Introduction Ferritic Stainless Steels
Ferritic stainless steels are classified in the 400 series, usually with 10% to 30% chromium content, and are often chosen for their excellent corrosion resistance and elevated temperature oxidation resistance. With greater strength than carbon steels, ferritics provide an advantage in many applications where thinner materials and reduced weight are necessary, such as automotive emission control systems. They are nonhardenable by heat treating and are always magnetic.
Typical applications for ferritic stainless steels include petrochemical, automotive exhaust systems and trim, heat exchangers, furnaces, appliances and food equipment to name a few.
Corrosion Resistance
Ferritic stainless steels can have good to excellent corrosion resistance depending on the chemical composition of the specific grade. Corrosion resistance is primarily determined by the chromium content. Those steels with molybdenum additions have improved pitting corrosion resistance. Ferritic grades are resistant to chloride stress corrosion cracking (SCC). The unstabilized ferritic grades can be susceptible to intergranular corrosion after certain high temperature exposures, including welding. For applications that require welding a post-weld heat treatment and a stabilized grade is recommended.
Formability
Ferritic stainless steels can be formed by blanking, bending, drawing, and spinning. Their low work hardening rate limits stretch forming somewhat. Caution should be used when forming ferritic stainless steels in thick sections as they have poorer notch ductility than other stainless steels.
Weldability
Ferritic stainless steels can be welded using the common fusion and resistance welding techniques. The metallurgical structure of the ferritic alloys imposes some restrictions on welding. The unstabilized grades can form martensite in the heat affected zone leading to a loss of ductility. The grades stabilized with titanium and/or niobium have improved ductility in the as-welded condition. Welded joints can exhibit low ductility and toughness due to grain growth, especially in heavy sections. Post-weld heat treatments will often improve the ductility of weld zones.
Oxidation Resistance
The ferritic stainless steels exhibit excellent resistance to oxidation, or destructive scaling, at high temperatures. This resistance is due to the formation of a tightly adherent chromium-oxide right film on the surface of the steel. The oxidation resistance is proportional to the chromium content. Additions of silicon and aluminum improve the oxidation resistance even further. The low thermal expansion coefficient of ferritic stainless steels allow these steels to resist spalling and perform exceptionally well even in harsh cyclic high temperature service.
