450 C (813 F) is high enough for graphitization to be a concern on the tube side. The weld is going to be less resistant to graphitization than the HAZ and could fail inservice. Probably wouldn't happen for awhile since you had some Mo pickup in the weld deposit from mixing with the base metals. If you're planning to run this a short time and replace the heat exchanger, you're probably OK for a year or two. If this is a new heat exchanger, you should discuss with the designer & customer, but probably will need to remove and reweld.