Novel powder metallurgy technique for development of Fe–P-based soft magnetic materials

Novel powder metallurgy technique (hot forging technique) is used for the development of high-density Fe–P-based soft magnetic alloys such as Fe–P binary, Fe–P–Cr ternary and Fe–P–Cr–Si quaternary alloys. In this process, mild steel encapsulated powders were hot forged into slabs, hot rolled and annealed to relieve the residual stresses. These alloys were subjected to in-house characterization, e.g. density and theoretically calculated porosity content at various stages. Microstructural study has been carried out to compare observed porosity with the theoretically calculated porosity. X-ray diffraction studies of these alloys revealed presence of only ferrite as product phase. Various soft magnetic properties such as resistivity, coercivity, maximum flux density (at 350 G magnetic field), retentivity and total magnetic losses were also evaluated and reported. These alloys were made by hot forging using two different kinds of dies, e.g. flat die and channel die. It was observed that the flat-die forged alloys had more porosity than the channel-die forged alloys. Addition of alloying elements such as P, Cr and Si increased the resistivity of Fe. The higher the alloying addition, the higher is the alloy's resistivity. Fe–0.7P–0.7Cr–1Si alloy showed a resistivity as high as 44.1 μΩ cm. Coercivity values of the alloys ranged from 1.0 to 2.2 Oe. Addition of Si and P helped in reducing the coercivity values of the alloys. The higher the Si, P content, the lower were the coercivity values observed. Combined addition of P and Si helped in reducing the coercivity values significantly, for example Fe–0.7P–0.7Cr–1Si alloy showed coercivity value approximately 1.0 Oe. It was observed in this investigation that maximum flux densities of the alloys were linearly related with their porosity levels. Total magnetic losses of these alloys varied from 6.0 to 7.8 W/kg. The total magnetic loss of Fe–0.7P–0.7Cr–1Si alloy was the lowest (6.0 W/kg) owing to its highest resistivity combined with its lowest coercivity amongst the alloys developed in the present investigation. Alloys developed in this investigation were capable of hot/cold working to very thin gage of sheet (0.5 mm thickness). These alloys could find their possible application in manufacturing of transformer core.