After making 100% TOFD to the welds of a propane sphere there were detected liner discontinuities such as localized cracks in the welds link of the dome to shell buds at several deep thickness (plates thickness 40mm).
After this first inspection the defects were mechanically removed and the remaining thicknesses were controlled by UT
The crack repairs have been done through filler welding at a controlled temperature and 100% UT control and magnetoscopy.
The sphere was then submitted to a PWHT at 620ºC during 2 hours following by slow cooling.
After PWHT, the sphere was again submitted to 100% of UT and magnetoscopy and there has been found new defects in zones out of the repaired ones.
These defects have also been removed and after being repaired they originate new defects in adjacent zones.
After metallographic replicas we found intergranular micro cracks
Now we are going to repair all the indications given by TOFD, even the ones which can be accepted by the codes.
We thing that the defects found have been induced by atomic hydrogen which is now molecular hydrogen arrested inside the micro voids, so we need to have the steel as much clean as possible to avoid more defects induction.
In principle, we think that with the PWHT, all the atomic hydrogen has been baked out; our problem is the remaining molecular hydrogen trapped into the micro voids which in our opinion, is irremovable by any preheating or PWHT temperature. For this reason we need to have the steel as clean as possible, to avoid more defects induction. In this sense, our intention is to repair all indications given by the TOFD even the ones accepted by the code. The cracks pattern is more stepwise shape than aligned.
Sphere material is BH36 (fine grain size) plates thickness 40mm.
During its service, before inspection, the sphere was contaminated with H2S. During operation of the sphere it was detected contaminations of H2S in the propane of about 1000 P.P.M
The cracks observed into of the casting material of some welds analysis, present characteristic morphologies to have been generated by SOHIC, where the presence of defects into the welds had a preponderant effect in the internal capture of molecular hydrogen, with consequent development of promotional effect of tensions concentration and nucleation of cracks.
The sphere has about 30 years and the material is BH 36.
BH 36 is a fine grain steel with weight of C, 0.01 to 0.2% by weight of Si, 0.2 to 1.5% by weight of Mn, 0.01 to 0.07% by weight of P, 0.006 to 0.015% by weight of S, 0.01 to 0.08% by weight of sol. Al, not higher than 0.004% by weight of N (N < where 0.002.
On repaired zones we found on weld deposit hardnesses between 200HV and 220HV and on base metal values within 145HV to 175HV
The sphere was totally PWHT even the zones not involved in repairing operations.
Maximum pressure operation of the sphere is 14.8 Bar
Maximum temperature operation of the sphere is 50ºC
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Luis Marques
