Study of the microstructure of FeCr25Ni20C0.40 tubes after carburization phenomena

C. LANZ; A. JUAN; G. BRIZUELA; S. SIMONETTI

Congreso; Advanced Materials World Congress; 2015

Reformer tubes are generally made from cast creep resistant austenitic steel. Their actual service life varies from 30,000 to 180,000 h, depending on the service conditions. Due to prolonged exposure to high temperature, the microstructure of the material is subjected to degradation. In this work we studied austenitic FeCr25Ni20C0.40 tubes of an industrial cracking furnace, after 25,000 h of service at 900-1200 ºC. We analyzed the changes in the microstructure due carburization process using optical and scanning electron microscopy. A complementary DFT theoretical study was also performed. The changes in the alloy electronic structure after service are addressed. The chemical bonding analysis is also considered in order to explain the failures observed experimentally.Under operation conditions, the material undergoes changes in its structure and composition; there has been a depletion of the constituent elements of the alloy produced by oxidation and carburization. This phenomenon leads to the degradation of the alloy by the formation of a powder consisting of fine particles of metal and carbon. It was also observed intragranular carbides. Carbides become visible in the grain edge generating internal tension due their big size. The existence of porosities and cracks contribute to reduce the effort propagation towards the substrate, since they act as points of nucleation and crack propagation.The theoretical calculations help us to interpret the changes observed experimentally by studying the alloy electronic structure and the chemical bonding after the carburization phenomena. The atomic orbital occupations of the metallic bonds close to the carbon atoms are affected. The mainly changes are presented in Ni 4s, Fe 4p and Cr 4p orbitals. An electron transfer to the carbon atoms from the Ni, Fe and Cr nearest neighbour atoms is observed. The strengths of the Fe-Ni, Cr-Ni, Ni-Ni and Fe-Cr bonds nearest neighbours to carbon atoms are the most affected. The metallic bond weakening is mainly a consequence of the new C-Ni, C-Fe and C-Cr interactions formed after carburization process.