Research On The Processing Properties Of Water-atomized Iron Powder

Iron powder, one kind of powder which accounts for the most part in powder metallurgy industry, is the main raw material of iron-based powder metallurgy materials and products which are material-saving and energy-conserving. With the boom of national economy, the expansion of automobile and mechanical manufacturing industry brings iron powder a vast potential for future development in China. At the moment, iron-based powder metallurgy materials are going forward in the direction of high density, high strength and complex shape, which put forward higher and higher requirements of the processing performance of the raw iron powder. However, the high-end market of iron powder is monopolized by overseas enterprises since the development of iron powder in our country starts relatively late and consequently the foundation is rather weak. In order to enhance the overall performance of iron powder of our country, it is necessary to conduct a systematic research on the processing properties of iron powder.In this work LAP100.29water-atomized iron powder produced by the Laiwu steel group powder metallurgy limited company was selected.as the basic raw material. The effects of chemical composition, particle size, size distribution and addition of nanometer iron powder on the processing properties of the iron powder were studied by different treatment methods. The main results are as follows:(1) The relationship between the main impurity elements(C、Si、 Mn、P、S、hydrogen loss) and compressibility of the iron powder was studied by linear regression analysis. The results from unit linear regression are generally unreliable due to the low relevant coefficient of regression fitting. However, the results from multiple linear fitting by a homebrew are fairly trustworthy with a relevant coefficient over0.97. From these fitting results, it is deducted that the main factors affecting compressibility of the water-atomized iron powder is the content of P and C, while the content of Si and Mn in the powder is the minor factors.(2) With the particle size decrease, the flowability of the iron powder decreased while the apparent density increases. By using the Athy compaction equation to fit and analyze the compact density of the iron powder, all the powders with different size fit well with Athy Equation. The powder with larger particle size has higher compression coefficient and better compressibility. In the case of roughly the same total porosity, the green compact with smaller particle size has larger numbers of well-distributed pores.(3) Different proportions of two kinds (-80+100mesh and-325mesh) of water-atomized pure iron powder were mixed. By controlling the different particle size composition, the apparent density of iron powder varied in the range of2.84-3.42g·cm-3and the flowability varied from20to31s/50g. When the compress pressure is less than300MPa, the mechanism of compact densification is mainly decided by particles slippage and rearrangement. Over300MPa, the mechanism of compact densification is mainly decided by the plastic deformation of the powder(4)The addition of nano-sized iron powder decreases the compressibility of the water-atomized iron powder and increases the ejection force as well as the friction coefficient. With the addition of0.5-6wt%nano-iron powder, the flowability of the iron powder varied from25to30s/50g, the apparent density from3.20to3.49g·cm-3and the friction coefficient from0.138to0.157.The compressibility of the water-atomized iron powder became worst with the addition of2%nano-iron powder. The nano-iron powder, with less than2%, can be well-distributed in the matrix, while small subtle aggregates are formed when more than2%nano-iron powder is added.(5) Five iron powder samples with grain size58.8-108.4nm, signed, were obtained by thermal treatment of the high pure water-atomized iron powder at500～800℃. The results show that the grain size has little effect on apparent density or flowability of the iron powder. The powder compressibility becomes better with growth of the grain size. The yield strength of the powder is inspected by the equation of Ln(1/(1-D))=KP1/2+B, which draws a conclusion that the better compressibility is mainly due to the growth of grain size and consequently the decline of yield strength. Prior to the powder particles break, the powders with coarse grain have larger deformation than the powders with fine grain under the same compaction pressure, leading to the better compressibility of the powders with coarse grain.