TO DEVELOP AND CHARACTERIZE HIGH• DENSITY Fe-P BASED P/M ALLOY

Abstract

High density Fe-P binary, Fe-P-Cr ternary as well as Fe-P-Cr-Si quaternary alloys were made using hot powder forging technique. In this process mild steel encapsulated hot powders (blend of iron powder and iron phosphide powder with/ without ferro-chromium powder with/ without ferro-silicon powder) were forged into slabs. Mild steel encapsulation was removed after homogenization of these slabs. The slabs were then hot rolled to get highly densified sheets and wires. These highly densified products were annealed to relieve the residual stresses. Use of fine powder and high temperature processing in presence of H2 atmosphere facilitate densification. These alloys were characterized in terms of microstructure, porosity content/densification, hardness and tensile properties. Additionally, electrical resistivity, coercivity, flux density as well as total magnetic loss for these alloys have also been evaluated in order to assess suitability of these alloys for magnetic applications. Densification as high as 98.9% has been realized by adopting hot powder forging manufacturing technique. Powder forging using channel die (in which there was a side wall restriction to material flow in lateral direction existed) showed better densification than that obtained using flat die. Microstructures of these alloys consist of single phase ferrite only. However, occasional presence of undissolved ferro-silicon particles was observed in the microstructures of Si containing iron-based alloys. The double grain boundary, which is typical of phosphorous containing iron alloys indicating presence of prior phosphide phase, were also observed occasionally in the microstructures of the alloys made in the present study. Rolled and annealed microstructure showed lesser porosity content than the forged and homogenized one. Phosphorous causes more hardening of ferrite than silicon in solid solution with iron. Alloys containing 0.35wt%P such as Fe-0.35P, Fe-0.35P-0.35Cr and Fe-0.35P-0.35Cr-1Si alloys showed very high ductility with low work hardening coefficient (n=0.15) and therefore, showed excellent formability. The Fe-0.35P alloy showed. approximately 40% elongation in annealed state. Hot rolled Fe-0.35P alloy also showed very high elongation (-30%). Alloys containing 0.7wt% P such as Fe-0.7P, Fe-0.7P-0.7Cr and Fe-0.7P-0.7Cr- 1 Si alloys showed very high strength (as high as 500MPa). These later alloys however, showed limited ductility with slightly higher work hardening coefficient (n). It was observed that, the alloying addition such as Si and Cr to Fe-P based alloys caused increase in strength associated with the reduction in ductility. Alloying elements P, Cr and Si contributed to increase in electrical resistivity of ferrite by forming solid solution and thereby hindering the passage of electrons. Coercivity of these alloys was found to be varying between 1.0Oe to 2.20e. More is the addition of alloying elements such as P, Cr and Si (within their solubility limit in iron), less is the coercivity level of these alloys. Maximum flux densities of Fe-P based alloys are found to be linearly dependent with the porosity content of the alloys. As the porosity content falls, maximum flux density raises. The total magnetic loss of channel die forged Fe-0.7P-0.7Cr-lSi alloy, in which maximum amount of alloying was done, was the lowest owing to its lowest coercivity and highest resistivity amongst all the alloys developed in the present investigation. The total magnetic loss for the rest alloys developed in the present investigation was found to be comparable to that of Fe-1.7Si non-oriented steel. The alloys were capable of hot/cold working to very thin gage of sheet and wire.