Figure 2.69 is a schematic of the P/M part manufacturing process. The iron P/M parts production process begins with the manufacture of iron powder. Volume production of iron powder is accomplished in one of two ways: (1) the solid-state reduction of iron ore or mill scale and (2) the melting and water atomization of iron and low-alloy powders. The solid-state reduction processes, although the oldest powder manufacturing techniques, are declining in usage because of limited alloying potential
FIGURE 2.69 Schematic of powder metallurgy processing.
and the inherent disadvantages of reduced material compressibility. These solid-state reduced materials are well suited for lower-density applications, such as self-lubricating bearings and components requiring a high degree of strength in the as-pressed condition.
Powders produced by the melting and atomizing process use conventional steel-making technology to produce liquid metal that is then reduced to iron powder by impinging high-pressure water jets on a stream of the liquid metal. These water-atomized powders offer the advantages of greater compressibility, greater purity (through liquid metal refining techniques), and the ability to prealloy the powders for enhanced P/M part performance. An excellent review of the various powder manufacturing techniques is presented in volume 7 of the ASM Metals Handtopic (1998).
The objective of the powder-making process is to produce a raw material that offers a high level of compressibility with sufficient strength in the as-pressed condition to facilitate part production. As such, the iron powder is annealed to lower the carbon level to the lowest possible level and metallurgically soften the iron, yielding reduced compressive strength to give enhanced green density. After the powder manufacturing step, the iron powder is premixed with additives such as graphite, nickel, copper, and pressing lubricants. The graphite, copper, and nickel are added to enhance the mechanical strength of the P/M part after sintering. Pressing lubricants are added to the powder blends to facilitate ejection of the P/M parts from the die after the compaction step.
After the premixing stage, the powder is ready for compaction. This step involves the filling of a die cavity with the premixed iron powder and then applying pressure to the powder to consolidate the loose powder into a green compact. By varying the compacting pressure, the green density of the P/M part can be varied; typically, higher green densities result in higher levels of mechanical and magnetic properties.
The range of compacting pressures is usually 20 to 50 t/in2 of part surface area. The green compact is then transferred to a sintering furnace which operates at a temperature below the melting point of the iron with a protective atmosphere. A typical sintering temperature is 1120°C (2050°F) with sintering atmospheres consisting of blends of nitrogen and hydrogen. The sintering step removes the lubricant from the powder compact, causes metallurgical bonding of the powder particles, and alloys the various premix additives. At the completion of the sintering stage, many P/M parts are complete; that is, no further manufacturing steps are required.
A recent variation on the standard compaction process is warm compaction of iron powder. In this process, a specifically designed powder blend and compaction tooling are heated to approximately 138 to 149°C (280 to 300°F). This elevated temperature compaction results in higher green and sintered densities with a corresponding increase in the magnetic and physical properties of the P/M part. Warm compaction is ideally suited for high-performance applications where restriking or double pressing of the part is not practical.
The greatest percentage of P/M parts are used for structural applications requiring mechanical strength, fatigue endurance, surface wear, and so on. The Metal Powder Industries Federation has established material codes and material property standards for the range of iron P/M alloys used commercially. The interested reader is referred to MPIF Standard 35 for the range of P/M alloys and mechanical properties available by means of the powder process in both the as-sintered and the heat-treated conditions.
Standard 35 is intended to help potential application engineers in the design of new structural parts. However, it is somewhat limited in information concerning the magnetic performance of P/M materials. ASTM has published material standards for magnetic alloys.