An Approach to Determine the Initiation of Carburization in a 304H Stainless Steel Piping under Petrochemical Environment

Carburization and metal dusting are common degradation mechanisms of the construction materials in petrochemical environment. The crossover piping system, exiting the furnace convection section and distributing the heated ethane feed to radiant coils, is fabricated of 304H stainless steel and is protected by a chromium oxide layer from carburization and metal dusting.

A thorough metallurgical analysis demonstrated that some components of the crossover piping system in one of NOVA plants suffered from metal dusting and carburization. This manuscript will describe the methodology established to determine the initiation point of carburization basing on metallurgical analysis and theoretical modeling. The predicted starting point of carburization using this approach was coincident with a major process parameter change in the plant operation, which dramatically reduced the resistance of the aged 304H SS to carburization and metal dusting.

Fick’s Second Law of Diffusion was applied to describe the carbon diffusion into the pipe steel at the bulk, the heat-affected zone (HAZ) and the weld. Through the metallurgical analysis, it has been established that the depth of carburization was in a power relationship with the carburization times. These relationships allowed us to figure out the initial starting time of the carburization for the HAZ and the bulk steel, thus provide important information for the assessment of the remaining life.