FM 82H to Carbon Steel ID bore cladding issues

Date 08-30-2011 17:16

We have had a particular welding application that has given us fits over the years. It involves automatic ID bore cladding FM 82H to a carbon steel pipe that's approx 20" long (flat position). The weld is a continuous spiral clad (.056" stepover) and arc time is approximately 8 hrs. The problem is the requirements we are held to is .015" max porosity and .005" max tungsten inclusions. We have had success in the past with different lots (with different heat of bm and fm), but our current lot we are running is not going too well. About a year ago we did a project where we discovered a heat of wire that was triple melted and ran much cleaner than the wire we were using at the time (not triple melted), and we achieved close to 100% acceptance. We tried our best to duplicate the chemistry and processing of the triple melted wire, and are currently using the wire that we developed from this project to clad the current lot. We have found that while this wire runs very clean compared to the wire we used to use that was not triple melted; we are now getting porosity and tungsten inclusions. The indications are hit or miss with no real pattern in the part. We are currently at about 50% acceptance for the lot. The part receives a UT Cscan immediately after cladding. When using the old wire (not triple melted), parts were mostly being rejected after c-scan for lack of fusion at the bondline. The ones that made it through were rejected for porosity and inclusions. The recent lot of parts we ran all made it through cscan but are failing xray due to the porosity and tungsten inclusions. We did have two make two changes to the welding parameters to get the new heat of wire we are currently using to run better and eliminate the lack of fusion that we were initially seeing. We decreased the torch angle by 1 degree and lowered the voltage by a half a volt. We are using 50CFH 99.99% purity Argon. The flow rate seems high but we have welded well with it before. I thought I would throw this on here to see if anyone may have some experience with this sort of situation and be able to offer any advice. Thanks.

Brett

By Lawrence (*****)

Date 08-30-2011 19:04 Edited 08-30-2011 19:08

Hey Brett,

I don't see how tungsten inclusions and Heats/Lots/Melts of filler wire (GTAW I assume) have any relationship to tungsten inclusions... Tungsten inclusions come when the tungsten touches the work, or when it is overheated and "spits" into the work.

Now if your tungsten standoff (distance from electrode tip to work) is governed by arc voltage adaptive feedback systems (Z Axis) ; A change in filler, a change in tungsten electrode alloy, brand or a change in included angle of tip prep, this could cause a change in arc voltage which would in turn change the stand off distance of the electrode, possibly causing contact with the work, If your voltage settings that control z-axis distance are pre programmed.

50 CFH is not uncommon in oversized gas lenses... If that's what your using.

Porosity (and it sound like your indications are very small) might be related to oxides, which may be delivered via your filler. This may be why the triple melt was superior. Those oxides may be on the toes or the parts of your clad weld that are overlapping. What is being called poroisity may be a little lack of fusion in areas where those oxides diddn't melt?????? (just a possibility)

Improving filler quality or (maybe cheaper) an improved argon coverage system, like a custom trailing shield, might improve those "porosity" issues.

Just some thoughts

By bjbercaw (*)

Date 08-31-2011 13:08

Hey Lawrence,
Thanks a lot for your reply. I agree with you that heat of material should not have a direct relationship to tungsten inclusions. I was thinking more porosity. Here is a little more food for thought. The process is AGTAW with AVC. We are using an oversize cup so 50 cfh shouldn’t be too high. The tungsten is ground to a 40 degree taper with approx .050” blunt tip. I will try to attach a pic of the tungsten used on a part that showed a .020” tungsten inclusion at RT. In my opinion, the tungsten looks great. There is erosion at the side of the tungsten, but nothing that appears out of the ordinary. The tip looks fine measuring approx .050” as it did initially. The inclusions have us baffled for sure, so any further suggestions would definitely help because currently we are at a loss. Has anyone had any issues with RT mixing up tungsten inclusions for anything else such as carbide by chance?
We dug up a bunch of info from the previous successful lot we ran (100%) and found we machined the parts ourselves out of solid stock, where all other lots have come in as pipe. We don’t really know if there’s any significance to that yet but there is a night and day difference between that run and our current run. Those parts were also a totally different heat of base metal. Other differences between that lot and our current lot include:
- The wire used on both the previous successful lot and the current unsuccessful lot were both put through the VIM-VAR process. They were different heats however.
- good lot ran at 12V, bad lot ran at 11.5V (made adjustment to help even out toes)
- decreased torch tilt angle by 1 degree (made adjustment to help even out toes)

We went over our gas lines, which are all stainless. They all look good. Has anyone ran into any interesting gas issues? I am going to look into the last time our tank was filled but I know we buy the highest purity argon available and we haven’t had any other porosity issues. We have had the analyzer hooked up and everything looks good. We are purging the entire tube as well. If you guys think of anything else, we would definitely appreciate it. Thanks again for your suggestions.
Brett

[img][/img]

Attachment: tungstenxrayreject.pdf (535k)

By Hoo

Date 08-31-2011 14:13

Just a few thoughts...

You mentioned you're using AGTAW; what is the active agent you are using, and could the porosity be originating here?

Have you considered going to a cryogenically treated tungsten? I have seen them used in cases where inclusions had been problematic with good success.

I would also think of looking at a trailing gas shield, and possibly reducing the gas flow on the primary shield to 40CFH. Is there a specific area in the pipe where these defects seem to occur more often, or is it random?

By defaced (**)

Date 08-31-2011 15:16

"We dug up a bunch of info from the previous successful lot we ran (100%) and found we machined the parts ourselves out of solid stock, where all other lots have come in as pipe. "

We have noticed massive differences in weld quality between pipe and forged (solid billet) materials (we pretty much only work with mechanize TIG). From wetting to oxide and silicate formation, everything is vastly different, and much cleaner with the forged materials. While our application is very different from yours, those observations may assist you.

That tungsten looks fine to me. How much arc time on it? I have heard of shops weighing the tungsten before and after use as evidence that no tungsten ended up in the weld.

Have you looked at the RT films yourself?

We have run into many interesting gas issues. All the argon we use if off of HP180 liquid cylinders which typically are around 4ppm O2 right off the cylinder.

By bjbercaw (*)

Date 08-31-2011 16:47

I appreciate your reply. Arc time is approximately 8 hrs. Its funny you brought up weighing the tungsten before and after because me and another welding engineer just talked about that 2 minutes ago. I have looked at the films and while it looks like tungsten in the films (.020"), judging by the condition the tip is in, it just doesn't seem possible. The blunt tip still measures .050. We have one part that is irrepairable which we are going to try SEM to see if we can pinpoint the chemistry of it.

Did you determine why the forged material ran better than the pipe? Do you think it had to do with chemistry?

Do you care to share a few of the gas issues youve faced and how you fixed them?

Thanks,
Brett

By js55

Date 08-31-2011 15:30

Seems reasonable to me that if you can't visually see any material loss (I don't know what you mean by erosion) in the tungsten then you won't be able to see it an a radiograph either. They are both visually referenced evaluations. I'm guessing your dealing with something else.
Also, if your buying quality wire the porosity would most likely be more related to the base metal not the wire.

By bjbercaw (*)

Date 08-31-2011 16:52

Sorry, I mean typical tungsten wear. After an 8 hr arc time we consitantly see slight wear on the edges of the tungsten as shown in the pics. Good point about the wire. We are pretty confident we have the best wire we can buy.

"Defaced" brought up an interesting observation about forgings welding better than pipe. Has anyone else ever experienced this? We are going to try some testing on the two types of base metal.

Thanks again,
Brett

By Lawrence (*****)

Date 08-31-2011 20:25

That is some tough acceptance criteria.

I can't say more than I already have.

But I know somebody who can! :)

My advice 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.

Gregg, is their chief engineer and has an amazing career of experience with Automated GTAW with AVC controls in the aerospace industry.. If you contact them please tell them Larry from United directed you there way.
http://www.pwt-online.com/

By bjbercaw (*)

Date 09-01-2011 12:55

Will do, thanks a lot Larry.

By Lawrence (*****)

Date 09-01-2011 23:01

Let us know what you discover!