Specialty Irons CGI, ADI and SiMo
ADI’s Growth Potential
For applications that call for higher strength, ADI has become one of the more popular choices. The material has been used in a wide range of industries and is capable of entering still more markets in the future. Through 2008, U.S. ADI casting shipments are predicted to grow 18.1% annually, to take up an 8% share of total ductile iron shipments, according to Stratecasts Inc., Ft. Myers, Fla.
“The advantage of ADI is that you’re offering a high strength material,” said Kathy Hayrynen, technical director at Applied Process, Livonia, Mich. “You have the ability to thin down sections, which allows you to make lighter components out of a perceived heavy metal.”
The ability to create lighter castings, combined with ADI’s superior strength to weight ratio, has enabled the metal to be competitive for many jobs typically thought best for aluminum. With a 10% weight savings versus steel, ADI also is a prime candidate for conversions from steel fabrications.
“ADI can save the customer a lot of money and increase the casting facility’s profit margin,” said Joe Farrar, president of Farrar Corp. “It can turn components into castings that otherwise would not be possible, so it creates more casting jobs and more ductile iron jobs.”
During the austempering of ductile iron, the casting first is austenitized (typically in a range of 1,600-1,660F [871-904C]). The section size and chemical composition of the casting will determine the time and temperature for austenitizing. The goal is to achieve an austenitic matrix with a uniform carbide content. Then, the casting is quenched to the austempering temperature. (Typical range is 460-740F [238-393C].) The choice of temperature will determine the final properties of the casting. If higher strength or greater wear resistance is desired, a lower quench temperature is used. The lower the temperature, the longer it will take for the ausferrite to form.
One of the most important things in producing ADI castings is the use of high quality ductile iron. Austempering will help good ductile iron achieve improved mechanical properties, but it can’t cure bad ductile iron. Rather, the poor qualities of bad ductile iron likely will be magnified by the heat treating process.
Also, ADI is more difficult to machine because of its hardness. However, accommodations by the metalcasting facility can be made to alleviate the added machining labor. ADI expands during austempering, and this expansion is so consistent that it can be measured and predicted. Many facilities, like Farrar, take this expansion into account by machining the components as-cast with the knowledge that the casting will grow into its tolerances through the heat treating process.
CGI: Filling the Gap
CGI was developed in the late 1940s at about the same time as ductile iron, but while the use of ductile iron was propelled into high volume production over the next 25 years, CGI was left in the back of the closet behind the winter coats. Why? CGI initially was too difficult to reliably produce. Both ductile and compacted graphite iron have small windows of chemical balance to reach their proper mechanical properties, but CGI’s window is five times narrower than ductile iron, and the technology to successfully measure CGI was not available until the late 1980s.
Now that the technology for process control is there and more casting facilities have proven that high volume production of CGI castings is possible, some customers are specifying the metal for new production ramp-ups.
Although the pure mechanical strength of CGI is not as high as that of ductile iron, the superior castability, thermal conductivity and machinability makes CGI ideally suited for complex components with simultaneous thermal and mechanical loading, such as automotive cylinder blocks and heads. The higher strength of CGI relative to conventional gray iron has resulted in some car and truck engines specified in CGI.
“As engine technology advances, conventional gray cast iron is not strong enough to meet new requirements,” said Steve Dawson, president and CEO of SinterCast, London. “The higher strength and stiffness of CGI allows engine designers to meet performance and emissions objectives without increasing the size or weight of the engine package.”
Although CGI is used to produce bedplates, exhaust manifolds, flywheels, gear covers, brake drums and hydraulic valves, nearly two-thirds of long-term CGI production is cylinder blocks, mostly for diesel engines. For this reason, the vast majority of CGI casting firms are located in Europe, where 50% of all cars on the road have diesel engines. Currently, only 3.2% of the new vehicles in the U.S. use diesel engines, but J.D. Power and Associates predict that percentage to increase to 10% by 2012. It’s still a far cry from the market share in Europe, but in the larger U.S. automotive market, even a 2.5% increase could mean 50,000 tons of CGI cylinder block shipments. Plus, trends toward larger vehicles, longer driving distances and rising fuel prices are making diesel engines a more attractive choice for the American consumer.
“We have now proven that CGI can be done in high volumes, and series production is growing,” Dawson said. “In 1999, there was no CGI cylinder block production. Today, 30,000 are shipped a month. That growth will accelerate as new programs come on stream in both the passenger vehicle and commercial vehicle sectors.”
High SiMo Stands the Heat
Automotive engine manufacturers recently have been taking a closer look at high SiMo’s thermal properties as engines heat up.
Years ago, many alloys could not take the heat of engines, so automobile manufacturers would use extra fuel to keep things cool. Of course, this meant adding more emissions into the air. Tougher air emissions standards forced automobile designers and engineers to find a way to create a cleaner engine, but current alloys already were being pushed to the limit for fuel efficiency and more horsepower.
“Exhaust gases produced by modern engines are becoming hotter as they are designed to be as fuel efficient as possible while meeting stricter emission regulations,” said Tony Thoma, of Wescast. “These higher temperatures demand more capable alloys.”
The addition of up to 1% molybdenum and 5% silicon to ductile iron greatly increases high temperature tensile strength, stress-rupture strength and creep strength. High SiMo castings are cost effective when used in applications with temperatures between 1,200-1,600F (649-871C), which makes the material a popular choice for exhaust manifolds and turbocharger housings.
Care must be taken by the metalcasting facilty to ensure quality high SiMo ductile iron is produced.In particular, carbon levels should be kept in a tight range for a particular casting section thickness. Up to 5% silicon can be used, but increasing the silicon content provides improved oxidation resistance and increased strength at the expense of toughness and machinability.
The addition of up to 1% molybdenum is normal. More molybdenum enhances high temperature strength and improves machinability, but reduces toughness and may segregate to form grain boundary carbides. METAL