This work is focused on the failure analysis of a HP40 steel tube used in steam reformer units, which was operated at about 850 °C for 100000 h. The failure analysis was conducted experimentally and numerically using the Thermo-Calc software. The main results indicated that the failed tube presented slightly different contents of Ni and Cr from the nominal ones, which originated the absence of the interdendritic eutectic microconstituent, composed of austenite and M 7 C 3 carbides. This fact was also corroborated by Thermo-Calc results. This absence of eutectic facilitates the crack propagation on the dendritic zones and then the trangranular brittle fracture propagation through the austenite dendritic zones.
The present work focused on analyzing the phase transformation in Fe-Ni-Al alloys employing a diffusion couple of Ni/Fe-Ni-Al/Fe, and Calphad-based diffusion and thermodynamic software. Diffusion couples were prepared by annealing at 1100 °C for 200 h and then air-cooled. These couples were also aged at 750 °C for 100 and 275 h. Both numerical and experimental results indicated that the diffusion path between Ni or Fe pure metal and the Fe-33at.%Ni-33at.%Al alloy is not linear. The phases formed during the diffusion anneal at 1100 °C correspond to those shown in the Calphad-calculated Fe-Ni-Al equilibrium diagram. The aging treatment at 750 °C promoted the inverse precipitation β′ → β′ + α, which caused the softening of the alloy. Moreover, the normal precipitation reactions, α→α + β′ and γ → γ + γ′, were also observed to occur during the aging of diffusion couple at 750 °C, originating precipitation hardening.
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