Advantages and Disadvantages of Adding Titanium to Austenitic Stainless Steel
When chromium nickel austenitic stainless steel is heated to a temperature range of 450-800 ℃, corrosion along grain boundaries often occurs, which is called intergranular corrosion. Generally speaking, intergranular corrosion is actually caused by the precipitation of carbon elements in the Cr23C6 form from the saturated austenite microstructure, resulting in chromium deficiency in the austenite structure at the grain boundaries. So avoiding grain boundary chromium depletion is an effective way to prevent intergranular corrosion.
The elements in stainless steel are sorted according to their affinity with carbon, 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 generation of chromium carbide and the formation of grain boundary poor chromium due to precipitation, and ultimately effectively prevent intergranular corrosion.
Because titanium and nitrogen can combine to form titanium nitride, and titanium and oxygen can combine to form titanium dioxide, the amount of titanium added is correspondingly limited. In actual stainless steel production, to avoid intergranular corrosion, the amount of titanium added is mainly about 0.8%.
To avoid intergranular corrosion, stainless steel containing titanium must undergo stabilization treatment after solid solution treatment. After solid solution treatment, austenitic stainless steel obtains single-phase austenite structure, but the state of this structure is not stable. When the temperature is increased to more than 450 ℃, the carbon in the solid solution will gradually precipitate in the form of carbides. The formation temperature of Cr23C6 is 650 ℃, and 900 ℃ is the formation temperature of TiC. To avoid intergranular corrosion, it is necessary to reduce the content of Cr23C6 and allow the carbides to exist completely in TiC form.
Because the stability of titanium carbides is higher than that of chromium carbides, when stainless steel is heated above 700 ℃, chromium carbides will begin to transform into titanium carbides. Stabilization treatment involves heating stainless steel to a temperature between 850-930 ℃ and holding it for 1 hour. At this time, the chromium carbide will completely decompose, generating stable gray or black titanium carbide. The resistance of stainless steel to intergranular corrosion is optimized. In addition, adding titanium to stainless steel can also disperse and precipitate Fe2Ti intermetallic compounds under certain conditions, improving the high-temperature strength of stainless steel.
However, titanium is not entirely harmless in stainless steel, and sometimes titanium can also harm the performance of stainless steel. For example, there are easily inclusions such as TiO2 and TiN, which have high content and uneven distribution, which to some extent reduces the purity of stainless steel; It can also cause the surface quality of stainless steel ingots to deteriorate, leading to an increase in the amount of process grinding and easily causing waste products; The polishing performance of the finished product is not very good, and the difficulty of processing high-precision surfaces is very high.
The elements in stainless steel are sorted according to their affinity with carbon, 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 generation of chromium carbide and the formation of grain boundary poor chromium due to precipitation, and ultimately effectively prevent intergranular corrosion.
Because titanium and nitrogen can combine to form titanium nitride, and titanium and oxygen can combine to form titanium dioxide, the amount of titanium added is correspondingly limited. In actual stainless steel production, to avoid intergranular corrosion, the amount of titanium added is mainly about 0.8%.
To avoid intergranular corrosion, stainless steel containing titanium must undergo stabilization treatment after solid solution treatment. After solid solution treatment, austenitic stainless steel obtains single-phase austenite structure, but the state of this structure is not stable. When the temperature is increased to more than 450 ℃, the carbon in the solid solution will gradually precipitate in the form of carbides. The formation temperature of Cr23C6 is 650 ℃, and 900 ℃ is the formation temperature of TiC. To avoid intergranular corrosion, it is necessary to reduce the content of Cr23C6 and allow the carbides to exist completely in TiC form.
Because the stability of titanium carbides is higher than that of chromium carbides, when stainless steel is heated above 700 ℃, chromium carbides will begin to transform into titanium carbides. Stabilization treatment involves heating stainless steel to a temperature between 850-930 ℃ and holding it for 1 hour. At this time, the chromium carbide will completely decompose, generating stable gray or black titanium carbide. The resistance of stainless steel to intergranular corrosion is optimized. In addition, adding titanium to stainless steel can also disperse and precipitate Fe2Ti intermetallic compounds under certain conditions, improving the high-temperature strength of stainless steel.
However, titanium is not entirely harmless in stainless steel, and sometimes titanium can also harm the performance of stainless steel. For example, there are easily inclusions such as TiO2 and TiN, which have high content and uneven distribution, which to some extent reduces the purity of stainless steel; It can also cause the surface quality of stainless steel ingots to deteriorate, leading to an increase in the amount of process grinding and easily causing waste products; The polishing performance of the finished product is not very good, and the difficulty of processing high-precision surfaces is very high.
