How did this happen?

[SpankySmith]

Yesterday I made about a dozen necklace sized blacksmith crosses, I have orders for a half dozen because someone wore one to church after buying it at my show several weeks back, and then others wanted to know where she got it. A little after-show perk!

So today I went to polish the crosses on my scotchbrite wheel and 1" sander, then went to clear coat them after they'd cooled down. It was then that I noticed several of these had somehow become magnetic! What tha? This is a new one on me, can someone explain? They showed no magnetism prior to forging, all were made with generic 1/4" and 1/2" mild steel. I've made dozens of these before, have never seen that! How does that happen?

[Mr.M]

The grinding process frequently adds/creates magnetism. I think the demag units work by quickly reversing the poles repeatedly

[SpankySmith]

Is there some particular set or sequence of circumstances that contributes to it? Odd that I've never seen it happen before and ive done a ton of these.

[RogueRugger]

As Mr M pointed out, grinding or machining will sometimes induce magnetism. With no firm basis in fact, beyond the fact that I stayed in a Holiday Inn last night, I've always understood it to be the result of molecular alignment that happens during fine machining. A lot depends on the carbon content of the steel: tough to do with raw iron, and ability to induce magnetism increases with the carbon content.

[Charles R. Stevens]

If you are using A36, it may have a fair amoun of carbon (this is why some times its hard to drill)

[petere76]

Sparky,

Add heat....magnetism goes away.

Peter

[Frosty]

Iron and steel is magnetic and a bar becomes magnetic when the molecules align with enough of their poles pointing more or less the same direction. There are a number of ways to do it vibration is probably the easiest. Parts will become magnetized if the long axis is aligned with a magnetic field and it is subjected to something that will vibrate the molecules. The vibration will allow the molecules to align with the external field and where there's enough the bar becomes noticeably magnetic.

 

A fun way to magnetize iron is to hang a long bar from a string, it will align N/S in Earth's magnetic field then give it a good rap with a hammer and filings will stick to it or a compass will align with it whichever way it's pointed.

 

A good demag device is a simple AC electro magnet or determine which way N/S poles are in the piece you want to demag and spin it laterally in a stronger field. For jewelry sticking it to a dowel that fits a drill motor with a dab of wax and rotating it between the poles of a horse shoe magnet works nicely.

 

Frosty The Lucky.

[ThomasPowers]

Any steel cooling down through the curie point will take on the earths magnetic field to a degree too If you accidentally had them aligned during cooling you could have picked up a bit

[JNewman]

The bars may have been magnetic when you started.  I make a lot of prybars out of pre hardened stock for maximum strength, (the forged end gets hardened harder than the rest of the bar)   More often than not these bars  are magnetized when I sawcut them to length before forging.  They are still magnetic after forging.  

[Andrew T]

I have chunks of alloy steel from the scrap yard that were magnatized by the crane magnet.

After heating to red and cooling no more magnetism.

[Frosty]

The bars may have been magnetic when you started.  I make a lot of prybars out of pre hardened stock for maximum strength, (the forged end gets hardened harder than the rest of the bar)   More often than not these bars  are magnetized when I sawcut them to length before forging.  They are still magnetic after forging.  

 

Does the stock align N/S in the saw? Heck, maybe the last pass through the rolls at the factory is aligned N/S.

 

In truth I don't think about it unless being magnetic gets in the way, sometimes enough chips, dust and stuff sticks to get in the way so If it's small enough I set it on top of an electric motor or wrap an extension cord around it to demagnetize.

 

If a person really had to demagnetize something large, looping the cables from your AC welder around it and running a couple beads will do the trick nicely.

 

Frosty The Lucky.