ATI 409HP™ (UNS S40900) ferritic stainless steel was introduced by ATI Allegheny Ludlum in 1961 to provide improved oxidation and corrosion resistance for automotive exhaust systems in comparison to carbon steel. The alloy was designated "MF-1", indicating its end use: automotive mufflers. The good fabricability of this alloy, combined with its basic corrosion resistance and economy have significantly broadened the utility of ATI 409HP™ stainless.

Since its introduction, the ATI 409HP™ alloy has been widely used in automotive exhaust systems for manifolds, exhaust pipes, catalytic converters, mufflers, tail pipes and other components. ATI 409HP™ 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 ATI 409HP™ (UNS S40900) ferritic stainless steel in most applications is excellent. However, some industry applications have resulted in intergranular corrosion of weld heat-affected zones. The result of research into this problem led to ASTM's re-definition of ATI 409HP™ stainless steel. The old UNS S40900 with 0.08% carbon maximum and Ti>5xC was found to be inadequate, but there was no consensus among users how to define the new grade. Therefore, three new grades of ATI 409HP™ stainless steel were created,

ATI 409Cb™ alloy (UNS S40945) was developed to eliminate the problems encountered when brazing S40900 stainless steel that was stabilized only with Ti. The dual-stabilized S40930 version of ATI 409HP™ alloy usually provides the same benefit and now is used for brazing applications.