Fig. 1. Six zone pusher furnace for iron PM parts


Despite the doomsayers' dire warnings, rapidly accelerating changes in American manufacturing provide an excellent opportunity for growth if they are recognized and acted upon. Just as agriculture changed in the early part of the 20th century, manufacturing is being driven, at the start of this century, by technology and improved methods. Requiring fewer people to produce more products through enhanced product design and manufacturing methods, the face of manufacturing is changing rapidly. Often reported in the conventional media as declining domestic manufacturing, the real dynamics and incredible opportunities are overlooked.

Additionally, suppliers of manufactured goods are continuously looking for ways to gain market-share through ever-improving products. Requiring increased capability and performance and/or lower cost allows them to differentiate themselves from their competitors. This need is insatiable and speed is essential.

Although "capacity demand," more of the same, may be down due to improved efficiencies and mature markets, "capability demand," the need to be able to do something that cannot be done now, is the key to the new manufacturing era. The only way to satisfy the "capability demand" is the relentless introduction of new technologies into the manufacturing arsenal.

Powder Metallurgy (PM)

Powder metallurgy is a technology that allows the design of a part through the engineering of the geometry, materials, and processing to effectively match the part performance to the end-use needs. Few technologies have the flexibility and economic advantages of PM, but for all of PM's advantages, the physical properties often fall short when compared to wrought materials, which sometimes limits PM's application.

To close the performance gap with wrought materials, the PM industry has been continuously developing new materials and processes to meet the higher performance applications, converting previously machined wrought parts to net shape powder metal parts. PM can be found in automotive drive trains, power tools, and even lawn equipment, providing excellent performance at a significant cost advantage relative to other material technologies.

PM - Materials and Compaction

The primary means of performance improvement for PM is increased density as most of the physical properties of sintered powder metal parts are proportional to density; the nearer to full density, the nearer to wrought material properties. As PM densities approach full density, it has been found to not only equal, but also in some instances exceed, wrought material properties due to the higher purity materials used in the formulation of alloys.

Many efforts, both by private companies and industry consortiums, are underway and achieving considerable success. Improved powder compressibility has enabled parts to be pressed to higher densities without exceeding the tonnage limitation of compaction tooling. Die wall lubrication systems have been used to lower the internal lubricant content of a powder blend, resulting in higher density parts.

While conventional double press, double sinter processing and powder forging are commonly used, more advanced techniques, such as high velocity compaction (HVC), orbital forging (OF), and warm powder compaction (WPC), are being developed and installed as commercial technologies.

  • HVC uses a high velocity ram to transfer kinetic energy to the powder, resulting in densification in a constraining die. Through a series of pre-programmed high velocity impacts, a part approaching full density can be achieved.

  • OF adapts this wrought forging technology to a PM pre-sintered preform, resulting in a nearly full density part. Effectiveness of this technology is very dependent upon the ultimate part geometry, but for certain configurations, it is quite effective.

  • WPC, although slightly more mature than HVC and OF, is a relatively new technology that employs proprietary polymer lubricants in the metal powder blend that, when heated, provide improved internal lubricity of the powder, resulting in higher pressed densities. Many commercial applications have been successfully developed using WPC that would not have been possible in PM without this technology. The outlook for WPC is for continued growth in a wide variety of applications.


Fig. 2. Pusher furnace for sintering stainless steel

PM - Thermal Processing

Another avenue available for improved performance of powder metal components is the use of high temperature sintering. Typically, iron PM parts are sintered in a furnace at a "conventional sintering" temperature of 2050°F (1120°C). Employing "high temperature" sintering [approximately 2200 - 2400°F (1205° - 1315°C) for iron parts] gives the parts manufacturer two ways to capitalize on this enhanced capability. Not only are PM properties strongly dependent upon density, they are also increased by sintering at a higher temperature. A gear designed to be sintered at 2300 °F (1260°C) should have superior performance to the same gear, using the same material, but sintered at 2050°F (1120°C). Using high temperature sintering can be an effective technique to incrementally improve the performance of applications that may be just beyond the capability of conventional PM, potentially opening new market niche opportunities.

Another aspect of high temperature sintering that can be leveraged to a commercial advantage is the performance of alternative, lower cost material systems. Since it can be argued that 80% of a part's cost is driven by its design, material selection is critical to the final cost of a part. Conventional PM materials are typically comprised of iron with elements added such as nickel and molybdenum to enhance the material's performance. Through the use of high temperature sintering, materials systems using lower cost components such as chromium and silicon can meet or exceed the performance of higher cost, conventionally sintered nickel and molybdenum materials. This can have a tremendous impact on part cost, especially for larger parts.

High temperature sintering is typically performed in an atmosphere pusher furnace. Recent developments in controls and refractory packages have made available designs that are capable of producing commercial production on a reliable basis, far superior to models produced twenty years ago.

Shown in Figure 1 is a six zone pusher furnace, rated to 2600°F (1425°C), hydrogen/nitrogen atmosphere, and capable of over 200 pounds/hour (90 kg/h) net production. Incorporating an alloy preheat muffle, this furnace is designed specifically for iron PM parts, which can be de-lubricated in the furnace prior to sintering in the high temperature heat zone.

Figure 2 is another pusher furnace, specifically designed for the sintering of stainless steels. Parts for this furnace must be de-lubricated externally, then sintered up to 2600°F (1425°C), in pure hydrogen with a throughput in excess of 200 net pounds/hour (90 kg/h).

Conclusion

Due to global forces and the changing manufacturing environment, the powder metal parts maker is facing an increased risk of losing "conventionally processed" products as they become a commodity in the marketplace. By adding high temperature sintering to his manufacturing capabilities, a PM parts producer will have an additional, powerful tool to meet the needs of his customers for higher performance, lower cost parts by custom engineering the geometry, material and processing to meet the end use requirements of his customers. This will enable him to achieve and maintain a significant competitive advantage by producing high value engineered products as opposed to low value commodity parts.

Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: High temperature sintering, high velocity compaction, HVC, powder metal, pusher furnaces, sintering, warm powder compaction