Today's Industrial Products and Solutions November 2018 - Page 6

CBN Hones for Improved Surface Finishing of Machined Superalloys Flexible cubic boron nitride (CBN) grit hones are ideal for restoring surface integrity of machined superalloys By its very nature, high-nickel, iron or cobalt alloy steels including Monel, Inconel, Incoloy, Invar, Rene and Hasteloy are some of the most difficult materials to ma- chine. Because these superalloys are designed to deliver high strength at extremely high temperatures in components often found in “hot” sections of engines and turbines, the stresses and heat generated during machining are considerable. In this process, the edge of the cutting tool is exposed to considerable mechanical stress, strain and heat. High compressive and shearing forces attack this edge, increas- ing temperatures such that the cutting tools can begin to lose hardness. This can produce a “gummy” machining behavior and also cause work hardening. According to Janos Garaczi, president of Delta Machine Company, LLC., a company that specializes in machining complex, tight tolerance parts out of titanium, nickel alloys, stainless steel, aluminum, plastics and other exotic alloys, this is caused when chips do not break away cleanly from the workpiece. Instead, the material is just ‘pushed’ to one side. “It can be really difficult to turn or mill this type of ma- terial because it doesn’t produce a clear shearing action. Instead it just kind of pushes the material to the side,” explains Garaczi. “Gummy” machining is also the cause of heavy burrs in drilled holes or grooves. 4 “The burrs are one of the biggest problems,” adds Garaczi. “Even during ‘grooving,’ if you cut the material from one side, it just pushes the burr to the edge; and if you approach it from the other side, it just pushes it back. It doesn’t want to break off the material cleanly. As soon as TIPS Magazine • November ‘18 New BRM flexible hones are simplifying the task by incorporating one of the toughest, hardest materials on the planet- Cubic Boron Nitride (CBN). the tool gets a bit dull it gets a lot worse so sharp tools with the right geometry are key.” The general poor machinability ultimately compromises surface integrity of the parts, says Garaczi, which is prob- lematic given that is one of the primary causes of failure of these types of components. Given the critical nature of the applications in which these components are utilized, most machined superalloy parts must therefore undergo a surface finishing oper- ation to remove work-hardened layers and any heavy burrs. Doing so has additional benefits, including reduc- ing the risk of oxidation and stress corrosion cracking (SCC). Although this has traditionally been achieved through an expensive, messy and time-consuming grinding process,