While conventional sintering requires regulation of time, temperature, and atmosphere within the sintering furnace, sinter hardening further requires careful regulation of the cooling curve to yield the desired physical and mechanical properties.
That is the technical definition of sinter hardening, but the part designers need to know is that sinter hardening offers some compelling engineering advantages It eliminates the time, cost, and potential for defects that come with a separate heat-treating step. During sinter hardening, components are quenched in the sintering atmosphere instead of a traditional oil quench, rendering the part surface clean and free of oil residue.
This atmospheric quench is preferred over conventional oil quench heat treatment to optimize form and dimensional control since warpage and distortion can be an issue with oil bath quenches. Since sinter hardened parts can be manufactured to net-shape or near-net-shape, this alternative to a secondary heat treatment provides a strong economic incentive.
Sinter hardened parts are routinely produced with fine features. It is common for the company to utilize this method on complex parts with tight dimensional requirements including gears and structural components used in power transmissions and household appliances, to name a few common applications. Many complex components could not be produced using any other metal manufacturing method other than the sinter hardening process —at least not cost-effectively, reliably, or with repeatability.
The case examples described below examine three components manufactured at Atlas, which have been co-designed with their customers, to take full advantage of the benefits of sinter hardening, and the utilization of this process results in repeatable, precise manufacturing and economically priced components.
Gearing is a classic application for powder metal with the added benefit of offering the ability to create complex gear forms that cannot cost-effectively be achieved with machining. For a recent example, consider a compound gear manufactured for use within an automotive electric motor. The gear is shown in Fig. 1 is produced to American Gear Manufacturers Association (AGMA) level 7 quality standards. This gear combines 101- and 14-tooth gearing in a single net-shape component. Aside from the disparity in the number of teeth, the individual gears have a 3 to 1 differential in their face depth and outside diameters.
From a manufacturing standpoint, the component requires complex, multi-level tooling to produce the gear forms. Additionally, the features on this gear are very fine; the tips of the smaller gear teeth measure between 300 and 500 µm across—or about the same distance spanned by three to five particles.
On a piece-price basis, the design and ability to have it sourced as a sintered metal component offers significant cost savings over the machined alternative originally considered.
The gears, having an AGMA level 7 requirement, have very tight tolerances. The run-out specification is 0.00267 inches (0.068 mm) max, while the part’s overall length has to be held within ±0.00196 inch (0.05 mm). As shown in Fig. 2, the total composite error for these parts demonstrates consistent statistical results between the lower and upper limits.
