arc blow

By Blaster (***)

Date 02-03-2010 02:03 Edited 02-03-2010 02:14

First I would turn the amps up. I know this will create more magnetic force, but at 140 on a 5/32" electrode you have little arc density. The soft fluffy arc at such low current is easily bent by magnetic fields. Plus the arc will extinguish easily if you hold the short arc length you need to in order to counteract arc blow.

With a 5/32 7018 I would START with about 160 amps. Burry the tip of the rod into either bevel face to maintain control of the puddle. Angle the rod into either face when you are there. Scrub the tip against the bevel faces and backing bar as you move side to side. Use any fitup tolerance on the root opening to your advantage. If you are allowed a little over 1/4", use every bit of the tolerance.

Second, if you can't move the ground (my first choice), change direction of travel. If you are working on an overhead plate, weld from the run off tabs towards the center from either end.

If the plate is vertical, you obviously don't want to go down. Hopefully the ground is coming in from the bottom as that is where you want it. Vertical I assume you are getting back or down blow when nearing the top of the joint. You can weld a large chunk of plate to the run off tabs / end of the backing bar at the top of the joint to effectively extend the joint. Putting some steel mass beyond the run off tabs will give the magnetic field an area it can move into rather than bunching up at the end of your weld joint. Fit your run off tabs well, and if it is allowed, thoroughly weld them to the backing bar and to the edges of the plate (all sides).

Make sure your leads (both of them) are isolated from your work, the table, the pole, or whatever else may be used to hold your plate in position, to the extent possible.

These things should solve the problem if not completely, at least well enough to put in code quality fill metal.

By octavio morales (*)

Date 02-03-2010 04:41

thanks i'll try those techniques tomorrow. i actually have the arc blow on my horizontal and overhead. so what do you think about pre-heating the plate? the only reason i think that is because it only happens the first couple of passes. once the plate is hot it seems to run just fine. or after those beads, it just changes the magnetics of the plate. idk
thanks

By 803056

Date 02-03-2010 12:57

You did not mention how the joint was prepared, but it is a good guess it was ground to bright metal by grinding. Grinding can induce a localized magnetic field that can ruin a good day.

I suggest demagnetizing the base metal. The residual field sounds as if it is relatively weak so a couple of sharp raps with a 2-pound hammer might be sufficient to demagnetize the work piece. If that does not work, you can possibly demagnetize it using the arc-welding machine. You are most likely using a DC machine so you will have to use a "step down" technique. This has been addressed in previous threads. A quick search on "Demagnetizing" should result in several threads on the subject. You can try this one: http://www.aws.org/cgi-bin/mwf/topic_show.pl?pid=121249;hl=arc%20blow

Steel retains its magnetic properties up to the Curie temperature. That is on the order of the lower temperature of transformation, i.e., when the steel starts to see "red heat.” Those temperatures are well above the normal preheat temperatures recommended for most steels.

If you are working with relative thin materials and you have a magnetic yoke available, you can use the AC yoke typically used for magnetic particle testing to demagnetize the work piece. If you have an AC magnetic yoke simply place it on the area you wish to demagnetize and slowly move it away from the part while turning it (the yoke) side to side with a twist of the wrist as you draw it further from the work piece. Standard practice is to keep the yoke energized until you are about 3 feet from the part being demagnetized. Repeat the process several times moving from one position to the next position approximately 6 inches from the previous position until you have done the entire joint. Again this will work if the part is small or thin or if the field is weak (which seems to fit your situation).

Best regards - Al

By octavio morales (*)

Date 02-03-2010 20:32

thanks al
yes, im grinding to bright metal- DC machine
i'll try searching some threads for demagnetizing
thanks agian

By whiteyford M1A1 (**)

Date 02-12-2010 23:22

Hi
Lots of good advise on here!
If your welding test plates grind/tack them 1-2 days before you weld them. Works on weldments too. I might add trying a backstep technique, welding toward and into your previous start.
Just 2pennys worth:)
Randy

By makeithot (***)

Date 02-04-2010 02:00

thats a good one Al, thanks for that.

By makeithot (***)

Date 02-04-2010 01:55

One of the simpilist ways I know is to run a couple passes with a smaller electrod & back step the weld if possible. After a couple of passes you should be able to go back to the larger rod for the cap and all is well..

By ironsport (*)

Date 02-06-2010 03:28 Edited 02-06-2010 03:30

preheat can work real well like right up to the interpass temp ac would help also

By sparkin (*)

Date 02-10-2010 20:54

Not sure if this will help but I build a lot of fence out of used and usually very magnetic oil field pipe.
I run into arc blow from magnetic pipe all the time.
I have tried every trick in the book and the one thing that works every time is grounding to the earth.

For some reason top rail on a pipe fence usually creates the most problems. I have found that if I put my ground clamp on a post that is concreted in the ground the arc blow disappears immediately.
It doesn't matter if I weld toward the ground or away from it, it doesn't matter what procedure, rod or amps I'm running; it works every time.
It may need to be noted that my posts are at a minimum of 4ft in the ground.

I have read a lot of threads on here of guys fighting arc blow and wonder if you could drive a rod or bar into the ground (earth) and pig tail it to your ground clamp or just to the piece of work with its own cable and get the same result?
I may be totally off or just real lucky but it seams that the earth's natural magnetic field balances out or redirects the disrupting field in the work.
Hope this helps, if not, can anyone explain to me why this is working for me?

By NMWELDING (**)

Date 02-11-2010 02:35

I once had to weld 2 lengths of pipe together in an oil well being drilled. It didn`t have to be a code weld or anything, just weld it together to get the bottom joints out of the hole. I didn`t have AC on my welder just DC but I still tried to eliminate some serious arc blow by wrapping my ground leads around the pipe{tried it both ways} but that didn`t help. I just ended up goobering it real bad,I had a horrible time trying to get some weld on there,and was very embarrased at what little I accomplished. But all I had to do was stick it together to get the remaining joints out of the hole.The weld {if you want to call it a weld}held but I`ll never know how. Any thoughts on how arc blow in a situation like this can be minimized?

By metalmelter Date 02-12-2010 02:04

sounds like ur talkin bout welding magnatized pipe. if so a method i have used with success is to ground out a 1/8 5p+ in the bevel have someone max out the machine for a lil bit and then turn machine off. break rod loose fire machine back up and weld as normal

By mtlmster (**)

Date 02-17-2010 18:40

I've welded on magnitized oil pipe quite frequently, usually just building junk, but like NMWELDING said, wrap your ground around your pipe (possibly several times) 5-10, both ways. Sometimes it works, and sometimes it doesn't. I've never really understood arc blow, but I know one thing, It can make what should be an easy fix into a real pain in the butt!

Good Luck,
Steve