Magnetic Particle Indications
Magnetic Particle Indications
(OP)
We often find magnetic particle indications that run in the long axis of tubing made from 4340. It is sometimes dense and runs the entire circumference. When we sand the indications, they remain intact. Is this a common occurance for tubing material (i.e. are these swage marks)? Is it possible that ferrite stringers are more common in tubing materials than bar stock?





RE: Magnetic Particle Indications
I would suggest taking a ring (cross section) out of one of the tube sections containing the surface inductions and perform a metallographic examination. This examination will be able to help you determine if these are surface imperfections or inclusions.
RE: Magnetic Particle Indications
Everything in your first 3 sentences makes it sound very similar to alloy phase segregation as observed when inspecting precipitation-hardening steels by MPI. However you say this is 4340 tubing which is a low-alloy carbon steel and not a 17-7PH.
Judging by your description though I would say that the mutlitude of axial indications all around the circumference are NOT caused by inclusions or surface imperfections. However they are real indications in that they are caused by magnetic permability variations within the metal. Whether these are due to phase segregations or some other cause, as metengr rightly says - only a metallographic examination would provide the answer. Severe deformation of a metal can give rise to magnetostriction which MAY in turn induce sufficient permeability variation to generate the flux leakage necessary to form the MPI indications.
Please post and let us know what your outcome is.
RE: Magnetic Particle Indications
RE: Magnetic Particle Indications
As you have seen this indication "often" have you had any investigative work done or have you accepted them as I did?
RE: Magnetic Particle Indications
While I understand your position, I would disagree with your resolution. We receive many boiler tubes and piping at our Power Plants. In situations where we perform wet fluorescent MT and indications are reported, follow-up action in the form of metallographic examination assures closure of the item. I do, however, like the catch-all phrase
"spurious indications due to metallurgical effects".
RE: Magnetic Particle Indications
In my first post I agreed with you that metallographic examination would reveal whether the indications were "non-spurious" IF sectioning and examination revealed discontinuities. However, as was the case on an occasion when an item was sacrificed to "get to the bottom of it", when NO discontinuities were observed all we could do was tick "macro defects" off the list of possible causes. The items still needed disposition and with no evidence of macro defects and with information from metallurgists and papers regarding phase segregation and other metallurgical phenomenon, we had no cause to reject them. Observation of indications requires interpretation of those indications. It would be wrong to reject them when further investigation than the mandatory test required revealed no observable cause. These indications tend to be multifarious very fine straight lines of varying lengths and intensity of response under UV light. Sometimes between the linear indications the particles could form a dusty look almost as if the the part was flux-saturated even though the technique was based on the then BS M35/BS 6072. They occur all around the circumference and sometimes even on the radial faces.
I am interested as to the outcome of Magnut's particular investigation and disposition. If you cannot resolve the problem Magnut you can borrow my catch-all phrase!
RE: Magnetic Particle Indications
RE: Magnetic Particle Indications
I emailed BINDT regarding the broken link for Mr Preston's paper which deals specifically with these indications in PH steels. Whilst the Editor could not immediately fix the link, he was kind enough to send me Mr Preston's paper as a PDF file. If you would like a copy please contact me and I shall forward it to you.
RE: Magnetic Particle Indications
These can be very prominent if the magnetic flux is very high. Reducing this flux will minimize these indications.
RE: Magnetic Particle Indications