Advantages and disadvantages of adding titanium to austenitic stainless steel

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When chromium-nickel austenitic stainless steel is heated to a temperature range of 450–800°C, corrosion along the grain boundaries often occurs; this is known as intergranular corrosion. Generally speaking, intergranular corrosion occurs when carbon elements precipitate from the saturated austenitic microstructure in the form of Cr23C6, thereby resulting in a chromium-deficient austenitic structure at the grain boundaries. Therefore, avoiding chromium depletion at grain boundaries is an effective way to prevent intergranular corrosion. In stainless steel, the elements are ranked according to their affinity for carbon; the order is titanium, niobium, molybdenum, chromium, and manganese. It can be seen that the affinity between titanium and carbon is greater than that between chromium and carbon. When titanium is added to steel, carbon preferentially binds with titanium to form titanium carbide. This effectively prevents the formation of chromium carbide as well as the depletion of chromium at grain boundaries that results from its precipitation. Ultimately, this helps to effectively prevent intergranular corrosion. Since titanium and nitrogen can combine to form titanium nitride, and titanium and oxygen can combine to form titanium dioxide, there are corresponding limits on the amount of titanium that can be added. In actual stainless steel production, to prevent intergranular corrosion, the amount of titanium added is typically around 0.8%. To prevent intergranular corrosion, titanium-containing stainless steels must also undergo stabilization treatment after solution treatment. After solution treatment, austenitic stainless steel exhibits a single-phase austenitic structure; however, this structure is unstable. When the temperature rises above 450°C, carbon in the solid solution gradually precipitates in the form of carbides. The formation temperature of Cr23C6 is 650°C, while that of TiC is 900°C. To prevent intergranular corrosion, the content of Cr23C6 must be reduced so that carbides exist entirely in the form of TiC. Since the stability of titanium carbides is higher than that of chromium carbides, when stainless steel is heated above 700°C, chromium carbides begin to transform into titanium carbides. Stabilization treatment involves heating stainless steel to a temperature between 850–930°C and holding it there for 1 hour; during this time, the chromium carbides completely decompose to form stable gray or black titanium carbides, thereby improving the steel’s resistance to intergranular corrosion. Furthermore, the addition of titanium to stainless steel can, under certain conditions, lead to the dispersed precipitation of Fe2Ti intermetallic compounds, thereby enhancing the high-temperature strength of the stainless steel. However, titanium isn’t completely harmless in stainless steel either; sometimes it can also adversely affect the properties of stainless steel. For example, inclusions such as TiO2 and TiN are likely to be present; their high content and uneven distribution reduce the purity of the stainless steel to a certain extent ； It also deteriorates the surface quality of stainless steel ingots, leading to increased grinding requirements during processing and making it easy to produce defective products ； The polishing performance of the finished product is also not very good; it is extremely difficult to process surfaces with high precision.