API 571 Certification (Corrosion & Materials) Practice Test

Which type of stainless steel is particularly susceptible to sigma phase embrittlement?

1. 400 series Stainless Steel

2. 300 series Stainless Steel

3. Ductile Iron

4. Nickel Based Alloys

The correct answer is: 300 series Stainless Steel

The 300 series stainless steel is particularly susceptible to sigma phase embrittlement due to its composition and the conditions that promote the formation of the sigma phase. This series, which includes grades like 304 and 316, is primarily austenitic and has a high nickel and chromium content. When exposed to certain temperatures, particularly in the range of 900°F to 1600°F (482°C to 871°C), the presence of chromium can lead to the precipitation of the sigma phase, a brittle intermetallic compound. The formation of sigma phase tends to occur in the heat-affected zones of welds or in areas that are held at these temperatures for extended periods. Sigma phase significantly detracts from the mechanical properties, such as ductility and toughness, making the material more prone to cracking under stress. Understanding this susceptibility is crucial for materials engineers and those involved in the design and maintenance of components made from the 300 series stainless steel, particularly in high-temperature applications.