Since its development in the late 1940s, ductile iron has become one of the most frequently used metals across industrial applications. This is due in large part to the strength and durability it provides, as well as the ease with which it can be cast and machined compared to other forms of cast iron. It is for these reasons that ductile iron, also known as nodular iron, is used for producing heavy-duty piping used in water and sewer lines as well as automotive components and other purposes.
What gives ductile iron its many advantageous qualities are its composition and metallurgy. In addition to raw iron, ductile iron also contains a number of other materials that give it unique properties compared to other forms of cast iron. When it comes to properly applying ductile iron, the first step is to understand what it is made of and how it is produced. This provides manufacturers with information they need to determine whether ductile iron is the right material for their specific needs. Read on to learn about how ductile iron is made as well as the qualities that make it so versatile and unique.
The Composition of Ductile Iron
The main component of ductile iron is ordinary pig iron. This is generally defined as iron produced by a blast furnace that has a purity of at least 90 percent. The low concentration of residual elements means it has a relatively consistent chemistry that encourages reliable results when used in industrial processes.
What makes ductile iron different from gray iron and other forms of traditional cast iron is the presence of spheroidal nodules of graphite. These are microscopic, round particles of graphite that are evenly distributed throughout the iron. By comparison, traditional gray iron contains tiny flakes of graphite. The difference between round and flat graphite particles is what provides ductile iron with many of its most important qualities.
Because the graphite is spheroidal, these particles are able to slide over each other with greater ease than flakes, which tend to become locked together. This means a greater degree of ductility and flexibility when forces are exerted on the iron. Because the graphite in gray cast iron is in the form of flakes, forces are more likely to cause cracks along the edges of the flakes.
In addition to spheroidal nodules of graphite, ductile iron also contains smaller percentages of other elements. The most prominent of these are carbon and silicon. Typically, the carbon content of ductile iron is between 3 and 5 percent, with silicon making up less than 3 percent. Trace amounts of other elements such as manganese, phosphorous, sulfur, and copper also are present. However, the exact ratios of these trace elements depend on the grade of iron being produced.
How is Ductile Iron Produced?
Creating ductile iron is a relatively simple process with a few critically important steps:
- Melting Down Raw Iron: The pig iron is melted down in a furnace between 1,400 and 1,500 degrees Celsius (roughly 2,250 to 2,730 degrees Fahrenheit). In some cases, the pig iron also may be mixed with iron scrap or recycled steel scrap.
- Adding Magnesium: This is perhaps the most important step of the process, as magnesium is used as the nodulizing element. This means magnesium is what changes the natural flake structure of graphite into the spheroidal nodular shape that gives ductile iron its many unique characteristics.
- Removing Impurities: Before casting with the molten ductile iron, there may be impurities in the form of oxides and sulphides that must be removed from the surface. Usually these are simply skimmed off the top.
- Casting: After the impurities are removed, the molten ductile iron can be poured into molds for casting.
- Annealing: Many products made using ductile iron, such as piping, are subjected to an annealing process to further enhance qualities such as durability and flexibility. This involves a regimen of controlled heating and cooling cycles that serve to help stabilize the molecular structure of the metal.
Why Ductile Iron is So Common
The unique properties of ductile iron make it one of the most commonly used materials across a wide range of industries. Some of the most important qualities that make ductile iron production so prevalent include:
- Strength: Ductile iron is very good at bearing heavy loads, making it exceptionally well suited for heavy-duty use. It offers tensile strength that gives it high resistance to breaking or splintering when subjected to extreme tension.
- Durability: Resisting wear is another key advantage ductile iron offers over other forms of iron. It can experience a high degree of force without permanent deformation and resists fatigue, which makes it capable of delivering much longer lifespans compared to other metals.
- Ductility: The presence of spheroidal nodular graphite means ductile iron is more flexible than other types of cast iron. This means it is good at absorbing impacts without fracturing and can be cast into a wider range of shapes that give engineers more options when designing castings.
- Machinability: With ductile iron, castings can be machined to meet final specifications and tolerances much easier than with other types of iron. With spheroidal nodular graphite, there’s less chance of ductile iron splintering or cracking when machined, giving manufacturers more flexibility in terms of designs.
- Corrosion Resistance: Even without specialized coatings, ductile iron performs very well in terms of resisting corrosion when used outdoors or in environments where it may be exposed to moisture or chemicals.
- Strength to Weight Ratio: The relatively light weight of ductile iron does not come at the cost of giving up strength. This means ductile iron is a good substitute for aluminum in applications where lighter weight is called for but durability remains a high priority.
Why Expertise Matters with Ductile Iron
One of the most important aspects of working on ductile iron castings is having a foundry partner who understands the material and how to get the best results from it. An experienced ductile iron casting partner will know how the metal’s unique chemical composition impacts its performance and will have the right processes in place to make the most of it.
This is why many manufacturers choose to work with Hiler Industries. As a foundry with more than 75 years of expertise in the industry, we have helped countless customers accomplish their goals when it comes to ductile iron casting. We have extensive experience in casting ferrous ductile iron as well as HiSiMo ductile iron and austempered ductile iron. In everything we do, our focus is always on delivering the highest degrees of quality and craftsmanship for our clients.
At the core of our success is our skill in the shell mold casting technique. This process uses extremely fine, resin-coated sand to create molds that capture more fine details and complex part geometries than other casting methods. We were among the first foundries in the United States to adopt this process, and we continue to look for new ways to enhance and improve our procedures using state-of-the-art technology. In addition, our ISO-9001:2005-certiifed quality systems give customers added peace of mind that comes from knowing their projects are in the right hands.
To learn more about our expertise when it comes to casting using ductile iron, take the first step today and connect with one of our experts. We will take the time to talk to you about your next project and discuss ways we can bring value to your products and processes.