Heat Treating Processes Applied To PM Parts

The majority of P/M parts heat treated today are neutral hardened and tempered. This treatment was used to develop the properties shown in MPIF standard 35 (Ref 6). These steels are hardened primarily for wear resistance and improved core strength. They contain 0.5 to 0.8% C and usually have densities exceeding 6.8 g/cm3. Most production P/M parts are normally hardened in batch-type sealed quench furnaces (rather than continuous belt furnaces) because they are net-shaped parts and should not touch or impinge upon one another during the heating cycle. This would tend to cause distortion and soft spots. The most widely used atmosphere for heat treating is endothermic gas with both methane and air additions to control carbon potential. Due to the rapid penetration of atmosphere into the interconnecting porosity, carbon diffusion is quite rapid and short cycle times at relatively low process temperatures are normally specified.

Medium- to high-density parts (6.8 to 7.2 g/cm3) are better suited for batch-type sealed quench furnaces where the temperature profile can be easily controlled. With this equipment, parts can be loaded into the basket in the arrangement that best minimizes distortion. Also, control of carbon potential is more precise in batch equipment. The one drawback to a batch furnace is the longer transfer time to the quench. Many of the newer furnace designs have automated internal handling equipment to transfer the hot load to the quench. This can prolong the time to quench, causing a loss of temperature, and subsequent surface hardness.

When quenching for maximum hardenability, a fast oil is recommended. Quench oils with a General Motors quenchometer rating of 10 s or less is considered fast. Load size is also a critical factor in attaining uniform hardness. Most sealed quench furnaces are designed to quench 0.5 kg of steel per gallon of quench oil. For P/M parts, it is recommended that 11.5 to 15 L (3 to 4 gal) of quench oil be used to quench 0.5 kg of sintered steel. Overloading is probably the most common problem in attaining uniform heat treated properties.

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