Induction Hardening

Spur gears, bevel gears, splined hubs, and cams are ideal components to utilized P/M production techniques. These parts usually require hard, wear-resistant surfaces in some areas, with retention of the ductility of the sintered matrix in the remainder of the part. Induction hardening is commonly specified for these applications. The process can be placed in an automated machining line to reduce handling and be a cost-effective hardening treatment when high volumes of parts are being produced. Because the inductance of P/M materials is typically reduced due to porosity, a higher power setting is normally required to reach a given depth of hardening compared to that used for a wrought material of similar composition. Furthermore, because the heat rapidly dissipates, a rapid transfer to the quench is mandatory.

As with wrought steels, the response to hardening by induction is dependent on combined carbon content, alloy content, and surface decarburization. This latter variable can be a major concern with P/M parts. Conventional belt-type sintering furnaces (depending on their construction) can allow decarburization to occur as the parts leave the hot zone and cool slowly through the 1100 to 800 °C (2010 to 1470 °F) temperature range. Newer designs are now able to compensate for this problem by installation of controlled cooling through the transformation range.

In most instances, induction-heated P/M parts are quenched in a water-based solution containing some type of rust preventative to forestall internal corrosion. In those applications where induction hardening is considered, densities above 90% should be specified. With a decrease in density, the resistivity of the steel increases and magnetic permeability decreases. For this reason, integral quench coils using a high-velocity spray quench are generally used to attain maximum surface hardness in the P/M part.

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