and the nitrogen content in austenitic stainless steel is increasing.For high corrosion resistance, the study was carried out in two aspects: high concentration of nitrogen and low concentration of oxygen. The thesis analyzed the processes of nitrogen absorption and removal. The nitrogen solubility forecasting formula was deduced considering factors concerned such as temperature, nitrogen pressure and alloy composition. Behavior of nitrogen in the steel was analyzed during solidification. These analyses provided theoretical foundation for the obtaining of high nitrogen 316L stainless steel. At the same time, the basic theory of deoxidation was detailed. The deoxidizing capacities of different deoxidizers were analyzed. And combined with other experimental results, the scheme of deoxidation was carried out to obtain low oxygen high nitrogen 316L stainless steel.This experiment made a vacuum melting of 316L stainless steel by using a high-frequency vacuum/high-pressure induction furnace in the atmosphere of nitrogen gas, whose pressure were 0.1, 0.2, 0.4, 0.6, 0.8 or 1.0MPa. The temperature was controlled within the range of 1497 ~ 1638 and the crucibles used were made of MgO. In order to obtain the low-oxygen and high-nitrogen stainless steels, Al and Si-Ca alloy were used as deoxidizers respectively, and also the two kind of deoxidizers was used together. The study concluded the nitriding kinetic equation in the experimental conditions. Under the pressure lower than 1.0MPa, nitrogen solubility in 316L stainless steel obeys Sievert's Law. The predicted nitrogen content is close to the experimental value. Nitrogen content in 316L stainless steel was 0.635% under 1.0MPa. When we used Al and SiCa as complex deoxidizer, the sample with the lowest oxygen content, 0.0019% was obtained. The deoxidation effect of different deoxidizer was analyzed theoretically.The precipitates and inclusions in stainless steels were studied with scanning electronic microscope and energy dispersive X-ray detector. The eventual experimental results showed that: The main type of inclusions in un-deoxygenated 316L stainless steel was silicon dioxide before nitriding, and the number of inclusions was large. The inclusions in the high nitrogen stainless steel with aluminium deoxidation was mainly AlN. These large AlN inclusions were irregular block in shape. AlN is formed from the chemical union of Al and N. By adding SiCa, the number of inclusions decreased and their size reduced. As a result of calcium treatment, most of silicon dioxide was was reduced to simple substance silicon or modified to complex inclusions, and the main type of inclusions changed to be complex nitrogenous compounds that were complicated. At the same time the typical inclusion Cr2N existed in the high nitrogen stainless steels. From the observation, the precipitatedσphase was found, and theσphase was reduced in the high nitrogen stainless steels by comparison with stainless steel before nitriding. The decrease ofσphase was attributed primarily to nitrogen alloying.The Tafel polarization curve was used to test corrosion resistance of 316L stainless steel in 0.9% NaCl solution at 37℃. What's more, we verified the corrosion process and results by A.C. impedance method. The relationship between the corrosion resistance of 316L stainless steel and nitrogen content was studied. At the some time, the influence of different methods of deoxidation to the corrosion resistance of high-nitrogen 316L stainless steel was discussed. The experimental results were summarized as follows. Nitrogen had a significant role in the improvement of corrosion resistance in stainless steel. If the nitrogen content is less than 0.413%, the corrosion resistance of 316L stainless steel increases with the increase of nitrogen content. The 316L stainless steel with 0.413% nitrogen content showed the best corrosion resistance. If the nitrogen content is higher than 0.413%, the corrosion resistance of 316L stainless steel decreases with the increasing nitrogen content. The influence of oxygen content to the corrosion resistance of High-nitrogen 316L stainless steel was obvious. The low oxidation high nitrogen stainless steel with aluminium deoxidation showed excellent corrosion resistance. Calcium brought great harm to the corrosion resistance of high nitrogen 316L stainless steel, its study of principles needs further. |
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