Comparing Dry MT Results - With And Without Contrast Paint

Date 01-23-2015 21:32

Good photo. Thanks for sharing.

Now, I do have a question for you: How easily is the contrast paint removed compared to PT developer when welds must be repaired?

Just curious.

He Is In Control, Have a Great Day, Brent

By thirdeye (***)

Date 01-23-2015 22:43

The application is very light, just enough to cover (fog) the area. Even though it is called "paint", it does not have body like regular spray paint does, and it dries in a few seconds. It's main function is to provide contrast, but it does improve mobility for the magnetic particles and fills small scratches from grinding. As far as cleaning, power brushing works well as does a flapper sanding disc, there is a little dust involved, but you don't get the gummed up flapper disc's like you do with regular paint.

I caution welders when it is used on repair excavations, or on MT of a root pass.... on one hand it improves the ability to detect small discontinuities, but they have to be mindful when cleaning. For final MT or detection of cracking on in-service components, it is a great product. When excavation photos are needed, the contrast paint is a good tool for that. It also shows surface porosity and undercut very well. On bigger jobs it's also handy to tell at a glance which joints have been MT'd and which ones still need it.

By welderbrent (*****)

Date 01-24-2015 11:10

"On bigger jobs it's also handy to tell at a glance which joints have been MT'd and which ones still need it."

Good point. That can be a problem since powder residue is not always visible especially from a distance. It washes away in the rain, blows away in winds (at least winds like we get here in AZ), gets wiped away during additional work on the parts, etc. So do the paint markings we put by the joints 'MTOK'. Not that our records should not confirm the completion of MT but for a quick confirmation when the welding foreman or other workers ask it's nice to be able to glance over and say 'yes, done'.

Thanks for your response.

He Is In Control, Have a Great Day, Brent

By pipewelder_1999 (****)

Date 01-26-2015 23:54

I have used that before.

Another place the "developer" method comes in handy is leak tests on a gas tight component that is verified by liquid "leak test".

I did some 3rd party inspection where I was to inspect for "any evidence" of leakage on duct work and air pollution control equipment. On joints with multiple faying surfaces, it would take a considerable amount of time to both wait for the leak and also get back up in a lift to look (the procedure that was approved did not account for joint configurations that may take a day or so for capillary action to do its thing. Spraying the joints or at least starts/stop and corners would greatly help things become visible faster. On some of the above cases, I never saw any liquid but actually discovered cracks that became evident from shop "dust" gathering at the crack ever so faintly.

In addition, with the liquid leak test, you could wipe it off, spray it again, and see the exact location of the opening.

Have a nice day

Gerald

By thirdeye (***)

Date 01-27-2015 14:00

Gerald, you are right about PT developer and tiny leaks. I have a customer that builds small exchangers which require a PT of the tubesheets following welding, and for convenience, they have them on sawhorses in the hydrotest bay. Following acceptance, they are brought up to pressure and held for the AI who usually visits around 10AM. He has borrowed a can of developer many times when he needs help in detecting a small leak.

By thirdeye (***)

Date 01-28-2015 17:28

Al, the primary advantage is to improve contrast, however when the background is improved a secondary advantage is the fact small or really tight indications are easier to see. In come cases like a rough or pitted surface (typical during in-service inspections) the contrast paint really improves mobility of the particles, and eliminates some false indications like scratches or scars. About 10 years ago we conducted a series of tests with different products and under different test conditions, utilizing 4 or 5 MT technicians. We looked at sensitivity, time required for the complete exam, and accuracy. The test featured a test plate with surface breaking indications along a fillet weld toe, a butt joint test plate with surface a breaking indication in the root area which had been ground flush, a production piece that contained stress cracking from heat, and an item that had been in-service and containing a joint that had transverse cracking in a machined area which also had some surface pitting. We had several colors of dry powder (yellow, gray, red) to choose from, contrast paint, and some wet visible MT spray in black.

In all cases, the indications were detectable without contrast paint. But with contrast paint and the white background, the indications were easier to see and we found the examination took less time... both advantages. An unexpected comment from some technicians was they felt that they had more "confidence" in the test (and in MT in general) when using contrast paint, because the indications were "so much easier to see". Using an 8X magnifier, if there was a loss of sensitivity on the same areas without and with contrast paint, it was slight. Now, I do have the leg lights on all yokes (in AC mode) and we use fluorescent tube lights when working in DC.... so it's easier to light (and view) an area that has been prepared with contrast paint, verses a dull finish on some steels, or a glare on steel that has been flapped to shiny. Another thing I have noticed is when doing overhead MT on a painted joint, the white background makes it easier to insure you are getting a good puff of dust where you need it.

Here are a couple of more photos of cracking that show the sensitivity.

An unrelated advantage of contrast paint is that it really shows off surface porosity and undercut.

By 803056

Date 01-28-2015 22:57

Nice response and there is no argument that the paint improves the contrast.

Thanks - Al

By eekpod

Date 01-29-2015 12:04

Wouldn't you have to have that tested and written into your MT procedure??
It's been a while since I've been in the MT spec but I thought you weren't allowed to test through paint? I know it works but I'm talking about meeting the requirement of the specification?
Maybe it was just a company requirement I had, I don't remember all the specifics.

By Superflux (****)

Date 01-29-2015 13:54

Contrast paint does NOT increase sensitivity, only "Interpretability".
I've use the the "scratch the weld with soapstone"to do the latter.
Sometimes, ya just gotta go outside the comfort zone to make it happen.

By 803056

Date 01-29-2015 14:31

A number of standards require the contractor to demonstrate the NDT procedure is capable of detecting a known defect of a specified size. Provided the proposed procedure is capable of detecting the specified defect, all is fair.

The paint being discussed here produces a very thin film of paint that provides a high contrast background that enhances the ability to see the MT indication.

A number of contractors include a statement that the welds have to be in the "as welded" condition, free of paint to avoid the potential of thick shop paint obscuring very small discontinuities. The paint will also interfere with making good electrical contact when using direct magnetization, i.e., using prods and passing high current through the test piece. It is unfortunate that there are too many (one is too many in my opinion) that fail to differentiate between indirect and direct magnetization, i.e., Yoke or coil versus the use of prods.

Training and experience is needed to recognize the difference between a layer of paint that thick enough to interfere with the test results and the ability to detect very small discontinuities. There is no doubt that a part coated with a three part coating system will adversely affect the test results. However, a part that has a light coat of shop paint that is intended to prevent rusting during storage will have little adverse effect on detecting the type of discontinuity that is of interest, i.e., indications 1/16 inch long or more, overlap, etc.

If the discontinuity of interest is the early initiation of fatigue cracks, then I agree, any paint can be detriment to the test results.

The bottom line is that the Level III responsible for developing the NDT procedure must have the training and experience to know the limitations of the test method and the nature of the discontinuities of interest. MT, using a yoke, is one of the few methods that can be effective used to detect overlap or toe cracks along the toe of the weld even when there is a coating of shop paint. It is a tool, like any tool, that has to be used properly to get meaningful results.

In any case, the welding standard invoked by the client must be followed. If the standard states that the NDT must be performed before any coating is applied, then that is the requirement. If the fabrication standard includes no such restriction, then the Level III has more latitude to perform the MT even when the component has a thin coating. Is the MT being performed on new construction or is an in-service inspection being carried out? Many clients are going to balk at having to spend the time and money removing paint from 100's of feet of welds to perform the in-service inspection unless it can be shown the paint is interfering with the inspector's ability to detect the smallest discontinuity of interest.

I use MT to detect and monitor cracks in the welds joining very heavy members in large press frames. These presses have been in service for 50 years or more. The paint is a couple of layers thick, but smooth and tight. Cracks are detectable using a yoke long before the paint is cracked or shows visible signs a crack is present. Is there a chance a small crack is present and not detected? Absolutely, but the welded structure is visually inspected and monitored on a regular basis. Cracks that are detected are monitored to determine the rate of crack propagation. FEA is used to determine when a crack is approaching the critical size and when a repair or replacement is necessary. Just because there is a "small crack" present does not mean the structure is in imminent danger of catastrophic collapse.

MT is a tool that can be used to detect surface breaking as well as slightly subsurface discontinuities. It has limitations, i.e., it is not very effective at detecting porosity or discontinuities not oriented perpendicular to the lines of magnetic flux (hence the need to conduct two tests). Two tests are conducted, the second test is conducted to produce a magnetic field that is perpendicular to the initial test, to optimize the probability of detecting all the surface breaking and slightly subsurface discontinuities of interest.

If the test is capable of detecting slightly subsurface discontinuities, surely a layer of paint is no worse than a thin thickness of steel. One can use a Ketos ring to get a feel for the depth of a subsurface discontinuity that can be detected. The individual making the decision must appreciate the length to width ratio, orientation, size, and depth of the discontinuity will influence the probability of detection. One must remember that there is no test that will detect all discontinuities and in some cases a test can be more sensitive than necessary.

Best regards -Al

By thirdeye (***)

Date 01-29-2015 15:02

eekpod, Here are three excerpts from the customers specification which deals with contrast paint, they have other MT spec's which do not reference it. The first two are in reference to your question about a procedure, and the last paragraph is along the lines of thin paint like Al mentioned:

"Surface of most parts tested to this specification normally require no special conditioning except as
necessary to remove scale, slag, oil, embedded sand, etc. The surface must be free of any deterrent
to particle mobility."

"A light spray of whiting (white) or white contrast paint will be applied to all materials being tested. A
No.8A (red) powder (or equivalent) shall be used along with the white background to accentuate all indications
being noted. When M/T’ing a back gouged joint, be sure that all the contrast whiting is removed from
the joint before welding is to resume".

"This method may be used on parts with a thin coat of paint, providing the detection of discontinuities
is demonstrated to be substantially equivalent to that of parts not painted. However, at points where
the yoke legs contact the test surface, the paint must be removed to bare metal."

By eekpod

Date 02-24-2015 11:23

Thirdeye, looks like your all set, thanks

By Bill M

Date 04-07-2015 14:53

Sorry for late reply on this

We had an inspector spray some developer to do a MT on a backgouged root recently, however the preheat torch was underneith and idling...

He is OK but looks funny without his eyelashes!

This stuff is extremely flammable

By babakaiff Date 04-15-2015 14:56

Sorry on the very very late reply.

I am trying to receive clarification on ASME BPV Section VIII Div. 1 Edition 2013 UG-99 (k): Painting and Coating.
It states that "pressure-retaining welds of vessels shall not be painted or otherwise coated either internally or externally prior to the pressure test."

We are using WCP-2 for magnetic particle contrast paint before the hydro testing procedure is completed.

Anyone have any technical data that states that WCP-2 is considered painting or coating, or that WCP-2 has no effect on the ability to retain water in a hydrostatic test.

Thanks for your help, any information is greatly appreciated.

By thirdeye (***)

Date 04-16-2015 15:13

Regardless of the small amount needed (very light spray), I think it would be hard to prove that contrast paint is not a "paint" or a "coating". The same goes for PT Developer. That said I can't recall an AI requesting that contrast paint be cleaned prior to hydrotest, and one AI I work with actually prefers that PT developer be left on certain joints as it makes tiny leaks easy to see.