Chuck will be the best to speak on the gas.. (I don't have my Avesta Manual handy) But I would guess a small amount of active gas would be preferred. 2% O2 is most common.
Your Volt/amp relationship also seems a little strange to me.
25 volts is in the spray transfer range, but the 60 inches per minute is not.
Is your Mig gun making a big droplet that the operator simply guides to the hole and the weld is done?
Are you using a spot timer or something similar or is the operator triggering?
Unless your doing a million of these things GTAW sounds like something to consider.
I agree with Lar (Lawrence),
I would recommend a 98%Ar/2%O2 gas mixture for this application. The straight argon you are using will not give you the arc stability that you need. I also agree that your parameters are a bit suspicious. To be honest, a pulse MIG would be much more to your liking if you have those capabilities, due to the 625. The solidification cracking is probably due to the lack of ferrite in the diluted weld metal. If you do not have pulse capabilities, you need to get into a full spray mode (your IPM seems too low). I do not think switching to a tri-mix is the answer. The nickel is a pure austenitic steel that is prone to solifidication cracking if heat input and interpass temperatures are not monitored correctly. Also, dilution with the base metal should be kept to a minimum. I agree that MIG might not be the most desired process. It appears that you are having cooling down rate differences that are causing the cracks. If the 625 is cooling down faster than your plug welds, it is likely you will experience some cracking at times.
Keep in mind the high Cb of 625 is not going to help the crack tendencies of the 347 either, which is highly crack sensitive on its own without help. The Cb in 347 is designed for a 1% max, in part to help minimize cracking tendencies. It will certainly be more than that with dilution from the 625.
I'm just starting to dig into this, but from what I know, we are using a Miller XR Control power supply rated at 450 amps. Burn back is 0, run-in speed is 30 and spot time is 1.5 seconds. I agree that the pulse would be a better idea. I don't think this Miller is capable of pulse, but we do have an old Linde power supply that is capable of that. I wonder if there might be another suitable filler that would help decrease the cracking susceptibility. Thanks.
Due to the fairly small thickness of these materials, will using spray transfer be feasible? Might the arc forces be too high with this mode of transfer and cut through the sheets instead?
The thought of using GTAW was brought up but due to the position of these plug welds and how many there are, GMAW seemed to be quicker.
Describe your soldification cracking more precisely.
I am assuming that 347 is being chosen because of its high temp properties. Is this so.
309 is not without hight temp properties of its own, and would not demonstrate near the crack tendency of 347. If 309 is inadequate what about 625 filler(NiCrMo-3).
Bottom line, recommendations will tend to be related to minimizing or eliminating cracking, and not pertaining to your particular service.
Again, I can almost guarantee you that your cracking is due to the difference in cooling rates. It is possible that the spray mode is not suitable if these thicknesses are that thin. "GMAW seemed to be quicker". Quicker isn't always the best. Solidification cracking is usually described as hot cracking or center line cracking, and that is almost always due to the difference in cooling rates of the two metals.
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