Failed X Ray

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hey im new I have a quick question for the forum but first hear my storyI have only been welding for 6 months (3 months on pipe) I took the asme 9 6g test with a 6" schedule 40 pipe...i prepared the coupons as instructed by the instructor then i tacked them but the coupons werent properly aligned so he made me do it over...i tacked at the 6,2 and 10 o' clock went on to my root, I welded on 72amp, made a near perfect root he checked it out he had one minor discrepancy but told me to move on to fill....i cleaned the root by trying to remove every dark spot with the grinder and when i was satisfied i used the metal brush to finish smooth it...as i moved on to fill i was scared that i might bust through my root like what happen to me earlier that week so i only turned the amp up to 74....after fill i noticed the fill was kinda leaning to the bottom so i went over the top again......i shaved off the excess and slag and drew my lines with grinder to proceed to cap...i turned the amp to 81 then proceed to cap he checked my cap and told me it was a lil higher than standard so shave it down i did so..he came back and passed me...i was confident with result.That was fine and dandy but now the x ray results came in and i failed but other classmates of mine had caps that was completely grind flat and inside of their pipe was dirty but they pass....do you think me filling again over my fill probably caused slag entrapment?...i scratched ahead of my weld travel everytime to avoid porosity, any lil splatter i grinded off, i cleaned up inside the pipe with a file and wire brush and it was smooth as a baby butt....i dunno what cudda happen any suggestions??
Reply:An Xray of a weld shows the density.  Its about like looking at an xray of a bone.  Lets say you look at an xray of your leg and you see the bone is cracked.  You can see the crack (break) in the bone as a dark line and it stands out pretty well since the bone shows more clear or more lighter on the film.Why the film looks like that is because the density of the bone stops more of the xrays from getting through to the film so the bone shows up as being lighter.  The crack in the bone (or the break in this case)  is less dense (or not dense at all) and the xrays pass fairly unrestricted so the film is more effected and shows more dark.On a weld the density of the metal slows or blocks the passage of xrays so the thicker the metal the lighter it shows on the film.  A very thick cap shows up very light compared to the surrounding thinner metals.  Voids (or places that are less dense) shows up more darker (or even as black if severe enough).  In this case the pipe should look fairly dark on the xray film and the thicker cap is fairly light - but any voids (usually called slag) inside the weld shows up as dark or black areas or lines (defects).Voids or imperfections in the root pass can be bevel that was not caught, gas pockets, suck backs, internal undercut, etc, etc.  All these defects shows up as black or dark lines or areas in the film.Undercut along the edges of the cap (places the metal is thinner than it should be) shows up as dark lines or areas on the xray film (defects).  Remember density.  When an xray hand says the weld was clean or clear, he is simply saying there was no black or dark areas (no defects). lolIt would be impossible for anyone to tell you why you busted the xray without either seeing the repair marked on the pipe (wrote up) or hearing from someone what the problem was, or looking at the film and knowing what to look for and what the code allows, and what code the weld was under.  Guessing doesn't help anyone, nor does it prove anything." do you think me filling again over my fill probably caused slag entrapment? "  Again who can say without being there and watching you do it.  I fill over top of fillers all day long every day on xray pipe without trapping slag - how?  Because I know when I'm not getting it.  How do I know that?  Because I'm standing there watching it clean as I weld.  I'm not guessing if I'm getting it or not  - I know when I'm getting it because I'm watching what I'm doing and I'm seeing it come out.  If I were to find myself in a situation where I don't know if I'm getting it or not on an xray weld then I stop and fix what the problem was.  I don't keep going and guessing if I'm keeping it clean.  That would be insane.  That would mean I need to get lucky to pass the xray.  That I can't recommend anyone try to do.Good luck manLast edited by slowhand; 04-30-2011 at 10:51 PM.
Reply:Radiographs are neat.  If you failed, you might want to look into seeing the actual radiograph and it will give you a good idea of where you went wrong.  When I was at the vessel shop and had to do tests that were radiographed, I would head to QC to see how I did!  UT's are neat as well; as opposed to a visual picture, the test is done via bargraph.Why you are welding pipe after 3 months of welding is beyond me.  You should be training to fabricate and/or gouge things apart and putting them back together before you learn to pipe weld.  I cannot think of any company that has hired anyone to pipe weld fresh out of school.Last edited by mb_welder; 05-01-2011 at 10:46 PM.