Advantages and Disadvantages of adding Titanium to Austenitic Stainless Steel

When the chromium-nickel austenitic stainless steel is heated to the temperature range of 450-800℃, corrosion along the grain boundary often occurs, which is called intergranular corrosion. Generally speaking, intergranular corrosion is actually caused by the precipitation of carbon in the form of Cr23C6 from the saturated austenitic metallographic structure, which makes the austenite structure at the grain boundary depleted in chromium. Therefore, avoiding chromium depletion at grain boundaries is an effective way to prevent intergranular corrosion.

The elements in stainless steel are sorted according to their affinity for carbon, and the order is titanium, niobium, molybdenum, chromium and manganese. It can be seen that the affinity of 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 formation of chromium carbide and the precipitation of chromium depletion at grain boundaries. Can effectively prevent intergranular corrosion.

Because titanium and nitrogen can be combined to form titanium nitride, and titanium and oxygen can be combined 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%.

In order to avoid intergranular corrosion, the titanium-containing stainless steel must be stabilized after solution treatment. After the solution treatment, the austenitic stainless steel obtains a single-phase austenite structure, but the state of this structure is not stable. When the temperature rises above 450℃, the carbon in the solid solution will gradually precipitate in the form of carbides, of which Cr23C6 The formation temperature is 650℃, and 900℃ is the formation temperature of TiC. To avoid intergranular corrosion, it is necessary to reduce the content of Cr23C6 so that the carbides exist completely in the form of TiC.

Because the stability of titanium carbides is higher than that of chromium carbides, when stainless steel is heated above 700°C, chromium carbides will begin to transform into titanium carbides. The stabilization treatment is to heat the stainless steel to between 850-930°C and keep it for 1 hour. At this time, the chromium carbides will be completely decomposed to produce stable gray or black titanium carbide, and the anti-intergranular corrosion ability of the stainless steel is optimized. In addition, the addition of titanium to stainless steel can also disperse and precipitate Fe2Ti intermetallic compounds under certain conditions to improve the high-temperature strength of stainless steel.

However, titanium is not completely harmless in stainless steel, and sometimes titanium can harm the performance of stainless steel. For example, inclusions such as TiO2 and TiN are prone to exist. They have high content and uneven distribution, which reduces the purity of stainless steel to a certain extent; it also deteriorates the surface quality of stainless steel ingots, resulting in increased grinding volume in the process, which is easy Resulting in waste; the polishing performance of the finished product is not very good, and the processing of high-precision surfaces is very difficult.

Advantages and Disadvantages of adding Titanium to Austenitic Stainless Steel

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