Nickel Chromium Iron Welding

By Saleen302 Date 05-04-2011 13:02

We have a single j-groove weld joint that once welded is about 0.285" thick. The bast material is a Ni-Cr-Fe Alloy (600) and the filler wire used is Ni-Cr-Fe based wire. This part is machine welded using 8 layers of weld. Each layer is visually inspected and penetrant inspected prior to welding the next pass. We've been having an intermittent problem with the 5th or 6th pass on these parts. Every other pass seems to be free of any defects, but when the 5th layer is inspected there seems to be some porosity. You can also see small bumps on the weld surface. Sometimes the parts come out fine, but other times we'll get a lot of parts that have this problem. During welding, all the parameters seem to stay constant - voltage, wire, current, etc. We don't see any fluctuation there.

Originally we felt it was a shielding gas problem, so the front end parts of the torch we switched out. We've taken a look at machine parameters but haven't seen anything out of the ordinary yet. The problem seems to happen at random, so we are not quite sure what direction to be looking in.

I'm wondering if it's a chemistry issue, but it's odd that it doesnt' happen on any other layer than the 5th or 6th.

Here are the parameters we are using..
3/32" tungsten (2% thoriated)
195 amps DCEN primary
160 amps DCEN bkgrd
(amps sync'd to oscillation)
9.4 volts
47ipm wire
3.19 ipm travel speed
30CFH argon shielding gas

Any ideas on which direction to go next?

Thanks

By Lawrence (*****)

Date 05-04-2011 13:51

Not enough to really go on... If you have images of the defects that would help.

Your background pulse current seems a bit on the high side, which could be adding unnecessary heat.

Off to top of my head I would say you are having surface oxide contamination issues. Your current range is pretty high not to be using a trailing shield to keep inert coverage over the cooling weld. Pics of your weld faces after they leave the gas coverage will answer that.. Need to be very clean (Silver/light straw).

Also... If your cleanup after your inter-pass inspections isn't perfect your going to run into issues. Dye penetrant and developer are not going to help your situation if there is even a spec of residue.

My best advice though is to contact PWT Precison Welding Technologies. These folks are the absolute best consultants in the world on issues like you are having. They are automated GTAW/PAW/EB gurus and also make custom trailing shields for any Machine/Auto GTAW operation. They have great experience with Inco, Haynes, Hastelloy base metal and fillers and pulse GTAW parameter design.
http://www.pwt-online.com/

By Showdog75 (*)

Date 05-05-2011 00:58

This may seem hard to believe but I've had similiar situations MANY times while making Inconel welds[347 stainless to carbon] using a Polysoude orbital machine. What we found to be our issue was using cheap Chinese gas lenses.We freaking pulled our hair out on this issue more than once. Maybe the cheap lenses had been contaminated in the manufacturing process. We switched out the bad parts for a know part[Weldcraft]. They may as well be made in china for all I know but they didn't contaminate the welds imediately our nice gold beads came back and the nasty sooty welds were gone. We even went so far as to have all the argon purity checked and the whole argon supply lines checked for leaks , etc.. Seems like something so small couldn't be such a big deal but believe me it was. Btw this was on multi pass transition welds[6 to 12 passes] and the issues usually reared there ugly head once the welds were flush then the nastyness began.Hope this helps.

By swsweld (****)

Date 05-09-2011 04:48

Is the 5th and 6th pass also the flush passes? You may need to have better argon coverage if you are almost flush. You may be getting some draft on the surface that you weren't getting before possibly from the fan on the machine. 30 CFH seems to be plenty but you might try a slight increase, even a small draft can cause big problems especially on the flush and cap passes.

Is there a cleaning issue from the penetrant testing? Probably not if it only shows up on the 5th and 6th passes but still something to look at.

Not much of a decrease between the primary and the background current. Might try lower background current. The flush and cap usually don't need as much current with no real walls involved. Too much current makes it weld "squirrely" or "cooked" that might lead to the bumps that you mentioned but not necessarily the porosity.

With 195/160 it is almost in straight current and the puddle has very little cooling action.

I don't know your actual travel speed at the tungsten but you might be traveling too fast or oscillating too wide/fast, out running or out weaving your coverage. The groove sort of catches the argon but when you are welding at the surface not so much.

Look at your cup sizes and tungsten stick out also. Largest diameter cup and not too much stick out at the surface and cap.

These are just some general things that come to mind.

Keep us posted on your progress please.

By Saleen302 Date 05-10-2011 13:47

Thanks for the input.

I think where we are stuck is the fact that the problem is very intermittent. We run a series of parts with absolutely no problems and then run another set where these defects show up. The machining, prepping, cleaning, etc - all those steps are followed to the t. In regards to the resulting weld bead, even on the ones that show defects, the bead appearance actually looks good. The resulting bead isn't black, gray, or anything that would indicate contaminated shielding gas.

To back up a little bit, when we first saw this problem, we were using a WT-27 torch with a conversion kit that allowed us to use the smaller front gas lens parts. We had been using this setup for a while with no problems. Once we started seeing these defects, we switched the front end parts back to the large diameter gas lens - the one that is about 90 bucks from weldcraft. (part 49V24). This didn't seem to improve things much. However, I just looked at the mesh screens on the gas lens and they do look a little worn. I'm not really sure if this is causing the problem or not, but I'm going to buy a replacement screen kit from arc-zone.com and put some new ones in. At least that's cheaper than spending 90 bucks for a new lens. I'd be surprised if this made a big difference because i'd assume that if the screens were worn, you'd see problems with the previous weld passes. However, I guess I can understand from the previous comments that for the initial passes, the groove holds the gas coverage pretty well, but once you start to get closer to the cover passes, if there is any restriction in the gas flow from the gas lens, it might create a problem for those passes since there's no joint to hold the gas. Is that basically the idea?

In regards to cup sizes, stick out, etc this is what we are running..
7/16" min cup size
30CFH minimum
7/16" max stickout, but use 3/8" when groove dimensions allow

By Lawrence (*****)

Date 05-10-2011 15:26

This is what you need

By Metarinka (****)

Date 05-10-2011 16:20

lawrence nailed it. The other thing I was gonna ask about was control of interpass temperature. either the weld is getting too hot and later passes are leaving the trailing gas too hot. OR the contamination is cumulative. I.E the first pass may only have 200 ppm of contamination, but after 5 passes that's now closer to 1000.

I would look into a better trailing gas solution. Also you can send it to a met lab to determine what the contamination actually is. having a lab run a contamination sample on an Scanning electron microscope is simpler than guessing, we used to have to do this to meet customer req's

By Saleen302 Date 05-10-2011 19:13

Thanks for the feedback. Our interpass temperature is limited to 250F and that's controlled fairly well.

That is a good point in regards to the cumulative contamination. After each pass the welder does address any surface contamination by blending if they see it. It certainly is more apparent as the layers build up. On the final passes, the "floaters" are visible without even using a 5X. We may try replacing the gas lens screens initially and see if that makes any difference. If that does not, we might look into a trailing shield. I've seen those shields from PWT before and they do work pretty well so we'll certainly look at it.