Tooling alloy started in aerospace

Modern Machine Shop, March, 1996

This specialty alloy was invented to meet the demanding specifications for landing gear components of U. S. Navy carrier-based jet aircraft. Companies working with this material began to discover that the alloy had certain outstanding properties that recommended it as an effective upgrade for conventional tooling materials.

With a little modification in mill processing, the developers were able to customize a variation of the original alloy, which has been used by tool makers and machine shops. The new material, known as AerMet-for-Tooling alloy, is made to tool steel tolerances, but is tested more extensively than other tool steels. Its chemical composition remains the same as that of the grade used in aerospace applications.

The alloy characteristics that have been valued in aerospace applications are now available for tooling applications requiring extraordinary high-strength and exceptional fracture toughness. In a growing number of tooling applications, this steel has been found to be a substitute for traditional tool steels to prevent the premature cracking or breaking of tools under heavy load or impact, or to solve those problems when they do occur.

What has made this alloy so distinctive as a tooling material is the combination of ultra high-strength with fracture toughness its developers believe is superior to that of any steel currently available. It can be heat treated to 280-300 ksi (280,000-300,000 psi) tensile strength while exhibiting fracture toughness in excess of 100 ksi [square root of inch] at 280 ksi. It can be heat treated to a hardness of 53.5-55.0 Rc, and exhibit Charpy V-notch impact resistance in excess of 30 foot-pounds longitudinally. It has fatigue resistance and ductility that is well in excess of that for any tool steel, combined with tool steel hardness, developers say.

While commonly used tough tool steels such as AISI S7 alloy and AISI Type A6 alloy provide three to five foot-pounds transverse Charpy V-notch impact toughness, the AerMet alloy provides at least 25 foot-pounds in the same evaluation for toughness. In addition, though not a common tool steel requirement, the material offers exceptional resistance to stress corrosion cracking.

This alloy is fully air hardenable, and shows very low levels of distortion when heat treated. This very low distortion is important to shops constructing tools with complex shapes and/or critical size tolerances. The AerMet alloy tooling material is a nickel-cobalt steel strengthened by carbon, chromium and molybdenum. Its nominal analysis: 13.4 percent cobalt, 11.1 percent nickel, 3.1 percent chromium, 1.2 percent molybdenum, 0.23 percent carbon and the balance iron.

In applications as a tooling material, the new alloy has achieved remarkable results. Tooling Dynamics of York, Pennsylvania, for example, had difficulty finding a durable tooling material for punches used in high-speed, two-stage coining of bearing races from C-1018 steel strip, 0.06 inch thick. For three years the company endured costly tooling problems. Punches made from AISI D2, L6, AISI S7 and AISI H13 wore out prematurely. They wouldn't cut or provide enough wear resistance. They would shatter, crack, even explode. When the shop switched to AerMet 100 alloy, it tripled production from 60 to 180 parts per minute, increased parts per punch 20 times from 15,000 to 300,000, while running at only half the tonnage capacity of the press.

Norwalk Powdered Metals, Inc. (Norwalk, Connecticut) had serious problems when it used L6 tool steel for the punch base plate of its mid-range powder metals briquetting press. The plate was subjected to 40 tons per square inch pressure with each stroke of the press. The bearing load on a critical section of the plate was 23,000 psi. As a result, cracking and deterioration limited production to 100,00 to 150,000 parts. The company then made a baseplate of the new alloy. At last report, that plate had produced more than one million powder metal parts, or up to 10 times more than the previous tool steel.

Precision Propeller, Inc., an Indianapolis manufacturer of marine turbo propellers, went through a succession of tool steels for a mandrel-type application subjected to repeated, high-impact hammer blows. The company needed a high-strength shaft to hold propeller blades in place for straightening. Shafts made of conventional tool steels broke in a month or less, enduring 200 to 300 hammer blows a day. Shafts subsequently made of the new alloy still in use after a year of service, so far lasting at least 12 times longer than tool steel shafts.

Other case histories show that AerMet-for-Tooling alloy may be considered a candidate alloy for shrink rings, swaging tools and a variety of tooling applications.