Guidelines for Heat Treating PM Parts

Whether heat treating P/M parts is done in-house or by an outside commercial heat treater, some guidelines need to be followed to ensure that parts are properly treated. In the past, heat treatment of P/M parts was primarily specified to improve wear resistance. With the advent of high-compressibility powders and high-temperature sintering, heat treatments are now added to provide improved dynamic properties as well. Recommendations for heat treating porous P/M parts are:

• Always degrease parts prior to heat treating. In many manufacturing plants, P/M parts are dipped in a rust preventative after sintering. Also, many P/M parts are machined prior to heat treatment and can retain some of the lubricant. These oils can contaminate the heat treating atmosphere and cause discoloration.

• Do not use dense loads. Overlapping parts can cause soft spots and distortion. For best results, parts should be single spaced in layers.

• Use highly agitated quench oil with a quench-severity rating (H) of 0.7 to 1.0.

• When tempering, hold at 205 °C (400 °F) for 2 h before proceeding to higher temperatures. This minimizes the evolution of smoke and prevents ignition.

If it is planned to send parts to an outside commercial heat treater, a technical review of the part requirements and prior processing history is recommended. Many commercial shops have had bad experiences with powder metals because they did not fully understand the implications of porosity and tried to treat the parts as they would wrought steel. It is important that a good communication network is established with the heat treater so that they can be alerted to any structural changes in the P/M part caused by changes in powder lot, tooling, or sintering cycle.

A good procedure is to send sample lots to the heat treater for them to establish process capability. After the print requirements are met, the heat treater should run the parts in the same furnace under the exact same conditions as were established. Parts fabricators should give as much consideration to the heat treating process as would be given to primary processes. Reworking because of improper heat treatment not only adds more cost, but can also significantly affect the physical properties of the P/M part.

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