Coated Tools of High Strength, High Tough Steel Produce up to 100 Times More Powder Metal Parts
A major producer of powder metal parts has enjoyed as much as 100 times more productivity by adding long-term wear resistance to tooling already made from steel with the optimum combination of high strength and fracture toughness.
Alpine Pressed Metals was challenged by the usual severity of powder metal parts manufacturing - the adverse effect on tooling of high press loads and the abrasive wear encountered in high speed powder metal compaction.
The Ridgway, PA, producer solved his initial tool failure problems by switching from conventional tool steels to AerMet®-for-Tooling alloy, a specialty grade developed by Carpenter Technology Corp., Reading, PA.
AerMet-for-Tooling alloy, Carpenter reports, offers an excellent combination of high tensile and shear strengths, high fracture toughness and resistance to fatigue. Along with these fine tooling attributes, however, improved wear resistance was also desired.
To obtain longer wear, Jim Paladino, Alpine engineering manager, turned to Metalife Industries, Inc., a Reno, PA, specialty industrial chrome plating company. Metalife applied a thin coating
to Alpine's tools in two stages. In the first, a chrome film was bonded molecularly to the metal substrate. Then, in what's called a polyseal process, a melted polymer compound was added for lower friction. Results, Paladino said, have been "spectacular".
Alpine's tooling problem started with an inner bottom punch used to make iron-base powder metal bushings for the seat adjustment in certain automobiles. The bushing was 0.900" long with a 0.650" OD and a 0.400" ID. The punch had an intricate shape and a thin wall. It was 7¼" long with a 0.500" OD, a 0.400" ID, a key on the OD and a 25m taper across the face.
Parts were molded on two 30-ton Dorst & Yosh compacting presses at the rate of 700 to 800 parts per hour. An average of 32 tons per square inch pressure was applied per each compacting stroke.
Initially, the punches were made from AISI Type S7, AISI Type D2 and 4% vanadium powder metallurgy cold work tool steels. These punches warped in heat treatment, making it impossible to grind relief in the punch ID. Worse, they would break off near the top end, sometimes after making as few as 10,000 pieces. Occasionally, but unpredictably, they lasted longer before failing.
"We couldn't get any repeatability," Paladino complained. "It was like a crapshoot. We wanted to produce 150,000 to 250,000 parts a week, but we never knew what to expect."
Alpine then asked Dietech Tool & Die Co., Inc., St. Mary's, PA, to make its punches from Carpenter's AerMet-for-Tooling alloy. Dietech, working from 1¼" bar stock, lathed the tools close to size, and heat treated in accordance with Carpenter's schedule. The punches were then double aged, finish ground, face detail burned on a CNC Ram EDM, polished and delivered to Alpine.
Using the AerMet alloy punches, the company eliminated all breaking problems and distortion. The Carpenter alloy is an air-hardenable steel that is virtually free of distortion when properly heat treated. That is a bonus for anyone who must make parts with complex shapes and/or critical size tolerances.
Then, to withstand all service conditions, Alpine installed new punches made of AerMet-for-Tooling alloy that were coated by Metalife. Paladino was astonished because at last count, he had produced more than one million bushings with these tools - and they were showing no signs of wear.
The consequences of the upgraded tooling were considerable. Alpine eliminated tool breakage and the costly press downtime required to pull worn tools; as well as the time to insert, reset and requalify replacement tools. Paladino had suffered production interruptions that cost his company thousands of dollars.
"The one thing we could not afford," he stated, "was a shutdown that could cripple us on a high volume job like this one."
Metalife, employing a proprietary process, produces a first-stage covering of extremely hard and dense chrome that averages 0.0001" in thickness. To achieve a molecular bond with the metal (both ferrous and non-ferrous), the surface is cleaned with a petroleum- or detergent-based solvent, or acid etched if scale is present. Final cleaning is accomplished by vapor honing, using a liquid slurry of fine abrasive particles that is mixed with air and applied through a hand gun under 90 psi pressure.
Individual tools or components can be fixtured to electrodes, for DC current, in a variety of ways; special fixturing is used on inside diameters, sharp angles, keyways, grooves, etc. Then the items to be plated are immersed in the Metalife solution. Plating thickness and growth rate are controlled by time exposure and amperage charged through the tool or part.
Since the Metalife plating becomes part of the parent metal, observes Bob Jacoby, company sales engineer, it does not chip, crack, plate or peel. The coefficient of friction of the parent metal is generally reduced by nearly one half.
AerMet-for-Tooling alloy also was reported to plate more uniformly than any of the conventional tool steels previously tried.
The plating, reports Jacoby, resists corrosion attack by almost all organic and inorganic compounds except sulphuric and hydrochloric acids. When the Metalife plating is subjected to 30% solution of copper sulphate, no breakdown is evident after 16 hours exposure.
Following the molecular chrome process, Metalife applies its MLP (Metalife polymer) treatment to further reduce friction, enhance the lubricity of treated parts, and improve corrosion resistance.
The MLP treatment uses a long-chain organic polymer that adheres extremely well to the chrome plated surface. It is impregnated into the microporous structure of the plated surface, sealing off the surface and imparting time-release lubricity.
The polymer compound is petroleum-based, containing wax and plastic. It contains a polishing compound and chrome particles which bond to the chrome plating. The compound is melted on the surface to be treated and rubbed into the micro-cracks of the plating to achieve a seal and polished finish.
Metalife has coated a variety of tools used to make powder metal parts, including: dies, core pins, plungers and tooling with delicate face details and intricate configurations. The double coating treatment, notes Jacoby, has safeguarded many tools from galling. In addition, it has helped powder metal parts "release" from the tool after they were under many tons of pressure. Finally, because of the coated tool's low friction, the powder metal does not "pickup" and transfer to the tool under pressure.
Metalife reported two additional successes where powder metal parts producers used tools that had been made from AerMet-for-Tooling alloy covered by its dual coatings:
A. - One manufacturer was making stainless steel parts with a tubular plunger, 9" long with 3" OD and 2" ID, made from AISI D-2 tool steel. Design of the tool caused a pressure differential and alignment problem. A long thin wall led to cracking problems. After producing 12 to 100 pieces, the plunger cracked and broke.
AerMet-for-Tooling alloy solved the tool breakage problems, but the tool made of this alloy galled after a few hundred parts. The same tool, coated by Metalife, has produced 40,000 to 50,000 parts, with no sign of wear or breakdown.
B. - In another case, a PM parts manufacturer was compacting double gears in a 40-ton press. The gear was 4½" OD x 1.9" ID. The tool's teeth ran parallel to each other on a vertical axis.
Tools of conventional tool steel cracked after 2,000 hits. One tool that had been made of AerMet-for-Tooling alloy eliminated the cracking. The same tool coated, was still going strong after producing 40,000 to 50,000 gears.