Advantages and disadvantages of adding titanium to austenitic stainless steel

Thread Content

When chromium-nickel austenitic stainless steel is heated to a temperature range of 450–800°C, corrosion along the grain boundaries often occurs, a phenomenon known as intergranular corrosion. Generally speaking, intergranular corrosion occurs when carbon elements precipitate from the saturated austenitic microstructure in the form of Cr23C6, 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. The elements in stainless steel are ordered by their affinity for 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 between chromium; when titanium is added to steel, carbon preferentially bonds with titanium to form titanium carbide. This effectively prevents the formation of chromium carbide as well as chromium-depleted grain boundaries resulting from precipitation, thereby preventing intergranular corrosion. Since titanium can combine with nitrogen to form titanium nitride, and titanium can combine with oxygen to form titanium dioxide, there are corresponding limits on the amount of titanium that can be added. In actual stainless steel production, in order to avoid 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 steels acquire a single-phase austenite structure, but this structure is not stable. When the temperature rises above 450°C, carbon in the solid solution gradually precipitates in the form of carbides; the formation temperature for Cr23C6 is 650°C, while that for TiC is 900°C. To avoid intergranular corrosion, it is necessary to reduce the Cr23C6 content so that the 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 is not entirely harmless in stainless steel either; sometimes it can undermine 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 some 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, and it is extremely difficult to manufacture surfaces with high precision.