Magnetic Particle Inspection

[metboss]

Hi all,

We have placed order for 13%Cr tubing (API 5CT L-80) which has premium threads cut on both sides of it.
As per our specification, it calls for wet fluorescent MPI of threads. Now, vendor has come up with clarification that wet florescent MPI would cause contamination of iron oxide particle with stainless material. They are now proposing Liquid penetrant inspection (LPI) in lieu of MPI.
As far as we know, 13%Cr material is a martensitic stainless and it still attracts a fair magnetism since it is ferromagnetic in nature and moreover, wet florescent MPI is more sensitive than LPI for threaded area.

What we believe that LPI costs less compare to wet florescent MPI and that’s why, they are proposing LPI.

Please advise your comments

Regards

 

[metengr]

The argument is silly regarding not performing MPI of 13Cr steel because this is a martensitic stainless steel. MPI is performed routinely on this material, and should be performed.

 

[EdStainless]

Yes, silly reason, this material is barely stainless in the first place, and secondly they should be cleaning it after testing.
Either method can be fine, as long as the sensitivity is specified. Make sure that you have called out a good specification and sensitivity level for the testing.
I can't see the cost being significantly different unless a shop is set up to do one method more efficiently than the other.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

I never enjoyed MagnaFluxing cut threads. All the sharp edges and corners and tearing especially in the root of the threads often meant non-stop "indications." Worse, once in a while there was a real problem hidden in the mechanical mayhem.

A process like fine glass beading did a nice job deburring all the ugliness, but is generally forbidden prior to mag particle testing

 

Do a better job of cutting threads cleanly and the inspection method becomes less critical.
Nevertheless, if they were my threads I would request fluorescent MT.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[WKTaylor]

Metboss... my 2-cents...

Your vendor's argument for LPI VS MPI fall short of technical validity... but may still be 'OK' in the larger scheme of things.

Your opening statement "...13%Cr tubing... which has premium threads cut on both sides of it." ... suggests threads are cut on the ID and OD the tube end[s?].

In my experience, MPI on interior surfaces... especially female threads within deep hollow parts and attaining the proper axial/circumferential magnetic polarization... is pretty tricky... whereas FPI is relatively easier/reliable to accomplish on all 'wetable' surfaces... assuming accessibility for follow detailed visual examination [problem for both methods, this situation?].

CAUTION.
NDI Semantics [aerospace?]. Liquid Penetrant Inspection [LPI] is typically a low-sensitivity inspection [level 1 or 2-MAX] procedure/materials for relatively gross defect detections in ordinary white light [and would only be used for quick/dirty field examination in aircraft]. Fluorescent Penetrant Inspection [FPI] methods/materials typically have various levels of sensitivity up-to-and-including level 4 which can detect very small/subtle defects... fluorescent dyes have much finer particles for penetration and UV-fluorescence for 'enhanced visual detection'.

NOTE.
As suggested, cleaning before/after NDI/NDT is essential part of the inspection process.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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[racookpe1978]

Your parts, your specification. But I ask why Mag Particle exam after cutting threads (ID or OD, but particularly ID (female) threads? If you have questions (doubts) of the metal forming or metal consistency in the threaded area, do the dye pen check prior to thread cutting.

Properly cutting (shallow) threads with proper lubrication and sharp tooling in a new part should not create "new" metal flaws or below-surface indications (tears) that would be found by a NDE dye pen exam.

 

[metboss]

Thank you all for your swift response. We instructed our vendor to proceed with WFMPI which can detect fine cracks with better visibility of sharp indications under UV light.

 

[blacksmith37]

When we first wrote the 13Cr spec we only considered MT for SEA ( special end area) ,( maybe it changed since I retired ). But in visiting many mills and inspection shops around the world , I can't remember anyone doing LPI for SEA. I wonder how competent and experienced your vendor is.

 

[metboss]

Thanks Blacksmith37.