Cracks appear after PWHT of a Fire Tube

[dennjay]

A 30" Dia. fire tube from a Treater has collapsed and was sent to reapir. this fire tube has been collapsed few times before.
Once MPI was carried out we found only one crack on mitre weld. Then we ground the crack and re-welded and sent to oven for stress relieving.

Once the tube arrived from the oven, we did MPI again. Then we found more cracks in un-repaired areas. Most of them are on Cir-seams and in tube-to-tube sheet areas. Some of cracks are 3 - 6 inches in length. The material is SA 516 gr.70. Can anyone tell me what are the causes of these cracks to appear? Quick response would be greatly appreciated.

 

[metengr]

There is really not enough information to provide help. Was this 30" diameter tube post weld heat treated in the past? Please describe what you mean by collapsed? What was or is the service conditions of this tube? Was it new?

Off-hand, when I have seen components that were fabricated w/0 stress relief and subsequent cracks are found it could be a sign of hydrogen or delayed cracking associated with the welding process. These cracks can be subsurface and driven by PWHT.

If this fire tube was in service and repaired, we nee to know the service conditions.