Permeability at very low level 1

[InfoDrain]

We have a problem with an apparent change in the magnetic properties of a stainless steel component which forms part of a magnetic circuit. The problem can be accounted for by a drop in material permeability

The stainless steel is heat treated to achieve the required strength. The BH curves for the steel show significant differences for various heat treatments as the curve approaches saturation but I can find no indication as to the effect of heat treatment at very low levels - a few micro Tesla.

I suspect that at these levels the curves are not affected by heat treatment, the differences only appearing as saturation is approached. Anyone help me on this please?

Jim

 

[drdick]

What grade of SS is used?

If heat treat for strength increased hardness and/or decreases grain size, then permeability has probably dropped. The effect can be pronounced at low flux levels.

Other factors to consider are the potential for magnetization (biasing) during cooling due to orientation with respect to the earth's magnetic field, or induction coils.

Can you change magnetic circuit performance by bringing a ferrous material (or magnet) into contact with any portion of the magnetic circuit affected by the heat treat?

 

[InfoDrain]

Drdick

Thanks for your interest in my problem. The stainless steel is BS S143 which apparently is martensitic and is to Rolls Royce spec MSRR6647. I'm not a materials expert so can't really elaborate on that.

The prescribed heat treatment is as follows:
1050degC (1922degF) for 1 hour.
Argon Cool to 0degC (32degF) and hold for 2 hours.
850degC (1562degF) for 2 hours
Argon cool to 0degC (32degF) and hold for 2 hours
350degC (662degF) for 2 hours.
Argon cool to room temperature.
Final hardness value to be 350HV min.

The component in question consists of a BS S143 pole piece supporting a coil of 2900 turns. The coil is excited at 1kHz at a current of a few mA, hence the low flux level. A target of the same steel approaches the pole and the inductance change in the coil is measured. Whilst this used to work OK in the past, recently the inductance change for a given target displacement has dropped out of spec. Having checked everything else I can think of, I am exploring the possible effects of variations in heat treatment on permeability at low flux levels. There are many published BH curves which do show that heat treatment has a considerable effect on permeability as saturation is approached but give no indication of the low level response.

Thanks again.

 

[ISUClone]

Eddy Current tests work similar to what you are seeing. Inducing a low magneic field into a part can and will show differences in heat treat. this is one of the reject criteria for many eddy current tests. So low flux levels are different for varring heat treatments.

 

[InfoDrain]

ISUClone

Thanks for the tip. I will look into eddy current testing techniques for more clues.

Jim

 

[drdick]

Magnetic core materials, typically low carbon steels or permalloys, can be annealed to bring out various magnetic attributes, including initial (low level) permeability. In general, those anneals that promote grain growth with slow cools result in higher permeability and lower hardness. If you need a hard pole face, perhaps the pole structure should be two piece so you get the inductance as well as a harder pole face.

 

[InfoDrain]

Thanks for the replies. It's given me some useful areas to explore.

Jim