May 2010 / Volume 62 / Issue 5|
By Alan Richter
Courtesy of Sandvik Coromant Insert geometries for effectively turning high-temperature superalloys.
No fun! That’s the general sentiment when turning nickel-base, high-temperature superalloys, particularly Inconel but also Hastelloy, Waspaloy, Rene, Haynes, Incoloy and a host of others. That’s because the high nickel content—half or more—does not conduct heat well, so it builds up at the cutting edge and doesn’t transfer into the chips.
“The rule of thumb is you want 80 percent of the heat to leave with the chip, 10 percent stays with the part and 10 percent stays with the insert,” said Patrick Loughney, turning product specialist for Sandvik Coromant Co., Fair Lawn, N.J. “You don’t want to get the material so hot that you’re workhardening it.”
Rather than the nickel, it’s the chromium in high-temperature alloys that has a tendency to workharden, as well as become abrasive, according to Mike Gadzinski, national training manager for Iscar Metals Inc., Arlington, Texas.
To effectively turn these materials, applying inserts with the proper geometric features is critical. “The geometry, in our opinion, makes much more of a difference than the coating or the substrate,” Gadzinski said.Keep it Positive
He emphasized that effective geometry begins with a highly positive top rake and rake face, as well as a positive, sharp cutting edge, to reduce tool pressure and minimize the amount of heat generated in the cutting zone. That allows a slightly higher cutting speed. “The flipside of the coin is that, unfortunately, the sharper and more positive your cutting edge is, the less tool life you’re going to get, but it’s the lesser of two evils at that point,” Gadzinski said.
Loughney noted that an insert’s entry angle has a direct impact on tool life, as well as chip thickness. A CNMG insert has a large entry angle of 95°, which makes it prone to notch wear and the DOC. That leads to premature insert failure. By switching insert shape to SNMG and selecting a toolholder with a 45° entry angle, there will be less heat at the cutting edge because a larger portion of the cutting edge is applied. That creates a chip thinning effect, resulting in longer tool life and less heat transferred into the workpiece.
In addition, Loughney said, “Everybody runs into issues when they want to machine into a square shoulder.” That’s the domain of the semifinishing and finishing inserts, whereas roughing requires a strong edge and a strong insert shape. “When it comes to roughing the material, it’s always better to use a round insert,” he added, noting that ceramic ones are “your best bet” when roughing Inconel.
According to Gadzinski, a 0.032 " corner radius on a typical diamond or square insert will be suitable for 75 percent of the applications. “It’s only when you get into very small-diameter parts that you need a smaller corner diameter,” he said. “Our general recommendation is to go with as large of a radius as you can tolerate and still stay within the part specifications.”
Gadzinski added the DOC taken should be greater than the insert’s corner radius to prevent the chip from curling too fast and pushing the insert away from the part. As suppliers forge near-net-shape workpieces closer to a part’s final dimensions, a smaller radius than would typically be dictated by the part print might be required.
In addition to radius hones, Thomas E. Frakes, president of Mastertech Diamond Products Co., Mentor, Ohio, noted that appropriate edge preparations on the company’s polycrystalline cubic boron nitride and PCD turning inserts include various K-lands and chamfers. A hone style that can be effective when turning high-temperature alloys is the waterfall hone, he added. “Instead of a conventional full radius hone around the edge, it creates a shape on the edge that looks like water falling over a rock,” Frakes said. “It creates a small chamfer with a radius hone on the cutting edge, which will increase as it goes along to strengthen the edge.”
To help prevent chipping and reduce the tool pressure when turning nickel-base alloys with the superhard materials, Frakes recommends a neutral to -5° rake angle. He added that coatings, such as titanium nitride and titanium carbide, are often applied to PCD and PCBN inserts as edge wear indicators so operators know which edge has been used when indexing. The coatings also provide lubricity and extend tool life.Forming Chips
Because heat-resistant alloys tend to produce long, stringy chips when turned, chipbreakers are generally employed to manage chip size. Similar to a tool’s rake angle, a positive chipbreaker is needed to shear the material as it forms the chip, according to Loughney.
Courtesy of Mastertech Diamond Products
“Anything that has a lot of nickel will normally give you problems with chip control,” Gadzinski said. “You’re never going to be really happy with it.”
He explained that chip formers are designed to break a chip at either a high DOC or a low feed rate. It depends on the proximity of the chipbreaker facet to the cutting edge as to how the chip coming off the cutting edge hits the facet, causing the chip to curl back over itself. Typically, a low feed requires a shorter distance from the cutting edge to the chip former’s facet.
“The selection process is based on whether you’re fine finishing or roughing,” he said. “If you find the right combination of DOC and feed rate, almost all chipbreakers are going to work fairly well as long as they are sharp enough.”
Although Frakes noted that most of the chipbreakers found on Mastertech inserts are in PCD for machining nonferrous materials, mostly aluminum, they are also put into PCBN inserts. “That’s normally because our customers will run from less hard materials and then turn into hardened journals and welded areas,” he said, “and they want to break the chip up in the softer sections.”
When turning nickel-base superalloys, the material will start to string but break into manageable pieces because it becomes so hard, according to Frakes. “It workhardens as you turn it and then the chips break up.”Cool Turning
High-pressure coolant can also aid chip control. According to Loughney, Sandvik Coromant’s tool bodies with built-in coolant nozzles direct high-pressure coolant to the tool/workpiece interface without any adjustment. “A lot of operators, when they have an adjustable nozzle, the first thing they do is adjust it,” he joked.
Loughney said the toolmaker witnessed “dramatic differences in chip control at 1,000 psi” when it conducted tests applying a sharp-geometry CNGP insert under normal and high coolant pressures. “We’ve also seen up to a 30 percent better tool life,” he said.
He added that the high-pressure coolant will help during interrupted turning by keeping the insert cooler so it won’t experience as much thermal shock as moves in and out of the cut.
In an application turning Inconel 718, Mogas Severe Service Ball Valves, Houston, effectively applies flood coolant to cool the insert, part and chips (see sidebar on page 60). “Typically, few cutting tool manufacturers would recommend using coolant in an operation like this with ceramic tools because of thermal fracturing of the ceramic when getting the tool hot and cold, hot and cold,” said Craig Kippola, sales engineer for Ingersoll Cutting Tools, Rockford, Ill. “But they do a very good job of this.”
And that’s essential because of the high amount of heat in the cut and the chips. “They’re on fire,” Kippola said. “Even with the coolant on it you can see the sparks flying.”
Similar to the negative cutting geometries sometimes found on superhard PCD and PCBN for superalloy applications, Kippola noted that “extremely hard” ceramic inserts have a negative land to provide edge strength.
Based on field reports, Iscar’s Gadzinski concurred that coolant pressure, as well as coolant selection, are major factors when turning heat-resistant alloys. However, high-pressure’s coolant role is not so much in breaking the chip but in enhancing lubricity and—more importantly—dissipating the heat. “It’s not that they’re so hard to cut, but what do you do with all the heat?” he asked. “If you can’t get the heat out with the chip, the only thing you can do is get rid of as much as possible with coolant.”
However, with superhard tools, coolant application isn’t an issue because superalloys are predominantly machined dry, according to Mastertech’s Frakes. Instead, high-pressure chilled air controls and clears chips. When the operations are run wet, he noted that the pressures can be as high as 3,000 psi. “The coolant pressure is high enough that it breaks chips up during machining.”
Nonetheless, Frakes emphasized that machine tool rigidity is the No. 1 issue when turning challenging materials. “The machine tool has to be capable of keeping the toolholder and cutting tool rigid in the cut so there isn’t vibration, which dramatically decreases tool life,” he said.
Although boosting productivity trumps extending tool life, controlling insert life is important. An end user turning superalloys should change or index an insert at the end of a radius or end of a part rather than in the middle of a cut to avoid creating stress points in the part, Loughney explained. “A stress point would be a possible failure point,” he said. “Especially in aerospace, you don’t want that to happen.” CTE
About the Author: Alan Richter is editor of Cutting Tool Engineering. He joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
Courtesy of Ingersoll Cutting Tools
Pushing the envelope when machining Inconel
Courtesy of Ingersoll Cutting Tools
Courtesy of Tungaloy America
Superalloy chip control
turning superalloys, such as Inconel 618 and 718, Tungaloy America Inc., Wood Dale, Ill., recently introduced the AH905-grade insert with HMM chipbreaker. The ribbed chipbreaker diminishes heat in the gullet to keep it from penetrating the insert, according to Brian Sawicki, the toolmaker’s senior manager of engineering and products. Tungaloy reports that the chipbreaker sits flush against the shim seat to improve stability and extend tool life, has a sharp T-land to reduce cutting force and the rake surface geometry minimizes heat transferred from chips. “The chipbreaker produces less surface contact with the chip,” Sawicki added.
He noted that the insert grade, which is the company’s hardest for turning high-temperature alloys, is designed for continuous cuts with a DOC from 0.020 " to 0.125 " per workpiece side and a 0.004- to 0.012-ipr feed rate.
Ingersoll Cutting Tools
Iscar Metals Inc.
Mastertech Diamond Products Co.
Mogas Severe Service Ball Valves
Sandvik Coromant Co.
Tungaloy America Inc.
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