Modifying Wrenches is common practice in certain industries like aviation maintenance. There are tight areas where a conventional wrench wont fit.
Hastelloy W is usually a great choice when you know its steel but cannot determine the exact metal type.
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.
This video shows tips, tricks, settings, and techniques for TIG welding 18 ga carbon steel lap joints
This video shows tips and techniques for tig welding 18 ga outside corner joints along with flash tacking, different cups, and chill bars
