Sky Dynamics Corporation works exclusively with the stabilized grades of stainless, i.e. 321 or 347, and also the 600 Series Inconel. Many manufacturers promote 304 series stainless for exhaust use. This grade has proven moderately successful for automotive use, but it is not suitable for the extreme requirements of an aircraft environment. If type 304 is exposed to high temperatures, (800-1600 deg F) for prolonged periods, carbide precipitation may occur. Type 304L (L meaning low carbon) is a step better than 304, but is still not as satisfactory as the stabilized grades.
 
    Type 321 stainless is similar to 304 but contains a titanium addition of at least five times the carbon content. Titanium is added to reduce or eliminate chromium carbide precipitation resulting from welding or exposure to high temperatures.
 
    Although titanium is quite effective as a stabilizer, maxiumum stabilization is best accomplished by annealing parts in the 1800-1900 deg F temperature range in an inert or vacuum atmosphere. Sky Dynamics Corporation typically heat treats all critical parts of an aircraft exhaust as part of its manufacturing process. In welding applications where carbide precipitation occurs and annealing is possible, harmful carbides can be eliminated by annealing followed by rapid cooling. This also relieves the residual stresses at the weld area.
 
    Type 321 stainless is used extensively for aircraft parts such as exhaust systems where operating temperatures of more than 800 deg F are encountered and where corrosive conditions are not too severe. Resistance to scaling or oxidation is important because loss in metal thickness reduces the strength of the part and results in early failure. Type 321 stainless resists scaling and vibration fatigue under such service conditions.
 

Welding Stainless Steel

    When welding 321 stainless, nearly all of the titanium is lost in the weld bead. For this reason, a type 347 welding rod with a columbium addition is recommended because the loss of this element is not harmful to the structural integrity of the weld.

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