Machining Cast Iron
Increasing use of ductile iron, CGI, and austempered ductile iron will require changes in cutting tool technology
By Don Graham
Product Manager, Turning
Gray cast iron has been a staple in automotive manufacturing for many years. It's used in a variety of components including engine blocks, cylinder heads, differential housings, shafts, flywheels, brake drums, and disks. The sheer volume of gray iron used in this segment of industry is staggering. There is nothing so predictable as change, however, and trends within the automotive market segment indicate that the use of gray iron is declining for a number of reasons, including rising fuel costs, emission regulations, and the availability of new materials.
As the desire for lightweight components grows in order to achieve better fuel economy, there is an associated increase in the use of ductile iron and austempered ductile iron. Additionally, just on the horizon there is an anticipated growth in the use of Compacted Graphite Iron (CGI), especially in the trucking industry, which relies upon diesel engines.
The change in material use is both government and environment-driven. To reduce particulate emissions, manufacturers have to employ higher combustion temperature and pressure. Today's gray-iron engines are not strong enough to withstand the higher temperature and pressure required to meet government requirements. Also, vehicle fuel efficiency will be aided by reducing the weight of the individual components.
The EPA regulation compliance requirements for cleaner diesels begin in 2007; consequently the trucking industry is actively engaged in the development of CGI. In 2004, however, only 0.4% of all new cars sold in the US had diesel engines. Memories of the loud engines, black smoke, and smelly fuel of diesel engines first introduced 20 years ago have kept the US car market demand very low, as compared to the diesel-equipped car sales in Europe—now 60% of the total. But, if fuel prices continue to rise, we may see consumers make the leap of faith to diesel.
In addition to these changes, there has also been a paradigm shift in machining processes. Over the past 10 years, manufacturing process speeds have steadily increased. Until recently, there were few application-specific cutting tools, and most machine tools came with few spindle options. As cutting tools have been increasingly designed to meet specific machining processes, high-performance machine tools are now offered with an increasingly large array of spindle options. High-speed machining is very well suited for price-driven, high-volume component production as seen in the automotive industry.
Compacted graphite iron has the benefit of combining the heat dissipation of gray iron with the strength and modulus of elasticity of ductile iron. However, it has a graphite structure that resembles coral and a chemical composition with the reputation of being somewhat hard to control. As manufacturers, particularly in the trucking industry, look at CGI as the material of choice for diesel engines, they face quality challenges and the possibility of unpredictable tool life and downtime, all of which can kill projects and profitability. But if the proper tools are used to machine these materials, the stories about CGI's temperamental machinability may turn out to be worse than the facts.
Although PCBN has proven to be an excellent cutting tool material that can last forever (almost) in gray iron, it is, unfortunately, subject to chemical dissolution when used to machine ductile irons and CGI. In these cases, the cutting tool material interacts with the higher ferrite content in the iron, thus creating very quick wear of the cutting tool. This problem drove the need to develop a cutting tool grade that could deal with the chemical reaction.
These new irons require tools that provide an increase in abrasion resistance yet are still very tough—two properties that are hard to come by in one package. To meet this challenge, a number of cutting tool manufacturers have focused on developing tools that offer an optimum mix of these properties.
Most of these inserts are coated grades that combine very hard CVD coatings, a hard, deformation-resistant substrate, and a substrate-coating interface that is cobalt-enriched for good edge toughness. These tough, CVD multilayer grades are designed for abrasion resistance and cutting speed, enabling them to be used to machine abrasive castings that can come embedded with sand, surface inclusions, oxides, and other materials that cause quick wear of cutting tools.
Field test results of such CVD-coated grades have been significant. In an internal, interrupted roughing application on a ductile-iron support component, tool life tripled as compared to a tough ISO-P20 grade. In a test using our company's TK2000 cast iron grade, when compared to our older TP100 grade in a machine clutch application, the new TK grade produced 67% more pieces per tool. When you factor in both tool cost and tool-change time, this improvement could mean a savings of almost $20,000 per year.
Gray iron is still going strong, despite the increase in ductile iron and CGI. There will continue to be plenty of gray-iron applications left in the automotive industry, especially in the area of brake drums and disks. But, the situation is a bit different today as the machines are being run at far higher speed rates than in the past—about 1000 fpm (305 m/min) faster than 10 years ago. The higher temperatures that result from high-speed machining can dissolve the coated carbide of the cutting insert in a manner very similar to the chemical reaction that occurs between PCBN and ferrite. To prevent this destruction of the coated carbide, and to keep heat out of the substrate, manufacturers are using ever-increasing thicknesses of aluminum-oxide layers in the coating. Additionally, and importantly, these layers have an improved structure that imparts increased toughness and wear resistance.
New heat-resistant grades designed specifically for gray irons machined at higher speeds—speeds up to 1800 fpm (549 m/min) are entering the market. These high-speed coatings contain a thick layer of aluminum oxide and a very hard form of titanium carbonitride, which allows consistently high wear resistance to maintain productivity. In a field test using a new coated heat-resistant grade to rough-machine a gray iron brake disk component at speeds ranging from 2160 to 1315 fpm (658-400 m/min), tool life doubled and productivity improved by 20%, as compared to a silicon-nitride ceramic insert. This productivity increase is due to higher edge toughness, which allows an increased feed rate as compared to ceramic.
Together, all of these factors have increased the need for better cutting tools that can handle stronger, more abrasive materials, and higher cutting speeds, while providing consistent tool life and ensuring part quality—surely a lot to ask of one insert.
The use of ductile iron makes it possible for manufacturers to produce lighter and smaller components without sacrificing component strength; and CGI offers up to two times better mechanical and fatigue strength than gray iron. Unfortunately, these benefits don't come without some challenges.
These irons contain more silicon than gray iron, which increases their abrasiveness, and causes cutting tools to wear out much more quickly. Additionally, the structure of these irons is different than gray iron, which has graphite particles in the form of flakes, making it relatively easy to cut. Ductile iron contains graphite particles in the form of spheres, which are surrounded by ferrite. In some instances, machining ductile iron can require three times as many inserts as are needed to machine gray iron.
Another consideration when machining gray iron is that shops like to use coolant to control the dust and dirt caused by graphite in the iron, as well as to keep the cutting temperatures from getting too high. High heat input can cause a part to expand, resulting in the removal of more material than was intended. Unfortunately, coolant can decrease tool life when temperature fluctuations cause thermal cracking in the tool. Temperature-resistant coated grades that are suited for dry machining situations can handle increased speed and feed rates, allowing the part to be cut before there is too much heat generated in the process, and providing an alternative to ceramic inserts. In a dry machining field test on a brake disk where ceramics were being used on gray iron, the use of our new TK1000 grade was able to improve tool life by 60% and increase productivity by 20%.
A trend throughout the industry—from the OEMs right down to the Tier 3s—is the increase in lights-out or untended machining that is common to high-volume production. Customers want to be able to load up a machine, and then let it run all night by itself with only a robot tending to it. Hence, the tools have to be reliable enough to produce consistent parts time and time again, with little-to-no variability.
One of the biggest complaints from customers in the past has been the batch-to-batch variation in machinability. It was not unusual for them to get 100 parts/insert corner in batch one, then only 50 to 60 parts/corner in batch two. Some of this relates to the changes in composition of the iron, and some of it relates to the weather—factors that are out of the machinist's control. However, one thing many tool manufacturers have tried to do is to develop tools that can handle these variations in machinability. The goal is to offer tools that will give the customer 100 parts today, 100 parts tomorrow, and 100 parts the next day—even if machinability changes. Increased toughness and strength, combined with abrasion-resistance, will allow these new grades to adapt to the variations in material, as well as being able to handle interrupted cuts of near-net-shape component machining. The designed-in reliability eliminates the need to monitor and adjust the process for material variability.
The sheer volume of gray iron needed for the automotive industry continues to keep this a crucial market segment, despite the fact that much of this work is going overseas. To battle this trend, cutting tool manufacturers will continue to develop application-specific tools that will help to innovate processes. These new tools will meet the demands for lighter and stronger materials such as the ductile irons, CGI, and even aluminum and magnesium, which are markets that are bound to grow.
This article was first published in the February 2006 edition of Manufacturing Engineering magazine.