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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.

Use ER/E NiCrMo-3 for comlete weld and PWHT would be a better choice than your three posited scenarios.

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...

In the dozens of ASME Audits of our Quality Systems, I've never had an been asked to provide licensing for software; only the hand calcs were checked.

In my experience at least two petrochemical companies required lethal service for particular vessels in H2S environments.

Add possible lack of a Welding Procedure to follow and lack of QC.

ER/E309 is fine for the weld. So would be ER/E316. ER/E310 has a greater possibilty of hot cracking due to lack of ferrite in deposit.

B31.3 governs. I'm not sure why you need a "pressurvasion layer" on SS pipe components and not sure what it is.

The difference is primarily minimum UTS/MYS and allowable C and Mn contents.

Are the 7mm plates subject to vibration from the mixer? If so you should considerthe effects of fatigue on the weld joint.

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.

There is no PWHT requirement for the Alloy 800 to SA-516 70 welds under ASME VIII, Div.1.

It's a bit light on Cr; closer to a 1%Cr-1/2Mo alloy which is a common tube alloy and would likely be OK at 450C.

At 450 C and above P11 would be a good starting point provided corrosion service permits.

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.

Section IX does not require impact testing. Other Codes or the purchaser based on the minimum design temp and stress may do so.

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...

U2 stamping also depends on the Jurisdiction requirements.