Wow,,, that's harsh...
How confident are you that the pre/post heat procedure and electrode identification and storage requirements were diligently followed?
Wers the root passes inspected for cracks prior to fill?
Were the welds UT'd prior to stress relief?

Tim

The preheat sounds low for the 75 mm thickness you are welding. The PWHT of 300 C is at best a tempering operation or hydrogen bake out. It isn't hot enough for a stress relief. 600 C for 1-hours isn't sufficient for 75 mm thickness. The hold time is typically 1 hour per 25 mm of thickness.

Low hydrogen electrodes should be baked at a higher temperature for one hour before welding to ensure they are free of moisture. I suggest baking them at 500 F for one hour (I'll let you convert F to degrees C).

Verify the welders did not allow the weldment to cool below the minimum preheat temperature while welding. It is mighty uncomfortable to weld at that high a temperature and welders are humans. They like to make their life as comfortable as possible, i.e., let the base metal cool below the required preheat.

The SMAW low hydrogen electrodes need to be removed from an unopened hermetically seal contain and placed directly into an electric holding oven to ensure they are moisture free. If they are exposed to the air for a half hour or more they can pick up enough moisture to be a concern. Again, bake them at high temperature for an hour before using them.

Once the assembly is welded, move it directly into an oven for a full post weld heat treatment, i.e., stress relief at 600 C. It may be helpful to perform a couple of intermediate stress relief operations as you weld (maybe after each 25 mm of weld thickness). Heating to 300 C is not sufficient for in-process stress relief of final stress relief.

I concur with the use of the 8018-B2, but I would prefer 8018-B2L. The ER70S-2 is questionable if you are required to match the mechanical properties of the base metal. There is a 4130 filler metal available for Lincoln and ESAB. You might consider using that filler metal.

Good luck with the next one.

Al

Can you provide a joint design?

Was the crack along HAZ or Centerline?

Was the crack detected right after welding the root or after completion of the entire weld?

Was preheat dropped prior to PWHT ?

What was the cooling rate after Welding and PWHT.?

Was the item attached to any operating machinery ?

Was any in-process inspection performed on the root pass or intermediate passes.?

If it was a saddle on connection, was the run pipe prepared to allow the hole to line up precisely with the ID of the branch?

With a tight groove angle, concave root contour, and a deep depth to width things can be difficult. I imagine this is a saddled on nozzle and not through the shell.