Weld splitting.

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I'm having problems with a groove joint with a spacing of 3/16 backed with ceramic. The plates are 1" and 1 3/4" respectively, the weld plan calls for a FCAW (1/16) root pass followed by 2 angle welds then filled with SAW.The root pass makes a nice thick bead under the ceramic but is cracked up top. I'm following AWS for machine parameters, and all the tests we did at the shop were fine. The big difference being that the piece is very large (154 feet). I removed surrounding tacks to give it some breathing room and heated it to 200 centigrade but the top still cracks, even in some spots where it looks fine, it fails the MRI.I'm thinking it might just need more wire, because the weld is concave. But at the same time, we are 2 FCAW certified guys who tried this weld and we are afraid to loose penetration if we try to let the weld fill. Anyway, I'd like to hear you opinions before we call in an engineer tomorrow. This thing has to pass a complete X-ray.thanks.
Reply:Don't know the answer but with a nick like 'Grytviken', you'd have to be swedish?
Reply:What material are you welding?  What filler metal?  What is the width of your weld deposit, and what is the depth of it?  How are you applying and measuring your preheat?  What parameters are you running?
Reply:I snooped around more. Seems that concave welds are prone to cracking. It's all mild steel. We went down to .52 and the problem has resolved itself for the most part. Usually it happens near the end of the weld though, we grind em out and run the MR machine along the rest to make certain we didn't miss anything. If I may another question, is there any way around getting a blasted fish eye at the end of every root weld?
Reply:Originally Posted by GrytvikenI snooped around more. Seems that concave welds are prone to cracking. It's all mild steel. We went down to .52 and the problem has resolved itself for the most part. Usually it happens near the end of the weld though, we grind em out and run the MR machine along the rest to make certain we didn't miss anything. If I may another question, is there any way around getting a blasted fish eye at the end of every root weld?
Reply:Originally Posted by LlundbergDon't know the answer but with a nick like 'Grytviken', you'd have to be swedish?
Reply:That's a high restraint joint when one section has more mass than the other. The 1 3/4" side is still growing when the 1" has stopped and may be starting to contract.Cracks can develop at the weld stops from shear. I've never been around anything like a 154' weld, but with HSLA the larger section can be soaked with heat longer or heated while welding to try and get it to keep up with the smaller section. I would think mild steel should just move around OK, but maybe not.(edit) +1 to the crater fill above, or just stop the weld with a few short trigger pulls and peen & grind right way.MattLast edited by Matt_Maguire; 09-10-2011 at 01:26 PM.