What is the black soot that appears on 316 SS at temperatures of ~600 °C?

               
Update: 07-01-2020
               
Abst:

First you should have to be very careful about any weld […]

First you should have to be very careful about any welding in the temperature range of 425 degree to 700 degree as in this range SS is subjected to Sensitization effect. In simple language if your SS is exposed to this temperature, it loss is corrosion resistance properties. It get rusted very fast if you keep SS in this temp.

Stainless steel becomes corrosion resistant due to formation of non-reactive chromium oxide (Cr2O3) film that adheres tightly to the surface of metal. This film acts as a barrier, and protect metal against corrosion. while heating this film got damaged and lead to rusting.

You can do is after welding you can immediately reduce the temp. by using water. (Yes in SS welding you can use water to cold SS even during welding)

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