Weld Terminology 1

[rmillercwi]

I am writting a report to the customer regarding repair methods of cracked fillet welds and butt welds found in the belly pans of steam drums. Carbon steel to carbon steel.

Cracked welds of this nature are fairly common. I know how to fix it the correct way, however the contractor typically does nothing more of a repair than simply welding over the cracks. (again and again and again)

The question:
Is there a definition of this type of weld repair? Where as the the cracked weld remains as-is and simply covered up with more weld. The closest I come up with is calling it a plug weld.

 

[metengr]

For weld repairing steam drum liners there are various approaches; defect removal and weld repair (not weld repairing over cracks, this does nothing), or use a top hat repair where in this case a formed piece of steel sheet (saem thickness as the liner) similar to a top hat configuration is placed over the cracked seal weld (the weld used to join liner sections in the steam drum) and is completely fillet welded to the surrounding liner material.

 

[mthakur]

Hi,
I am not sure whether the question was regarding the liner repair. I have the similar problem with our steam drum and would like to have more input to it. Apparently during fabrication, heat treatment was not effective and now we have cracks in almost all the nozzles. Each shutdown these cracks are measured for compared with critical size which is a very expensive proposition. Does any one have similar experience of having this problem, and the behavior of these cracks with time. Do these cracks also grow in size and instensity and what is the mechanism (the shell material is carbon steel.)
regards

 

[metengr]

mthakur;
The nozzle cracks are probably corrosion fatigue, based on what I have seen from prior steam drum nozzle cracks on one of our B&W units some time ago. This unit was cycled, which really drives the crack propagation rate.

The cracks are not necessarily self limiting in depth as you would expect from a thermal fatigue damage mechanism because you have corrosion that helps propagate the cracks. The corrosion fatigue cracks normally initiate along the toes of the nozzle attachment welds.

Repairs should be performed (remove the cracks and weld repair using one of the NBIC alternative welding methods) along with blend grinding to reduce stress concentration at the weld toes.

 

[mthakur]

Thanks for the info, in our case all the cracks are sub surface which looks like the case of thermal fatigue. Its interesting to know about corrosion fatique in steam drum, can you please elaborate more about it and corrosion mechanism.
regards

 

[Mr168]

In regards to the original post, there some exceptions in regards to welding over cracked material known as a weld overlay. Its commonly used to remedy stress corrosion cracking. The overlays typically extend well beyond the crack boundaries, and the toes are feathered into the base material. It sounds as though your "repair" man was just running a bead over a crack, the technical term for which is "garbage".