Machining superalloys more productively

STAINLESS steel, patented in a variety of forms about 100 years ago, was the initial step toward modern heat-resistant superalloys (HRSA). In the first stainless steel alloys, chromium was added to steel to resist oxidation and corrosion – basic stainless steel alloys have minimum chromium content of 10.5 percent by weight. Subsequently, nickel also was added to improve stainless steels’ hardness and toughness. The percentage of nickel grew as the alloys were applied in increasingly harsh environments, and nickel eventually became the materials’ main alloying element. Today’s familiar HRSA Alloy 718 – known commercially as Inconel 718 – has nickel content of 50 to 55 percent, chromium 17 to 21 percent and other elements 10 percent, with the remainder being iron. Modern HRSA and titanium-based alloys provide excellent strength, heat and corrosion resistance and reliability.

When confronted with these new challenging workpiece materials, manufacturers first try to apply familiar machining practices. However, they only truly gain maximum productivity when they incorporate tools and techniques engineered for use with these specific materials and operations.

For example, in the mid-1980s Seco established what it called its Alpha Group of scientists and engineers to find ways to machine stainless steels more productively. The group worked with a number of stainless steel makers to develop new carbide grades and geometries and also specific cutting methods for stainless steel. In the 1990s the effort was expanded to include higher-performing HRSA materials. 

In addition to carbide grades, coatings and geometries, tools have been developed to optimize HRSA machining productivity in specific segments of the metal cutting process. Aimed at rough machining operations, Seco’s CS100 sialon ceramic grade, for instance, features high chemical inertness, abrasion resistance and toughness, allowing it to achieve long and consistent tool life. Typical rough turning application parameters include cutting speeds of 150 m/min. to 305 m/min., feed rates of 0.2 to 0.4 mm/rev, and depths-of-cut of 0.5 mm to 3.75 mm.
The CS100 grade is complemented by Secomax CBN170, a tough and wear-resistant PCBN grade designed for continuous finish turning in nickel based superalloys.

The CBN170 grade incorporates a whisker ceramic binder that enhances tool life and thereby reduces the number of machine stoppages required to change cutting edges. It is intended to satisfy exacting surface finish, tolerance and length of cut requirements in finishing operations on nickel-based superalloys. CBN170 tools are engineered to operate in continuous cutting situations, employing coolant, at depths-of-cut up to 0.5 mm and cutting speeds of 300 m/min. to 400 m/min. The grade’s CBN content is 65 percent by volume, with a 2-μm grain size. Inserts are provided with a 25-μm edge hones. 

Other developments intended to increase tool life and productivity in HRSA machining include technology such as Seco’s Jetstream Tooling high-pressure direct coolant (HPDC) system, which delivers coolant close to the cutting edge. The jet of coolant lifts the chip away from the rake face, improving chip control and tool life and enabling application of more aggressive machining parameters. In some cases the rapid cooling of the chip makes it brittle and more likely to fracture.