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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.
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
My Friend JD designed and cut the parts to convert a Harbor Freight Hand Truck to a very functional welder cart for a lincoln power mig 210mp