Degassing Window for Jet-Milled HPMS NdFeB Powder Recycled from Wind-Turbine Magnets: Hydrogen Desorption and Magnetic Recovery

Hydrogen processing of magnetic scrap (HPMS) is a promising short-loop route for recycling end-of-life NdFeB permanent magnets; however, the resulting hydrogenated powder requires vacuum degassing before reuse. This study defines a phenomenological degassing window for HPMS jet-milled powder from end-of-life wind-turbine magnets by screening degassing temperature (650–900 °C), heating rate (1–3 °C/min), and dwell time (5–60 min) under vacuum. Pressure–temperature desorption traces were deconvoluted into three Gaussian components (R2 = 0.999), associated with hydrogen release from the Nd2Fe14BHx matrix and NdH2.7/NdH2 grain boundary phases, and correlated with SEM-EDS microstructural analysis and magnetic properties (Hc, Mr, and M(1700 kA m−1)). Among the heating rates tested, 2 °C/min gave the most favorable combination of properties. Under this condition, the coercivity reached 434.1 kA/m after degassing at 900 °C, which we tentatively attribute to the recombination of α-Fe and Nd into Nd2Fe14B, while M(1700 kA m−1) peaked at 99.8 A·m2/kg at 880 °C. Coercivity recovery depends primarily on the peak degassing temperature rather than on the time spent above 800 °C. The oxygen content increased from 0.46 wt.% after HPMS to 0.59 wt.% after jet milling and 0.66 ± 0.06 wt.% after degassing at 900 °C, which makes oxygen control a key limitation of the route. The results reveal a trade-off between coercivity and M(1700 kA m−1) that can be tuned within a single degassing step, and a degassing window distinct from that established for powders intended for re-sintering. The suitability of the recovered powder for bonded magnets remains to be demonstrated.