IMPROVED MACHINABILITY OF COLD FORMED STAINLESS PARTS BOOSTS PRODUCTIVITY AND TOOL LIFE
21 July 2003 - Carpenter Technology Corporation

Typically, most stainless steels with excellent heading characteristics do not machine well when secondary fabrication is required. Similarly, a stainless alloy designed to provide exceptional machining properties is not a likely candidate for cold heading.

Ready Rivet & Fastener, Ltd., of Kitchener, Ontario, however, found that it could get superior cold forming and machinability performance from a single stainless grade used to mass produce ball studs for an automotive application. The shop increased productivity by two thirds and tool life by 50 percent by using 302HQ-FM® stainless (S30431), an alloy developed by Carpenter Technology Corporation.

Originally, Ready Rivet tried making the ball studs from the conventional 302HQ stainless steel (S30430) obtained from various sources, with little or no success. At worst, the shop was unable to make the parts to customer specifications because of the alloy’s poor machinability. It experimented with cutting tools and coolants, as well as materials. Machinists struggled with heat buildup and declining tool life, achieving no better than mild success, at best.

Then, when asked for technical assistance, Carpenter regional metallurgist Bob Mohr, suggested that the shop try 302HQ-FM stainless, a modification of Carpenter Stainless Custom Flo 302HQ.

This variation of the alloy has excellent cold headability, plus machinability approaching that of Type 303 stainless. It was designed primarily for cold headed parts that needed to be drilled, slotted, broached or otherwise machined.

Ready Rivet made its ball studs by starting with 0.315” round Type 302HQ-FM stainless heading wire in annealed condition. Using a cold header, the shop cut the heading wire to 0.680” lengths and extruded down to 0.235” dia. (a rather severe 44 percent extrusion). The part was then upset in two blows to form a 0.470”-dia. washer about one third up the length of the stud.

After the cold headed studs were cleaned in a washer, they underwent secondary fabrication on a Towa shaving machine. They were fed into the machine, chucked and spun at high speeds. Two carbide form tools, on opposite sides, plunged into the headed studs to form a 5/16”-dia. ball on one end and cut a neck from 0.320” down to 0.155” dia. – also a very severe reduction.

Since switching to Carpenter 302HQ-FM stainless heading wire, Ready Rivet has been able to reduce cycle time by approximately 40 percent; which means it has increased hourly output of stainless studs by almost 67 percent. Even after increasing its shaving machine speeds by 40 percent, the shop has been enjoying a 50 percent gain in tool life. After passivating, the parts exhibit the bright finish required for the application.

“This stuff cuts better than the low carbon steel we use to make similar parts,” commented Tim Brennan, Ready Rivet vice president, manufacturing, referring to 302HQ-FM stainless. “The difference between the conventional material and this improved alloy is amazing,” he added. “You could tell even from the sound that the 302HQ-FM stainless was cutting easier. The original material would rumble when the work was running; the upgrade stainless would squeal.”

Carpenter 302HQ-FM stainless has a machinability rating of about 75 percent of Carpenter Project 70+® stainless Type 303 in conventional cutting type applications. Due to its low work hardening rate, this stainless grade is superior to Project 70+ stainless Type 303 in roll threading and cold form tapping machining operations.

Annealed 302HQ-FM stainless is resistant to atmospheric corrosion, foodstuffs, sterilizing solutions, many organic chemicals and dyestuffs, and a wide variety of inorganic chemicals. For optimum corrosion resistance, surfaces must be free of scale and foreign particles, and finished parts should be passivated.

Typical analysis of the alloy is: carbon 0.06 percent max., manganese 2.00 percent max., phosphorous 0.040 percent max., sulfur 0.14 percent max., silicon 1.00 percent max., chromium 17.50 percent, nickel 10.00 percent, copper 1.75 percent, balance iron.

http://www.cartech.com

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