Secondary Operations

A variety of secondary manufacturing and finishing operations may be required to complete the part, to improve properties, or to calibrate dimensional tolerances. Because die-compacted parts have residual porosity that may affect the response to these secondary operations, several guidelines are provided in this section.

Repressing. Die-compacted parts can be repressed in a second tool set in order to reduce the amount of porosity or reduce dimensional variation. By increasing the part density, critical physical and mechanical properties will be improved. This secondary densification step can be performed on the entire part or just specific features, for example, gear teeth. The sizing of P/M parts for improved dimensional tolerances is widely performed, especially for the manufacture of bearings.

Impregnation and Infiltration. Several procedures are used to fill the residual porosity in conventional P/M parts. Oil impregnation adds an internal lubricant to the product, which is useful in bearing and wear applications. The part can be resin impregnated to seal the pores for pressure tightness and improved machinability or to prevent the intrusion of undesired plating chemicals. Infiltration with a lower melting metal than the base material (such as copper-infiltrated steel or copper-infiltrated tungsten) is also used to seal porosity, but most often to increase mechanical properties or create unique composite structures.

Steam treating, also known as steam oxidizing, is a low-temperature (540 °C, or 1000 °F, 1 to 2 h) heat treatment process in which P/M steel parts are exposed to superheated steam. The process can be conducted in batch, pressurized furnaces, or continuous belt furnaces. The steam reacts with the iron surface converting it to an adherent, protective blue-grey iron oxide (Fe3O4). Because the steam can penetrate the porosity, the oxide layer can be 0.020 to 0.050 in. deep depending on the processing conditions. Steam treating enhances the product by:

• Increasing wear resistance

• Increasing surface hardness

• Improving corrosion resistance

• Increasing compressive yield strength

• Providing low-pressure leak tightness

Formation of this oxide layer unfortunately reduces tensile strength and ductility ^10 to 20%, depending on the material system and processing conditions.

Heat Treatment. Due to the presence of residual porosity in die-compacted P/M parts, heat-treat practices should utilize gaseous or noncorrosive liquid media (such as quench oil rather than water or fused salts). In addition, because the porosity allows for the penetration of gaseous media (i.e., carburizing gas), special steps must be considered when trying to develop a carburized case in P/M steels. The surface (or entire part) must be high enough in density to prevent the carburizing gas from penetrating the pore network. A density of 7.2 g/cm3 or greater is desired. Copper infiltration can also be used to seal the part for a case-carburizing treatment. Heat treatment of P/M steels is most effective in improving mechanical properties when parts have a density greater than 7.0 g/cm3.

Finishing. The variety of finishing operations--machining, plating, deburring, joining--must also consider the effects of residual porosity. Improvements in machinability have been achieved through resin impregnation (seals the porosity) and by adding machinability aids to the original powder blend. Surface finishing may also require resin impregnation if the part density is low enough to allow entrapment of plating or finishing chemicals. These liquids can cause internal corrosion of the P/M part if allowed to penetrate the open porosity network. For welding and brazing, precautions are necessary to ensure a sound joint (Ref 8).

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