I am not sure how to approach this problem. If you have a carbon steel pipe that is being heated on one end at a constant temperature of 1500C. Can you figure out how much distance in pipe will it require before it cools to 790C? The thermal conductivity of Carbon steel is 54 W/mk. Another situation is what if there is insulations on top of the carbon steel, what will happen in that situation. I am pretty sure with insulation the length of the pipe will increase to reaches 790C.