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This work presents the characterization of the external oxide scale of a centrifugally cast HP steel tube, modified with Nb and Ti, after 90,000h of operation in a steam reforming furnace, showing different aging states as a function of the column height and exposure temperature. The objective was to relate the oxide microstructural morphology to its magnetic response for three different microstructural aging states. The morphological and compositional characterization was performed by high resolution scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction. The magnetic response of the external surface was analyzed by magnetic force microscopy and scanning magnetic susceptometer. The results showed the formation of a multilayer oxide scale with different compositions along its thickness. In addition, a microstructural modification was identified in the scale-matrix interface, due to the depletion of chromium. It was also observed that the magnetic response is dependable both on the oxide scale thickness and the chromium carbides depleted zone. Magnetic force microscopy was able to identify the origin of the magnetic response resulted from the steel tube surface.

The nickel-base alloy 718 is widely used due to its superior performance at high temperatures, combining properties such as high strength, high corrosion and oxidation resistances and good formability and weldability. Its properties can be further improved by controlled additions of alloying elements such as Ce, W, Mo, P, B and Y. The present work aims to evaluate the effect of the yttrium alloying on the phase transformation in alloy 718. Thermodynamic calculation, scanning electron microscopy, electron backscattered diffraction, hardness measurements and differential scanning calorimetry tests were conducted. The Ni17Y2phase was observed in two situations: associated with the Nb-rich particles or as single particles. The addition of yttrium has a remarkable effect on grain size control. The differential scanning calorimetry tests indicated that the alloy with the high Y content showed earlier precipitation of the γ” phase (Ni3Nb), which is the main hardening contributor, resulting in an increase in hardness.