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root cracking

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  • root cracking

    I have a question for experienced pipe welders out there about root pass cracking when the root is left to cool before the hot pass or fill pass and cap.

    I have been told by a verteran to never let the root cool right off and to at least hot pass if its coffee time or the end of the day. So this is always what I have done.

    A month ago I welded and 20" steam header and when I got the root in the 2 fitters from Eastern Canada said it was coffee. I said no way and explained about what that veteran had said about letting the root cool. They laughed at me and said that they leave them all the time and its no big deal. I was hesitant but regretfully I went for coffee that day. Since then I have learned that I will no longer care what supposed experienced welders/fitters from Ontario or anywhere say. I will do things my way when I have to put my name to a weld at the end of the day.

    Long story somewhat short...the plant is now shut down for 3 weeks of maintenance and I will be doing all the flanges on the steam lines that drop down to this header. All 8 joints will be x rayed and I am wondering if maybe they might shoot that 20" joint from last month and find that indeed the root cracked slightly on me. What is your take on this? Have you seen roots crack on x ray from cooling off before the hot pass?

  • #2
    If the pipe was made from a material requiring preheat, then yes I would finish the weld and/or maintain an interpass temperature until the weld was completed. If you were rooting a 20" pipe without preheat, I would be surprised if there was much of a rise in overall temp of the joint by the time you were finished. A 20" header is one big heat sink. I think you may want to read what the weld procedure says. That should be the determining factor. The old boy that told you about that may have been passing on a bad habit of getting away with out preheating where it was required.
    Flash me! I'm a welder.

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    • #3
      All our written welding procedures call for a maximum of 5 minutes from the completion of your root pass to the start of your hot pass. I've always adhered to this requirement and never had a problem with x-rays. Rapid cooling will definitely cause a crack to form in your root. It's hard to say what your 20" joint will display if x-rayed, too many variables to make a determination or guess.
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      • #4
        What was the wall thickness of the 20"?

        6010, 7010, tig?

        What's the name of the plant you're working at?


        • #5
          I always but my got pass in right away I vet pipe welder tell me if u leave ur root in there unsupported it can mess with grain structure of pipe. I just did a steam line last week and put in my roots and hot passed b4 capping and I passed xray no problem.


          • #6
            Always rely on your experience and training and NEVER listen to a fitter!! They think that they know everything and they worked with a guy kinda crap. If they knew they'd be doing it. I always hot pass everything no matter what time it is you can always have a break later. Think of it this way is coffee more important than losing your job? Just saying. Jef


            • #7
              Your work is your name.
              Did you grind down your root before you started the fill pass? If there were any cracks, you would've spotted them.
              If you want to clear the conscience, talk with QC and explain what you did and they can take it from there. Better they know about it now than down the road.


              • #8
                short answer is to follow the procedure written for the project.

                What your really talking about is "cold cracking". Hot cracking happens as the puddle solidifies and you can see it happening.

                What your dealing with is the tensile strength of the material as it cools and contracts. If memory serves regular mild steel constracts at 0.0000059" per square inch (I might be off a bit but I think I am close, it is also depending on reaching the tranformation stage which welding does). Now this doesn't sound like much but it is what held the old horse drwan wagons wheels together after they heated and slipped the rings on the outside. As the metal cools and shrinks there is all of tension across the face of the weld. In the puddle state there is no strength in the weld. As it cools it develops its strength. Some alloys can take several hours to a few days to reach their full tensile strength. Your little root pass even if it is 1/8" 6010 or 7018 isn't going to be a match for the strength of the pipes as they cool. The hot pass is stretching out the cooling time as well as adding throat to the weld. This increased thickness is giving you the strength needed to resist the crack as the pipes try to tear the weld apart.

                Hope this helps.


                • #9
                  Root Cracking

                  Leaving for coffee after Root Pass.

                  You didn't give us much info but I will assume you TIG rooted cause it's a steam line.

                  If you didn't TIG root - run man run........

                  Everybody here assumed 7018 I would imagine a 20" steam line is probably 1 1/4 or 2 1/4 chrome moly at a minimum. ER80XX, 8018 or 9018, etc. IMO you should have at least put in a Hot Pass and maybe some fill in some areas (kinda like "Bridge Tacks") maybe like 5 or 6 places around the pipe
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