Greetings. I just finished a job at a local chemical plant. The carbon steel tank we were repairing had become highly magnetic from the procedures they use to extract chemicals from salt brine. It was so magnetic that my chipping hammer could stick to the side of it and hang there. All grinding dust stuck to the side also. I had extreme difficulty starting the arc and wandering arc/arc blow. I managed to get a good looking weld on it that passed inspection but with much difficluty. I've never welded on steel this magnetic in my life. Any suggestions? We may be doing more work there and I'd like to have an answer on how to solve this problem. Welding rod was E6010 root with E7018 fill and cap, 3/32" rod diameter was used to repair small fittings and attachments. Thanks.
By JDAC
Date 10-02-2008 05:42
The problem is known as Arc Blow, it is well documented. Have experienced similar problems, although not to the extent you describe, on long pipe lengths that have been stored for long periods in the north-south whereby the pipe itself becomes a magnetised by the earth's magnetic field. Long term storage of pipes should be east-west to prevent this.
The simplest solution is to randomly disorientate the magnetic dipoles again by exposing the weld zone and surrounding area to an AC magnetic field. The ideal tool for this is an AC Yolk, as used in magnetic particle testing.
Other methods include vibration or hitting the area with a hammer, which is not advisable in most cases!
Hello awspartb, where I live Reynolds Metals use to have a smelter, pot lines, etc. for the production of aluminum alloys. These pot lines were "highly" magnetic, strike an arc in one spot and watch an arc that looked like a lightning strike hit somewhere else. They had their best results from welding with AC current and E7014 or possibly E7018 AC electrodes. Are you nailed down to using a specific WPS? Could you possibly speak with them about using AC current? If so you might have an easier time by going this route. As the other gentleman pointed out, possible using an AC yolk in the area that you are welding might be the answer to neutralizing or reducing it's effect. I believe you also received a suggestion to check out other threads concerning "arc blow", I don't recall how long you have been associated with the forum, but there are a number of threads that discuss possible methods for reducing the effects of arc blow and different approaches to help deal with it. Try using the search function, there's plenty of reading on this topic. Best regards, Allan
The AC yolk sounds interesting. I'm familiar with arc blow and how to correct the problem but this was above and beyond being cured by those techniques. The chipping hammer actually STUCK to the side of the vessel and stayed there. Thanks for the suggestion about the AC rod also. I'll look into it the next time we work there.
I would assess the metal thickness and geometric configuration against a possible head shot. If the roof vs the floor has potential for tie in points for the cables, and you can identify the poles of the magnetism, it may be possible to head shot it in alternating fields and demag the whole thing. Do remember this though, your going to have to have some meat on the tank for this to work.
What size are the diameter, height and wall thicknesses of the tank?
Giovanni S. Crisi
Sao Paulo - Brazil
Tank is around 10 feet diameter X 10 feet high with rounded edge and flat bottom. Top is open with a 1" thick manway door and numerous fittings and penetrations. Wall thickness of tank was 1/2" carbon steel but was corroded to around 3/8" where we were working on it. The magnetism got much stronger towards the bottom of the tank. It held HOT salt brine.
This is a very special case that goes well beyond the magnetism that is sometimes found when welding with DC eletrodes and that is corrected by switching to AC or changing the position of the earthing clamp.
In this case, the hammer actually sticks to the tank wall, which demonstrates that the magnetic field is awful strong (not to say damn strong, because the AWS Forum doesn't allow swearing). Being a special case, in my opinion a special solution should be envisioned to get rid of the magnetic field.
The tank is a small one, so small that it doesn't fall under API 650 but under API 12D (Small welded pre-fabricated tanks). The wall thickness is low (3/8 in). I would suggest to weld it with oxyacetylene. Magnetism has no influence on an oxyacetylene flame.
Does someone have a better idea?
Giovanni S. Crisi
Sao Paulo - Brazil
forgive me if I'm wrong but if you heat the localized area above it's curie temp (the point at which it is no longer magnetic) and then allowed it to cool, would it loose the residual magnetism?
Interesting idea. The rest of the part still being magnetized probably would re-magnetize the heated area when it cools. Ever try to demagnetize (1) end of a magnet?
well magnetic force is also a function of distance I imagine if you demagged on the order of 10"+ in every direction the force would be very small at the weld zone. Then again this would probablycause serious metallurgical issues, the curie temp is quite high
Wrap the welding leads around the tank. Five to ten wraps should work. Hook the leads to an AC welder set on the highest amperage it will produce. I usually use the stinger and the largest rod I can get to make and break the arc on a piece of scrap. You only have to activate the arc for a couple of seconds. Break the arc and turn the machine down 10 to 20 amps and repeat the process until you reach the lowest amperage setting. Now continue, but instead of reducing the amperage, you're already at the lower setting, remove the wraps of welding lead one wrap at a time until all the welding lead is removed.
Repeat the entire process, except the welding leads should be wrapped around the tank 90 degrees to the direction of the wraps in first step described above.
This is known as degaussing. When you complete the process, a paper clip will not stick to the tank and you will not have any arc blow or residual magnetic field remaining in the tank. I've used this process to degauss 12 ton girders using a 300 amp AC welding machine and it works very nicely and the beauty is that it only takes about a half hour to complete.
The sketch attached was presented a few months back with a similar problem. We never did hear if the person that asked the question told us if he tried it. If you implement our suggestion(s), it would be great to hear whether it accomplished what you wanted it to do or if it was a total failure. I bet everyone here would like to know if it worked as well.
By the way, the strength of the induced magnetic field has to be greater than the strength of the residual field, so if 10 wraps of the welding lead is insufficient to do the trick, you can increase the number of "turns" of the welding lead around the vessel.
As for the AC yoke, it will work on small parts or localized areas, but I doubt it will be successful on a large vessel.
As for my method, it works for the Navy and they do entire ships with larger coils and higher amperage. So, as you can see, I am only borrowing an idea from someone else. No original thoughts on my part, just a different application for a tried and true method used by someone else.
Best regards - Al
It does work, I have seen it. I was ging to wait till monday at work to explain it so I didn't leave anything out(it is also covered in some MT training books). Looks like Al got it covered well.
Thanks Carl.
Best regards - Al
Al!
Please allow just two words*:
"Most excellent!!!"
Best regards,
Stephan
*Rather an exception with me!;-)
I run into this on ocation working with used oil field pipe. The only thing I have found so far that helps is change polarity.
Tom
AC changes the polarity 60 times per second. No need to switch the welding leads.
Point of interest; The setup I described should be fine for tanks, pipe, and other thin walled structures. However, if you are demaging something thick and heavy, switch to a DC machine. Everything stays the same, except the power supply and the fact that everytime you reduce the amperage by 20 amps, switch the leads to change the polarity. The process is slower, but DC "penetrates" thick parts better than AC.
One more point, break the circuit by breaking the arc with the welding rod. If you are using a engine drive don't switch amperage ranges with the range switch unless you have a burning desire to replace the range selector when it arcs under load!
Thanks for the compliment Stephan.
Best regards from Vagas Baby!
Al
Yep I had heard of wrapping the leads around and even tryed it with DC machine but it didnt work, They had left the part out about using AC.
My problem there is I run DC portable welder like most others I have met. I will have to give it a try like you describe for DC.
Without demaging DCEN the weld blows into the work DCEP goes everywhere .
Tom
We tried all the standard arc blow control techniques . The AC version is something new to me. MANY THANKS!!
Once again, DC will work just fine if you give it a shot of high current using DCEP, then reduce the current by 20 amps and give it a shot with DCEN, reduce amperage another 20 amps and give it a shot of DCEN. Repeat this, dropping the amperage 20 amps each time you switch polarity. Like so:
DCEN - 250 amps
DCEP - 230 amps
DCEN - 210 amps
DCEP - 190 amps
.......
.......
DCEN - 50 amps
DCEP - 30 amps
DCEN - 10 amps
Then when you get down to 10 amps start removing one loop of welding lead at a time from the work piece. Every time you remove one loop, switch polarity until there are no more loops around the work piece.
You have to do this procedure by wrapping the leads around the tank length wise first to remove any circular remanent magnetic field. Then repeat the process wrapping the leads around the diameter of the tank (or work piece) to remove any longitudinal field. Using DC is fine, it just takes longer because you have to reverse the polarity by switch the leads manually. Start with up to 10 loops of cable, but no less than 5 (minimum).
The piece is magnetized immediately, so there is no reason to leave the circuit energised for more than a couple of seconds.
You have to start with high current to over come the residual magnetic field in the tank (or work piece). That is, the field you induce has to be greater than any residual field in the part from previous sources. Each time you kill the power the residual field in the piece is less than it was when the current was flowing. Each reduction in current produces a field that is stronger than the remnant field, but less than the strength of the previous field when the current was flowing at a higher level.
As I said previously, AC should work fine for thin walled components (1/2 inch or so). Thicker parts, over one inch should probably be done using the step down DC technique.
I probably shouldn't be giving away all my trade secrets. I've made good money demagnetizing large girders and such for companies.
Good luck and let us know what you find works for you.
Best regards - Al
How to keep it from becoming remagnetized, that a lesson for another day. ;)
Al
I actually just heard of this technique last week in a NDT class, but I've never tried it. That's basically how it's written in the mag particle books for de-maging parts.
good information!
Our pleasure. That's what we're here for, and to throw a couple of friendly jabs at each other occasionally.
Best regards - Al
Use an ac buzz box welder as a degaussing machine. With a portable that has a 230/240 vac plug the buzz box into it. Set for 100 amps and dead short the work and electrode leads to the tank and then turn on. Then weld with the generator weld as normal.
