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Steel Pipe defects

Steel Pipe defects

Steel Pipe defects

(OP)
I was using a t-mic wall thickness gauge to verify the wall thickness of column pipes that were delivered in the field.  The pipe was either API 5L-X42 or ASTM A252 Gr. C pipe, 66"Øx0.875".  This section of pipe is 40'-0" long and is made up of five 8'-0" long cans.  The pipe was rolled and welded, not spiral welded pipe.  I checked the thickness of each can as I went down and all of them were coming out at roughly 0.880" thick.  The last can measured 0.413".  I moved the mic over about 2" and it read 0.88".  I started checking a bigger area around this initial bad reading and found a somewhat random spattering of good and no good readings.  The measured thickness is 7/8"; there is no pitting.

The gauge was consistently reading bad where the bad readings were found and consistently reading good where the good readings were found.  There were a couple of spots where the t-mic provided a good reading with the mic axis running one direction and a bad reading with the mic axis perpendicular to the first spot.  Maybe the edge of an internal imperfection?

All the bad readings ranged from .390" to .430" – right about half the thickness.  We cut out a portion of the pipe so we could see if we could find some cracks or delamination, but it was impossible to see anything on the torch-cut edges.  Oddly enough, even though the steel in the coupon tested no good in the pipe, after it was cut out, it tested fine everywhere.

I don't know what to think about this.  The bad spots seem to be so small that they probably won't cause a problem, but so many of the locations tested badly.  Because the bad readings are all roughly the same, I think the delamination, cracks, or other imperfections don't run through the thickness, but rather parallel the surface of the steel.  Does this seem reasonable?

Have you had a problem similar to this before?  Is this a common problem?  Do you have any suggestions of testing we should perform, or questions I should ask the pipe supplier?
 

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