Superalloy

Hastelloy, Stellite, Monel, and Waspaloy are not as common as Inconel 718, but when a job comes in requiring one of them, the parameters that work for standard steel will not work at all. These materials combine work-hardening, abrasiveness, and low thermal conductivity in ways that punish mistakes. Here is what I have learned about drilling each one. Common Characteristics of Exotic Alloys All four of these materials share properties that make deep hole drilling difficult: ...

I get more questions about drilling small holes in Inconel 718 than almost any other topic. A recent Practical Machinist thread about drilling 3mm holes 160mm deep in Inconel 718 at HRC 40-45 got over 3,500 views. The user was struggling with drill breakage, centerline deviation, and surface finish. The root cause in that thread was the same one I see most often: coolant pressure at 8 bar (116 psi) when the job needs 100 bar or more. Here is what I have learned about getting this application right. ...

Inconel 718 is a nickel-based superalloy used in aerospace and power generation components. It retains its strength at high temperatures but is extremely difficult to machine. The material work-hardens rapidly and retains heat at the cutting edge. I have drilled hundreds of holes in Inconel 718 across multiple jobs. Every one was a battle against heat, work-hardening, and tool wear. Here is what works for me. Cutting Parameters for Inconel 718 The cutting speed is the single most critical parameter. Above 25 m/min, the edge degrades rapidly from heat. Below 15 m/min, the tool rubs instead of cutting and work-hardens the surface. I start at 20 m/min and adjust based on tool life. ...

Waspaloy and Rene 41 are nickel-based superalloys used in turbine engines and high-temperature applications. They are among the most difficult materials to deep hole drill. These alloys retain strength at high temperatures, work-harden rapidly, and produce extreme cutting forces. I have drilled both materials for aerospace components. Rene 41 is slightly harder than Waspaloy at room temperature — about 40 HRC versus 38 HRC. Both are unforgiving. Here is what I have learned. ...

Heat management is the main challenge when drilling high-temperature alloys. These materials retain heat at the cutting edge instead of letting it dissipate through the chip. The heat at the cutting edge can exceed 1000 degrees C in the cutting zone. This heat degrades the tool edge rapidly, and once the cutting edge goes, the tool fails within seconds. I’ve learned that the key is removing heat as fast as it is generated. If you let the heat accumulate, the tool fails within seconds. Here is my approach to keeping the cutting zone cool when drilling superalloys like Inconel 718, Waspaloy, and Rene 41. ...

I use the slowest speeds and highest coolant pressures for cobalt chrome drilling — and I still expect to change tools frequently. Cobalt chrome and other superalloys are among the most difficult materials to deep hole drill. They are hard, abrasive, and retain heat at the cutting edge because their thermal conductivity is a fraction of steel’s. I have broken more gun drills in cobalt chrome than in everything else combined, and every break taught me something about what this material demands. ...