The graphitization tends to be in the HAZ next to the welds. For your application, I would suggest moving the cut line away from the existing welds at least 1 inch, but preferably 6 inches just to be sure there are no welding issues. You can confirm the cut line location by having metallography performed on a sample of the old tube when it comes out. An elevated "preheat" would not be enough. You have to get above the lower transformation temperature, which is 1335 F typically for carbon steels.