Since its introduction, the Type 409 alloy has been widely used in automotive exhaust systems for manifolds, exhaust pipes, catalytic converters, mufflers, tail pipes and other components. Type 409 stainless steel sheet has also found many other applications such as in culverts, home heating systems, automotive thermostats and fuel filters, electrical transformer cases, caskets, heat exchanger tubing, and various farm equipment components. Plate has been used as blades and vanes in standby power generation turbines and as jet engine sound-deadening devices at airports.

The performance of Type 409 ferritic stainless steel in most applications has been excellent. However, some industry application have resulted in intergranular corrosion of weld heat-affected zones. The result of Allegheny Ludlum's research to investigate and redefine the balance of stabilizing elements is the AL 409HP stainless steel amounts of titanium and columbium which provide resistance to sensitization and intergranular corrosion which might occur in the heat affected zones of welds or in base metal following other thermal exposures. A low incidence of titanium nitride surface imperfections is also a characteristic of this alloy. The balance of stabilizing elements is designed to optimize weldability without need for post-weld annealing to restore ductility. Formability and toughness are also improved in comparison to the standard Type 409 alloy.

This AL 409HP alloy is a proprietary stainless steel composition. Like all Type 409 alloys, the AL 409HP stainless steel contains nominally 11 percent chromium for greatly enhanced corrosion resistance in comparison to carbon steel. It is an excellent choice for all automotive exhaust system applications where 11 percent chromium provides sufficient resistance to oxidation and corrosion. Hot end exhaust applications may require an 18 percent chromium alloy.

Information provided by Allegheny Ludlum