Solution treatment, stabilization treatment, and stress relief treatment of austenitic stainless steel

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What are the characteristics of austenitic stainless steel? At room temperature, it is in the austenite phase; the Cr content is greater than 13% in order to raise the electrode potential of iron and thereby enhance the material’s corrosion resistance. The Ni content is also high – it helps to increase the electrode potential of iron, and it also acts as an element that promotes the formation of austenite, ensuring that the steel remains in the austenite phase at room temperature. The most common austenitic stainless steel has a WCr content of 18% and a WNi content of 9%, which is what is commonly referred to as 18-8 stainless steel. This type of stainless steel also contains strong carbide-forming elements such as Nb and Ti, with the aim of preventing intergranular corrosion. To achieve optimal corrosion resistance in austenitic stainless steels and eliminate work hardening, solution treatment, stabilization treatment, and stress relief treatment must be carried out. Solution treatment. Austenitic stainless steels should not have too high a carbon content, as this can lead to the formation of (CrFe)23C6 during cooling, thereby affecting the properties of the austenite structure; therefore, the carbon content in austenitic stainless steels is generally kept below 0.1%. Although the carbon content is controlled, carbides still form during slow cooling. To eliminate carbides, austenitic stainless steel is heated to 1000–1150°C to dissolve all the carbides into the austenite phase; thereafter, it is cooled rapidly so that the carbides do not have time to precipitate, resulting in a single-phase austenite structure. This is known as solution treatment. Stabilization treatment. It is mainly to prevent intergranular corrosion. After solution treatment, austenitic stainless steels containing Ti and Nb are reheated to 850–900°C and held for 1–2 hours, then air-cooled to room temperature. This is known as stabilization treatment. During this process, the Cr-containing carbide Cr23C6 essentially dissolves, while TiC and NbC are partially retained. This prevents Cr23C6 from precipitating intergranularly, which would otherwise result in a chromium-deficient intergranular region and a reduced corrosion resistance, thus avoiding intergranular corrosion during use. The morphology of Cr-containing intergranular carbides is shown in Figure 2. Stress relief treatment. It is mainly to eliminate the internal stresses generated by cold working and welding. Residual stress can induce stress corrosion; therefore, stress relief is beneficial for improving a steel’s resistance to stress corrosion. However, the stress-relief treatment temperature needs to be controlled and should not be too high, otherwise Cr carbides will precipitate, reducing the intergranular corrosion resistance. The stress-relief tempering temperature is generally 300–350°C; if the steel does not contain Ti or Nb, the heating temperature should not exceed 450°C. For ultra-low carbon steels as well as stainless steels containing Ti or Nb, heating at 550–950°C followed by slow cooling is required.