Engineering PM Parts Without Pricey Alloys

Powder metallurgy (PM) is a cost-effective method for producing precision-metal consumer and industrial products.

The basic PM process uses pressure and heat to form precisely shaped parts. Powder is squeezed in dies at pressures to 50 tons/in.2 into engineered shapes such as gears. The “green” parts then travel through a high-temperature, controlled atmosphere furnace where they are sintered — metallurgically fused without melting — to bond the particles together.

This results in high-strength parts used in everything from cars and lawn mowers to surgical instruments and aircraft turbine engines. And it saves valuable raw materials by recycling unused powder and eliminating costly secondary machining though net-shape design.

But PM parts makers are being hit by spiraling energy costs and volatile commodity prices. Traditional sinter-hardening PM steel contains molybdenum, nickel, and copper, and prices of these alloying agents have seen significant price increases in recent years. For instance, nickel prices have doubled and molybdenum is up about 400% since 2003.

To lessen price sensitivity, Hoeganaes Corp., Cinnaminson, N.J., has developed new formulations that lessen the pricey constituents yet retain excellent mechanical properties.

Sinter hardening
Sinter hardening is an increasingly popular method for improving PM part properties. According to Hoeganaes officials, it eliminates the need for secondary quench-hardening and reduces part distortion. Also, parts quenched in oil baths retain a considerable amount of oil in their pores. If tempering above 400°F, oil-quenched parts must first be heated below 400°F to burn off the entrapped oil. And sinter-hardened parts do not need an oil-removal step prior to finishing operations such as plating.

Traditional sinter-hardening PM steels have high levels of Mo, Ni, and Cu along with high-carbon content to form martensitic microstructures. For instance, Hoeganaes’s Ancorsteel 737 SH combines good compressibility and excellent hardenability, but at current prices, the 1.25% Mo and 1.4% Ni in the powder make it somewhat less attractive to OEMs.

Fortunately, as sinter hardening has become more common, furnaces have been redesigned to cool parts faster. This means lower alloyed steels can be used for sinter-hardened components, especially small parts that tend to cool faster than large ones. Therefore, parts makers needed an alloy composition that minimizes price-sensitive elements while taking advantage of the ability to sinter harden in modern belt furnaces.

Answering this demand, Hoeganaes developed Ancorsteel 721 SH, a prealloyed steel powder specifically developed for sinter hardening. It contains less molybdenum (0.9%) and nickel (0.5%) than 737 SH. With good compressibility and dimensional stability, 721 SH is a good choice for small to medium-size sinter-hardened parts that do not need the slightly higher hardenability of 737 SH.

Comparing properties
Here’s a look at how low-alloy 721 SH and more-traditional 737 SH compare, keeping in mind that processing considerations (accelerated cooling) strongly dictate mechanical properties of the new alloy.

As the accompanying chart shows, 721 SH has better compressibility with a green density of 7.1 gm/cm3 or better at a compaction pressure of 690 MPa (50 ksi). A higher density at similar compaction pressures plays a significant role in letting the new alloy match and even surpass the mechanical properties of its higher-alloy counterpart.

Other charts highlight mechanical properties of the alloys at a compaction pressure of 690 MPa. The two alloys perform similarly with 2% added Cu and 0.9% graphite. Both have the hardenability to reach or exceed 30 HRC.

A major difference between the two is the dimensional change after sintering. Current die dimensions would require modification to account for greater part expansion with 721 SH. However, one benefit of lower alloy content is less retained austenite in the sintered microstructure, and this leads to better dimensional consistency in parts.

Additionally, elongation, ultimate tensile strength, and yield strength are comparable in level and observed trends for parts with 2% Cu and 0.9% graphite for both alloys. These results suggest that 721 SH is a practical sinter-hardening alternative to 737 SH.

Results are similar with 1% Cu and 0.7% graphite. With conventional cooling rates (~ 0.7 °C/sec), 721 SH has lower hardness and inferior mechanical properties. But at higher cooling rates (1.6 to 2.2 °C/sec) properties match or exceed those of 737 SH.

A look at the microstructures backs up these results. When 721 SH mixed with 1% Cu and 0.7% graphite is slow cooled at 0.7°C/sec, the new alloy largely transforms into a pearlitic (with some bainite) microstructure, which explains the lower mechanical properties. At faster cooling rates, however, the microstructure almost fully transforms into martensite; increasing from approximately 15% martensite, 85% bainite/pearlite to 87% martensite, 13% bainite/pearlite at the highest cooling rate, based on a point-count method. This clearly shows the significance and necessity of using accelerated cooling to fully exploit this lean sinter-hardening alloy.

Therefore, incorporating advanced cooling systems in modern sintering furnaces yields almost fully martensitic microstructures within the new alloy. The relative amount of martensitic transformation under accelerated cooling of 721 SH provides mechanical properties rivaling those of 737 SH at a reduced cost.

As a result, users now have a choice. When alloy prices are low, it is easy to use heavily alloyed materials to ensure martensitic microstructures regardless of section size and cooling rates. As rising prices force a reassessment of alloy selection, it may be more cost effective to invest in modern furnaces and reduce the content of high-priced alloying elements.