Failure of Recuperator with Austenitically Welded Pipes

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 coarse-grained and acicular, and the microstructure of the welding seam had become predominantly martensitic as a result of the mixing of the weld metal with the fused pipe material. The chrome steel pipe had become partially transformed to martensite or bainite at the transition to the weld. Thus, the failure occurred due to typical contact corrosion wherein the alloyed welding seam represented the less noble electrode. The martensitic structure may have contributed to the failure as well. Due to the typical nature of the failure, no recommendations were made.