Martensitic steel heat treatment, to get an even magnetic performance?

[Peddy58]

Hello metallurgists out there,
I have designed a inductive linear sensor using a 4mm dia 416 martensitic steel shaft as the sensing part. Measurement lengths of 25mm to 300mm. The sensor operates by detecting the change in permeability of the 416 shaft as it enters an inductive coil.
The shaft is centreless ground from 416R 7/32” Round Bright S/S H8, from the mill ( Sama), to a 4mm dia h7 12CLA for use in our sensor. I think the material from the mill is solution annealed before processing at the grinders. The material is not heat treated after grinding. I are experiencing, what appears to be a material permeability change per mm, which is affecting the performance of the sensor. I think this might be caused by work hardening during the grinding process?, or it could caused during manufacture at the mill.
Does anybody know what heat treatment process ( higher holding temperature/time and reduction rate?), I might try in order to reform an even distribution of the martensite/ferrite?, in the machined shaft component, and then I would then get an even permeability performance.? I can withstand a lower toughness performance from the material.

Peddy58

 

[MagMike]

Peddy58: Check out this discussion in the Magnetics Engineering Forum:
thread340-241390
The paper that remetaper posted is particularly helpful

-Mike

 

[Peddy58]

Thanks for the pointer Mike
I will have to put on my mechanical head to digest the report from Valbruna. at first glance it looks very interesting... regards
Peter.

 

[remetaper]

Peddy58,
The mill SAMA supplied a 416R 7/32 centerless ground bars toll. h8. Please verify in his certificate chemical analysis, heat treatment ( H+T or Annealed ?) and mechanical properties ( Rm, Rp0,2, A%). I think SAMA supplied according to following process: " hot rolling +annealing + pickling +cold drawing + centerless grinding with a minimum of stock removing. Probably, no straightening after cold drawing. And no magnenitic annealing if you had not required this one.
In case of hardening and tempering , the magnetic situation is worst.

Supposing these 4/32 roud bars had - FOR INSTANCE- a mag.permeability 400 - 600 and supposing these values were good for your sensor, you have to consider that:
1) a reduction from 7/32 to 4 mm round ( i.e. 48% of reduction of area) using a centerless grinding process could reduce magnetic permeability to 200 -250 if this process had been a little bit heated the surface of bar. I did a lot of test with Ferritic grades in order to evaluate the influence of this heating generated by a poor grinding process and I can confirm this behaviour.
2)If the 4 mm round bar had been even slightly bending, the magnetic permeability could fall to same values as above
3)If 4 mm round bar had been burnished ( reeled ) a reduction of 30% of mag. permebilty will happen.
4)Higher Roughness CLA causes a reduction of mag.perm.
In order to give you a better answer, it would be usefull to know:
a) did you require any magnetic properties of 7/32 bars ?
b) which magnetic permeability values on final 4 mm bar ( or sensor ) you want ?
c) if you wanted to restore magnetic properties of 4 mm round bars , you could tempering 600° c x 4 hour or more if higher magnetic permeability were required . After this heat treatment , 4 mm bar would be ground avoiding any heating or bending with a minimum stock removing from finished dimension. A Hydrogen or vacuum annealing could be chosen but the surface status after this heat treatment must be evaluated. Grinding with care after annealing is a good way to avoid troubles.
However, the right temperature of final magnetic annealing MUST be only chosen when information about chemical analysis, condition of delivering of 7/32 bars ( Rm , Rp0,2,ans so on ... ) are available.