Inconel® refers to a family of trademarked high strength austenitic nickel-chromium-iron alloys that have exceptional anti-corrosion and heat-resistance properties. These alloys contain high levels of nickel and can be thought of as super-stainless steels. Inconel alloys are used for a variety of extreme applications including navy boat exhaust ducts, submarine propulsion motors, undersea cable sheathing, heat exchanger tubing and gas turbine shroud rings.
For many years, Inconel has been used for Formula One and Champ Car exhaust systems. More recently, several Winston Cup racing teams have utilized Inconel for producing ultra-light, high-durability exhaust headers.
Burns Stainless recommends Inconel 625 alloy for exhaust systems due to its excellent strength, corrosion resistance and fabricability. This alloy also exhibits high creep and rupture strength; outstanding fatigue and thermal-fatigue strength; as well as excellent weldability (though the guy welding it might have a different opinion!). Inconel 625 contains molybdenum and columbium, which stiffens and strengthens the nickel-chromium matrix without precipitation hardening treatments. Some hardening however does occur when heated to intermediate temperatures (1200 F to 1600 F) increasing room temperature strength. Also, this alloy retains over 75% of its room temperature strength at 1200 F. This alloy is available in a wide variety of forms including tubing, sheet, bar, plates and castings. Burns Stainless typically stocks welded and drawn Inconel 625 tubing. The tubing specification is SAE AMS 5581, Nickel Alloy, Corrosion and Heat Resistant, Seamless or Welded Tubing.
Inconel 625 can be welded using conventional stainless steel TIG welding techniques. Inconel Filler Metal 625 rod is used to weld Inconel to Inconel as well as to dissimilar metals including stainless steel. Inconel weldments are high strength and are highly resistant to corrosion and oxidation. Many welders describe that welding Inconel as "dirty". In other words, the weld pool appears to be under a "skin" and is not well defined. In addition, the weld pool is somewhat "sluggish" as compared with steel or stainless steel. These characteristics tend to result in a "coarse" appearing weldment as compared to stainless steel. Welding Inconel is not necessarily more difficult to weld than stainless, just different. By following the welding procedures outlined in the header construction tips article, successful welds with Inconel 625 are possible.