The Influence of Phase Transformations on Creep Resistance in Fe-Ni-Cr Alloys for Reformer Tube Applications

Heat resistant steels of the HP-series have widespread uses in the petrochemical industry in pyrolysis and reformer furnaces. The alloys are carbon-rich Fe-Ni-Cr alloys, with additions like Mn, Si, Nb, Ti, W... The typical microstructure of as-cast HP alloys is an austenite matrix with intergranular eutectic-like primary chromium carbides of the M₇C₃ type and niobium carbides of the MC type. Upon ageing, phase transformations occur. Intragranular secondary M₂₃C₆ carbides precipitate, which is thought to restrict dislocation motion, and intergranular M₇C₃ transforms into M₂₃C₆. Under certain thermal conditions, a partial transformation of the primary niobium carbides into a nickel-niobium silicide called G phase can occur. These phases may play a critical role during creep, but neither their role on mechanical properties nor the mechanisms of phase transformations are clearly identified. The aim of this study is to understand the role of each phase or phase transformation in the creep resistance of HP alloys. Consequently, a critical review of phase formation and transformations in such alloys is presented using a set of experimental and modelling techniques (electron microscopy, Castaing microprobe, creep tests at high temperature and neural networks modelling of mechanical properties...).