[termik]

I have the following problem. I use an annealing furnace for stainless steel tube heaters. The material I use is typically 304 and 316L. After testing these elements in water, they rust after 20-30 minutes. I excluded all processes before (iron contact with heater surface, tube quality). I have found that there must be something wrong with the atmosphere, because when I try to anneal the parts in another furnace they turn out fine. I use propane and air 1:10. What could be the problem?

[josephmarkgreene]

Have you analyzed the surface of the tubes for carbon content? Probably, your process is inadvertently carburizing the stainless steel. Stainless steels depend on their chromium content for corrosion resistance. Carburizing or nitriding binds chromium as carbide or nitride, and thus the corrosion resistance suffers.

Vacuum is a superior atmosphere due to the absence of reactive species. Otherwise hydrogen or hydrogen blends are widely used to keep stainless steels bright during heat treatment. Quick cooling isn't essential, but helpful by way of preventing precipitation of carbide from the traces of carbon allowed in the standard composition. For stainless steels stabilized with Ti, Ta, Nb, etc., this is less critical because these elements bind the carbon and prevent it from precipitating chromium.