Heat Handlers: Turning Performance

Nickel-base superalloys can withstand hot and tough conditions, but are difficult to machine. With the right approach, machinists can keep their cool when cutting them.

Metal components used high in the sky and deep in the ground experience high-temperature, high-stress environments. For those applications, part designers often call on heat-resistant superalloys (HRSA), which are classified into nickel-, iron- and cobalt-base groups.

According to Pat Schuur, owner of metal distributor Schuur Metals Inc., San Clemente, Calif., the primary applications for nickel- and cobalt-base superalloys are for the aerospace, oil and gas and petrochemical industries. “That covers about 90 percent of HRSAs,” he said.

Superalloys provide high mechanical strength and resistance to surface degradation at temperatures of 1,200° F and above. That’s because HRSAs have high tensile, creep-rupture and fatigue strength; good ductility and toughness; and enhanced resistance to oxidation and hot corrosion, stated Rick Frank, metallurgist for metals producer Carpenter Technology Corp., Wyomissing, Pa.

Compared to other superalloys, iron-base superalloys cost less but are less tolerant of alloying additions and have less favorable mechanical properties and lower temperature limits. Cobalt-base superalloys cost significantly more than the others and typically cannot be age hardened to high strength levels. However, cobalt is an important alloying addition to nickel-base superalloys because it extends the maximum temperature they can be used at by reducing the solubility of the age-hardening phase.

Nickel-base metals represent at least half of the market for superalloys, with 718 Inconel, Waspaloy and Udimet 720 being the most prominent members of that group, according to Sean Holt, aerospace applications manager for Sandvik Coromant Co., Fair Lawn, N.J. He added that aerospace applications account for 60 percent of that group’s usage. Nickel-base superalloys represent about half of the weight of a typical aircraft engine, which has two hot areas: combustion and turbine.

This article covers applications for nickel-base superalloys, the properties of nickel and elements alloyed with it to produce the metals, tools and techniques for effectively cutting them and cost considerations when selecting the appropriate workpiece material.

Application Areas

In addition to aerospace engines, other common applications for nickel-base superalloys include power-generation-turbine parts, aerospace fasteners, automotive manifold bolts, diesel-engine exhaust valves and hot-working tools and dies, according to Carpenter Technology.

Paul Dickinson, new product engineer for Winsert Inc., Marinette, Wis., noted that the foundry is involved in numerous exhaust-gas recirculation applications using nickel alloys. “Basically, it’s making sure customers have a material that they can use to make their equipment, such as an engine running on diesel, EPA compliant,” he said. In addition to producing superalloys, the company also machines them based on customer requirements.

When copper is a major alloying element, the nickel-base superalloys are used to make electronic components, such as fuses and shunts, noted Gregory Zahm, quality and safety manager for Vista Metals Inc., Bristol, R.I., a warehouse distributor of bar, rod, wire and strip metals. One frequently used metal the company offers is Niclal 37 and 38, copper-manganese-nickel alloys that Vista sells in coil form or cuts to length. “For shunt applications, it takes high energy through a piece of equipment without any surges that may short out the equipment,” he said.

Zahm added that cobalt-nickel alloys provide magnetic shielding, so component-shielding shells and canisters are often made of the material, and copper-nickel alloys’ corrosion-resistance properties make them suitable for parts found in fumigation spraying equipment.

Because of their combination of strength, toughness and environmental resistance, some of the alloys are also suitable for other lower-temperature applications, such as medical implants.

Nickel and Alloying Elements

Although most superalloys contain more than 50 percent nickel, some do not. For example, Carpenter Technology reports its Pyromet A-286 alloy is still considered a superalloy with only 25 percent nickel.

In some applications, Zahm pointed out that the superalloy’s hardness and electrical properties increase as the percentage of nickel increases, which is beneficial for fuse or shunt applications.

Commercially pure (99.6 percent) nickel 200 is ductile and has good mechanical properties, high electrical and thermal conductivity and a low hardness, stated Ulbrich Stainless Steels and Special Metals Inc., North Haven, Conn., in a white paper about nickel-base superalloys. Although it has a relatively low workhardening rate, nickel strip can be cold worked to moderately higher strength levels while maintaining ductility.

Courtesy of Sandvik Coromant

When turning heat-resistant superalloys, accurately targeted high-pressure coolant provides chip control while enhancing cooling at the tool/workpiece interface and enables higher cutting speeds.

The paper also stated that nickel 200 is best at resisting corrosion in reducing environments, which have little or no free oxygen, but can also be used under oxidizing conditions and in caustic environments that develop a passive oxide film on the alloy surface. Nickel also can withstand sulfuric acid at low or moderate temperatures, anhydrous hydrofluoric acid at elevated temperatures, organic acids of all concentrations (if aeration is not high), caustic soda and other alkalis and nonoxidizing halides.

Nickel 201, the low-carbon version of nickel 200, is similar in terms of corrosion resistance, according to the Ulbrich paper. It is preferred for applications involving exposure to temperatures higher than 600° F.

For most applications, alloying elements help to impart and maintain the desired properties at elevated temperatures, according to Carpenter’s Frank. In addition to nickel, superalloys contain various combinations of iron, cobalt and chromium, with smaller amounts of other elements, including molybdenum, niobium, titanium, aluminum and minor additions of beneficial elements such as boron and zirconium.

Refractory elements such as molybdenum, tungsten and niobium, with their large atomic diameters, increase high-temperature strength and stiffness by straining the nickel/iron base matrix, according to Frank. Larger additions of molybdenum increase this solid-solution strengthening effect. Other alloying additions, such as chromium and aluminum, also contribute to solid-solution strengthening, but to a lesser extent.

Frank added that titanium, aluminum and niobium are added to the nickel or nickel/iron matrix to form an intermetallic Ni₃ (Al, Ti, Nb) phase during age-hardening heat treatments. The resultant gamma prime or gamma double-prime phases are the main strengthening agents in superalloys. Although other elements, such as boron, zirconium and magnesium, may be added at less than 0.1 weight percent, the beneficial effects are quite potent. These elements segregate to and stabilize grain boundaries, significantly improving hot workability, high-temperature strength and ductility. Small additions of carbon may be made to form carbides that restrict grain growth and grain boundary sliding during high-temperature exposure.

Numerous other elements, including silicon, phosphorus, sulfur, oxygen, nitrogen and a larger number of tramp elements (like lead, bismuth and selenium) must be tightly controlled in superalloys to avoid detrimental effects on high-temperature properties, Frank stated. These minor and tramp elements are controlled during raw material selection prior to melting, as well as during the melting and remelting processes.

Machining Guidelines

A number of wear mechanisms occur when machining HRSA. According to Sandvik Coromant’s Holt, the two major ones that have a detrimental effect on chip formation are built-up edge and, because the surface is prone to workhardening, notch wear.

The latter effect, also called notching, is found on the main and secondary cutting edges and is more prominent when cutting nickel- and cobalt-based alloys, the toolmaker reports. On the main edge, notching is seen as chipping at the DOC and is mainly mechanical wear. Notching occurs on the secondary edge where an insert exits the workpiece and negatively impacts the surface finish. Notching on the secondary edge is caused mainly by chemical wear; aluminum-oxide and PVD tool coatings are recommended to minimize it.

BUE and notching on the main and secondary edges can be minimized by applying round inserts with positive, reinforced geometries. “Round inserts are the strongest inserts available, and with a round insert you’re getting out of the area of notch wear by varying the entering/lead angle, depending on the depth of cut selected,” Holt said. “You should never be using a typical CNMG insert to machine superalloys as this creates a 95° entering angle, thus increasing notch wear on the insert.”

Courtesy of Carpenter Technology

Carpenter Technology’s vacuum arc remelting furnaces remove impurities from ingots produced in primary melting.

Although machining hardened materials tends to cause the most notching and increases the cutting temperature, it’s easier to effectively break a chip in harder nickel-base alloys because the material has less elasticity, Holt noted. “By fixing the coolant nozzle in the correct position, a parallel laminar coolant jet is created with high velocity to segment the chip in harder materials.”

He added that a heat-treated HRSA part with a hardness of 44 to 52 HRC is usually finish-machined, whereas the part is roughed in a softer state with a hardness of about 26 HRC. Softer materials are much gummier, he added.

Other wear mechanisms when cutting HRSA include plastic deformation, chip hammering and top slice, which is only found in ceramic tools. According to Sandvik Coromant, plastic deformation is thermal wear caused by high temperature and high pressure on the cutting edge.

An insert that resists flank wear and has high hot hardness helps reduce plastic deformation. “Wear on the insert should never exceed 0.008 ” because that’s when you start to create plastic deformation in the insert and create negative residual stress in the part,” Holt said.

Chip hammering is a form of mechanical wear created when chips hit the edge line outside the cutting zone and mainly occurs when machining softer, more ductile nickel alloys. It can occur at the top and bottom of an insert, and changing the feed rate and DOC may redirect chips to reduce wear. Because they have a higher edge-line toughness, PVD-coated inserts rather than CVD-coated ones are recommended for nickel alloys, especially when roughing, according to Holt.

Top slice is caused by high cutting pressure and vibration and must be controlled when surface quality and burr minimization are important. To minimize top slice, Sandvik Coromant recommends lowering the cutting pressure by reducing the chip area in stable conditions, and reducing the engagement angle with programming techniques in vibration-generating unstable conditions.

Holt pointed out that when cutting nickel-base superalloys, trochoidal milling and trochoidal turning can be effective because they eliminate the high level of contact between the insert and workpiece and prevent wraparound. (Wraparound occurs when plunging or profiling corners with round inserts because of the high angular engagement, creating high cutting pressures. Therefore, the feed must be reduced.)