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I found, inside the steam drums, one crick for about 80% around the welding of two side downcomer.

The materials are:

steam drums MARREL AM 60
piping A105 diameter 460 mm

I want to know the cause for this problem.

Can somebody help me?

by

Paolo,
Welcome here.
1. Look into the piping material again. The one you mentioned (A105) is for fittings/flanges. Confirm again.
2. Please mention whether this observation has been picked up during in-service inspection or just after welding.
3. Any photograph would help.

Dear Paolo,

1. Principally, the materials involved in construction of steam drums and associated piping are high strength low alloy steels which are susceptible to crack growth during and after welding if not handled poperly.
2. During construction phase, all the hot work involving welding, post heating & post weld heat treatment must be carried out very carefully to avoid any chance of crack development.
3. Proper NDTs are performed on weld joints before and after post weld heat treatment which include UT and MPT mainly.

The cause of this failure appears to be the presence of residual stress in the weld & HAZ region which have eventually resulted into crack.
For proper understanding the actual cause of this crack, I advise you to look into the following:
a. Welding material used. Conduct Positive Material Identification
b. Hardness of the failed part along side of the cracked area. See if this lies in the acceptable range
c. See if applicable NDTs were conducted at the time of construction especially after PWHT
d. Inspection of other such areas which still appears to be sound

If you need any further help, never hesitate to ask.