A recuperator used for preheating the combustion air for a rolling mill furnace failed after a relatively short service time because of leakage of the pipes in the colder part. The 6 % chrome steel pipes used for the warmer part connected by means of welding with austenitic electrodes to the unalloyed mild steel pipe of larger diam. Visual inspection showed corrosion and deep, trench-like erosion over the entire circumference of the seam on the side of the thicker mild steel pipe. Examination using the V2-A solution for picral etch showed the microstructure of the unalloyed pipe had become...