Microstructural Characterization of Magnetic Regions in a Cast Austenitic Heat Resistant Steel

The development of magnetic regions in an initially non-magnetic cast austenitic heat-resistant steel has been examined. Samples were extracted from different regions in a failed steam reformer tube. The samples had been exposed to different average temperatures over the 7-year life of the tube, resulting in different levels of damage. An as-cast sample of the material was also available as a control. After standard metallographic grinding and polishing of the samples, magnetic etching with ferrofluids was used to determine the presence and location of the magnetic zones. After locating these regions, energy-dispersive spectroscopy and electron backscatter diffraction in a scanning electron microscope were used to characterize the composition and structure of the magnetic regions. While not conclusive without further experimentation, results indicate that these magnetic regions are caused by localized chromium depletion from the Fe-Ni-Cr austenitic matrix. Unfortunately, due to experimental difficulties, the precise nature of the magnetic regions was not determined as intended, and further study is recommended.