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Up Topic Welding Industry / Welding Fundamentals / magnetism in heavy wall chrome
- - By jimmasoncwi Date 05-14-2013 05:13
What causes magnetism in heavy wall chrome and do I get rid of it?
Parent - By Len Andersen (***) Date 05-14-2013 13:42
jimmasoncwi ( Ladies and Gentlemen ),
    Below is a good answer. Relative to not magnetic steel alloys 18 8 304 stainless steel was in USN minesweepers. I have heard of wrapping welding cord with electricity flow as a way to control such. I hope this is helpful 
                      Sincerely
Len Andersen weld@spemail.org
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Topic thermal arc blow By CWI555 Date 04-18-2009 19:37
There is a difference between thermal arc blow and magnetic arc blow. The first thing you need to is identify which is your case.
You've stated it's thermal arc blow, but then you've stated "I've tried turning down the heat and moving the ground clamp w/ no success"

That later sentence implies you suspect magnetic arc blow rather than thermal arc blow. I believe the other responses reflect that implication.

With that said, let's give a rough definition of the two most common forms in ascending order of probability.

Thermal arc blow: As you advance along the plate, the gap between the rod and the surface of the weld pool is ionized. That space becomes more conductive and facilitates transfer of the arc/heat from the rod to the plate. This condition sets up "thermal back blow" as the arc attempts to maintain itself through the ionized hot space rather than the cold space/cold plate. Some level of thermal back blow is not detrimental and effectively unavoidable. However; when you try to run to fast, or without enough heat, you're trying to transfer the arc through a zone of higher resistance with the inherent weld quality issues that evolve from that condition.

Magnetic arc blow: MAB is due to magnetic lines of force/eddys in an unbalanced condition. It is also the reason thermal arc blow (TAB) is hard to diagnose. The inherent pushing of the arc due to magnetic lines of force/flux can push the arc away from the ionization zone and into a more resistive area which by default increases the arc blow effect. Going back to what causes MAB when those lines of force get out of balance, they tend to leave every increasing levels of residual magnetism. In effect the situation just makes itself worse the longer it's left unaddressed. After some time, TAB is no longer a concern.

Fixing MAB is relatively straightforward in that you either eliminate residual magnetism, and or balance the magnetic flux conditions. You've already received some valuable comments in that regards. Defining the difference between TAB and MAB and in particular, the point in which one begins to amplify the other, that is not so easy.

Some additional information;
Most articles I've read claim MAB can only be caused by 50 gauss (as determined by gauss's law for magnetism) but the evidence I've personally witnessed and researched defies that common logic in that it does not take into account the lower levels in which < 50 gauss flux lines amplify the inherent effects of TAB without leaving > 50 gauss in residual magnetic flux lines. It also doesn't take into account the materials in question. Nickel in particular is also ferromagnetic with a low curie temperature. That low curie temperature when welding materials high in Ni, causes the magnetic domains to align and snap the moment the metal drops below the curie temp. Unlike Fe which has a high curie point, the lower curie point of Ni effectively gives more time for alignment of the magnetic domain fields.

These are just some of the points to be considered. I'll desist on theory at this point. But I do strongly suggest you identify which is your case (TAB or MAB) before proceeding.

Regards,
Gerald
Parent - By 99205 (***) Date 05-14-2013 15:22
The nickel content of the stainless your using, will cause magnetism.  Yes, nickel is magnetic.  You can check this by using a high powered magnet.
Parent - - By PhilThomas (**) Date 05-16-2013 12:29
There are a couple tricks you can use to try to abate the problem before going to the trouble of trying to completely degauss the weldment:

1.  Use an inexpensive gaussmeter to find the "null" points along the length of the joint and weld the root at those points.  Don't mark all of them, because there is a good chance that as you weld, they will move.  Just find one, weld it, then look for another.

2.  Add a piece of flat bar to the back side of the weldment along the root of the weld.  This can, in some cases, provide an alternate path for the magnetic lines of force and allot the root of the weld to be much less affected.

3.  As mentioned earlier, sometimes wrapping the weldment with the welding lead can work - if the weldment isn't overly large.

If these don't work, you can have an NDT company hook up one of the really big MT units to the weldment and degauss it.

HTH
Parent - - By 357max (***) Date 05-22-2013 04:15
Using two welding machines; Use an alternating current weld output welding machine and dead short work and electrode lead to workpiece. You can dequass while welding with a direct current welding arc.
Parent - - By Tyrone (***) Date 05-29-2013 10:29
Hey 357max,
I just read this post, hence the late question.

Can you explain in more detail how you use 2 weld machines to degauss while welding?

Tyrone
Parent - - By aevald (*****) Date 05-29-2013 14:39 Edited 05-29-2013 15:40
Hell[o] Tyrone, I believe that he is referring to using an AC machine to degauss. He is doing this by turning the machine on and adjusting it to a fairly low setting, then he is attaching the ground of that machine on one side of the weld joint and the stinger lead on the other side of the weld joint. This sort of connection should help to neutralize the magnetism somewhat. Then he is using his DC unit(other machine) to perform the actual welding. Hope that makes a bit more sense. Best regards, Allan

Allan, I fixed it for ya :wink:
John Wright
Parent - - By welderbrent (*****) Date 05-29-2013 15:20 Edited 05-29-2013 21:24
Allan,

I think you missed a keystroke on your intro.  :confused:  You don't generally start a conversation that way.

Tyrone, I agree with Allan's interpretation of the previous post. 

Have a Great Day,  Brent
Parent - - By ssbn727 (*****) Date 05-29-2013 19:59
Just out of curiosity, what does "coversation" mean?:eek::twisted::lol::roll::lol::wink:
Parent - - By welderbrent (*****) Date 05-29-2013 21:23
See now, here I am, politely pointing out a mistake to Allan, and I go and make one myself.

Thanks Henry.  :lol:  Did you have a good Memorial Day Weekend Henry?  I hope so, you deserve to be recognized for all you have done and are doing.

Have a Great Day,  Brent
Parent - By ssbn727 (*****) Date 06-02-2013 07:13
Thanks Brent!

Yes I did... I went to the Oakland area of the city of Pittsburgh for the annual parade and sort of marched with my new powered wheelchair...

We then continued to honor the vets who have fallen from past wars at the Soldiers and Sailors Museum which is an incredible place of reverence...

Every Vet should go to the Soldiers And Sailors Museum at least once in their lives in order to witness what is probably the most comprehensive collection of both historical items as well as thorough displays of all the conflicts this country found itself involved in through out our relatively brief history as the Greatest Nation in the World...

All vets will appreciate the attention to detail involved with the displays as well as some of the films and photographs being shown at the museum also...

It is the only museum that has almost as thorough of a complete history of the "Silent Service" as the Submarine Museum in Groton, Connecticut which is the best IMHO.

I have visited the Soldiers and Sailors Museum previously but, this is the first time I stayed long enough to really appreciate the significance of the history found there.:cool:

I most definitely suggest that every one who served pay the museum at least one visit and experience it because they will not be disappointed in the least!

You had me worried there Brent... I thought for a minute that I missed out on an important addition to the English language! :eek::twisted::lol::roll::wink::cool:

I hope you enjoyed yours as well Brent.

Respectfully,
Henry
Parent - By aevald (*****) Date 05-29-2013 17:09
Thanks Guys! I would have swore that I had that right, guess not though. Certainly don't start posts that way intentionally. Apologies and regards, Allan
Parent - - By 357max (***) Date 05-30-2013 00:39
Tyrone ~ Aevald is correct about the connections. One thing to add is if the machine has a duty cycle of say 200 amps at 25 volts (5000 watts) at 20% duty cycle. The machine could be set at 150 amps and a dead short would be 1 volt (150 watts) easily 100% duty cycle. Beware if the machine has a switch, tap transformer. Changing the amperage while under load would cause arcing between switch points. A moveable shunt or moving coils would permit changing output while under a load. Lots of moveable coil and moveable shunt design  AC machines can be had from estate, yard sales or craigs list for a very small cost.
Parent - By Tyrone (***) Date 05-30-2013 10:31
Brilliant!  Can't wait for the next mag. issue to try this on.
Thanks for the details.

Tyrone
Up Topic Welding Industry / Welding Fundamentals / magnetism in heavy wall chrome

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