Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

Fe–35Ni–25Cr–0.4C alloys with different compositions are aged between 750 and 1150°C up to ∼10,000 h. As-cast microstructure contains interdendritic carbides of type M 7 C 3 (‘Cr 7 C 3 ’) and MC (‘NbC’). At service temperatures, M 7 C 3 transform into M 23 C 6 (‘Cr 23 C 6 ’) within hours. Then, a hardening precipitation of secondary intragranular M 23 C 6 occurs over hundreds of hours, the nose of the ‘temperature-time-hardening’ curve being around 1000°C. G phase forms after long aging; its solvus temperature and formation kinetics depend on silicon content. Z phase is observed after long aging at 950°C or above. G and Z phases form at the expense of MC. Very long aging causes nitridation under air, with first a transformation of M 23 C 6 into chromium-rich M 2 X carbonitrides (X = C,N), then of MC into chromium-rich MX carbonitrides.