Ferritic stainless steels are alloy steels that are composed of at least 10.5 percent chromium and contain mostly a body-centered cubic (BCC) solid solution phase. Ferritic stainless steels are typically used in applications that require high corrosion resistance and moderate strength. In particular, they are well-suited for use in high-temperature environments because they are generally non-magnetic and tend to resist stress corrosion and chloride-induced pitting.
Although ferritic stainless steels are not as strong and impact-resistant as austenitic stainless steels, they offer greater resistance to many corrosive environments and have higher temperature creep and thermal stability compared to other stainless steel alloys. Additionally, unlike austenitic stainless steels, ferritic stainless steels are not hardenable by heat treatment and must rely on cold work instead. Typical applications for ferritic stainless steels include chemical process plants, food processing equipment, automotive trim, medical components, and power generating components.
At elevated temperatures, ferritic stainless steels can become less corrosion-resistant and more brittle, which is why these types of alloys are typically used for lower temperature applications. The most common grade of ferritic stainless steel used for high-temperature applications is AISI 442, also known as 442HQ. This grade of stainless steel is highly resistant to chlorides and provides excellent mechanical properties at temperatures up to 800°F (427°C). Other grades may also be used depending on the specific environment, such as AISI 446, AISI 439, and AISI 444.
When using ferritic stainless steels, it is important to note that weldability is often poor and welds are prone to hot cracking. This is why proper heat treatment is necessary. The proper heat treating procedure is to preheat the material to 1000°F (538°C) before welding and then anneal the parts after welding at a lower temperature of 2100°F (1149°C).
Due to their low strength, ferritic stainless steels are not used for components that are loaded in tension. In such cases, austenitic stainless steel bar or forgings are typically used. For components that have to be exposed to high temperatures for extended periods, ferritic stainless steels are the ideal choice for achieving excellent corrosion resistance and long-term stability.