Steel Blocks Develop Magnetic Properties 1

[DTW]

We machine a family of parts from steel blocks. A recent change in both material and process have caused the blocks to develop magnetic properties as they are machined. Generally, the magentic field is strongest closest to the location of the most recent cut. The forgings are approximately 22" x 16" x 16". Material is ASTM A646, AISI 4140, normalized, quenched, & tempered to 311-369 BHN.

Anybody know what might cause this?

 

[Iskit4iam]

This was explaned to me once as the cutter causing alignment of molecules at the shear interface similar to what happens when you drag a needle across a permanent magnet. Never dug into it but I've seen it before.

 

[EnglishMuffin]

Very common phenomenon. It is an example of stress induced magnetization, or piezomagnetism, and is apparently caused by preferential orientation of the previously randomly oriented magnetic domains. There is also an inverse effect called magnetostriction, where you get a stress developed by applying a magnetic field. But giving something a fancy name doth not an explanation make. At least if you have the scientific name, however, you can research the subject should you so desire. There are a lot of very complex and recent papers out there, which probably means that nobody quite understands it yet. I remember an old Magyver episode in which someone had dropped a key (or something) down a drain, and he took a long steel rod, banged it on the ground a few times, and then proclaimed "now it's a magnet", and immediately retrieved the key (or whatever). If only life were that simple. Another somewhat related example which comes to mind is a problem the Germans had with their V1 missiles. The residual magnetic field of the metal skin messed up the compass used for direction control, and their low tech yet somewhat un-Germanic solution was to have a whole bunch of guys banging on the things with mallets before launch, until the field was neutralized.

 

[rhodie]

Great post EnglishMuffin. It's stuff like that that makes these forums great.

 

[Barry1961]

Getting a little of the topic here but I have magnetized many scewdriver tips by holding the tip pointed toward the north pole and striking it with a hammer. It is not a strong field but good enough to hold a small screw. Since I don't watch TV unless I am fixing one I have not seen the Magyver episode but it seems to loosely based in reality.

You can also track the magnetic tail of the earth by attaching a magnet with a shiny surface to a single oriented thread strecthed between floor and ceiling and shining a ruby laser on it and watching the track on a wall. A thread from an old mountian climbing rope works well for this since they are almost total free of twist.

Way off the track, sorry

Barry1961

 

[EnglishMuffin]

Barry1961: I am highly skeptical that the direction in which you pointed the screwdriver during the striking process would have had any measureable effect on the resulting field. Of course, I could be wrong.

 

[ietech]

Seems easy, point the screwdriver south, east, or west and see if the same results occur. I'm sure it's just the striking, providing stress that causes the field ---

 

[Rich2001]

Barry1961 is correct. Take an iron or steel bar and align with magnetic North/South and striking it will result in a magnetized bar. You can also heta the bar in lieu of striking it.

BTW, if you want to demagnetize a specimen completely you align it east-west and then use what ever method to demag, decreaseing AC field, heating above its Curie temperature etc.

 

[EnglishMuffin]

Would anyone care to divulge where this alleged effect is documented scientifically, as opposed to anecdotally ? It is not clear, if it exists, that it is necessarily related to piezomagnetism. Part of my skepticism results from the fact that the earth's magnetic field is extremely weak compared to the field required to pick up a screw, for example. But I'm always ready to be convinced.

 

[EnglishMuffin]

Actually, I have just gone through all my screwdrivers, and was hard pressed to find one that wasn't magnetized. I found that the newest one had no detectable magnetization, and tried hitting it in various orientations with both a copper and steel hammer, but could not produce any noticeable magnetization. Where are you Magyver? Of course, I don't want to wreck a perfectly good screwdriver by hitting it excessively hard. But the fact that all screwdrivers become magnetized after spending the majority of their lives in no particular orientation suggests to me that orientation has little to do with the effect, although I will admit that this is not a conclusive argument.