Microstructural Investigation of the Magnetic Behavior of a Modified HP-NbTi Heat-Resistant Cast Austenitic Stainless Steel

A previous work has shown that a modified HP-NbTi heat-resistant stainless steel exhibited a different magnetic behavior after 90,000 hours of operation in a steam reformer furnace exposed to temperatures between 500 °C and 550 °C or between 950 °C and 1000 °C. In this work, in the sample exposed to the lower temperature range, a thin ferromagnetic layer was detected by magnetic force microscopy in the matrix around the chromium and niobium carbides. However, for the sample exposed to the higher temperature range, no ferromagnetic response was detected whatsoever. The understanding of the influence of microstructure on magnetic behavior is important for the development of non-destructive inspection test methodologies. For this reason, the aim of this study is to perform a microstructural analysis to assess the different magnetic responses. Transmission electron microscopy allowed identifying a Cr-depleted zone in austenite at the Cr23C6-matrix and (NbTi)C-matrix interfaces in the sample exposed to the lower temperature range. Therefore, the austenitic region, with a lower Cr content, becomes ferromagnetic. In the sample exposed to the higher temperature range, the chemical composition of the austenitic matrix was homogenous, and the sample was completely paramagnetic.