It is well-established that arc welds on joints between dissimilar sulfur content stainless steels will be significantly displaced to the low sulfur side of the joint.  The tendency for this is probably largely driven by Marangoni flow but it also appears possible that the arc is displaced towards the low sulfur side of the joint.  This work uses optical spectroscopy to explore the possible shift in arc position during welding of dissimilar sulfur materials.

 Experimental

GTA welds were made on a joint between Type 304 stainless steel (S = 0.003 wt%) and Type 303 stainless steel (S = 0.293 wt%).  A spectrometer was used to measure the intensity of light emitted by the arc as a function of radial and vertical position.  The spectrometer was oriented such that it collected light from a small region looking parallel to the weld travel direction.  The spectrometer collected data from 350 – 700 nm, which allowed for line intensity measurements of several argon lines (both neutral and ionized species, ArI and ArII) as well as emission from some constituents of the steel (primarily neutral manganese, MnI).  The data were analyzed to determine arc symmetry relative to the welding electrode.

Results

As expected the welds shifted significantly to the low sulfur side of the joint.  However, the spectroscopic data showed that the arc is symmetric with respect to the welding electrode (in the direction normal to the weld travel direction) with no tendency to shift towards the low sulfur side of the weld joint.  The apparent shift of the arc, seen visually, is shown to be primarily emission by manganese atoms that are evaporating from the steel.  The MnI emission does follow the shape of the weld pool and accounts for the apparent arc shift that is observed.