We orbital weld solenoid valves and have been experiencing centerbead cracking along the circumference. Needless to say this is a bad thing. We pulse the current from 150 amps to a background of 5 amps at a 70% duty cycle DCEN, Ø 3/32 2% EWTH electrode, with about 10.5 Volts. The joint is a square groove butt joint that occasional has a .010 root opening. Problem seems intermittent. For those of you great people unfamiliar with this alloy it has .040 max Phosphorous, .30 Sulphur, .065 max Carbon plus the normal ferritic grade stuff. The completed weld is Flat with a little toe undercutting. Weld bead is wide at 3/16" with about .030 penetration. Any thoughts??? I am tending to favor joint profile problems, poor fit-up, contamination etc... I know the sulphur and phosphorous can cause cracks but our alloys have stayed the same.
Do the center bead cracks have a relationship to those occational 0.010 root openings?
I'm hoping you mean .030 sulphur. And its probably alot less.
My initial thoughts are, square butt autogenous welds need to be fit up closed or you will experience thinning or sinking of the weld bead that can lead to centerline cracking for those susceptible alloys.
That may very well be the case since you mentioned a little toe undercut and that you sometimes have as much a .010 gap.
3/16" also seems wide for what I am assuming is your BM thickness. What is it by the way? .030?
And it seems to me, though I've never done it, that if you aren't welding them full pen, and you have a gap at some locations of the joint, that those portions of the fit that are tight will have no give during shrinking and can initiate cracking of those areas immediately adjacent where there was a gap.
I just confirmed that the Sulphur contant has an acceptable range of .20 / .35 %. I know that is high, but like I said we have been welding these parts for years with "No ?" problems. It may very well be that our process was on the cusp of capability or that we never noticed the cracks before. Our inspection department does not look for cracks and this joint is not a pressure bearing joint so it is not pressure tested or visually inspected except for an occasional glance. The joint is a punch hole in the stamping, (possibly an area to look at, Punch surface contamination?) with taper. This is where the sloppy joint consistency comes in to play. The wall thickness is .100. There is a shoulder on the mating part that gives the joint backing as well as positioning.
You may have found your solution. The high sulphur would not necessarily cause cracking as long as for the most part certain variables remained within certain tolerances. However, the suplhur will still form low melt eutectics that are pushed along the solidification front until final solidification at the weld center. And this low melt region is just waiting for those tolerances to be violated (the cusp as you put it) in order to manifest.
As a guess, your PJP autogenous weld shrinks (as all welds do), but the unpenetrated portion underneath it will not (or will not want to), therefore the weld joint will be in tensile stress. When you combine the low melt sulphur regions with gapped weld metal thinning you may exceed the weld metals ability to prevent cracking. My guess would be that the inititation point would be where a thin weld region interfaces with an ungapped shrinkage stress region. This to me would be the point of highest combined stress and thinning.
Sounds good anyway.
JS, I agree with what you're saying. We have the same issue when laser welding high sulphur machined castings to 1018 covers. Our problem areas form along the grain boundaries where the sulphur and Phosphors form these low melting intermetalic alloys. It increases as the gaps get larger. On two of our welded products we started adding nickel based filler to help combat this condition. It took make hours of WPS development and field testing, but the end result was much better.
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