| Abstract:Iron-based materials prepared from iron and steel powders account for a large proportion in powder metallurgy industry. They are widely used in automotive, household appliance, construction machinery, etc. Premixed steel powder prepared by premixing technique, characterizing in environment friendliness, excellent microstructure and dimensional stability, is the optimal raw material for fabrication of high-precision, high-performance iron-based materials. Currently production and sales of premixed steel powder is monopolized by Europe, USA, Japan and other developed countries. In order to expand application of domestic iron-based materials in the manufacturing of automotive components and the like, research and development of fabrication technique of pre-mixed steel powder matters much.Manganese, a novel alloying element in iron-based materials, is characterized by abundant resources and excellent solution strengthening and hardening effects. But when it is added into iron-based material in the form of elemental manganese powder or high Mn content pre-alloyed powder (FeMn), it is easy to be oxidized due to its high affinity for oxygen, resulting in poor mechanical properties of sintered components. Therefore, investigation of manganese carrier on oxygen content control has practical significance in promoting application process for manganese-containing iron based materialsIn this work, a pre-mixing process with high manganese and graphite bonding ratio is developed. For Fe-0.5Mn-0.5C premixed steel powder, the effect of four kinds of organic binder on alloying elements bonding ratio was surveyed. The bonding ratio of non-polar thermoplastic elastomer for graphite is as high as96%, while the bonding ratio of polar polyacrylic binder for Mn reaches92%. Compared with mechanical mixed Fe-0.5Mn-0.5C powder, flowability of the premixed powder improves from25.3s/50g to24.3-24.7s/50g; apparent density of both keeps equivalent, highlighting that the apparent density of the premixed powder can be adjusted between3.03and3.32g/cm3 Effects of different Mn carriers and Mn content on the properties of the premixed steel powders were studied. The Mn carriers are FeMn powder (79.0wt.%Mn) and Mn-containing low alloy steel powder. The results show that reducing the particle size of FeMn leads to increase of Mn bonding ratio. When the particle size of FeMn decreases to-800mesh, Mn bonding ratio increases to98%. When manganese was added in the form of Mn-containing low alloy steel powder, compressibility of Fe-Mn-C premixed steel powder decreased with manganese content increasing. When manganese content increases from0.5wt.%to1.89wt.%, the uniaxial600MPa green density decreases from7.07g/cm3to6.96g/cm3.Oxygen content was examined in the processes of premixing, dewaxing and sintering. When the manganese content is0.5wt.%, the oxygen content increases in the premixing and dewaxing stage. After dwelling at1120℃for30min in H2atmosphere, iron oxides and part of iron-manganese complex oxide are reduced, with oxygen content of0.21wt.%existing in the form of MnO, which is almost equivalent to the oxygen content of Fe-0.5C system. With Mn content increases in the Fe-Mn-C premixed system, the oxygen content in the sintered sample increases a little. When the manganese content reaches1.89wt.%, compared with Fe-0.5C, oxygen content in the sintered part increases nearly0.1wt.%, compared with Fe-0.5C sintered part. Fe-1.89Mn-0.5C system with composite manganese sources has the advantages of higher green density, more homogeneous composition and lower oxygen content.Bonding mechanism between binder and mixed powder are proposed. The polar ends of polar polyacrylic binder adsorb onto iron-based powder and FeMn powder, the non-polar ends, on the one hand, adsorb onto graphite, on the other hand, entangle one another, resulting in adsorbing FeMn powder and graphite onto the iron-based powder. The non-polar thermoplastic elastomer binder adsorbs FeMn powder and graphite onto the iron-based powder maily through Van der Waals attraction force. |
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