The present invention relates to homogenous iron-based powder mixtures of the kind containing iron or steel powders and at least one alloying powder. More particularly, the invention relates to such mixtures which contain an improved binder component and which are therefore resistant to segregation or dusting of the alloying powder.
The use of powder metallurgical techniques in the production of myriad metal parts is well established. In such manufacturing, iron or steel powders are often mixed with at least one other alloying element, also in particulate form, followed by compaction and sintering. The presence of the alloying element permits the attainment of strength and other mechanical properties in the sintered part at levels which could not be reached with unalloyed iron or steel powders alone.
The alloying ingredients which are normally used in iron-based powder mixtures, however, typically differ from the basic iron or steel powders in particle size, shape, and density. For example, the average particle size of the iron-based powders normally used in the manufacture of sintered metal parts is typically about 70-80 microns. In contrast, the average particle size of most alloying ingredients used in conjunction with the iron-based powders is less than about 20 microns, most often less than 15 microns, and in some cases under 5 microns. Alloying powders are purposely used in such a finely-divided state to promote rapid homogenization of the alloy ingredients by solidstate diffusion during the sintering operation. Nevertheless, this extremely fine size, together with the overall differences between the iron-based and alloying powders in particle size, shape, and density, make these powder mixtures susceptible to the undesirable separatory phenomena of segregation and dusting.
In general, powder compositions are prepared by dry-blending the iron-based powder and the alloying powder. Initially, a reasonably uniform blend is attained, but upon subsequent handling of the mixture, the difference in morphology between the two powder components immediately causes the two different powders to begin to separate. The dynamics of handling the powder mixture during storage and transfer cause the smaller alloying powder particles to migrate through the interstices of the iron-based powder matrix. The normal forces of gravity, particularly where the alloying powder is denser than the iron powder, cause the alloying powder to migrate downwardly toward the bottom of the mixture's container, resulting in a loss of homogeneity of the mixture (segregation). On the other hand, air currents which can develop within the powder matrix as a result of handling can cause the smaller alloying powders, particularly if they are less dense than the iron powders, to migrate upwardly. If these buoyant forces are high enough, some of the alloying particles can escape the mixture entirely, the additional phenomenon of dusting, resulting in a decrease in the concentration of the alloy element.
U.S. Pat. No. 4,483,905 to Engstrom teaches that the risk of segregation and dusting can be reduced or eliminated if a binding agent of "a sticky or fat character" is introduced during the original admixing of the iron-based and alloying powders in an amount of about 0.005-1.0% by weight. Specifically disclosed binders are polyethylene glycol, polyropylene glycol, glycerine, and polyvinyl alcohol. Although the Engstrom binders are effective in preventing segregation and dusting, they are, by definition, limited to substances which do not "affect the characteristic physical powder properties of the mixture such as apparent density, flow, compressibility and green strength" (Column 2, lines 47-51). Accordingly, the practical application of iron-based powder mixtures would be greatly enhanced by the provision of binding agents which not only effectively reduce segregation and dusting but also improve the green properties of the powder as well as the properties of the final sintered articles.