Flow behavior of magnetic steel powder

Flow occurs in most powder-based processes, opposed by various cohesive forces. Magnetism is often overlooked for metal powders. Here, flowability and magnetization were measured for a dual-phase steel powder in size fractions from 20 <= D/mu m <= 40 to > 200 mu m. The finest fraction did not flow through a Hall flowmeter, then flow time increased continuously with particle size from 12 +/- 1 s for the next fraction (40 <= D/mu m <= 45) to > 28 +/- 0.5 s for > 200 mu m. Drying had little effect. Key metrics derived from shear tests gave no overall relationship between flow behavior and particle size. Magnetism was considered the most likely reason for this behavior. Magnetometry showed a remanent magnetization of 3x10(5)A m(-1), which causes similar to 5 mu N cohesion between 200 mu m diameter particles. X-ray diffractometry showed that the powder contained 77 wt%-80 wt% of (magnetic) martensite. Liquid bridging, van der Waals forces and friction (in the Hall flowmeter geometry) contribute 50 mu N, 0.08 mu N and 4 mu N, respectively, to cohesion in 200 mu m particles. These results can be used to help explain flow behavior in other magnetic powders and allow optimization of powders and/or powder-based processes.