Magnetized sheet problem

[SeanP]

Had a new one today cutting some parts from a sheet of 8mm ar450, 2 holes and a outer cut of about 3', the nozzle is destroyed on just one part, cursing for 4 parts before spotting the magnetic pull on the sheet, it seems to be in certain areas, enough to hold this strip underneath!

Job stopped for now, anyone had this before or any ideas if it can be got around?

[acourtjester]

Have you thought about talking to your vendor, maybe they will replace the sheet. Kind of a puzzle as to how it got that way, is it the whole sheet or just in one section.

[SeanP]

It's strange that strip holds in quite a few places on the sheet but not all over.
In this case the customer supplied the sheet, yes I think we will have to see about a replacement, never had anything like it before, there is always something to catch you out!

[AREA]

Wow, that's odd. I suppose if a magnetic lift was used at some point to move the sheet it could have magnetized the plate in places where the mags were attached.

I wonder if heating those magnetic areas with a torch would eliminate the magnetization, I have ruined many magnets with heat in the past.

[cstroke]

I thought sometimes you can demagnetize something by striking it or dropping it???
Correct me if I'm wrong.

[ScottRa]

AC driven coils can be used for degaussing, but I don’t know big it would have to be for plate steel. With magnetic tape and small tools or parts you’d just move them in a circular motion by the item, gradually moving further away from the part.
I’m curious, was the magnetic field directly affecting the plasma in the torch head and damaging it?

Scott

[SeanP]

Thanks for the ideas, I tried walloping a part laid on a pallet, that didn't help, reading up you really need to get it red hot to do any good.
Yes the nozzle was getting well burnt out, I have seen similar when picking out parts with a magnet and getting to close to the torch especially on finecuts.
Latest plan is he will pay for a nozzle for each part if that's what it takes.

[Dfrick23]

Hey guys, 90% of the material we cut for the past 4 years is AR500 plate and I will say magnetism has been an issue the whole time. What I have found is that it is a two fold issue. Part of the heat treating process imposes the field within the sheet, aligning the molecules and the lift magnets in the mills locks it in and intensify the fields in certain spots. I have spent hours trying to find a work around. I emailed hypertherm a while back, and inquired about a mild steel shield cup to help isolate the field from the nozzle ( found this solution from a Japanese research paper) they said use a oxy torch instead... I just laughed. Thanks right.. I had mentioned something about would a larger cutter help, they said maybe a little but not worth the expense and I would just have to live with short consumable lifespans.. fantastic im glad we got this all taken care of right.. so more research... I ordered some magnetic film and traced the field onto the sheet to be cut next, what i found then was basically when you cut parallel or at an angle to these fields (which intensify even more with the added heat) it pushes the plasma arc over into the nozzle and blows out the side of it.. I have a bucket full of trashed 45 amp nozzles from cutting 3/8 ar plate. Sometimes only cutting a few inches before blowing a nozzle.. talk about expensive cuts and a lot of post grinding to save the piece.. raising the cut height does help a little but by far the best thing I've done is ignore the ever so helpful hypertherm email and switch from a 45xp to a more powerful pmx85. I now cut 3/8" ar500 plate at 65amp and have exponentially increased consumable life. The deflections from the fields are still there, it goes from cutting straight to a bevel area and back to straight so there are still spots to grind straight later but it doesn't often destroy the nozzle or workpiece and throw rooster tails and lead to swearing anymore. I think the added amperage and more powerful plasma arc can resist the push pull effect from the magnetic field. So that works decent for me.. it would be nice if hypertherm would acknowledge the issue and make mild steel shield cups for the power max line of cutters.. you know its a pretty strong field when you can't slide smaller pieces around on each other because they are "stuck" together. But the solution is A waterjet, B laser, or C bigger cutter.. don't have time for a jet, or money for a fiber laser so C it was.. heck the assist gas for the laser would have been way cheaper than all the nozzles I burnt with the 45xp.

[weldguy]

Dfrick23 that is very interesting, thanks for sharing. I wonder if you could modify the shield with a steel sleeve or something to see f your idea would work.

[SeanP]

Thanks Dfrick23 a lot of that sounds familiar! wish I could step up to a 85, but I'm on single phase 240 and the 45 is as big as I can go.

I did mean to report back on this problem, I left if on the rack for a week or more in disgust but pulled it out for another try, I did up the air pressure to 70psi in the unit settings but I also slowed down quite a bit, and to my surprise had very little bother with the rest of the job, another 30 parts on the same nozzle.
Had a bit of fun getting the angle on these jaggy parts, they are some blades for a recycling plant.

Video of setup here:
https://photos.app.goo.gl/gyzwrYUtF5DvbwJ17

[djreiswig]

Nice jig. Looks like it did the job quite well.

[ScottRa]

I found some good reading by searching for “degaussing large steel plate”. It can even cause problems with welding. There was mention of how certain alloys are comparable to the alloys used for permanent magnets. There are large commercial degaussers for industry. They look about the size of a welder so I would just guess equivalent price wise. Ships were degaussed by dragging a heavy cable under the hull and then applying a 2000 amp pulse. This was to keep them from triggering magnetic mines.

Scott