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The role of titanium and niobium in stainless steel

Austenitic stainless steel, composed of chromium and nickel, is prone to a type of corrosion called intergranular corrosion when heated in the temperature range of 450 to 800°C. This phenomenon occurs along the grain boundaries. Intergranular corrosion is commonly attributed to the precipitation of Cr23C6 in the form of carbon, leading to chromium depletion in the austenitic matrix. Preventing chromium depletion along grain boundaries is an effective strategy to mitigate intergranular corrosion.

It has been observed that intergranular corrosion can be averted by introducing certain elements into the steel in a particular order of affinity to carbon. This sequence is as follows: Ti, Zr, V, Nb, W, Mo, Cr, Mn. Both titanium and niobium exhibit greater affinities for carbon than chromium does. Upon their incorporation into the steel, carbon preferentially combines with them to form titanium carbide (TiC) and niobium carbide (NbC). This preemptively prevents the precipitation of chromium carbides, thereby averting chromium depletion along the grain boundaries and effectively inhibiting intergranular corrosion.

Additionally, titanium and niobium can form titanium nitride and niobium nitride when combined with nitrogen, and titanium can also react with oxygen to produce titanium dioxide. Austenitic structures can also dissolve a certain amount of niobium (approximately 0.1%). Taking these factors into consideration, in practical production, the amounts of titanium and niobium to prevent intergranular corrosion are generally calculated using the following formula:

After solution treatment of steel containing titanium and niobium, a single-phase austenitic structure is obtained. This structure exists in an unstable state. As the temperature rises above 450°C, carbon within the solid solution gradually precipitates in the form of carbides. The temperature of carbide formation is 650°C for Cr23C6, 900°C for TiC, and 920°C for NbC. To prevent intergranular corrosion, it is essential to reduce the content of Cr23C6, allowing carbides to exist solely in the form of TiC and NbC. Due to the greater stability of titanium and niobium carbides compared to chromium carbides, when the steel is heated above 700°C, the transformation of chromium carbides into titanium and niobium carbides begins.

Stabilization treatment involves heating the steel to a temperature between 850°C and 930°C and holding it for 1 hour. At this point, all chromium carbides decompose, leading to the formation of stable TiC and NbC. This treatment enhances the steel’s resistance to intergranular corrosion.

Stainless steel is fortified with titanium and niobium, which, under specific conditions, disperse and precipitate metal intermetallic compounds such as Fe2Ti and Fe3Nb2. This results in an enhancement of the steel’s high-temperature strength. Given the high cost of niobium (70 times that of titanium), titanium-strengthened stainless steel is more widely adopted. However, titanium-containing steel does present some drawbacks. For instance, TiO2 and TiN exist as inclusions, with uneven distribution and high content, which diminishes the steel’s purity. The quality of ingot surfaces is compromised, necessitating additional grinding steps and significantly increasing the risk of producing a large volume of defective products. Furthermore, the finished products exhibit subpar polishing performance, making it challenging to achieve high-precision surfaces.

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