MT and Porosity

By ctacker (****)

Date 01-24-2008 21:37

I think if it did, it would be fuzzy and look more like a subsurface discontinuity! I don't have enough experiance in it yet to know but I am pretty sure you'll get the same answer from someone more experianced.
Also, you should see porosity with VT and save the MT for locating cracks!

By CWI555

Date 01-24-2008 22:50

Yes it can depending on surface or subsurface, flux density, surface condition, size etc.

By thirdeye (***)

Date 01-26-2008 18:24

In addition to what Gerald said .... Prior to MT inspection, we prepare most weldments with contrast paint. I can't tell you the number of times smaller pores are missed with the initial VT, but are almost impossible to miss after painting. (the same goes for undercuts)

~thirdeye~

By CWI555

Date 01-26-2008 18:52

Nice picture, makes your point clear.

By jmdugan10 (*)

Date 02-01-2008 16:47

MT will not find pin holes, and porsity is ify at best. The flux field doesn't escape from a round indication, no difference in poles, no flux field. PT is best for verification on round indications.

By hogan

Date 02-01-2008 17:14

being that mt is evaluated to visual criteria for aws d1.1. i would think that a rejectable indication would hold powder. not to mention if it would most likely be seen first, seeing how it would need a visual inspection prior to ndt.

By CWI555

Date 02-01-2008 20:42 Edited 02-02-2008 21:55

Wow, didn't know that. I guess ASTM E125 should strike porosity from table 1, ASME should remove it from Section V, AWS should remove it as well.

E709 defines an indication as 16.1.1 Relevant Indications--Relevant indications are produced by leakage fields which are the result of discontinuities.
If all of those groups will allow MT with proper procedure considerations to search for Porosity, I guess they are unaware of this definition. By your
statement the flux field doesn't escape.. I suggest writing your paper and sending it in if you've proof of that. you'll make a fortune.

The only thing I'll agree with you on is it's not suited for small pores. But the idea that you can't get a flux leakage field on all of them I'll not agree to.
It would not be allowed in any of the mentioned codes as an MT indication relys on a leakage field (as you stated, poles being set up) to create the indication to start with.

By jmdugan10 (*)

Date 02-04-2008 13:13 Edited 02-05-2008 14:13

I'm not saying that porosity CANNOT be detected by MT. All I am suggesting is it is not the best method for detecting it. No one NDT meathod is the end all be all of detection. MT is best for finding linear indications. If a flux field does occur in a porosity pocket the indication will be weak at best. This is especially true if you are performing fluorescent MT. If you use dual sensitivity or dry you can atleast clearly see the porosity visually, but under just a UV light those indications could easily be missed. Circular fields will not show much of an indication.
You can use a set of plyers to break a nut loose but a wrench is a better tool for the job. I would suggest PT for finding porosity. Also in the 2007 Spring issue of inspection trends in the "Best Practices" page 34 it states that MT is a marginal application for detecting porosity.