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Thin 4130 chromoly tubing can be welded without a preheat as long is its .120" or below in wall thickness.
This chunk of 4130 absolutely needs a preheat of around 500F.
4130 is a hardenable low alloy steel and can have hard and brittle zones if allowed to cool quickly.
Preheat slows the cooling rate and by using a preheat as well as er70s2 filler rod, a ductile weld can be made.
Sometimes 4130 chromoly is welded with matching 4130 filler rod when it is heat treated to full strength after welding.
In this case, the part was stress relieved in a furnace after welding and was not required to be strengthened by heat treatment.

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