Power parts: Heat-resistant superalloys: Drilling Performance

Extracting oil and natural gas from hot, corrosive and otherwise demanding environments, such as when hydraulic fracturing or deep-sea drilling, is helping to make the U.S. energy-independent, but stresses the metal components used to get the job done. Fracking gear goes 7 miles or more into the ground, where it is subjected to pressures up to 25,000 psi (1,724 bar) and temperatures as high as 500° F (260° C). In addition, deep wells contain “sour” crude, which is highly corrosive because of its sulfur content. Even parts for above-ground applications in the energy industry experience enough stresses to make conventional metals wilt.

Therefore, designers of turbine blades, rotors, valve bodies and manifolds, pump parts, vanes and a host of other energy parts turn to exotics—primarily heat-resistant superalloys (HRSAs). “They use superalloys that can tolerate heat, corrosion, abrasive wear—just about anything,” said John Forrest, vice president and national sales manager for toolmaker Tool Alliance, Fort Myers, Fla. “Some precision components, such as for a generator or turbine system, are used in a stable environment, while others are subject to extreme conditions, such as of being part of a deep-well drilling system.”

Courtesy of Sandvik Coromant

Effectively directing high-pressure coolant to the cutting zone enhances machinability and extends tool life when cutting HRSAs, such as when making energy industry parts.

Three groups of HRSA materials exist: nickel-, cobalt- and iron-based alloys. According to a white paper from Sandvik Coromant Co., Fair Lawn, N.J., nickel-based ones are the most widely used, with common types including Inconel 718, Waspaloy and Hastelloy X. Cobalt-based superalloys, such as Haynes 25 and Stellite 31, display exceptional creep and corrosion resistance at high temperatures, similar to the nickel alloys, but are more expensive and more difficult to machine. Developed from austenitic stainless steels, iron-based superalloys, such as Inconel 909, can provide low thermal-expansion coefficients, but have the poorest hot-strength properties of the three.

Lance Hughes, industry specialist – oil and gas for Mitsubishi Materials USA Corp., Fountain Valley, Calif., added that HRSAs, such as Inconel 718 and 625, Duplex and Super Duplex, also provide antimagnetic, abrasion-resistance and high yield-strength properties and are suitable anywhere a seal surface needs to be protected against corrosion. “These parts are used in extreme environments,” he said, “and HRSAs help these products maintain their integrity.”

Machinability Matters

The structural characteristics that make HRSAs desirable for extreme applications also make them significantly more difficult to machine than run-of-the-mill metals. Nickel, for example, effectively resists high temperatures, but can also be quite gummy, reducing its machinability and increasing the prevalence of built-up edge, noted Jim Wyant, application engineer/project development for Greenleaf Corp., Saegertown, Pa. While most of the HRSA applications he sees—in the energy industry and others—are for nickel-based metals, there are many for cobalt ones, with iron alloys a distant third.

In addition, the high yield strength nickel provides causes significant heat development during machining, Hughes noted. This ultimately compounds the machining challenge.

Courtesy of Mitsubishi Materials

Mitsubishi Materials recommends its iMX exchangeable-head endmills when machining HRSAs, such as Inconel 718. Both the head and holder are made of cemented carbide.

While some are more common than others, a slew of metals are under the HRSA umbrella and contain varying percentages of up to 10 or more alloying elements, such as chromium, molybdenum, tungsten and titanium, within the same alloy group. This means machining parameters and behavior significantly vary within each group. “Every material brings its own set of obstacles to the manufacturing process,” Wyant said. “The speeds, feeds and DOCs for each material can be quite different.”

Even the same HRSA behaves differently when machined, primarily depending on whether or not it was heat or solution treated and the way in which it was produced, which includes forging, casting and as bar stock.

“Cast materials can add another machining variable that must be taken into consideration during the machining process,” Wyant said, noting many HRSAs are cast.

In addition to having a finer grain that enhances strength compared to castings, forgings have stress in them because the forging process “hammers” a material’s internal structure, said Scott Walker, president of machine tool builder Mitsui Seiki (U.S.A.) Inc., Franklin Lakes, N.J. “When you machine forged materials, they have a tendency to move around on you.”

Castings typically have a hard, visibly mottled surface, which reduces machinability and can cause notch wear on cutting tools. Bar stock is the easiest form of the three to machine, as notching is not as big a problem.

Thin is In

When notching is an issue, employing chip thinning techniques can help reduce DOC notching when roughing, Wyant said. Chip thinning is essentially using a tool’s lead angle to spread the chip over a larger section of the insert, effectively reducing tool pressures. While this helps reduce DOC notching, in many cases it allows for increased metal-removal rates because an increase in the feed is required to maintain the proper chip thickness.

Wyant added, however, that part manufacturers can increase the feed rate quite a bit when machining a material like steel, but that’s not necessarily the case with an HRSA. “With a high-temperature alloy, you can take advantage of the added benefit of increased feed rates but only to a certain point because the material will start pushing back due to its ability to withstand extreme conditions,” he said.

Courtesy of Greenleaf

Greenleaf says its WG-600 coated whisker-reinforced ceramics enable faster cutting and better resist wear and heat than carbide tools when machining HRSAs.

In addition to notching, HRSA materials tend to workharden. This requires machining them, particularly forgings, at relatively slow cutting speeds. “Speed is probably the thing we have to watch most in machining these exotic materials,” said Tool Alliance’s Forrest, noting the cutting speed for HRSAs ranges from about 75 to 150 sfm. “Speed creates heat, and heat damages tools and can workharden the material.”

He added that when a workpiece surface workhardens on a part that requires multiple passes, each subsequent pass in an already difficult-to-cut material will be even more challenging. “These multiple passes wreak havoc on your tool life.”

To lessen heat generation, prevent workhardening and effectively machine these materials when making parts for energy applications and others, Mitsubishi Materials’ Hughes recommends reducing the machining parameters four to six times compared to those for cutting alloy steels, such as 4130 and 4140. Although the reductions are not as drastic when finishing HRSAs, a 75 to 80 percent reduction in the sfm when rough turning and a 30 percent feed rate reduction when general roughing is common.

According to Forrest, peel milling is a technique that can be effective when tackling HRSAs, depending on the application. Instead of fully engaging the milling tool, such as an endmill, and taking a conventional roughing cut, the tool takes lighter cuts at higher speeds and feeds. “The radial engagement is fairly light but the feed rate is high,” he said. “Multiflute tools can be effectively used to maintain high-ipm feed rates.”

Tool Development

A high flute count helps when milling HRSAs, noted Steve Shofler, president of Superior Tool Service Inc., Wichita, Kan. In one application trimming Inconel stampings, for example, the toolmaker produced a 3⁄8 “-dia. carbide endmill with 10 flutes. The high number of flutes enables more teeth on the cutter to share the workload while taking small bites out of the workpiece.

“You almost have to file it away,” he said. “Basically, it takes a lot of patience because you have to cut it real slow.”

Shofler added that heat- and wear-resistant coatings, such as AlTiN and AlCrN, prove beneficial when cutting HRSAs.

The tool coatings might have special additives, such as silica, to allow a substrate to more effectively tolerate the demanding machining environment, Forrest pointed out. But regardless of a coating’s composition, all are moving in the direction of high wear resistance and durability, enhanced lubricity and good thermal stability.

Rather than recommending a specific coating for cutting HRSAs, Hughes emphasized the importance of a thin coating deposition, such as PVD, to maintain a sharp edge for shearing rather than rubbing the metal and minimizing heat generation. Positive cutting geometries are also effective.

“The high yield strength of these alloys creates an inordinate amount of heat during machining,” Hughes said, “and the sharper the edge, the lower the cutting forces required and, subsequently, the lower the friction and heat.”

An appropriate substrate is also critical when cutting “nastalloys” for energy applications. When applying carbide cutters, Greenleaf’s Wyant recommends one with a micrograin structure to provide the required hardness and wear resistance in such an abusive environment. Coated carbides further boost productivity by enhancing the overall substrate characteristics, allowing for increased cutting parameters and extending tool life. “Coatings supply the insert with further wear resistance and lubricity, allowing the chip to flow across the insert a little smoother,” he said.