I've been told that SS welds should not get grey; IOW, the fillets should be that yellowish/bluish/iridescent color that only occurs in my weld about 50% of the time... I know that the grey color indicates too much heat in the joint (and obviously not enough practice on my part), but I'm interested in the downside of this. Does the grey color this mean the joint is weaker? Do we know how much weaker? Any rules of thumb about this? Any references or guides I should be reading?
I appreciate the response, but I wasn't asking how to get rid of the grey color, but rather what the downside is of a weld that exhibits it. IOW, is the strength of the weld compromised, and if so how?
the use of stainless... On the subject of backpurge for mainly seam (butt) welds colour is a good indication of oxygen level. They had a colour chart for underbead colour which they had calibrated to parts-per-million (ppm) of oxygen in the shielding atmosphere. I take it they had deliberately made argon with known small impurity levels of oxygen, welded in carefully controlled conditions (no additional contamination of backpurge gas) and recorded the colours they got. I do TIG stainless so was very interested in this. Note this is for backpurge, whereas you seem to be referring to bead surface. I took it bead surface is always black because the area of shield, though good where it is, is small in extent and the weld is still hot when the shroud moves on with the weld. I understand that a "cool" weld is good and that a thin black oxide on the weld bead surface is good, as compared to a thicker black oxide coating. For backpurge, "light straw" is the darkest oxide colour on stainless which is acceptable in food-grade applications. I do see this in my more ornamental applications, where I also see dark straws to these "iridescent purples" which I take it you are mentioning. Obviously metallic silver with no oxide is the ideal. The "light straw" is associated with very little loss of corrosion resistance, in controlled tests. So I hear, at "institute" meetings...
you wrote: "but I'm interested in the downside of this Does the grey color this mean the joint is weaker? " I do TIG weld stainless, and this is my experience with it.
if you could maintain the original stainless silver color it would be the best , it can easily be achieved when spot or stitch welding larger pieces so the heat is drawn away quickly.
gold purple and light blue colors are acceptable, none or little of the stainless properties lost. gray / black = metal oxidized/ some of the nickel /chromium particles are burnt up, there for corrosion resistant properties decreased.
the correct term is: carbide precipitation which occurs at 800-1400 F in the weld and the heat effected area.
once it turns black (metal is sugaring) it also becomes porous, in which case the weld strength have decreased/
so to sum it up dark gray colored bead will be the first sign of rust on your stainless weld.
I am sure Ernie will jump in and correct me, and give a more scientific answer.
Have a look at : APPROX ARC LENGTH. Gap between tungsten and weld surface
My major error was that I was holding my tungsten too far like a flame thrower. This would cause the welds to become black and it would also take longer time to heat up the weld. After I started paying attention to the gap my welds started to look more metallic with pretty raku rainbow effects. This came at the expense of spending more time re grinding the tungstens than welding but it was well worth it. I felt that I somewhat graduated last night when I welded a 10mm hard bolt to a 10mm stud that was sheared off flush with turbo inlet flange. This was accomplished without welding the stud to the flange. After some prolonged back and forth the stud came out. So mind your gap.