When chromium – nickel austenitic stainless steel is heated to the temperature range of 450-800 C, corrosion often occurs along the grain boundary, which is called intergranular corrosion. Generally speaking, intergranular corrosion is actually carbon element precipitated from Cr23C6 in the metallographic structure of saturated austenite, resulting in poor austenite structure at grain boundaries. Therefore, it is an effective way to prevent chromium poor from grain boundary to prevent intergranular corrosion.
The elements in stainless steel are sorted according to the affinity of carbon affinity, and the order is 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 give priority to titanium bonding to form titanium carbide, which can effectively prevent the formation of chromium carbide and precipitation, resulting in grain boundary chromium depletion, which can effectively prevent intergranular corrosion.
Because titanium and nitrogen can combine to form titanium nitride, titanium and oxygen can form titanium dioxide. Therefore, the amount of titanium added is limited. In order to avoid intergranular corrosion in the actual stainless steel production, the amount of titanium is mainly about 0.8%.
In order to avoid intergranular corrosion, stabilization treatment must be made after solid solution treatment of titanium containing stainless steel. After solution treatment of austenitic stainless steel for single-phase austenite, but the state is not stable, when the temperature is raised to more than 450 DEG C, solid solution of carbon in the form of carbide precipitation will gradually, of which the formation of Cr23C6 temperature is 650 degrees Celsius, 900 degrees is the formation temperature of TiC. To avoid intergranular corrosion, the Cr23C6 content is reduced, so that the carbide is completely in the form of TiC. Because the stability of titanium carbide is higher than that of chromium carbide. When the stainless steel is heated to 700 degrees, the carbide of chromium will start to transform to titanium carbide. Stabilization treatment is to heat the stainless steel to 850-930 degrees, and to keep it for 1 hours. At this time, chromium carbide will be completely decomposed to form stable gray or black titanium carbide, and the intergranular corrosion resistance of stainless steel can be optimized. In addition, the addition of titanium in stainless steel can also diffuse and precipitate Fe2Ti intermetallic compounds to improve the high temperature strength of stainless steel under certain conditions.
However, titanium is not completely harmless in stainless steel, and sometimes titanium will also harm the properties of stainless steel. For example, TiO2 and TiN are their high content of inclusions, and uneven distribution, to a certain extent, reduce the purity of stainless steel will make the surface quality of stainless steel ingot; the variation of grinding process leads to increased, it is easy to cause waste; the polishing performance of the finished product is not very good, very high processing difficulty and high precision surface the.
Source: China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)