New strategies and solutions take on ISO S materials
The ISO S classification of workpiece materials includes heat resistant superalloys (HRSA) and titanium alloys. The hot hardness and strength of these materials prompt their use in a wide range of critical aerospace, energy and other applications. The alloys’ beneficial properties, however, also produce machining characteristics different than those of traditional irons and steels. In response, makers of cutting tools have developed products and application strategies that address material machinability and enable reliable, consistent and relatively economical processing of ISO S group alloys. These toolmakers also now seek to educate manufacturers on the new tools and strategies as well as convince machinists to rethink any outdated machining techniques that, most likely, will not apply to today’s advanced materials.The ISO S classification of workpiece materials includes heat resistant superalloys (HRSA) and titanium alloys. The hot hardness and strength of these materials prompt their use in a wide range of critical aerospace, energy and other applications. The alloys’ beneficial properties, however, also produce machining characteristics different than those of traditional irons and steels. In response, makers of cutting tools have developed products and application strategies that address material machinability and enable reliable, consistent and relatively economical processing of ISO S group alloys. These toolmakers also now seek to educate manufacturers on the new tools and strategies as well as convince machinists to rethink any outdated machining techniques that, most likely, will not apply to today’s advanced materials.
The term machinability describes a metal’s responses to the machining process. Machinability includes four basic factors: the mechanical forces produced in machining, chip formation and evacuation, heat generation and transfer, and cutting tool wear and failure. Excessive effects of any or all of these factors can cause a material to be deemed “difficult to machine.”
Machinability issues arise with regard to tool life, process time and reliability and part quality when HRSA and titanium alloy machining is attempted with the same tools and techniques used over many decades on, for instance, steels and irons. Only in the last few years have tools been developed with nickel-based and titanium-based alloys in mind. Machining these relatively new materials is not necessarily more difficult than machining traditional metals; it simply is different.
For example, the usual approach to machining a “difficult” material is to proceed cautiously and use less-aggressive cutting parameters – including reduced feed rates, depths-of-cut and speeds. However, with cutting tools developed specifically for these high-performance workpiece materials, a basic rule is to, instead, increase depths-of-cut and feed rates. Tools engineered to handle these more aggressive parameters include fine-grained carbide grades that provide good high-temperature edge strength and coating adhesion, with particular attention to resistance to notching caused by work-hardened workpieces. Ceramic and PCBN tools have also been developed for roughing and finishing of these high-performance alloys (see "Continuing tool development" below).
Regarding specific machinability factors, HRSA present mechanical or force-related issues that are not vastly different to tough irons or steels. There is a major difference, however, in the generation and dissipation of heat. Heat is generated when metal cutting deforms the workpiece material, and chips generated in cutting processes can carry heat away. However, the segmented chips produced by these materials often fail to do the job well. In addition, the heat-resistant materials themselves are poor conductors of heat. Temperatures in cutting zones can be 1100˚ - 1300˚ C., and when heat cannot be dissipated, it builds up in the tool and the workpiece. The result is reduced tool life and even deformation of the workpiece and changes in its metallurgical characteristics.
To help solve this problem, a change in perception about cutting tool strength is necessary. Sharp-edged cutting tools are generally considered to be weak, but one way to control build up of tool temperatures is to use sharp cutting tools that cut the material more than deforming it, thereby generating less heat. Executing this strategy requires tools engineered for edge strength, applied on machine tools with sufficient power, stability, and vibration resistance.
Tendencies toward strain and precipitation hardening also complicate the machining of HRSA. In strain hardening, material in the cutting zone becomes harder when subjected to the stress and high temperatures of the cutting process. Nickel- and titanium-based alloys exhibit greater strain hardening tendencies than steel. In precipitation hardening, hard spots form in a workpiece material when high temperatures activate an alloying element that was otherwise at rest. With either tendency, the structure of the material may change significantly after only one pass of a cutting tool, and a second pass will have to cut through a much harder surface. A solution is to minimize the number of passes. Instead of removing 10 mm of material with two 5-mm-deep cutting passes, for example, it would be better to use one pass at 10 mm depth-of-cut. In many situations single-pass machining is not possible, but it is the theoretical goal.
This approach also requires rethinking the finishing process, which traditionally involves multiple passes at small depths-of-cut and light feed rates. Instead, machinists should look for possibilities to increase the parameters as much as possible. Doing so can improve tool life as well as surface finish.
A slightly deeper depth-of-cut for a finishing pass also positions the sharpest part of the cutting edge below any strain- or precipitation-hardened areas of the part. However, too deep a finishing pass may generate vibration and negatively affect surface finish. Finding the optimum balance between aggressiveness and caution is the key.
With today’s tools and strategies developed specifically for nickel- and titanium-based alloys, machining can be accomplished essentially without technological problems. The ongoing challenge is not simply machining the workpiece, it is machining the workpiece correctly in a given time at a given cost. The goal is to improve process reliability and production economics.
Considering the high cost of advanced workpiece materials and the components made from them, machining processes must be totally reliable. Manufacturers cannot afford to produce scrap parts while seeking a reliable machining process. Using appropriate tools and machining parameters help ensure consistent machining results.
Regarding machining parameters, increasing depths-of-cut and feed rates contributes to productivity. Higher cutting speeds also can expedite part processing, but that opportunity has yet to be fully exploited. The speeds employed today in nickel- and titanium-based alloys are still lower than those used with steels. But current research is focused on developing cutting tool properties that will allow even higher cutting speeds while still maintaining reasonable tool life.
In addition to cutting tools, other components of the metal cutting process such as use of a high pressure direct coolant (HPDC) system can also help increase productivity. If cutting speeds for an ISO S material is 50 m/min., HPDC can permit cutting speeds as high as 200 m/min and thereby quadruple output.
Tool life is another element of productivity that can be viewed from a new perspective when machining HRSA. The traditional measure of tool life counts minutes of cutting before required replacement. Another measure is cost.
If, for example, producing a certain workpiece takes 2 hours and tools must be changed every 20 minutes, then 6 tools must be purchased to complete the part. Along those lines of thinking, the goal would be to reduce tool cost and get 30 minutes of tool life instead of 20.
Tool cost, however, is a very small portion of the overall value of the parts when processing costly components made from HRSAs or titanium alloys. A more relevant measure is tool utilisation, also called a tool’s utilisation index. When comparing two sample tools, if one lasts 10 minutes and produces one workpiece, the tool cost is one tool per workpiece. Another tool, applied in a different way, might last only 5 minutes, but produce two parts. Even though the second tool’s life in minutes is half that of the first tool, the output of parts is doubled. The goal is to create the maximum number of correct workpieces in the shortest time at an acceptable price. Considering the high cost of parts made of HRSAs, the tool utilisation index is a better gauge of true productivity.
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As is always the case, the key factor in maximising the benefits of newly developed metal cutting technology is the knowledge of the best way to apply it in a particular operation. As progress in high-performance workpiece materials such as HRSA and titanium-based alloys continues, toolmakers will also continue to engineer new ways to maximize productivity in machining processes for the new alloys. Manufacturers will benefit from awareness of the availability of the new tools as well as the toolmakers’ comprehensive knowledge of the best ways to apply them.
Stainless steel, patented in a variety of forms about 100 years ago, was the initial step toward modern 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.
