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News

New hope for the machine shop or department that is locked into a stainless steel

Carpenter Technology Corporation : 08 May, 2006  (New Product)
There's new hope for the machine shop or department that is locked into a stainless steel that is just right for the part it is making except for one shortcoming. It has poor, profit-draining machinability. Valve components machined by G. W. Lisk from Carpenter Micro-Melt Type 431 FM stainless steel and from Carpenter Project 70+ 15Cr-5Ni stainless for high-temperature applications. G. W. Lisk Co., Inc., Clifton Springs, N.Y., for example, found a highly productive solution to that dilemma. Machinability had been a problem with two different alloys specified for two critical parts it had been making for use in high-temperature applications.
There's new hope for the machine shop or department that is locked into a stainless steel that is just right for the part it is making except for one shortcoming. It has poor, profit-draining machinability.

Valve components machined by G. W. Lisk from Carpenter Micro-Melt Type 431 FM stainless steel and from Carpenter Project 70+ 15Cr-5Ni stainless for high-temperature applications.

G. W. Lisk Co., Inc., Clifton Springs, N.Y., for example, found a highly productive solution to that dilemma. Machinability had been a problem with two different alloys specified for two critical parts it had been making for use in high-temperature applications.

The well-known manufacturer of solenoids, valves, linear variable differential transducers and flame arresters was enduring poor results trying to machine two different valve components, one from stainless Type 431 and the other from Type 15Cr-5Ni stainless.

The Type 431 grade (UNS S43100) provided the high strength, corrosion resistance, toughness and hardness required for the first part. The 15Cr-5Ni alloy (UNS S15500) offered the high strength, heat resistance and corrosion resistance needed for the second part. But the machinability of both stainless steels was unacceptable.

From 12-ft.-long stainless Type 431 bar stock, Lisk machine operators worked 0.687-in. dia. x 2-in. long valve shafts on a Nakamura Tome-TW-20 twin-spindle CNC machine. They performed 10 machining operations on the part, including drilling, turning, boring, milling, slotting, deburring and cutoff.

Machining cycle time per part was 5 min. 6 sec., which was too slow a production rate for the plant to ship the required 1,500 parts per week for four months. It took anywhere from 93 to 131 hours to make 1,000 parts, depending on consistency of the bar stock from one shipment to the next.

Carbide tools such as end mills, t-slot cutters and drills broke frequently, or required excessive maintenance. Burrs created in milling the t-slot had to be cleaned off with an abrasive brush, at substantial cost in labor and productivity.

Unable to keep up with demand for the components, Lisk asked Bob Mohr, regional metallurgist for Carpenter Technology Corp., Reading, Pa., whether he had any ideas for improving machinability of the Type 431 stainless.

Mohr, with a Carpenter technical service team, suggested that the shop try an improved variation of the alloy made by powder metallurgy with increased sulfur content. With Lisk's agreement, Carpenter Powder Products produced a special grade called Micro-Melt Type 431-FM stainless.

Carpenter's Micro-Melt powder metallurgy processing operations convert gas-atomized powder into 100% dense finished products. This processing, in harmony with the sulphur addition, gives the alloy the much improved machinability obtained in this application.

Sulphur is increased from less than 0.03% in the standard alloy, to a substantially higher level in the upgraded alloy. The rapidly solidified, gas-atomized powders that are produced at the beginning of the Micro-Melt process have a finer, less segregated as-cast microstructure than that obtainable by the conventional cast/wrought method of steelmaking.

Thus, 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 sulphides. This microstructure is ideal for one seeking good machinability.

As a substitute for the conventional stainless, the newly developed alloy was a resounding success. While providing much improved machinability, it retained all the other essential properties demanded of the material.

Lisk operators working the Micro-Melt Type 431-FM stainless were able to reduce part cycle time from 5 min. 6 sec. to 3 min. 27 sec. By another measure, the time required to machine 1,000 parts was slashed to 55-58 hours, down from the best previous experience of 93 hours. The great variation in machining time required with the standard alloy was eliminated because of the highly consistent performance of the upgraded alloy.

Tool life was doubled using the Micro-Melt PM material, which machine operators found significantly easier to cut. Time required to remove the burrs around the T-slot has been reduced to a minimum.

Discounting the higher cost of the improved alloy, which is made by premium processes, Lisk Purchasing Manager Steve Cheney estimates that it has helped the company reduce machining costs for this part by approximately 40%.

From the outset of its first production run, Lisk had difficulty with the machinability of its stainless Type 15Cr-Ni. Several operators wondered aloud whether they were using the right material, such was their disappointment with its machinability.

The contractor making the second part for Lisk machined it from 4-ft.-long x 1-in.-dia. cut lengths of 15Cr-5Ni stainless on a CNC turning center. Bars received from the steel supplier normally had to be cut and sent to a commercial heat treater for aging and heat treatment before machining.

This is a precipitation hardening alloy known for its very good combination of high strength, hardness and corrosion resistance. It has been used most commonly for aircraft structural components such as landing gear and flap tracks, as well as other critical parts requiring its special combination of properties.

Turning, drilling, boring and tapping this standard grade of 15Cr-5Ni stainless was painful and unsatisfactory. Machining results were generally poor. Carbide tools broke prematurely or required frequent maintenance. Downtime for tool sharpening was burdensome, labor costs escalated and productivity suffered.

Apprised of these problems, Mohr suggested that the shop try Carpenter's upgrade of the conventional alloy specifically designed to improve machinability. The alternative alloy, known as Carpenter Project 70+ 15Cr-5Ni stainless, offers all the attributes of the original alloy while conforming with existing specifications such as AMS 5659 covering bars, wire, forgings, rings and extrusions.

Lisk agreed, seeing an immediate difference when it received 12-ft. lengths of the Carpenter alloy already aged to condition H-1150 (1150F/621C) at the mill. That eliminated the need for outsourcing aging and heat treatment.

Since using the alloy with improved machinability, Dave Phillips, CNC department supervisor, estimated that the shop has more than doubled tool life, increased productivity by about 35%, and saved a similar percentage in costs for labor and machine downtime for maintenance and tool sharpening.

Unlike the resulphurized Micro-Melt Type 431 FM stainless, which was customized for the shop, the Carpenter Project 70+15Cr-5Ni stainless variation is commercially available. They both demonstrate that cost-efficient alternatives can be considered for materials lacking in machinability the characteristic that is most important to machine shops.
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