Investigação do efeito da adição de alumínio, titânio e zircônio nas propriedades mecânicas a quente de ligas Ni-Cr-Nb utilizadas em fornos de pirólises

Abstract

This study was motivated by the failure analysis of pyrolysis furnace coils that ruptured after 43,370 hours of operation. The investigation focused on the interaction between carburization, creep, and ductility loss, employing finite element analysis to evaluate thermal stresses, magnetic permeability measurements, and NACE TM0498 testing to determine carburization depth, along with microscopy and X-ray diffraction techniques. The results showed that tubes operating at higher temperatures experienced severe degradation, with extensive carbide formation, significant ovalization, and brittle fractures associated with the accumulation of residual stresses. These findings highlighted the need to improve the materials used in such critical components. In response, this work assessed the effects of adding aluminum, titanium, and zirconium to Ni-Cr-Nb alloys, aiming to enhance mechanical properties and increase resistance to carburization and creep degradation. Four alloys produced by static casting were studied, with compositions of 0.4C-30Cr-40Ni-1Nb; 0.4C-30Cr-40Ni- 1Nb-3Al; 0.4C-30Cr-40Ni-1Nb-3Al-0.2Ti; and 0.4C-30Cr-40Ni-1Nb-3Al-0.2Zr. These were subjected to carburizing atmospheres at 1150°C for 480 hours and creep tests under a constant load of 17MPa at the same temperature. Microstructural characterization included optical microscopy, SEM, EDS, EBSD, and dilatometry. The addition of aluminum effectively promoted the formation of more stable protective oxide layers, increasing carburization resistance, while titanium and zirconium had limited influence. Regarding creep performance, aluminum-free alloys showed superior behavior in non-carburizing environments; however, the combination of Al with Ti or Zr delayed the transition to critical damage stages under carburizing conditions, extending rupture time and demonstrating potential for severe service applications. This study underscores the relevance of integrating scientific knowledge with industrial practice, showing how a detailed understanding of failure mechanisms can guide the development of more robust alloys and optimize operational strategies—preserving structural integrity, reducing maintenance costs, and improving the efficiency of critical industrial processes such as hydrocarbon thermal cracking.