Solenoid Core Magnetism

[DennisP]

I have a situation with a customer. We make solenoid cores for them out of SD 200G, which is a 12L14 equivalent, with an electroless nickel plating. Cores made from one heat of material seem to perform fine. Cores from a different heat of material do not seem to have the same magnetic properties and, consequently, do not wortk as well. The valves need to open against 29 psi minimum pressure. The valves that have the issue will only open against 22-23 psi. The material certs would indicate that there are only mionor differences in chemistry of the heats of material. After some research I tested the hardness of parts from each lot. The good lot has a hardness of HB 122. The poorer performiong lot has a hardness of HB 142. Is this enough of a ahrdness difference to cause an issue withthe magnetic propertuies of the cores? I understand that 12L14 is not an optimum material for the application, but it has been in use in this application for 30 years and this issue has never been seen before. Does anyone have any suggestions on what could be happening here?

 

[israelkk]

When you say solenoid core what do you mean? The plunger? or all metal parts of the solenoid. Can you post a drawing/section cut of the solenoid. How can you tell that the problem is with the core? Maybe the problem is with return spring, the dimension tolerances etc. I assume you are referring to a solenoid valve.

I would suspect that the solenoid has no force reserves and it is sensitive to tolerances, air gap changes and some mechanical friction. Without complete mechanical and magnetic analysis of the solenoid and external and internal loads, pressures and inertias it will be impossible to give an intelligent answer.

I would check the C.O.T of the 12L14 to make sure it is in the annealed condition, if it is not, that may be the reason for the higher hardness. If the core was supplied in the cold rolled or hot rolled condition instead of the annealed condition this may explain the change in the hardness and the decrease in magnetic properties.

 

[DennisP]

Thank you for the comments. I am a novice in the field of magnetics and appreciate the help. You are correct in that this part works right on the edge of acceptability to begin with. If our customer swaps the core with one from a different heat of material, it performs as it should. This would lead me to believe that the core is most of the problem. This part is the core of a miniature electromagnetic air valve. The magnetism induced draws a flat plate away from the orifice allowing air to flow. Unfortunately, I do not have a drawing and I'm not sure my customer would appreciate seeing it in a public forum anyway. Since you mentioned annealing, might it nbe possible to improve the magnetic properties of the suspect parts with an annealing process?

 

[sreid]

No matter how long the original steel has been used, it's never too late to try the correct material.

http://www.cartech.com/products/wr_products_magnetic.html?leftn=our_products&lefto=empty&lefti=empty

 

[EdStainless]

I would try annealing a sample of the new material and seeing if it helps. I could explain why I think that it will, but let's see.

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Plymouth Tube

 

[israelkk]

DennisP

I would check the dimensions of the core added to the heat treatment. Maybe someting is not as the drawing specify.

Sreid

The magnetic alloys offered by Carpenter and far more expensive than 12L14 and much harder to machine. 12L14 is the best iron based low carbon alloys allow to machine. I assume the whole existance of the valve depends on the cheap price.

 

[DennisP]

Thanks for the comments. I think I can convince my customer on a plan to try and solve the problem.

 

[sreid]

If magnet grade steels can't be used due to cost, I find 1018 to work well. In general, the lower the alloying content, the better the magnetic properties.

 

[israelkk]

sreid

The machining cost of 1018 is more expensive than 12L14. 12L14 is especially designed as a super free cutting alloy for mass production on automatic machines.

 

[sreid]

I am well aware of 12L14 free machining steel. I had always though that 12L14 would be a bad choice for magnetics due to the lead, sulfer, etc. An internet search shows that to not be true.

More directly answering DennisP's question, the hardness of the material can reflect the internal stresses in the material and internal stresses can reduce the magnetic propertities of the steel as has already been said. Anneling the parts may fix the problem.

 

[dgallup]

12L14 is widely used for magnetic components but the chemistry allows huge variation and consequently large differences in magnetic response. Even with similar chemistries different lots can have significant magnetic response. Annealing MAY help but mostly in the shape of the initial BH curve & reducing the Hc & Br. I suspect the root problem is insufficient Bmax (saturation flux density) which will not improve significantly with annealing.

Critical components should be made from materials with controlled magnetic properties as mentioned before. Otherwise, this kind of problem will repeat randomly. The customer has to decide if it is cheaper to test & scrap complete assemblies or build in the quality at the design stage.

 

[DennisP]

Thank you all for your responses. Further discussion with my customer reveals that they have an application in a different assembly that will perform to spec using these parts. I've learned a great deal from the discussion and appreciate the help.

Regards