Need more magnatism from stainless steel 3

[cooperjer]

I'm working on a project which involves an inductive sensor detecting a flag. The flag is currently made of 430 SST and is 12mm wide. The sensor is a 12mm sensor with a nominal sensing range of 2mm. The flag is on a roller and rotates at an unknown speed with the sensor mounted over the roller.

At times the sensor misses pulses. Testing has shown that the sensor misses more pulses when set at 2mm than when set at 1mm from the flag. Some discussion with co-workers indicates that the sensor is having a hard time picking up the 430 SST, hence the closer the sensor needs to be to sense the steel. However, the manufacturer indicates that "magnetic stainless steel" is equivalent to structural steel when it comes to sensing.

Is there a more ferromagnetic steel than 430 SST? Marks Handbook for MEs indicates that 446 has more carbon and no NI, but does that mean it is more ferromagnetic?

Thank you.

 

[metengr]

Switching to a 446 SS is not the answer in terms of enhancing magnetic properties. Actually, there are ferritic stainless steels that can exhibit enhanced magnetic properties over standard Type 430 SS. The attached web site below is from Carpenter that links you to one of their technical articles on "soft magnetic" ferritic stainless steels. This may be of interest to you

http://crswnew.cartech.com/wnew/techarticles/TA00045.html

 

[cooperjer]

Metengr,

It appears as though the linked web site is a nice article about magnetic stainless steel and a little advertisement for the company's own Chrome Core series of SST. Do you have personal experience in comparing the Chrome Core 8 to a 430 SST? Does the 20% increase in saturation magnetization seem to be a significant difference in test?

Sorry to sound like a skeptic.

Cooperjer
Mechanical Engineer

 

[metengr]

I work in the Power industry so I have no reason to promote or endorse suppliers for business reasons. I had heard of these ferritic stainless materials from one of our Instrument Controls guys, and this was the reason I provided the link for you. We have used the Chrome Core materials for solenoid switches and they work great.

 

[EdStainless]

Is your sensor really based on Ferromagnetic response or induction?
Some sensors have a constant field and a field sensor. When a ferromagnetic material comes close it 'bleeds off' some field strength and this is detected. Higher saturation material helps.
In some sensors a field is set up and a material moves through and the induced field in the material is detected. In this case you need a material that is ferromagnetic and has good electrical conductivity.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[cooperjer]

The sensor is the latter of the two examples given.

Based on the information from the web site provided by Metengr and some logic I would draw the conclusion that the ratio of carbon to other alloys determines the "saturation magnetization."

Comparing the ratios of carbon to other alloys (based on info in Mark's Handbook for MEs) 430 SST has 1:141.7, and 446 SST has 1:127.5. Assuming my logic is true, 446 SST would have a higher saturation magnetization and thus be easier for my sensor to detect.

Is my logic true?

Thank you,

Cooperjer
Mechanical Engineer

 

[metengr]

cooperjer;
Considering that this is a rather unique problem (which I must admit, I don't get involved with magnetic saturation or permeability of metals on a daily basis), I will take on the challenge. Armed with my reference material, I offer the following regarding your logic.

- there are basically two types of magnets - soft or permanent. Permanent (hard) magnetic materials exhibit a large amount of residual magnetism after exposure to a strong magnetic field. Soft magnetic materials become magnetized by a relatively low strength magnetic field, and when the applied field is removed, they revert back to a state of low residual magnetism. Soft magnetic behavior is essential in any application where changing electromagnetic induction occurs- such as solenoids, relays, motors, generators, transformers and magnetic shielding…. (excerpts from ASM Metals Handbook, Desk Edition).

Magnetic soft materials are - high-purity iron, low carbon irons, silicon steels, Ni-Fe alloys, Fe-Co alloys, ferritic stainless steel and ferrites.

- magnetic permeability. According to my source "Physical Metallurgy Handbook", the magnetic permeability of AISI Type 446 SS is 400-700. Comparing this with a typical AISI Type 430 SS (no special effects), the magnetic permeability is reported to be 600-1100.

- the effect of C in "soft magnets". Carbon, oxygen and nitrogen are considered as impurities in soft magnets. These elements will tend to locate at interstitial sites and pin magnetic domains that can result in a "hardening" of the magnet. In effect, hysteresis losses increase significantly with carbon additions under an induced magnetic field.

- commercial irons that have little carbon (less than 0.025%) have the highest saturation index in relation to iron that contains carbon. The addition of carbon significantly reduces the saturation index from 2.15 T to about 1.6 T. The only alloying element in iron that actually increases the saturation induction is cobalt.

The bottom line is that your correlation does not appear to be valid for soft magnets.

 

[sreid]

An "Inductive Sensor" implies that the sensing mechanism is the change in inductance caused the presence of the steel. There has to be a threashold for making the decision. At 2 mm the change in inductance is dominated by the air gap and the sensor is apparently marginal in making the decision at this gap. A change in steel type will probably not improve this much.

Unless you can't go to the 1 mm gap for some mechanical reason there are hundreds of commercially available "Proximity Sensors" one of which will work in your application.

 

[cooperjer]

Thank you for the information. I should pick up a copy of "Physical Metallurgy Handbook." With this information it appears as though I have the best SST for the job. I think it's time to find a new sensor for the job.

Sreid, the 1mm gap is not a physical limit, however, the error involved in setting the sensor could often times cause the sensor to be beyond the 1mm distance. I believe I can get the same sensor with a nominal sensing range of 4mm, which should help the problem.

Again thank you both very much.

Cooperjer
Mechanical Engineer