Grinding and Liquid Penetrant examination 1

monirri · Feb 16, 2011

Hi. I was told in my Liquid penetrant course that surfaces should not be grinded prior to testing, cause that would cause litlle round indications to hide. Now i am working in the pipeline industry, and a quality control engineer with far more experience tells my, after I finish the LP test, that the final weld should be grinded so that spaces between welding passes won´t hide real indications. You know, that would mean that the joint should be "blended" to be smoother, cause wipers don´t always reach that spaces, so penetrant remains, and it looks red. That would be bad for joints, specially if the final pass is ugly or if it is made with 6010 or 7010, because of the "waves they let". What is the best point of view? It is necessary to clean toroughly with the wiper again and again to avoid grinding? Thanks in advance

unclesyd · Feb 16, 2011

I don't know where your instructor is coming from as you state you have a lot of trouble checking stringer. There are areas that no ammount of cleaning with present a surface for good PT inspection
Most welds will need some touching up by grinding or sanding prior to a PT test due to the reasons you state. Remember to grind normal To weld bead direction, cross wise. Never turn a grinder on edge to dress the toe of the weld where you usually find cold lap. Never try to make a weld bead pretty.

monirri · Feb 17, 2011

Unclesyd, can you explain that about grinding normal? why? Something i have to tell you is that PT we use it here 98% for fillet joints, requested by the client. So, the grinder will only reach the weld in the longitudinal way. Thanks in advance.

ndeguy · Feb 17, 2011

wire brush, blow away detritus and cleaner application and dry should be sufficent surfacec preparation initially. In case of difficult interperetation, then light grind, clean and retest required. Depth of red colour gives some indication of whether trapped penetrant due to geometry (light red) or bleed-out (deep red) from toe crack, but grind after test if required is acceptable

unclesyd · Feb 17, 2011

There are sometimes you have grind parallel with the weld bead as you case. You have to be very careful in grinding using a fine grit well and making the bead as smooth as possible. When grinding longitudinally we use a sweeping motion starting in middle to cross the toes of the weld bead without leaving a trench. If you material is CS a carbide rotary burr will work in some places. Make sure you have enough horsepower.
I may get pounded one but there is no set way to PT test a particular component. It is up to the technician to be able to slightly modify a PT procedure to get the highest quality inspection. The only caveat is to appraise the owner and get permission as to what you are going to do..

deco0404 · Feb 18, 2011

you grind the caps on pipelines to avoid misinterpretation of defects during radiographic interpretation.

ie the gap between two stringers may be on or close to the edge of the root, and what may well just be a gap, can look like a serious root defect....thats all.

Penetrant is probably too sensitive ( not sure thats the right word......) for carbon pipeline welds, the carbon can hold too much penetrant at times. Why not use MPI, much quicker and easier

weldtek · Feb 18, 2011

Any surface preparation must be performed in a manner that doesn't 'smear' the metal. If you've ever tried to grind aluminum with a disc designed for use on ferrous material you'll know what I'm talking about. That smearing action can result from other activities such as wire brushing, sand blasting, and, in some cases, grinding.
Use of the proper media and technique are important. For example using a hand held SS welders wire brush is not likely to smear metal on a ferrous application, but, use of a stiff braided wire wheel on a large power tool will likely smear metal where ever it's used.
As in most things there's no one right way that covers all situations.

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Grinding and Liquid Penetrant examination 1

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