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,

UNS S40910, S40920, and S40930, all with reduced (0.03% maximum) carbon but with somewhat different stabilization formulae. Under ASTM A240, orders for S40900 material may be fulfilled by supplying any one of the new 409 grades unless the purchaser states additional requirements. Composition requirements for the three varieties of ATI 409HP™ alloy are shown in the composition table below.