Machine Design

Ferrous P/M

Specifying powder-metallurgy (P/M) parts and their consolidation process used to be a simple process: Design the part, select the metal powders and lubricants that provide the required properties, compact the powders into a briquette, and sinter the briquette into its finished form. Through this procedure, millions of parts have been produced for applications ranging from automobiles to appliances and from business to farm and garden machines.

However, the needs of industries have changed significantly. Removing weight from all products has risen to primary importance. Energy, tooling, and materials costs now figure prominently in parts design, and productivity has emerged as the watchword of the eighties.

With these changes have come changes in powder-metallurgy technology. Through the many manufacturing processes, improvements have been made in the powders themselves -- improvements such as lower levels of inclusions and higher compressibility. In addition to conventional iron and steel metals, the list of available powders has been expanded to include new classes of tool steel, as well as materials such as cermets and alloys of titanium, nickel, and aluminum.

Accompanying these developments has been the growth of new consolidation technologies. As a result, design engineers need current information on which P/M technologies are viable, cost effective, and production effective, and which have potentially wide application.

Although powder metallurgy is used to fabricate parts from just about any metal, the most commonly used metals are the iron-based alloys. Low-density iron P/M parts (5.6 to 6.0 gm/cm≥), with a typical tensile strength of 16,000 psi, are usually used in bearing applications. Copper is commonly added to improve both strength and bearing properties. Alloy-steel powders are sometimes hot forged to high or nearly theoretical density to form parts with improved mechanical properties which, when heat treated, may have tensile strengths to 170,000 psi. Powder forging (P/F) is now established as a serious contender for parts formerly made as wrought forgings or machined from mill forms. New vendors with sophisticated automated equipment are increasing the overall P/F capacity as well as contributing to better quality of output.

Iron P/M or sintered iron-copper alloy strength can be varied by adjusting density, carbon content (up to 15%), or all three to satisfy specific design requirements. Each variable can be adjusted by the vendor to satisfy tolerances, mechanical properties, and other requirements of the part being made.

Low-density P/M parts are used in bearing applications because they provide porosity for oil storage. Impregnating sintered-metal bearings with oil usually eliminates the need for relubrication.

For higher strength needs, alloyed (frequently prealloyed Ni/Mo/Fe) iron, compacted to a higher density, is used. When carbon or other alloying elements are mixed with the iron powders and densities exceed 6.2 gm/cm≥, the parts are considered to be steel rather than iron. As carbon content is increased up to 1%, the strength of steel P/M parts increases, just as the strength of wrought steel increases with higher carbon content.

Additional applications can be accommodated by sealing the pores in iron P/M parts. The sealing materials used are copper, polyesters, and anaerobics; each requires a different processing system to impregnate the parts. Impregnation of sintered P/M parts is done for any of several reasons:

  • To serve in pressure-tight applications.
  • To improve surface finish. (Impregnated parts are platable.)
  • To improve machinability.
  • To improve corrosion resistance.

Although high precision has been achieved in P/M parts for many years, their application was once restricted because of mechanical property limitations. Now, however, mechanical properties can be increased in steel P/M parts by hot forging in closed dies. Properties of P/M parts forged to 100% theoretical density in production conditions are claimed to be equal, and sometimes superior, to those of wrought steels of similar composition.

Relatively complex carbon or low-alloy steel parts required in large quantities are ideal candidates for P/M forgings. Automakers have been among the first to use these full-dense, precision-forged components in transmissions, accessory mechanisms, and engines.

Adding carbon enables steel P/M parts to be heat treated to increase hardness, toughness, wear resistance, and strength. The addition of alloying elements in the iron-powder mix further enhances the properties of heat-treated steel P/M parts.

Ferrous P/M parts containing 0.3% or higher combined carbon can be quench hardened for increased strength and wear resistance. Surface hardness values of 500 to 650 Knoop, which are file hard, can be obtained by quenching.

In addition, ferrous parts can be carburized by standard means other than liquid salts. Low-density parts carburize throughout while high-density parts develop a distinct carburized case. Very-high-density parts respond favorably to fused salt carbonitriding, but density must be high enough to prevent salt absorption into the pore structure. Tempering for stress relief after quenching is also possible, although oil vapors generated by quench oil in the pore structure of P/M parts must be vented or dispersed.

TAGS: Metals
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