Ha, you ask a question then suddenly you’re an expert. The only heat treating, if you can call it that, they do on kart frames is to draw the temper out of the welds by localized heating to 1100F. Normalization of 4130 tubing happens at around 1650F. 850F should not effect a kart frame at all although, I can imagine a painter selling you a line of BS about 850F then cranking up the heat to make it easy to peal.
Still the same old kart under the pretty paint. Did I spell everything rite?
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience