what is main cause of such crack? 1

[fattema]

After only 3 moths in service Crack was observed adjacent to HAZ of nozzle to shell fillet weld of new constructed Methanol Drum (vessel). Crack occurred on shell perpendicular to weld bead. There is no cyclic stress.
Nozzle dia is 3” thickness 18mm
Shell wall thickness 6mm
Material Carbon steel
The drum shell is thinner than the pipe nozzle thickness which is 1/3 thickness of the pipe nozzle.

 

[desertfox]

Hi fattema

We need much more information on your problem one possible cause could be hydrogen embrittlement but there are lots more.
Look at this site:-

http://www.hobartbrothers.com/aboutus/understanding_weld_cracking/

desertfox

 

[EdStainless]

Look into the original NDT on this weld. Minor imperfections in the weld fusion or shape could add to high stress in these locations.
That is not to mention outright defects.

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Plymouth Tube

 

[metengr]

Usually, cracks oriented transverse to the weld line is delayed cracking caused by hydrogen embrittlement. I would perform a surface NDT using wet fluorescent MT of other similar welds just to be sure no other cracks are present.

 

[stanweld]

Agree with metengr. The original weld may not have been subject to any NDE; thus, the crack (probably subsurface) was never discovered. Cracks in nozzle to shell welds in carbon and low alloy steels are quite a bit more common than many might believe.