As a production method, casting offers an exceptional capacity for design—often eliminating the need for additional assemblies and fabrication. Here, we'll compare cast iron to cast steel to get a sense of how to get the best performance out of each material.
Many materials can be cast—including several types of metals and synthetics. Iron and steel in particular, however, feature excellent mechanical properties suited to a number of applications.
While they may appear similar on the surface, bearing a similar glinted appearance, it's important to know the advantages and disadvantages of each in terms of production and application. Knowing and not knowing can mean the difference between well-functioning, unforgiving strength and durability and fractured or deformed parts that will quickly lose their luster.
Cast iron vs cast steel
Iron and steel, as they're used in manufacturing, refer to many different alloys and grades. Both are ferrous metals comprised of primarily iron atoms. At this point, it's important to distinguish between the "iron" used in everyday products, and the scientific element known as "iron" (Fe). The elemental iron is the stuff that's found in nature—typically in an oxidized form that requires intensive processing, called smelting, to extract. Elemental iron, in its pure form, is actually quite soft and isn't much use to us.
What makes iron useful is to make it an alloy with the addition of carbon. In fact, carbon composition is what distinguishes cast iron from steel. Cast iron typically contains more than 2 percent carbon, while steel contains between 0.12–2 percent.
Before we get into more details, the table below provides a general overview of the advantages and disadvantages of working with each material. While there are many different types of iron and steel to consider, this table focuses on gray iron and carbon steel—two of the most common forms for each metal.
|Quality||Cast iron||Cast steel|
|Ease of machining||✔|
|Corrosion resistance||✔||✔ (stainless alloys)|
|Wear resistance||✔ (depending on application)||✔ (depending on application)|
Most people haven't encountered iron or steel in their molten state. (This is understandable. Cast iron melts at about 2300˚F and steel melts at about 2600˚F.) What you quickly discover when working with these metals in their liquid form is that they differ in their pourability and shrinkage rates.
Cast iron is relatively easy to cast, as it pours easily and doesn't shrink as much as steel. This means it will readily fill the complex voids in a given mold and requires less molten material.
Pouring steel can be more difficult. It is less fluid than molten iron and more reactive to mold materials. It also shrinks more when it cools, meaning more molten material needs to be poured—typically into an excess reservoir, called a riser, from which a casting will draw from as it cools.
Also, castings don't typically cool uniformly throughout their internal structures. Outer areas and thinner portions will cool and shrink at different rates than inner areas and bulkier portions—often creating internal tension, or stress, that can only be alleviated through heat treating. Steel is much more susceptible to shrink stresses, and in some situations, these tensions can lead to fracture.
Overall, steel requires more attention and inspection throughout the casting process, making it more resource intensive to manage production.
Depending on the final application, cast parts may need to be machined to achieve specific tolerances or to create a desired finish. At the very least, things like gates and runners need to be cut away and ground down.
Some materials are more difficult to machine than others. Machinability is measured in terms of how easy it is to cut or grind a material. As a rule of thumb, the presence of alloys and improved mechanical performance properties are inversely related to machinability.
Cast iron is typically much easier to machine than steel. The graphite structure in cast iron breaks away easier and in a more uniform manner. Though, harder irons, such as white iron, can be more difficult to machine. Steel isn't as easy to cut with the same consistency, and it causes more tool wear. Hardened steels, or those with higher carbon content, also increase tool wear. And, low-carbon steels, despite being softer, can get gummy and difficult to work with.
Damping ability can be a major variable to consider. When looking at how and where a material is used, effective damping can create sturdier, more reliable performance. A lack of damping ability can lead to excess vibration and noise, such as ringing or squealing.
The graphite structure in cast iron—especially the flaked formations in gray cast iron—is especially good for absorbing vibration. This makes cast iron ideal for engine blocks, cylinder housings and machine beds, where sturdiness and precision are important. Reducing vibration can minimize stress and prevent wearing of moving parts.
You might also be interested in...
Compressive strength is the ability of a material to withstand forces aimed at reducing an object's size. This is different from forces directed at pulling a material apart. Cast iron typically has better compressive strength than steel. Compressive strength is often beneficial in mechanical applications where pressure and containment are factors.
Cast steel components reduce assembly and fabrication
So far, it might seem that there are more advantages to using cast iron than steel, but where steel really shines is in its ability to resist sudden impacts. This has to do with its toughness properties—i.e. its ability to withstand high stress and strain forces.
Strength without ductility means a material will be very brittle and susceptible to fracture. This is the case with cast iron, which is why its range of application is limited. At the same time, high ductility, or the ability to deform without failure, isn't much use without the strength to withstand significant forces. A rubber band can withstand significant deformation without breaking, but the total amount of force it can take is ultimately limited.
While iron may be easier to work with in most casting applications, steel has an optimal mix of both strength and ductile properties, making it extremely tough. The impact-resistant qualities—and all-around load-bearing nature—of steel is what makes it desirable for so many mechanical and structural applications. It's why steel is the most widely used metal in the world.
Iron has better resistance to corrosion than steel. However, when left unprotected, both metals will oxidize in the presence of moisture. Given a long-enough time, they'll eventually flake away into nothing. If left exposed to the environments, painting or powder coating are recommended to protect both iron and steel surfaces.
If corrosion resistance is an important factor, alloy steels are likely a better option—specifically stainless steels, which have chromium and other alloys added to prevent oxidation.
Cast iron typically has better resistance to mechanical wear than steel, especially in friction-wearing situations. The higher graphite content in cast iron creates a graphitic dry lubricant that allows solid surfaces to slide against each other without deteriorating surface quality.
Steel, without the same graphite properties, wears more readily than iron, but may still be resistant to certain types of abrasion. Certain alloys can also improve the abrasion qualities of steel.
Cast iron is typically cheaper in terms of material costs, energy and labor required to produce a final product. Steel is more costly to purchase, and it requires more time and attention to cast. When designing cast products, however, it's worth considering long-term use and replacement costs. Parts that are more expensive to manufacture can end up costing less in the long run.
Different types of cast iron and cast steel
Cast iron is easy to mold into decorative designs
So far, we've compared the qualities of the most basic forms of cast iron (gray iron) and cast steel (mild or carbon steel). It's worth noting that the composition and phase structure of iron and steel can greatly affect mechanical properties. For example, the carbon in a standard gray iron takes the form of sharp graphite flakes, while ductile iron features more spheroidal graphite structures. Flaked graphite is what makes gray iron brittle, while round graphite particles creates better toughness, making it more suitable for impact-resistance applications.
Many alloys can be added to both iron and steel to achieve desired properties. Manganese, for example, increases toughness, and chromium creates better corrosion resistance. Varying carbon content is also what distinguishes between low-, standard- and high- carbon steels—higher amounts making for much harder materials.
Steel is also available in many prefabricated forms—such as sheets, rods, bars, tubes and beams—and can often be machined or assembled to suit a particular application. Depending on the product, and quantity required, fabricating existing steel products may be an option to consider.
To learn more about the benefits of using cast iron or steel in a future project, contact Reliance Foundry's Sales Department to learn more.
- American Foundry Society. "Machining Cast Iron Components." afsinc.org
- American Foundry Society. "Understanding Material Specifications for Steel Castings." afsinc.org
- Machine Design. "Cast Iron." machinedesign.com
- Nickel Institute. "Cast Irons & Cast Alloys." nickelinstitute.org
- Hosford, William F. Iron and Steel. Cambridge University Press. 2012.
- Woodford, Chris. "Iron and steel." explainthatstuff.com
- Vibration damping makes cast iron ideal for engine blocks: Andrew Taylor, CC BY 2.0, via Flickr