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There is a bunch of data that states 4130 chromoly tubing .120" and under does not required a preheat or post heat treatment. But a preheat won't hurt.
There are several different techniques that work for tig welding coped tubing.
This video explores a few and shows instructive arc shots.
Both Stick welding with 7018 and dual shield flux core are good processes but one is way more productive. If you are in business, that matters a lot.
Welding thick 4130 parts requires a preheat in order to prevent hard brittle areas. ER70 filler wire was used.