fazel1,
I have seen it done in order to verify the integrity of the root pass prior to welding it out. No code mandate that I
am aware of.
Regards,
RLS
The engineering reasons are primarily based on costs of repair should a defect occur in the root. RT after completion of the root and hot pass or after 3/8" deposited weld metal is often used for heavy wall pipe welds not accessible to be repaired from the ID surface.
We employ 200% RT on very high alloy ethylene cracking furnace tubes. The material welds like chewing gum and cracks with the slightest deviation of welding technique. 200% RT has earned itself as a standard step in weld QC in this case.
We might also employ 200%RT for common grade CS pipe welds where someone submits an approvable golden weld hydro waiver request.
fazel1, IMO it depends on the wall thickness, material, inspection criteria, PWHT requirements, and most of all the level of welding talent available. At the beginning of a job I'll RT after 3/8", final, and post-PWHT. If the talent is there based on weld quality of first few RT's, I'll only RT final and post-PWHT. My experience is that MT/PT doesn't do much for finding root concavity or LP problems. MT can be performed relatively "hot" though, so can RT if film is protected by insulation barrier. That said, always take the last RT at ambient temp. As Rob said, "Weigh it up against the benefit."
I have heard of one job where each weld pass had to be RT'd. I do not know if ASME Class 2 or Class 3 mandates this process. Needless to say this was a nuclear power job.
This is directly related to repair costs after the job goes into service (I cannot even imagine how high a repair cost could be...millions?)
Dear fazel1,
if you want to find economical way that is better you have qulified welder & qulified welding inspector that control during welding.also if the thickness of the material is more,if you can performe PT or MT is not bad but you should be know only open surface defects specify by PT and with MT ( DC current) you can find the defects up to 1mm under surface .So 200% RT is performed for alloy steel matreials that have PWHT,first step befor heat tratemaent and second step after PWHT.
mostly used on nuclear components, not only safely guarrantee the rootpass but will avoid any crack propagation in the future passes, so it becomes very economical/cheap approach.
er
