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Pros and cons of adding titanium to austenitic stainless steel

Posted: 04/03/2019 14:05:55 Hits: 60

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

The elements in the stainless steel are sorted according to the degree of carbon affinity, in the order of titanium, tantalum, 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 preferentially combines with titanium to form titanium carbide, which can effectively prevent the formation of chromium carbide and precipitation to form a grain boundary which is poor in chromium, thereby effectively preventing intergranular corrosion.

Since titanium and nitrogen can combine to form titanium nitride, titanium and oxygen can combine to form titanium dioxide, the amount of titanium added is limited. In order to avoid intergranular corrosion in actual stainless steel production, 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. (Since the austenitic stainless steel obtains a single-phase austenite structure, but the state of this structure is not stable after the solution treatment.) When the temperature is raised to more than 450°C, the carbon in the solid solution gradually precipitates in the form of carbides, among which the formation temperature of Cr23C6 is 650 °C, and TiC formation temperature is 900 °C. To avoid intergranular corrosion, it is necessary to reduce the Cr23C6 content and allow the carbide to exist completely in the form of TiC.

Because the carbide stability of titanium is higher than that of chromium, when the stainless steel is heated above 700 °C, the carbide of chromium begins to transform into the carbide of titanium. Stabilization treatment is to heat the stainless steel to 850-930 °C for one hour. At this time, the chromium carbide will completely decompose to form stable gray or black titanium carbide, and the resistance of the stainless steel to intergranular corrosion is optimized. In addition, titanium is added to the stainless steel, and under certain conditions, the Fe2Ti intermetallic compound can be dispersed to increase the high temperature strength of the stainless steel.

However, titanium is also not completely harmless in stainless steel, and sometimes titanium also jeopardizes the performance of stainless steel. For example, there may be inclusions such as TiO2 and TiN, whose content is high and unevenly distributed, which reduces the purity of stainless steel to some extent, and it also makes the surface quality of stainless steel ingots worse, resulting in an increase in the amount of process grinding, which is easy to cause waste. Moreover, the polishing performance of the finished product is not very good, and the processing of high-precision surfaces is very difficult.

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