Coercive Force as per ASTM A838-18 / Lower Saturation Induction 1

[CMStainless]

What is the ideal coercive force for fully magnetically annealed bars between diameters 6.35mm to 17mm for type 2 Alloy?

Also, is there a way to reduce the saturation induction of the bars through some changes in annealing? My values are always in the range of 0.9 to 1.1 Tesla

 

[MagBen]

ASTM A838 specifies Hc max < 3.4 Oe, ideally 1.8Oe, or even 1.6 Oe (if low is ideal for your application).

saturation induction is insensitive to structure, one can hardly change saturation by heat treatment. However when you got .9-1.0 T, material was not saturated, you should have got 1.4 T or so when fully saturated. Normally applications ask for high saturation. when you ask for a lower induction at some H level, that means you are asking for a lower permeability at that H level, this could be accomplished by annealing, e.g. decrease anneal temperature/time. Or, do not do magnetic anneal, instead use as mill annealed condition.

 

[hacksaw]

A838 is a ferritic stainless steel, commonly used in relays. What is your application? Relays?

 

[CMStainless]

Thanks MagBen.

The application is to make plungers for solenoid valves. However we only produce soft magnetic bars and it is machined by the user. The user needs the bars to have high permeability and low coercivity and the Br needs to maintained at 0.9T max. To give you an example, please see the attached BH curve.

@Hacksaw, the application for our bars is Plunger and the stopper for solenoid valves and fuel injectors.

 

[hacksaw]

Remanence occurs after saturating, and the field is lowered, apart from the changed reluctance of the magnetic circuit due to the plunger being in a new position, so Hi-Remanence is a function of adequate coercivity and low-remanence of the plunger in its latched state...

You see measured remanence of the core material goes to zero with zero coercivity....and is high with high coercivity.

Sounds like poorly dimensioned design, as machining tends to increase the coercivity, and your customers magnetic flux circuit is inadequate.

 

[CMStainless]

Sorry I was talking about Residual Induction(Remanence) not Saturation Induction, my bad.

The Saturation Induction is usually close to 1.5T.

If the Remanence is high it causes "sticking" of the plunger which is why we need it to be below 0.9T while the required Permeability should be around 1700.

We are able to achieve low remanence when permeability is below 1300. But low permeability leads to Higher coercive force.

This problem is only with grade 430FR. We are able to achieve the desired results in grade 430F..

 

[hacksaw]

it's all about the saturation curve when remanence is involved, though commonly described in terms of its "slope" or dc permeabilities.

As you recognize, material properties is the critical issue.

 

[MagBen]

It makes sense now. Asking for a lower saturation is a "weird" requirement.

It is hard to decrease remanence while still remaining high permeability and low cocercivity! you don't have much to do on material side unless you can change your design.

Decrease L/R ratio (shorten length or enlarge diameter or both) will increase de-mag factor, so lower remanence.
It seems a DC solenoid, right? you could add a reversal field to decrease the magnetic residual. maybe adding a capacitor in the circuit to de-mag applying positive and negative magnetizing directions with decreasing amplitude.
If it is AC, 430FR will be more beneficial due to high resistivity so lower eddy current losses. 430F maybe good enough for DC if hardness (so deformation) is not an issue.

 

[dgallup]

Best way to kill the effects of remanence is adding air gap. A little nonmagnetic plating or a shim will prevent sticking.

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[MagBen]

@dgallup: agreed, but that would decrease effective permeability drastically. Anyway, the solution is more of to change the design/circuit, rather than to decrease intrinsic remanence of material itself.

 

[dgallup]

It has minimal effect on opening since obviously there must be a large air gap if you are pulling in a plunger. You just need to preserve a little air gap once open to get rapid closing.

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[MagBen]

If the max force does not matter at the moment when the air gap is closed, this seems to a fine/easy approach. But just wanted to note that the force/effective permeability will decrease exponentially with gap.

 

[dgallup]

It's almost always the force when the armature is at it's maximum distance that's the limiting factor. Most solenoids make too much force when the air gap is closed. Frequently have to resort to current limiting drives. Fast closure is the name of the game in fuel injectors.

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[CMStainless]

Thank you for the information gentlemen.

However, these solutions are possible only at a later stage.

As I said, we manufacture the bright bars(Raw material) for these applications. The customer then cuts and machines the bar to make a plunger.
This is a relatively new product range for us and we are learning as we go.

The properties are achieved by magnetic annealing but they change drastically when we perform grinding or straightening after that.(For tolerance and surface finish)
And this problem is prevalent in smaller diameters only, around 10mm and lower. There isn't much that can be done with the chemistry. Is there any way to maintain the properties after annealing?

 

[hacksaw]

"The properties are achieved by magnetic annealing but they change drastically when we perform grinding or straightening after that."

"Is there any way to maintain the properties after annealing?"

Sounds like you need to sort out your manufacturing steps...