Novel scanning dc-susceptometer for characterization of heat-resistant steels with different states of aging

We have built a scanning dc-susceptometer for magnetic characterization of millimeter-scale samples. Using samples with few millimeter-thick slabs, the setup used is able to apply perpendicularly to the sample uniform dc magnetic fields up to 500 mT. It has a scanning range of 100 mm × 100 mm with micrometer resolution. In the present configuration, the dc-susceptometer is equipped with a pair of commercial Hall effect sensors forming an axial gradiometer. The measured output noise at 6.0 Hz is about 1.0 µTrms/√Hz in an unshielded environment and the magnetic moment sensitivity is 8.8 × 10−11 Am2. In particular, we used it to characterize heat-resistant steels, used as radiant tubes in steam reformer furnaces. We measured the cross-section of tube samples with progressing states of aging from as-cast to state VI. Measuring their susceptibilities, we detected a ferromagnetic layer at the external tube wall on aged samples. We were able to establish a relation between the magnetic response and the microstructural change of the steel.