QC of Magnetic Permeability of StainLess Steel 1

[Ron07663]

I have a requirement for a Deep Drawn Stainless Steel Component to have a Relative Permeability of less than 1.05. The austenitic annealed material meets this spec. but a high percentage of cold work could raise the value.

Finding a metal fabricator to tool and manufacture the part who also is familiar with the magnetic requirements and has a QC system in place for it has been a problem.

How should the requirement for low permeability be controlled and measured by our component vendor?

 

[rhodie]

Is there a correlation to be had between surface (work) hardening and magnetic permability?

 

[Ron07663]

rhodie:
Yes, depending on type of austenitic SS, the perm can increase significantly with cold work.
Typical numbers are published by the mills. This is one example- see page 4 :

http://www.alleghenyludlum.com/ludlum/documents/302_305.pdf

 

[rhodie]

That is very interesting, thank you!

 

[rorschach]

The best way to approach this is probably indirectly. call for the part to be bright annealed after drawing. check by hardness testing. either that or you have to obtain a magnetometer to measure permeability directly. In my past we had a assembly that used stainless screws in a magnetic sensor assembly, cold work from the heading and thread rolling made the sensors erratic. brass screws tended to have thier heads twist off (they were #2-56). we ended up having to use Ti6Al4V screws. (can you say expensive?)

 

[Ron07663]

This part is for a magnetic sensor assembly also. I am researching equipment to measure the magnetic characteristics as we would like to avoid post-draw anneal if possible. We are planning to qualify the material and manufacturing process. If OK, we will need a plan and procedure to do magnetic inspection on the finished parts. My concern is that the variation in chemistry of the SS may cause some parts to be out of spec.

Ron

 

[rorschach]

Your concern about chemistry is a valid one. you may want to consider specifying cartech's Custom Flo 302HQ, it has small amounts of copper added to lower it's strain hardening rate. see here:

http://crswnew.cartech.com/wnew/techarticles/TA00009.html

There are other 3xx series that have alloy additions that are intended to suppress strain hardening, but my mind is blank right now. If I think of which ones, I'll repost later.

Monel or Aluminum may also be an option

 

[Kenneth]

A Severn Gage (

http://www.severnengineering.com/lowmu.htm)

is a relatively inexpensive (

http://www.severnengineering.com/pricing.htm)

instrument for performing "less than" measuremnts of "feebly magnetic" materials. It's use is described in ASTM A342 and at the linked site.

 

[Ron07663]

We plan to evaluate a low perm digital meter by Foerster Instruments. It has a handheld probe and direct readout over the permeability range of 1.000 to 2.000. They call it a Magnetoscope.
Are you familiar with it and how does it compare to the Severn Gage?

Ron

 

[Kenneth]

No, I have never used the Magnetoscop (

http://www.foerstergroup.com/Maintenance/permeability_measure.html),

but I have used a Foerster %IACS meter and came away with the impression that it was a quality instrument. Out of curiosity, I went looking for a price (at

http://www.corvib-int.com)

on the Magnetoscop (to compare the $575 for the Severn Gage) and came up with a price of $9,975 (ouch!) for:

Operating Instructions
Magnetoscop 1.069 Measuring Device
Permeability Probe, X1
Calibration Standard MU 1.2
Batteries (5) AA
Carrying Case

The Magnetoscop provides a "value" (as opposed to the "less than / more than" standard comparsion provided by the Severn Gage). Please let us know how your evaluation goes.

 

[Ron07663]

I received two Magnetoscops yesterday and started playing with one today. So far, it looks good.
The unit is a microprocessor driven digital readout meter that comes with a probe and calibration standard. The standard we received is 1.223 u. On power up, you place the probe on the standard, enter the perm value (1.223) and the processor self calibrates. When done, the unit reads 1.000 when the probe is in air. When the probe is placed on the test piece (in our case austenitic Stainless Steel)the permeability value is displayed directly.

Next week we will verify correlation in the readings of the two units. Our plan is to consign one unit to our vendor and the other to our incoming inspection. We need to be able to reproduce each others reading.

 

[Ron07663]

The Magnetoscops are working out well. The two units are producing compareable results. The only complication that we found was that the permanent magnetic field that the probe emits will magnetize any ferrous inclusions in the stainless steel. This remnant field will affect any subsequent closeby permeability readings. To solve this, we had to add a demagnetize step between readings to the procedure.

Ron