409 is a titanium stabilized ferritic stainless steel which means that it is a steel alloy containing chromium. It contains less nickel and more carbon than 304 stainless steel. Ferritics are best suited for high temperature applications that require corrosion resistance and high strength. The principal use of 409 stainless steel is automotive exhaust systems and most catalytic converter shells are made of 409.

More workable and stable than 304, 409 will accept bending and heat cycling better than 304. It resists both atmospheric and exhaust gas corrosion. It is magnetic due to its higher carbon content. Through chemical reaction, it oxidizes to a slight brownish hue which aids in corrosion resistance. While it does not polish well, it offers the advantages of higher strength, lower cost and longer life due to its heat handling qualities.
Be sure to ask for 409 Stainless for your exhuast system to get the best combination of both long life, corrosion resistance and heat resistance.