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Can defects appear after Stress Releiving

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  • Can defects appear after Stress Releiving

    Friends, I am involved in making fabrication components for power industry generally of low carbon steels.Presently we got our weldments ultrasonically tested before and after stress releving. I am intrested in knowing that can I skip NDT after stress releving after 100% NDT before stress releving.
    Is there any chance of reappearnce of any defect after stress releiving in the weldments.

  • #2
    are you finding many defects before stress relief??? if not move the ultrasound to the end of the process, and only do a visual to the parts prior to stress relief.

    stress relief should not cause new defects to appear

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    • #3
      thermal stress relief can and will cause defects to appear. As the thermal expansion can cause cracks to open up or propagate.

      also other cracking types such as Hydrogen cracking will occur on a delayed time scale. It's not unheard of for parts to pass inspection only to crack days later. I'm not sure what type of inspection and code your following but generally there's a requirement to wait so long before performing NDT to allow for creep and cracking. Then again low carbon steels aren't too susceptible to hydrogen cracking.

      In the codes I work on it's usually up to the engineer, but there's no way we could get through a thermal stress relief without some additional NDT prior to shipping. If we aren't expecting high fallout we'll wait until more fabrication steps have taken place before inspecting again.
      In case of welding emergency
      use a bolt

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      • #4
        thanks very much to both of you.

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        • #5
          On "welded in stresses".

          Most of you have been welding for several decades. I repsect that.
          I on the other hand have been machining thru / across / around
          welds for decades.
          My first welding coach showed me how to make weldments,
          for tooling in general that were virtually "distortion free"
          and to some extent "stresss - free".

          Basicly other than say a pipe-weld, or any weld that will become
          one 100% joined object, stresses are needlessly caused by
          insisting on slam-butting components together and welding away.

          If you had cut thru the thousands of welds I have, you would see the "coined"
          profile of one componet mashed into the other as the welds cool.
          Remember I have built coining dies, we are talking in excess of 8 - 10 tons
          of force per inch of weld, to acheive this coining.

          On a T-joint a couple of stands of thin wire say 10 - 15 thou placed in-between,
          and the result will be distortion-free, and very little contractive stress
          introduced into the weldment.

          I realise there is a difference between one tool-room machinist
          making a fixture how-ever he pleases, and a pro with a print
          and a process to follow, just the same be aware of the cause
          of this type of stress.

          vg
          Last edited by vicegrip; 12-26-2009, 06:24 PM.
          sigpicViceGrip
          Negative people have a problem for every solution

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          • #6
            What thickness of material do you typically work with? Is your NDE Phased-Array UT? What are you trying to check for with your NDE?

            I can't think of, off the top of my head, a reason to perform NDE both before and after stress relieving. A skilled NDE technician should be able to detect flaws either way, although SR may "open up" some cracking and make it more obvious.

            BTW CS should only be susceptible to hydrogen embrittlement and cracking when operating above the Nelson curve (need poper temp and hyd. partial pressure).

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