Magnetic Particle Indications 5

No, these are not ferrite stringers. The axial-oriented indications are one of two possibilities - surface imperfections from forming operations or they are inclusions (nonmetallic compounds that are present from steel-making practices and align in direction associated with hot forming).

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.

Hi Magnut

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.

Is this welded tubing?

Just to recount, I used to work as an NDT tech at an aerospace sub-contractor and we often had similar indications. I seem to recall low alloy carbon steel tube as well as PH alloy steel machined parts showing similar indications, maybe for different reasons. After similar sanding and inspection under a magnifier I used a very sensitive PT method which revealed nothing. So the items were accepted on the basis of "spurious indications due to metallurgical effects". Both CR Preston and NN Peters in BINDT journal Insight have written papers on this (or similar) phenomenon. Unfortunately the link to CR Preston's paper on the BINDT website is broken and Peter's paper is earlier than their database archive starts.

As you have seen this indication "often" have you had any investigative work done or have you accepted them as I did?

ndeguy;
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".

metengr

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!

Good feedback from all of you and I do appreciate it. I kind of think that this grade is more suspect to ferrite or stringer indications than the more refined PH steels. I think that when we do find the condition, the most logical approach is to perform metallography and build some history. We recently had the same condition with 17-4 and found that it was banding with retained austenite. We had seen previous conditions of banding that were either ferrite stringers or non-metallic inclusions. The acid test (no pun) on this was that the austenite will not etch. The reason for my inquiry was that the occurence appears to be on the upswing. I do have photos but sure how to share them.

Magnut

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.

One thing we have encountered in alloy tubing is the presence of Lauders Lines or Stretcher Strain Lines.

These can be very prominent if the magnetic flux is very high. Reducing this flux will minimize these indications.

I haven't moved to the analysis part yet. We have been exploring the approach - magnetic particle at 1/2 amp value, using residual magnetism, degreasing and performing FPI.... We inspect aerospace parts and I don't have a confort level of accepting based on a catch all phrase. Last week - Air Bus reported a crack in an engine mount that was due to alloy segregation. I guess that is ok if they have 3 engines left!!! We plan to seek funding and qualify the effect of these indications with fatigue and mechanical testing in the indication area. It appears a lot of the dispositions to use this condition is based on the function of the part. If the part see's side loads or dynamic stress it is viewed in a different light than one that is static.