We are using API 5L X65 Grade SAW pipes of 24" dia and 14.3 mm WT. The
WPS has been qualified using E6010 (3.2 mm) electrode for root uphill
and E8010G (4 mm & 5 mm) for hotpass, filling & caping in downhill.
But during the welder qualification process, we observed some cracked
joints in RT. The parent metal has been checked for the chemical
compostion and is found to have % C -0.05%. All other chemicals are
within the tolerance as per API 5L.
On further investigations, it was observed in the micro that infact
several cracks have been developed in the weld metal (both
longitudinal & transverse). We increased the pre heating from 80C to
110C. Even though we havent found any cracks after that still we
couldnt effectively get the reason for the crack formation.
The weld metal revealed on the chemical testing the following results,
%C- 0.16, %Ni - 0.66, % Cr - 0.43 (I can give you more details if you
require). These % even though slightly on the higher side are still
within the acceptable limits and dont seem to me causing a serious
problem.
What I have observed one thing is the cooling rate, especially E8010
electrode is very fast. For eg on the filler passes it drops from
110-120C to 60-65C even up to 55C after cleaning.
My question is, do you think using E8010 electrode for a 14.3 mm thick
pipe (even if it is X65 grade) is advicable? What may cause such a
fast cooling rate in the pipes? (Inspite of practicably possible
shieldings have been provided) Most importantly what do you think is
the major cause of the cracking. I would like to your valuable
suggestion in this regards
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