G-phase formation in twenty-years aged heat-resistant cast austenitic steel reformer tube

NbC TRANSFORMATION DURING AGING IN HP40-Nb HEAT RESISTANT ALLOY

“…In Table 5, the results of the analysis for the sample with 96 h of aging at 850°C and the comparison with the Gphase composition can be observed. As it can be seen, the change in the chemical composition and, therefore, the transformation of the Nb-rich carbide towards Gphase (Ni16Nb6Si7), is notorious since the mean concentration value of each element is very close to that corresponding to Gphase [3]. In addition, no carbon was found in any of the determinations made on the sample for the aging condition already described.…”

“…G-phase is known as a Ni-Nb-rich silicide that mainly forms at grain boundaries. The general chemistry of G-phase is proposed as Ni16Nb6Si7 [3]. For HP40-Nb alloy, this transformation is reported to take place between 700 and 1000°C [4].…”

“…They contain in their heat exchange surfaces, which are necessary for the process, an arrangement of metal tubes that are generally vertical and make up, together with other parts, the surfaces where the reforming reactions take place. The reformer furnace tubes are the most critical components, as they are exposed to severe conditions in terms of high temperature and internal pressure for long time during service, and have inner diameter of 60-200 mm, wall thickness in the range 10-25 mm and length of 10-15 m and with a designed nominal life of up to 100,000 h [1][2][3][4][5]. Since service conditions involve high temperatures and moderate pressures, the materials used for reformer tube demand for higher creep and oxidation resistance.…”

“…There is scarce information about this silicide, which is formed at the interface between the matrix and NbC and continues inward. This transformation causes an increase in the volume of the unit cell and in the interfacial energy, creating compression stresses on the matrix and tensile stresses on the particles, promoting the nucleation of microcracks, which reduce the creep properties of the alloy [3,21,22]. In addition, the formation of the G-phase causes a consumption of Ni and Si from the matrix reducing solid solution hardening [1,11,23,24].…”