Tungsten Inclusions in FM82H Cladding

Date 02-14-2012 21:48

Hello bjbercaw, I am curious about the electrode to work distance and possibly the gas purity or associated issues that could be linked to your issues. Have you noted any coverage gas turbulence that could be associated with gas flow rates or tungsten stick-out/electrode to work combinations? If the weld pool is being agitated to the point where the electrode is experiencing bearding from the either the base metal or filler metal could that not possibly lead to power increase requirements that might cause for an unstable arc from the electrode that might promote tungsten spitting or instability?
Everything being perfect, if the base material or the filler material has properties that don't play well together with one another or with the GTAW process you might be needing to consider some changes to your procedure. Possibly you might have to incorporate electrode changes and renewals more frequently to help with electrode instability over a set time or the bearding effect after a particular period of time.
Everything that I have stated here has come from observations that I have noted from other types of welding associated with GTAW, I have no experience with the particular process application that you have inquired about so please don't beat me up too hard. I am here to learn too so I'll be interested to see where your thread ends up. Best regards, Allan

By bjbercaw (*)

Date 02-21-2012 16:04 Edited 03-07-2012 15:02

Hey aevald,
Our standoff runs around .130 during welding from the original bm. We use high purity argon 99.9995%. As far as turbulence goes, its definiately possible. Do you know why bearding occurs? Its interesting that we don't see it on every electrode, its sporadic. From what I have been able to dig up on it, it seems like its due to mostly elements from dirty bm or fm precipating up onto the electrode. I attached a picture of the bearding builing up on the electrode during welding. We have seen this formation drop into the weld puddle yet passed with flying colors through x-ray. Heres a little more background on the process:

- carbon steel pipe id bore cladding of 82h fm using GTAW
- 250A, 11.5V
- Xray requirement = .005 max tungsten inclusion
- cold wire
- 2% thoriated electrodes

We have recently switched taper angles on the electrode. We have been seeing heavier erosion on the side of the taper opposite the wire. the tungsten inclusions we have been getting range from .01 to .03. Its crazy though, when looking at some of the electrodes post weld that failed due to inclusions, its hard to believe a piece that large is missing.

[vid=youtube][/vid]

By MMyers

Date 02-21-2012 16:22

Touch start or high freq? I've seen tungsten get stuck and need to be broken free on touch starts. Sometimes they leave something behind, which if you're not paying attention, will end up in the weld.

We run alot of TIG and find that bearding (we call it fuzzing) is mainly based on shielding quality at the torch. Even if you're starting at 99.9995% gas at the bottle, that doesn't say anything about what's up stream or happening at the torch. To say the least, we go through alot of leak check and heat tape to ensure good shielding. If the weld isn't silver, it's not shielded properly (barring metallurgical effects like Ti and Al concentrations in Ni based wires). Though even with good shielding, there is an upper limit to tungsten life, it won't last forever and it depends on many different things including tungsten quality, cooling, shielding quality, and welding parameters.

I asked about hot or cold wire because start up of hot wire can be explosive and cause premature tungsten erosion, but since you're running cold wire, that's a non issue. My only other comment is that the voltage looks a little high, I'd expect it to be about 1.5 to 2.5 volts lower for mechanized cold wire TIG with 100% Ar shielding gas. I obviously don't know all the details of what you're doing, but if it works for you, then it works.

By aevald

Date 02-21-2012 20:17

Hello bjbercaw, it's interesting that you mentioned that you saw "bearding" on the opposite side of the electrode from the wire addition side, wonder if that would indicate a base metal issue with regard to your electrode bearding, could this be associated with whether wire is fed on the front or the trailing end of the weld pool? Meaning that the base metal is possibly shedding or emmitting material during the arc application and this is precipitating onto the tungsten.
Noticed that you also mentioned the use of thoriated tungsten... diameter? and would a change to another type of tungsten possibly be a more stable choice and possibly curtail the "spitting" that sometimes occurs with thoriated tungsten if it is used at an elevated current level for a given diameter? I have heard of hi-speed videography being able to capture quite a few different things relative to an arc. Any possibility that this could be introduced to your particular challenge to help you out with nailing it down?
I do have another remote thought to put into the mix, any possibility of "arc blow"? Since this is somewhat sporadic, is there any consideration for this? When arc blow comes into play with most processes there is generally a destabilization of the arc column, which to my way of thinking could contribute to tungsten transfer with higher current levels. Thinking out loud again and throwing things out for consideration or to spark comments from others that are likely much more knowledgable than I.
From the type of inclusion that you have discussed I would not think that shielding gas is associated with your problem. Look forward to more discussion from yourself and others. Good luck and best regards, Allan

By MMyers

Date 02-15-2012 15:54

Hot wire or cold wire?

By bjbercaw (*)

Date 02-21-2012 19:57

Hey Mike,
Good stuff. We are running high frequency start. We have had issues lately with excessive oxides in the weld (ill attach a few pictures). At first we thought it was from the base metal, but we are currently welding on an old heat of bm that ran really clean in a previous lot (100% yield), and we are still seeing the oxides. So now we are leaning more towards wire because we also switched wire heats from the previous successful lot. The large oxides sometimes stick at the toes of the weld, and when welded over top of, it creates lack of fusion which gets rejected at C-scan. Im not sure if these oxides could contribute to causing inclusions, but the arc can act pretty wild and flutter pretty bad when it hits these big clumps. Adjusting the voltage from 12 to 11.5 helps to agitate the puddle differently and spin the oxides off the toes better and move them to the crown of the weld where they can get machined off. This is the only way we have been able to pass C-scan. The main problem still seems to be the tungsten inclusions being rejected at X-ray. We have done quite a bit of testing of our electrodes the past 2 months including metollgraphic (for oxide dispersement), chemistry (for thorium content), and weld tests, but no matter what we can’t eliminate the inclusions 100%.
I may have forgotten to mention that this is an 8 hour continuous spiral ID bore cladding where if it stops at any point its scrap (cannot restart). We currently are trying a 20 degree taper with a .050" blunt tip on the electrode (previosly 40 degree, .050 blunt tip), in attempt to change the location of where the arc is located with respect to the tip. If you look at the pics on my first post of this thread, you will see a spiral looking line on the taper of the electrode. We noticed when this line intersects the tip of the electrode where the arc column is emitting, we get heavy erosion on the side opposite the wire. By changing the taper we have been able to move this line further up the taper away from the tip and hopefully help to minimize the erosion.

The linear locations of the inclusions are pretty sporadic across the 20” pipe. Some have been towards the start, mid, and end of the weld. We are also purging the entire ID of the pipe and running around 45 cfh at the torch w/ an oversized cup (1.25"). The gas itself is very clean, we have less than 1ppm oxygen at the torch and purge. We run all TIG welded stainless hardlines from a bulk argon tank. It feels like a contamination issue but when you are rejecting an 8 hr weld for a single .005" tungsten inclusion, it could be a fly farting on the way by for all we know. Its strange though we will weld 2 or 3 good ones in a row and then the next three will have inclusions. In 2010 we went 25/25 good ones in a row, granted that was with different bm and fm. At the time we were using VIM/VAR (triple melt) filler material, which ran very well. After we ran out of that heat, we had it duplicated the best we could and that's what we are currently using. Ever since we started using it though we have seen these inclusions and some porosity. We think we have the best wire money can buy, but maybe there is something in it that is just not wanting to cooperate. We have also tested various heats of thoriated electrodes and all seem to be giving us similar issues. Lanthanated electrodes is another option, but we have yet to investigate it much.

Since you brought up the shielding issue, I want to run another issue by you to see what you think. We have three AMI stations and one AMET station that we use to weld a specific seal attachment weld. We have recently discovered that we are getting porosity in this seal attachment weld when we use AMI 2,3 or the AMET station, but not when we use AMI 1. The porosity is 100% all the way around the part. The porosity doesn’t start until the 5th layer each time as well which is important to note. Our gas system is a continuous closed loop system made of stainless hard lines. We bring flexible stainless from the main loop off of each station. The AMI 2, 3, and AMET station are all along the same wall. AMI 1, however is across the room on another wall. We have done quite a bit of testing trying to figure it out. Praxair even came in with their ultrasonic leak checker. We have also tried using argon bottles to take our gas system out of the equation. Oxygen and water content is less than 1 ppm on all stations. Likely unrelated but you never know. We know its not a material issue because we have used the same bm and fm on AMI 1 and there was no porosity whatsoever. We are testing flow rates right now. We haven’t had much luck pinpointing the issue, but whats interesting is the issue arose around the same time as the cladding. I thought you might have some input. Here are some other links to previous threads on this topic if you are interested. Thanks again.

http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=220850;hl=\
http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=215476;hl=

Brett

By ozniek

Date 02-22-2012 12:56

Hi

Just a thought, to eliminate some variables.

1) To make sure that the base metal and preperation methods are not directly contributing, I propose you prepare your base metal like you always will before starting the overlay operation, but then doing the NDT without performing the overlay.
2) To make sure that the filler is not directly contributing, I propose you give a couple of wires to a metallurgical lab with a vacuum melting facility. (Many labs have these for making "buttons" of metal for chemical analysis using spectrometry.) Let them melt you a "button" from this filler, and also apply NDT to this button.

If you can do this to a couple of sampleas of each without finding W inclusions, then there will be something less to worry about when doing your other tests.

Regards
Niekie

By bjbercaw (*)

Date 02-22-2012 21:06

Hey Niekie,
We already x-rayed 6 pieces of bm and 6 spools of wire and did not find any tungsten. we also cut out one of the inclusions out of the cladding, took the cross section and ground back to the inclusion and evaluated it using SEM. Heres a link to the findings:

http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=220850;hl=

see first post.

We are currently trying to eliminate the large oxides we have been seeing in the puddle. We think it may be due to our base metal cleaning procedure. Will let you guys know how it turns out.