You do not have to preheat the 4130 tubing to weld it. It doesnt hurt, but its not necessary. Chromoly with a cross section of under I believe 3/16" does not need to be pre-heated. As mentioned there are different types and grades of chromoly as D Pozzi mentioned. Its similiar in theroy to the Temper of Aluminum i.e. 6061-T6, 6061 T-4 etc.