First, when you want to ask a question about how a code sees an aspect of your particular job, it helps to supply info like the applicable code, material type, welding process, etc.

Second, welding codes don't tell you how or what to weld. Those decisions are up to the applicable Engineer. The codes just tell you how to validate, regulate, and inspect the decisions made and work performed.

What Joey was hinting at is that the only one who can answer your question to begin with is the Engineer. If the Engineer requires that yes, this atypical weld is to be made, then you'll have to qualify a procedure to validate the chosen process and parameters. Most codes have a guideline chart to match preheat temps with base metal types/thicknesses, but the end result is a combination of the Engineers call and the validation test results.

Tim

but the code says "Seal welding of threaded connections may be performed either before or after final heat treatment".

Hey there was a memo....The Engineer already decided "Before"...... so how sure you are "not required" if you will not ask?