Nature of onset of permanent magnetism in 8620 armature?

[CSMDakota]

Hello,

I am on a team designing on a solenoid valve, we have had issues with durability of the armature, so are testing a prototype configuration with an 8620 alloy armature instead of the Silicon Iron original armature. The armature is case hardened in either material.

Once concern we have is whether the armature will be develop permanent magnetism over time, after being subjected to the magnetic field. We are trying to drive this phenomena by 1) running durability testing and 2) running over-current tests where we purposely hit the coil with double the normal current for a minute or so, then restore the control current back to normal and look for changes in valve performance. If we had onset of perment magnetism, we would expect longer valve closing times.

My question is, IF we were subject to the onset of permanent magnetism in our armature, would it be something we would see after a short term (mintues), mid term (days), or long term (years) operation in the field?

Thanks in advance,
CSMDakota

 

[MagBen]

you would see more or less "permanent magnetism", because once the current drops to zero, there is some residual induction. This residual will be there once it is magnetized. from this perspectve, it is a short term issue.

on hte other hand, this residual will be increased, when the alloy was not originally saturated, and when is now applied double current. The residual could also increase when the valve open and close again and again and the alloy is created consideralbe internal strains, or, is even created additional martensite structure. from this perspective, the "permanent magentism" is long term.

 

[dgallup]

I'm doing almost exactly the same thing! So far we are having good results with the 8620. I've got 150 million cycle durability so far. We also did the double current test without a problem. My biggest concern now is consistency between different lots of material. We have only tested a small number of parts all from one bar of material. I've got 4000 lbs of material on order.

Let us know how your tests go.

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[EdStainless]

It will all have some field retention, becoming permanently magnetized.
And yes it will very quite a bit between lots.
As long as the saturation levels are about the same they should function similarly.
Any change in operation speed would only be at the very start of actuation. It shouldn't have much impact on closure.

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Plymouth Tube

 

[MagBen]

If assuming the bar is carbonized on the surface, guarantee of a same carbonized depth is more critical than the chem. and process varaitions within lot-to-lot. The magnetics difference between outer and center of the bar could be large.

 

[CSMDakota]

Thanks everyone for the input, I appreciate your comments. We (well, the pirmary member group on the team) will be heat treating the material before assembly just for this product, so at least variation will be something we can control and monitor.