Post-Deperming Magnetic Stability of Naval Hull Steels Under Geomagnetic Exposure: A Preisach-Based Study of SS400 and HY80

Deperming reduces the permanent magnetic signature of ferromagnetic marine structures, but the state established by the treatment may change under subsequent environmental fields. This study compares post-deperming state persistence in SS400 and HY80 using measured major-loop characteristics, an M-B-variable Preisach model coupled to a two-dimensional magnetostatic finite-element model, and scaled-hull experiments. Under a common 5000 A/m protocol, the full-profile RMS change was 4.89–15.68 µT (50.7–162.6%) for SS400 and 0.015–0.053 µT (below 0.40%) for HY80. At a matched representative element, the normalized local Mx change was 51.38–164.70% for SS400 but only 0.01–0.13% for HY80, closely tracking the full-profile response across the 16 numerical cases. Experiment-matched numerical protocols preserved the same ordering. Under approximately 31.5 µT longitudinal geomagnetic exposure, the measured full-profile RMS change was 108.5% for SS400 and 3.15% for HY80. Because the experimental hulls differed in thickness and protocol and were measured in single cycles, the experiments are interpreted as independent trend validation rather than a controlled material-only comparison. The results distinguish immediate demagnetization effectiveness from post-deperming state persistence and support a substantial material-hysteresis contribution to magnetic-state stability.