Ductile Iron vs Cast Iron: Understanding the Key Differences and Applications

Iron is one of the most durable and long-lasting metals, providing strength to essential infrastructure projects for centuries. Yet it’s often assumed to come in only the familiar cast iron form. Ductile iron is also available and is better suited to many of today’s most challenging applications. 

But which one do you need? It’s hard to answer the ductile iron vs cast iron question if you don’t even know these are options, and which one is the best fit for certain parts and installations. 

If you’re planning major municipal, infrastructure, and construction projects that call for metal fabrication, choosing the right material is essential to ensure strength and durability. Whether you’re sourcing materials independently or working with a provider of metal fabrication services, understanding the differences between ductile iron vs cast iron is the first step toward making an informed decision. Start exploring which material best suits your project needs.

Why the Choice Between Ductile and Cast Iron Matters

Is ductile iron the same as cast iron? Ductile and cast iron vary in more than just name. Both are known for longevity and durability, but they differ greatly in their resistance to certain types of forces. 

Assuming they are interchangeable has led to costly rework when cast iron pipes or structural supports failed because ductile iron was needed instead. For example, aging and leaking cast iron gas pipes cause multiple injuries each year.

The difference between the two materials goes down to the crystalline structure of the metal. The brittleness of cast iron has labeled all iron alloys as hard but resistant to bending or welding. However, flexible iron is available in the form of ductile materials.

Using gray or white cast iron in place of a ductile alloy will significantly impact the lifespan, total cost, material performance, and maintenance requirements of the project. Ductile iron is more resistant to cracking, stops cracks from spreading far, better handles impacts and surges in pressure, offers great corrosion resistance, and, in general, outperforms cast iron in most construction and utility applications.

What Is Cast Iron?

Iron has been used for thousands of years, but it was limited in value in its raw form due to brittleness and moderate strength. Cast iron was the first attempt, invented around 600 BCE, to create a higher-quality alloy through the addition of carbon. It specifically takes the form of graphite. Carbon adds hardness to pure iron while also reducing corrosion reactivity thanks to the protective layer of graphite on the surface of the metal.

However, graphite in cast iron tends to form into flakes that interrupt the iron matrix. These graphite flakes significantly enhance the material’s durability, corrosion resistance, hardness, and strength. But when the stress point of the cast iron is reached, it can’t do anything but snap. Cast iron has little ability to bend or fold, limiting its workability and reducing its value in applications like utility structures and pressurized pipes.

While it was once widely used for bridges, concrete reinforcements, and major municipal utility services like water supply pipes, cast iron has been largely superseded in many applications by either carbon steel or ductile iron. Cast iron might have been the material of choice for the construction of the Eiffel Tower, but today, it wouldn’t be an engineer’s first choice for that job.

What Is Ductile Iron?

Ductile iron features nodular graphite formation throughout the body of the metal. Despite the long history of iron’s use, this alloy wasn’t invented until the 1940s. Very small amounts of magnesium, silicon, and cerium are added to the desulphurized iron, increasing ductility in the finished product. The added ingredients trigger the formation of nodules of graphite in the cooled iron rather than flakes.

Changing the structure of the carbon added to the iron alloy makes it ductile, which is a measure of a material’s ability to stretch and deform under tensile stress. Cast iron is particularly weak to this kind of stress, but ductile iron is much stronger against it. 

Ductile iron is widely used for the same municipal, industrial, and utility applications that once relied on cast iron, primarily due to its superior properties.

Comparing Microstructures: Flake vs. Nodular Graphite

There’s no need for a deep dive into every microstructure of iron alloys just to understand what makes ductile iron vs cast iron so different. It’s easy to understand why ductile iron material is so much stronger against tensile stresses just by looking at the shape of the graphite that’s a part of it.

How Graphite Shapes Influence Material Behavior

Consider how a ball joint, like your shoulder, allows your arm to move in all directions smoothly. Then, imagine replacing the mobile ball shape with a flat surface. It certainly wouldn’t move the same way. 

The nodular graphite in ductile iron acts more like the ball joint than the flat pieces in cast iron. Flake graphite is closer to a fragile piece of toast than a strong spheroidal shape. It’s no wonder that the nodular graphite distributes stress more evenly throughout the metal, enhancing the mechanical strength of any parts you make with it.

Impact on Strength, Ductility, and Brittleness

Drain, waste, and vent (DWV) pipes are often still made from cast iron since they’re not under pressure. Yet, it’s not a great idea for pressurized water pipes because the risk of cracking under stress is simply too high and contributes to the fact that an average of 700 water mains break each day across North America.

The ductility and increased strength of ductile iron protect it from pressure, shearing forces, and impacts. The nodules of graphite in the iron also affect fracture mechanics, slowing the spread of fractures that do occur to minimize the chance of failure. Since the material is less brittle and more flexible, it’s more likely to bend or stretch rather than break.

Mechanical Properties Side-by-Side

If you’re still asking, “What is ductile iron good for?”, comparing each material difference between cast and ductile iron may make the answer clearer.

Tensile Strength, Yield Strength, and Elongation

Tensile strength measures a material’s ability to resist forces trying to pull it apart. The tensile strength of ductile iron tends to range between 60,000 and 120,000 pounds per square inch (PSI). In comparison, you’ll find that cast iron tops out at 60,000 and drops as low as 30,000 PSI.

Yield strength is slightly more complicated. It measures the point at which a material is no longer deforming in a plastic way, or in a way that it can snap back from and start to deform permanently. Ductile iron offers a yield strength similar to that of mild steel at 40,000 to 45,000 PSI on average. Gray cast iron, in particular, is regarded as having no yield strength at all.

Ductility measures how well a material responds to elongation, and ductile iron offers rates of 18% or even higher in some alloys. Cast iron, in general, offers less than 1% elongation before breaking.

Hardness, Toughness, and Fatigue Resistance

Cast iron, especially gray alloys, offers a top Brinnell hardness (HB) rating of 290. In comparison, ductile iron typically reaches a hardness of only 260 HB, with some grades offering higher hardness through extensive heat treatment. If hardness is essential for an application like manhole covers, cast iron is likely the better choice.

However, hardness doesn’t equal durability in all applications. Material toughness and fatigue resistance also play a large role in how the metal you choose holds up under use. The increased resistance against bending and shearing forces makes ductile iron tougher in many cases. That’s why ductile iron is more commonly chosen for applications like municipal piping and long-term utility poles.

Corrosion Resistance and Durability

Ductile iron is also preferred for buried pipe installations due to its increased corrosion resistance and durability. The pipes can be lined with cement and encased in polyethylene, keeping moisture and chemicals away from the metal surface. Today’s ductile iron often provides a much greater corrosion resistance rating than older forms of the same metal, thanks to variations in trace metal content and manufacturing methods as well.

Machinability and Weldability

One of the reasons engineer Keith Mills invented ductile steel was to find a form of iron that offered greater flexibility in metal fabrication. While both machining and welding are still a challenge with ductile steel, they’re less difficult than with cast iron. That means you enjoy reduced production costs, lower labor costs, and fewer delays in your project timeline, especially since there’s less need for time-consuming heat treatments to reduce stresses in the metal if you use ductile iron. Ductile iron’s ability to withstand elongation opens up more manufacturing options than the casting required to give cast iron its name.

Cost Analysis: Initial Investment vs. Long-Term Value

Ductile iron tends to cost more upfront than cast iron, depending on the grade; however, it may be less expensive to install due to its ability to accept welding and greater design flexibility. You’ll also save more in the long run if the additional tensile or yield strength is needed and results in a longer-lasting part. 

However, you may not always need the extra strength. Some projects can benefit from using cast iron to save upfront, even if lifecycle costs are higher.

Real-World Applications: Where Each Iron Type Excels

Ductile iron offers many of the ductility and strength benefits of steel combined with the benefits of iron, such as a low melting point and good castability. That’s why you’ll find it has so many applications in infrastructure today.

Water and Sewer Infrastructure

Graphitic corrosion is a common issue that can lead to water main breaks. The iron in the cast pipes migrates out into the surrounding soil due to corrosion, leaving behind a fragile network of exposed graphite flakes. 

Even with constant water exposure and varying chemical and environmental factors, ductile iron tends to resist this effect more effectively. However, in certain situations, it may require additional cement inner coatings and polyethylene encasements, incurring extra costs. That’s just one of the many sets of pros and cons of ductile iron.

Construction Components and Structural Supports

The impact resistance and ductility of ductile iron make it a safer and more reliable choice than cast iron for critical construction components, such as structural supports. Brackets and beams can always suffer a sudden shock if another part of the structure collapses, while bending can occur over time due to weight. 

Ductile iron will resist the shocks and bend without snapping, preventing a catastrophic loss of a key building support. Cast iron is far less resistant to these damages, making it less reliable for essential supports. Yet, there are often opportunities to save by using cast iron for non-load-bearing applications, such as balustrades, decorative brackets, and balcony enclosures.

Municipal and Utility Installations

For manhole covers, drainage grates, and other utility enclosures that receive relatively little wear and tear, cast iron is still the material of choice. It’s simply not necessary to use ductile iron in most of these applications, except for utility enclosures that might receive direct impacts from vehicles and other moving objects. 

The hardness of cast iron makes it ideal for long-lasting use as a cover or grate. Many municipal utility installations can be more cost-effective if ductile iron is reserved for essential applications with tensile stresses, and affordable cast iron is used for the remainder.

Environmental and Sustainability Considerations

When deciding between ductile iron vs cast iron based on sustainability, consider primarily the machinability and weldability of the two materials. How they’re handled during manufacturing determines how much waste is produced and how much is recycled.

Recyclability and Environmental Impact

The production of both types of iron releases approximately the same amount of emissions, which manufacturers largely mitigate through the use of scrubbers. They also share a similar recyclability, with both offering 100% reclamation rates with minimal loss of strength. 

Ductile iron is often made with as much as 98% recycled material, while cast iron can come from either recycled or virgin sources. Check the source of the specific grade of metal you’re considering, if recycled materials are important to you.

Energy Consumption in Production

Ductile iron takes a lot more energy to produce. Cast iron requires various smelting and casting processes, and then the ductile alloy requires still more processing in a furnace to induce the nodule formation. If embodied energy is a major concern for your project’s goals, cast iron is likely a better choice whenever possible.

Choosing the Right Material for Your Project

Exploring the pros and cons of ductile iron and cast iron should give you an idea of which alloy is right for your project. If you’re still trying to decide, there are several factors to consider.

Factors to Consider Based on Application Needs

For crucial infrastructure components that must last 100 years or longer, even when exposed to water and other corrosive elements, ductile iron is likely the best choice. It’s also recommended for structural supports and construction features that could bend over time or due to sudden damage. Ductile iron is less likely to snap, providing ongoing strength even if the part is compromised.

Aside from sheer durability, the cost of ductile iron vs cast iron also matters. There’s little reason to use the most costly ductile iron for applications where tensile and yield strength aren’t needed. For example, cast iron pipes are still valuable for above-ground venting and drain uses where they’re easily replaced as needed. But for buried applications under high levels of pressure, it’s far smarter to go with ductile iron that won’t need replacement nearly as often. 

It’s all a matter of considering what you’re building and what risks each component faces before making a decision. Most municipal projects will require both cast and ductile iron in various applications, and attempting to settle for just one or the other could lead to durability issues or overspending.

Decision-Making Checklist

Try using this easy checklist to finalize your decision between cast iron vs ductile iron.

• Consider machinability and weldability requirements
• Determine the risks of the parts bending or twisting under stress
• Figure out if your schedule can handle the extra time needed for ductile iron heat treatments
• Verify impact and shock risks from moving vehicles and other hazards
• Explore what other manufacturers or contractors are using in your field
• Set aside extra time for the slightly more complex casting process required for ductile iron
• Bring your questions and concerns to The Federal Group for customized advice on your project.

Making an Informed Choice Between Ductile and Cast Iron

Ductile iron is a unique material that offers many of the benefits of mild steel without its cost. However, it’s not automatically the best option for every pipe, valve, and building support application. Some cases will still call for the hardness and lower cost of cast iron. It’s a matter of knowing what stresses are affecting each part or pipe you’re designing and when ductility is a key part of the project requirements. 

If you’re looking for help making the right decision on cast iron vs ductile iron, consult with The Federal Group. We’ll help you settle on the choice and aid in sourcing the right material, no matter the grade or alloy of iron you need.