G. W. Lisk Co., Inc. (Clifton Springs, New York), a manufacturer of solenoids, valves, linear variable differential transducers (LVDT) and flame arresters, was experiencing poor results when machining two different valve components, one from Type 431 stainless, and the other from Type 15Cr-5Ni stainless.
The Type 431 grade (LNS S43100) provided the strength, corrosion resistance, toughness and hardness required to machine the first part, while the 15Cr-5Ni alloy (UNS S 15500) offered the strength, heat resistance and corrosion resistance required to machine the second part. However, the company reported that the machinability of both stainless steels was unacceptable in these high-temperature applications.
Machining cycle time per part was 5 minutes, 6 seconds, which proved to be too slow a production rate for the plant to ship the required 1,500 parts per week for 4 months. Typically, 93 to 131 hours were required to make 1,000 parts, depending upon the consistency of the barstock from one shipment to the next.
From stainless Type 431 barstock measuring 12 feet in length, Lisk’s machine operators worked valve shafts (0.687 inches in diameter by 2 1/2 inches in length) on a Nakamura Tome-TW-20 twin-spindle CNC machine. A total of ten machining operations were performed on the part, including drilling, turning, boring, milling, slotting, deburring and cutoff.
Carbide tools, such as end mills, T-slot cutters and drills, broke frequently or required excessive maintenance. Additionally, burrs created in milling the T-slot had to be cleaned with an abrasive brush, a step that, according to the company, represented substantial labor costs.
Unable to keep up with demand for the components, the company consulted Bob Mohr, a regional metallurgist for Carpenter Technology Corp. (Reading, Pennsylvania), to find ways to improve the machinability of the Type 431 stainless steel. Mr. Mohr, along with a Carpenter technical service team, suggested that the shop try an improved variation of the alloy produced by powder metallurgy with increased sulfur content. Carpenter Powder Products then formulated Micro-Melt Type 431FM, a custom grade stainless.
The Micro-Melt powder metallurgy processing operations convert gas-atomized powder into 100 percent dense finished products. With the company’s particular application, this metallurgy processing, when combined with the addition of sulfur, improved the machinability of the alloy.
The standard alloy has a sulfur content of less than 0.03 percent, while the upgraded alloy contains a considerably higher level, according to Carpenter. Additionally, the rapidly solidified, gas-atomized powders that are produced at the beginning of the Micro-Melt process are said to have a finer, less segregated as-cast microstructure than would be obtained using the conventional cast/wrought method of steel making. After the powder is consolidated and processed into finished machining bar, the bar microstructure is also uniform, with fine grains and uniform distribution of small carbides and sulfide.
According to Lisk, the newly-developed alloy works well as a substitute for the conventional stainless steel. In addition to enhancing machinability, it retains all of the other essential properties that are required for various applications, the company says.
Working with the Micro-Melt Type 431 FM stainless, the company’s operators were able to reduce part cycle times from 5 minutes, 6 seconds to 3 minutes, 27 seconds. By another measure, the time required to machine 1,000 parts was also reduced—from 93 hours to between 55 and 58 hours.
Another benefit of using the custom alloy was an increase of about 200 percent in tooling life. Operators at the company also noticed that the Micro-Melt FM material was easy to cut. In addition, the time required to remove the burrs around the T-slot has been reduced.
Although the custom alloy costs more than conventional stainless steel, the company has found ways to more than compensate for the discrepancy in price. Steve Cheney, purchasing manager at Lisk, estimates that the upgrade reduced machining costs for this part alone by approximately 40 percent.