Stabilization Requirements for T409 (UNS S40900) Ferritic Stainless Steel

Type 409 (UNS S40900) ferritic stainless steel was developed more than 30 years ago specifically for automotive exhaust systems where it has subsequently found extensive use. Performance of this 11 Cr- Ti alloy for the most part has been good. However, recent failures of exhaust system components demonstrate that the T409 alloy can become sensitized in weld heat-affected zones and suffer intergranular corrosion. Results of a broad study of commercial heats illustrate that weld heat-affected zones of T409 are stabilized when titanium plus columbium or titanium alone meets the criterion:

Experimental procedures used to establish this criterion are described, including development of a test to detect sensitization. Also the benefits of stabilizing with titanium in combination with columbium are discussed.