welding of SS 316

[Hang Suin]

It happens that I need to weld piping in material grade UNS S31600, size 4inc schedule standard and I have been consulted that the back side of the weld needs argon shielding just as much as the front. That I would like to know is why is such a backing shield considered necessary?

[James Scrufy]

If the back side is not shielded with argon, it will be oxidized since a stainless steel oxidizes very quickly at elevated temperatures (much faster than carbon steels). it has to do with the fact that chromium oxide has a melting point much higher than the metal, unlike plain iron oxide which melts a little before the iron. So, iron oxide flows and floats out of the puddle, but chrome oxide stays put and gets in the way of the metal flowing properly.

The phenomenon is called "sugaring" and it is used to describe the appearance of the backside of a stainless steel weld surface that has been exposed to oxygen. In this case, chromium at or near the surface combines with the oxygen to form a thick, porous oxide layer, depleting the chromium content. If this depletion of chromium is great enough the corrosion protection is compromised because the resultant oxide layer is porous exposing the chromium-depleted areas to corrosive materials, leading to premature failure. At the same time, oxide debris provides crevices that also act as corrosion sites.