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 M7C3 (‘Cr7C3’) and MC (‘NbC’). At service temperatures, M7C3 transform into M23C6 (‘Cr23C6’) within hours. Then, a hardening precipitation of secondary intragranular M23C6 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 M23C6 into chromium-rich M2X carbonitrides (X = C,N), then of MC into chromium-rich MX carbonitrides.
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