This is a ferritic stainless steel that was developed for use in corrosive environments where carbon steel, even coated, is unsatisfactory, and where the higher chromium ferritics prove to be economically unfeasible.
Type 409 Stainless Steel (UNS S40900) is a stabilized ferritic stainless steel that provides improved oxidation and corrosion resistance compared to carbon steel. Ease of fabrication combined with its corrosion resistance and economy have significantly broadened its utilization in automotive exhaust and other non-automotive applications. ASTM A240 defines three (3) sub categories of Type 409 stainless steel; UNS S40910, UNS S40920 and UNS S40930. Each of these designations has a slightly different Ti or Nb stabilization requirement. Each of these compositions can be supplied as UNS S40900 unless the purchaser specifically identifies a specific composition. It is produced with enhanced formability characteristics known as “High Performance” or “Ultra Form” in the industry.
This alloy is much used in automotive exhaust systems. It may also be used as a relatively low-cost alloy where its non-hardening properties and its relative freedom from grain boundary precipitation can be used to advantage.
The alloy may be forged between 2100/1500ºF (1150/815ºC) and air cooled. The titanium addition will serve to reduce grain boundary precipitation during cooling from forging.
The material should be heated to 1440ºF (780ºC,) soaked and air cooled. The alloy cannot be hardened by heat treatment
The alloy will machine in a similar manner to type 405 stainless.
This alloy is weldable by all normal fusion and resistance methods. The titanium addition prevents hardening when cooling from welding, hence effects an overall improvement in weldability. When applications do not involve high-temperature exposure, a 308-308L filler metal may be used. When high temperature exposure is involved, a type 409Cb welding rod may be used.
409 stainless can be cut, blanked stretch formed and drawn without difficulty. The formability characteristics of Type 409 are very similar to carbon steel.
Type 409is readily welded using the conventional welding procedures common to carbon steel. These include high frequency, resistance, gas tungsten arc (GTAW or TIG), gas metal arc (GMAW or MIG), electron beam and laser techniques. Although autogenous welds are common, particularly in manufacture of tubing, a filler metal, Type 409Cb, is often employed.
The corrosion resistance of 409 stainless is about the same as Type 410 stainless steel. Type 409 offers improved corrosion resistance in comparison to carbon steels. However, its corrosion resistance is lower in comparison to other stainless steels with higher chromium content. Type 409 offers sufficient resistance to corrosion and oxidation to be used in many automotive and non-automotive applications.