Failure of a furnace outlet pipe in a benzene plant by internal oxidation due to improper welding practice

A furnace outlet pipe made of INCOLOY (R) alloy 800H to handle gaseous hydrocarbon in a benzene plant developed cracks in the weld heat-affected zone during operation at 595 degrees C. Microstructural characterization revealed that the cracks were of the ductile intergranular mode, which could be related to localized plastic deformation alongside the grain boundaries. The microstructure of the heat-affected zone was distinguished from the base metal by a coarser grain structure and intergranular oxidation in addition to higher hardness indicating the presence of residual stresses from the welding process. Intergranular oxidation was found to result in a mixture of Cr and Fe oxides enveloping a Ni-rich solid-solution adjacent to the grain boundary. Therefore, the observed ductile intergranular cracking could be related to localized plastic deformation in the relatively "soft" zone of Ni-rich solid-solution. Most evidence indicated that the failure occurred because of improper welding atmosphere leading to internal oxidation under relatively low oxygen potential, which is oxidizing to Cr and to a lesser extent Fe, and reducing to Ni. (C) 2009 Elsevier Ltd. All rights reserved.