Microstructures And Preoterties Of Copper(Nitrogen)-Bearing Antibacterial Stainless Steels

The people'healthy consciousness rises along with the improved living quanlity. Due to the wide fatal infection incidents happened in recent years, antibacterial stainless steels were developed and have met the raised public awareness of hygiene. They have found wide applications in food industry, hospitals, domestic appliances and public transportation and buildings, etc. Development of new type of antibacterial stainless steels with a combination of high properties and multi-functions is always the objection of many investigators. In this paper, microstructures and properties of copper (nitrogen)-bearing ferrite and austenite stainless steels were studied, using experiments and theoretical analysis.Followings are the main conclusions:The composition and growth mechanism of copper based precipitates in copper-bearing ferrite antibacterial stainless steel was analyzed with HRTEM and carbon replica extraction techniques. Copper based precipitates mainly consist of cooper and small amount of manganese and iron elements, having and approximate OR with the ferrite matrix, as identified by TEM. They also have multilayer structure which is identified as (111) twins and faults. Copper based precipitates enlarge by the growth of the layers along and [111]_ε-Cu preferred orientation. The special structure enables us to understand the precipitation and growth mechanism of copper based precipitates in ferrite stainless steel and to an extent enrichs the precipitation theory of copper in steels.The precipitation process of copper based precipitates in copper-bearing ferrite antibacterial stainless steel was investigated. The results show that the precipitation of copper based precipitates in ferrite antibacterial stainless steel is controlled by diffusion mechanism. The optimum aging range is between 750 and 800℃. The larger the precipitates are, the more the quantity is, the higher the antibacterial property will be. To secure a good antibacterial property, the precipitation amount should surpass 0.5%. The volume fraction of precipitates formed within 60 minutes can be predicted by a modified Avrami equation, This mold will benefit the addjustion of the etchnology and improvement of the techniques.EBSD techniques were applied to analyze the copper based precipitates in the surface of copper-bearing ferrite antibacterial stainless steel. Copper based precipitates precipitat along some preferential directions. The habit plane for copper based precipitates is basicly (110)α-Fe. Copper based precipitates on the surface of ferrite stainless steel are predominantly located within ferrite grains having [101] orientation. Cold deformation lead to a decrease of Goss oriented ferrite grains in the steel surface layer and an increase of {111}γoriented grains, and simultaneously, brings about a change in the precipitate morphology from rod to ellipse or sphere, as a result, a declined antibacterial property and improved corrosion resistance. After 800℃aging for 90 minutes, cold-rolled ferrite antibacterial stainless steel with 99.9% antibacterial property can be obtained.Nitrogen has been proved a beneficial element for improving both the antibacterial property and corrosion resistance of copper-bearing austenite stainless steel. The result of thermodynamics calculation reveals that nitrogen increases the activity coefficient of copper and the effect of nitrogen concentration on the activity coefficient of alloying copper can be predicted from the equation, ln f_Cu/f_Cu⁰= 0.53524+4.11x_N-0.48x_N². Where, f_Cu is the activity of copper in the present alloy, f C0uis the activity of copper in multi-component infinitely diluted solution and x N is nitrogen concentration. Nitrogen also increases the free energy difference of copper distribution in precipitation phase and in austenite due to an increase in the activity of copper caused by nitrogen, promotes the precipitation of copper, and as a result, enhances the antibacterial property. In addition, nitrogen addition improves the corrosion resistance by enriching in the passive film, compensating the harmful effect of copper based precipitates on the corrosion resistance. This type steel is more applicale to be made in industries due to short-term aging process and excellent properties.