ATI 439™, a titanium stabilized, 18 percent chromium alloy, is also known as ASTM XM-8 and by the UNS designation S43035, is a ferritic stainless steel designed to resist corrosion in a variety of oxidizing environments from fresh water to boiling acids. It may be used in either the annealed, cold formed or as-welded condition in many applications where other stainless steel alloys such as Type 304, Type 410, Type 409 and Type 430 are used. ATI 439™ alloy may also be used in many oxidizing environments where Type 304 is considered adequate in terms of general corrosion resistance but is subject to chloride stress corrosion cracking. ATI 430 Ti™ (UNS S43036) material is essentially equivalent to type 439 (UNS S43035) material. It is used for heat exchanger tube, etc. ("430Ti" may also refer to UNS K91800, ASTM F256, a controlled expansion alloy for glass-to-metal sealing. ATI Allegheny Ludlum no longer produces the K91800 material.)

The composition of ATI 439™ stainless steel has been balanced to provide a completely ferritic structure at all temperatures, to avoid the loss of ductility after welding and to provide resistance to intergranular corrosion. Similar ferritic stainless steels such as ATI 430 and ATI 434 alloys are susceptible to brittle martensite formation after welding and to intergranular corrosion sensitivity in the as-welded condition. ATI 439™ alloy does not require annealing after welding to restore ductility or to provide intergranular corrosion resistance.

Excellent resistance to stress corrosion cracking, good weldability, high thermal conductivity and low thermal expansion characteristics make ATI 439™ stainless steel an ideal consideration for many applications. For shell and tube heat exchangers where carbon steel shells are used with ATI 439™ tubes, the close match of thermal expansion coefficients may possibly eliminate the need for an expansion joint in the heat exchanger. ATI 439™ alloy has a low cobalt content, compared to Type 304, making it an attractive consideration for nuclear applications.

When aplication temperatures are too high for ATI 409 alloy, ATI 439™ alloy provides good oxidation and corrosion resistance for many automotive exhaust system components and residential furnace primary heat exchangers.

ATI Allegheny Ludlum currently supplies only an improved formability version of this alloy, known as ATI 439HP™. This is especially beneficial for the severe tube bending required for automotive exhaust manifolds.