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Advantages and Disadvantages of Titanium Addition in Austenitic Stainless Steel

文章出处:未知责任编辑:管理员人气:发表时间:2019-07-29 16:57 【

When chromium-nickel austenitic stainless steel is heated to the temperature range of 450-800 C, the corrosion along grain boundaries often occurs, which is called intergranular corrosion. Generally speaking, intergranular corrosion is caused by the precipitation of carbon in the form of Cr23C6 from the saturated austenite metallographic structure, resulting in chromium depletion of the austenite structure at the grain boundary. Therefore, avoiding grain boundary chromium depletion is an effective way to prevent intergranular corrosion.
The elements in stainless steel are ranked according to their carbon affinity, in the order of titanium, niobium, molybdenum, chromium and manganese. It can be seen that the affinity between titanium and carbon is greater than that of chromium. When titanium is added to steel, carbon will preferentially combine with titanium to form titanium carbide, which can effectively prevent the formation of chromium carbide and the formation of grain boundary chromium depletion resulting from precipitation, and ultimately effectively prevent intergranular corrosion.
Because titanium and nitrogen can combine to form titanium nitride, titanium and oxygen can combine to form titanium dioxide, so the amount of titanium element added is limited. In order to avoid intergranular corrosion in the actual production of stainless steel, the amount of titanium added is mainly about 0.8%.
In order to avoid intergranular corrosion, it is necessary to stabilize stainless steel containing titanium after solution treatment. Single-phase austenite structure is obtained in austenitic stainless steel after solution treatment, but the state of the structure is not stable. When the temperature rises to more than 450 C, the carbon in the solid solution will gradually precipitate in the form of carbide, of which the formation temperature of Cr23C6 is 650 C and that of TiC is 900 C. In order to avoid intergranular corrosion, the content of Cr23C6 should be reduced and the carbides should exist in the form of TiC completely.
Because titanium carbides are more stable than chromium carbides, when stainless steel is heated above 700 C, chromium carbides begin to convert to titanium carbides. Stable treatment is to heat stainless steel to 850-930 C for 1 hour, at which time the carbide of chromium will be completely decomposed to produce stable gray or black titanium carbide, and the anti-intergranular corrosion ability of stainless steel can be optimized. In addition, the addition of titanium in stainless steel can also disperse and precipitate Fe2Ti intermetallic compounds under certain conditions, which can enhance the high temperature strength of stainless steel.
However, titanium is not completely harmless in stainless steel. Sometimes, titanium also harms the properties of stainless steel. For example, inclusions such as titanium dioxide and titanium are easy to exist, which have high content and uneven distribution, and reduce the purity of stainless steel to a certain extent; they also make the surface quality of stainless steel ingots worse, resulting in increased process grinding and easy to cause waste products; the polishing performance of finished products is not very good, and the processing of high-precision surface is difficult. The degree is very high.

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