Contamination in the joint has proved the most likely cause of transverse cracking in carbon steel weldments in my exprience. The 3 equally spaced cracks might indicate contamination from tacking the pipe in place prior to weld completion with a hot crack in the cotaminated area due to welding...
Design temperature and construction code dictate impact testing requirements. If the contents are liquid oxygen, impact testing of the PQR would most likely be required.
Scenario 1. Poor choice. The carbon steel filler will produce a crack sensitive deposit though dilution with stainless steel.
Scenario 2. Poor choice because of the differential in expansion/contraction of 309 vs carbon steel. Also potential for stress corrosion cracking or knife edge corrosion...
Because you have not provided info as to material,length of service, service environment, I would advise making the repair. It is assumed that the corrosion is at the bottom of the vessel.
There have been enough forged bolt documentation to mount finding sourcing of bolts and checking chemistry as possible suspects. Also check failed bolt profile to determine any obvious flaws.
It could. Excessive dilution with the base metal. It appears that fracture initated at or near the fusion line and into the weld metal itself. There may also be some weld flaws which contributed to failure.
It's really thermal contraction and it is so small when operating at -33 C that it can be ignored at weld joints to dissimilar metals like low carbon steel to austenitic stainless steel. It should not be ignored in the pipeline design with regard to the length of the pipe and providing freedom...