Destructive Accumulation of Nitrogen in 30 Cr 20 Ni Cast Furnace Tubes in Hydrocarbon Cracking Service at 1100 C^⋆

Centrifugally cast furnace tubes of 30 Cr, 20 Ni, balance iron, exhibited certain unusual structural characteristics after failure in service at 1100 C. A common feature of these tubes is a relatively high nitrogen content. In one instance, weakening and rupture of the tube wall under normal operating conditions was associated with a nitrogen-rich phase in the alloy microstructure. Other cases were noted in which porosity and actual blisters developed within the walls of high-nitrogen tubes under conditions of severe carburization and overheating. Similar phenomena were produced in laboratory experiments and a mechanism involving the evolution of molecular nitrogen gas within the alloy is postulated.