What Is the Sigma Phase?

The sigma phase is an intermetallic compound composed of iron and chromium. It forms when chromium carbides precipitate during high-temperature heat treatments. This can occur in duplex stainless steels (DSS) with a ferritic-austenitic microstructure, such as 2205 DSS or 2507 DSS. This microstructure can cause corrosion and structural instability over time if left unchecked.

How Does It Form?

The sigma phase forms as a result of a two-step process. First, chromium carbides precipitate during high-temperature heat treatments between 500°C and 900°C (932°F to 1,652°F). Second, these carbides react with the austenite phases in the steel to form a new compound, which is referred to as “sigma” (σ). The exact amount of time required for the formation of sigma depends on the temperature and composition of the treated material.

Why Is It Important?

It’s important to understand how the sigma phase forms because it can cause corrosion and structural instability in duplex stainless steel components over time. To prevent these problems from occurring, manufacturers must ensure that their processes are designed to minimize or eliminate exposure to temperatures where sigma formation could occur. Additionally, they should perform regular inspections to monitor for any signs of corrosion or structural degradation caused by sigma formation.

Conclusion:

The sigma phase can be a major concern when working with duplex stainless steels like 2205 DSS or 2507 DSS because it can cause corrosion and structural instability over time if not properly managed. While understanding how this phenomenon occurs is important, preventing its formation in your manufacturing processes is even more critical for ensuring the long-term performance and reliability of your products. By designing your processes accordingly and performing regular inspections for any signs of corrosion or degradation due to sigma formation, you can ensure that your components remain safe and reliable for years to come.

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