I'm not sure why AWS does a few things but I can state this from experience.

1) It's difficult to control preheat/interpass temperature on plug welds. I was involve with welding A 514 Fan impellers which had plug welded blade liners. We had cracking problems often. The cracks would extend underbead to the A514. The plate was preheated in accordance with approved welding procedures. It was very tempting for welders to weld the plug all the way up. When this was done the base metal became very hot and the same affect as excessive interpass was reached (The base metal was annealed)

2) Small diameter circles in general are difficult to weld without cracking on heavier sections. I would guess that there is a great amount of strain develpoed in one localized area. This probably applies to plug welds. This strain can be reduced by preheat. Q & T Steel can be damage by excessive preheat and this cannat be determined by normal NDT methods.

3) The quality of welds on Q&T steel is difficult to obtain with normal methods. RT MT PT and UT do not assure a good weld. The process must be monitored to assure heat input limitations are not exceeded. I don't believe there is any NDT method that can provide this information.


Have a nice day

Gerald Austin http://www.geocities.com/pipewelder_1999/