What Are the Characteristics of Austenitic Stainless Steel?

It is an austenite phase at room temperature. The Cr content is greater than 13% to increase the electrode potential of iron, thereby improving the corrosion resistance of the material. The Ni content is also high. On the one hand, it can also increase the electrode potential of iron. On the other hand, it is an austenite-generating element, making the steel in an austenite phase at room temperature.

The most common austenitic stainless steel is wcr=18%, wNi=9%, which is commonly known as 18-8 stainless steel. This type of stainless steel also adds strong carbide-forming elements such as Nb and Ti to prevent intergranular corrosion. The intergranular corrosion microstructure is shown in Figure 1.

In order to achieve the best corrosion resistance and eliminate work hardening of austenitic stainless steel, solid solution treatment, stabilization treatment and stress relief treatment must be carried out.

Solid solution treatment. The carbon content of austenitic stainless steel cannot be too high, otherwise it is easy to form (CrFe)23C6 during cooling, which affects the performance of austenite, so the carbon content of austenitic stainless steel is generally controlled below 0.1%.

Although the carbon content is controlled, carbides will still be generated during slow cooling. In order to eliminate carbides, austenitic stainless steel is heated to 1000~1150°C to dissolve all carbides into austenite, and then cooled rapidly so that carbides do not have time to precipitate, thereby obtaining single-phase austenite. This is solid solution treatment.

Stabilization treatment. It is mainly to prevent intergranular corrosion. After solid solution treatment, austenitic stainless steel containing Ti and Nb is heated to 850~900°C for 1~2h, and then air-cooled to room temperature.

This is stabilization treatment. In this process, the Cr-containing carbide Cr23C6 is basically dissolved, while TiC and NbC are partially retained. This avoids the precipitation of Cr23C6 between grains, making the grains poor in Cr and reducing corrosion resistance, thereby avoiding intergranular corrosion during use. The precipitation morphology of Cr-containing carbides between grains is shown in Figure 2.

Stress relief treatment. It is mainly to eliminate the internal stress generated after cold working and welding. Residual stress can promote stress corrosion, so stress relief is beneficial to improving the stress corrosion resistance of steel.

However, the stress relief treatment temperature needs to be controlled and cannot be too high, otherwise Cr carbides will precipitate and reduce the intergranular corrosion resistance.

The stress relief tempering temperature is generally 300~350°C. If the steel does not contain Ti and Nb, the heating temperature should not exceed 450°C. For ultra-low carbon and stainless steel containing Ti and Nb, it should be heated at 550~950°C and then slowly cooled.