Introduction

As raw materials and processing methods continue to improve, more high-performance components requiring heat treatment are being converted to powder metallurgy due to its neat-shape capability and cost effectiveness. Powder metallurgy parts sintered to net shape after compaction typically contain a minimum of approximately 10% residual internal porosity in traditional press-and-sinter methods. With warm compaction, minimum residual porosity can be lowered to approximately 5%.

Unlike wrought steels, where hardening response is controlled primarily by chemical composition and grain size, P/M hardenability is significantly influenced by this interconnecting porosity, which makes the structure permeable to gases and fluids.

Because P/M processing can provide net-shape capability, the hardening process becomes much more critical when compared to near-net-shape wrought components where finish machining is usually applied after the harden-and-temper operations. With powder metallurgy, dimensional shrinkage normally occurs upon quenching and tempering. With simple shapes this shrinkage is predictable and can be accommodated in the compaction tool design.

However, as the P/M components become more complex in shape, such as the typical multilevel parts in production today, variations in porosity from one level to another can cause significant distortion in the part upon heat treating, which can affect the function or fit of the part in an assembly. When this occurs, a finish machining operation is usually required, which detracts from the cost benefit of powder metallurgy.

This permeability also influences the hardenability of the material. The interconnected porosity acts as an insulator and reduces the thermal conductivity of the material. When rapidly quenched from the austenite range, this permeability retards the cooling rate, resulting in mixed microstructure and inconsistent hardness measurements. These variations increase with part-shape complexity.

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