Magnetic Effects on Alloy Segregation

[ndtkansas]

I have found a condition that I hope can be logically explained. We inspect 17-4 material using magnetic particle. Periodically we find a condition known as alloy segregation or banding in the material. It generally is in the center 1/3 of the material running the entire length. Banding is usually retained austenite or un-tempered/un-hardened martensite. The banded area usually has high concentrations of nickel and other alloying elements. We have found that after the inspection, demagnetization of the part with an AC coil, verification of demagnetization using a field indicator and part cleaning with acetone, that if we dip the unmagnetized part in the magnetic particle fluid, iron particles are attracted to the banded area.
1. Is it possible that due to significant differences in permeability between the banded area and the surrounding martensitic structure, that different hysteresis exist thus making it impossible to demagnetize the banded structure?
2. Why wouldn’t the magnetism in the banded structure be detected with a field indicator? Is it due to the fact that the field remains circular thus no flux leakage and if so, why wouldn’t the field convert to a longitudinal field if was passed repeatedly through a coil?
3. Because of the extreme differences in permeability, is it possible the magnetic field in the banded area vs. the surrounding area would always be 90 degrees out of phase with each other?
It would be my goal to not only explain this to the NDT Layman but also mathematically explain the condition.

 

[MagMike]

It's possible that the nickel-rich and/or martesitic regions have higher coercivities than the other areas so they aren't demagnetizing as thoroughly. It would require larger demagnetizing fields to eliminate the magnetism. Also, the demagnetization field would be most effective if aligned with the magnetic orientation of the material producing the magnetism.

I don't know what you are using as a field indicator, so I'm guessing it is only set up to detect fields along one direction. It's plasuible you are missing the stray fields because the indicator is not oriented along the correct direction to detect the remaining fields.

I'm not sure what you mean by your 3rd question. If the material is cold rolled/drawn, then it probably has a preferred orientation. Is that what you are asking about?

 

[EdStainless]

in addition on #2 it may have a lot to do with the area of your field detector. This may be a localized issue and you detector may be averaging over a larger area.

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Plymouth Tube

 

[remetaper]

I fully agree with Edstainless about n°2.Pay atttention that Hc of 17.4PH is very high and depends on heat treatment condition. If magnetic particles are attracted to the banding area even after a demagnetization ( good or poor) it means that a large difference of magnetic permeability exists between steel matrix and banding area ( stringer ).
An idea about magnetic properties and permeabilty of 17.4PH can be seen in annex. This paper could be useful in order to understand both metallurgical and magnetic behaviour of this grade at different aging condition. Its magnetic permeabilty is included between 20 up to 200 approx depending on chemical balance and aging condition. This means that the stringer which produces magnetic indication must have a large difference in magnetic permeability when compared to steel matrix. Therefore, the first step is to know what is the composition/morfology of stringer/banding.Moreover,it could be very useful to know the dimension ( diameter )of inspected piece/bar and its heat treatment ( Condition A, h900,h1150 ans so on ).
Grade 17.4PH doesn't show any Austenite in cond A.Therefore, the magnetic indication will appear because the stringer has a strong magnetic permeability difference compare to steel matrix (i.e fresh and satured Martensite+Cu phase).
This banding could be:
1) Ferrite ( its magnetic permeabilty is > 10-20 time more than Martensite matrix) .Indications coming from Ferrite are usually found from center to 1/4 and 3/4 radius of bars. If chemical analysis of your steel ( see steel making certificate ) gave a high Cr/Ni equivalent, probability of Ferrite stringers are higher. Same situation if , during hot forging/rolling of bars, highest temperature were reached (zone of delta Ferrite under liquidus line)
2)Niobium-Carbide stringer ( its magnetic permeability is close to 1 as air,i.e. not magnetic ).Indication coming from Nb stringer are usually found well spreaded in whole section of bar/piece. Nevertheless, Nb stringer could appeare as a ghost line ( starting from 1/4 radius of bars and finishing at subsurface) if 17.4PH had been processed by Continous Casting where electromagnetic stirring didn't run in right way.

Finally, some evaluation about Austenite in this grade.
As said before, no Austenite (or retained Austenite ) in Condition A. ( 1050° C, air cooled). On the contrary,stable Austenite starts to form at 900 F ( 480 °C) if Cr/Niequivalent is very low and its amount will pick up in condition H1150 F ( 620°C ). This stable Austenite will be well spreaded in matrix of Tempered Martensite satured with Cu-rich phase and can be well evaluate by Rx -diffraction or ,approximately , by optical microscope. This structural situation doesn't give any magnetic particle indication.
On the contrary, if Ac1 point ( 630° C approx) were overcome, Austenite becomes unstable and transforms in Martensite when air cooled. This means that a new structure ( fresh Martensite, residual of stable Austenite and tempered martensie whose Cu-phase has lost its coerence with matrix)will be very difficult to de-magnetizing and you could find a not well defined indications.
However, if you could know the composition of stringer/banding by a SEM analysis , it would be easy to understand the reason why magnetic indication appears and corrective action to impose (or suggest) in order to avoid a "reply".
Finally, you obviously know the difference of magnetic indication caused by non metallic inclusion ( as Si-Al, MnS,ect ) and of metallic one as Ferrite, Niobium Carbide, Martensite stringer and "ghost line".

However, in case of doubt, inclusion can be see by naked eye or by loop or by LT when canceled magnetic indication. On the contrary, metallic stringer disappeares and no discontinuity can be found on surface if used the decribed above methodes ( LT and so on)
Sorry for my poor english.