The investigation combines microstructuralrncharacterization analyses and atomistic modeling to elucidate the mechanism ofrncarburization embrittlement of an iron-nickel-chromium alloy exposed to highrntemperature. It was studied an austenitic Fe-25Cr-20Ni-0.40C stainless steelrnobtained of the tubes of the industrial cracking furnace after 25,000 h ofrnservice at 900-1200 ºC. The service-exposed material presented bigger carbidesrnin the austenitic matrix comparing with the as-cast material, cavities betweenrnthe grains and micro fissures in grain edges, indicating that the materialrnpresent damage by creep. The theoretical calculations help us to interpret thernchanges in the alloy electronic structure and the chemical bonding. The atomicrnorbital occupations of the metallic bonds are affected during carburization.rnThe strengths of the, Cr-Ni, Fe-Cr, Fe-Ni and Ni-Ni bonds nearest neighbors tornthe carbon atoms are the most affected. The metallic bond weakening is mainly arnconsequence of the new C-Ni, C-Fe and C-Cr interactions formed afterrncarburization phenomena.