Casting offers an exceptional ability to design details, often without additional fabrication and assembly. Many materials can be cast, including many types of metals and composites, but steel in particular has excellent mechanical properties and is suitable for a wide range of applications.
While cast iron and steel may appear similar on the surface, they each have different advantages and disadvantages from production to application. Knowing these pros and cons and making the proper selection can mean the difference between unbearable strength and durability and broken or warped parts that tarnish quickly.
Carbon content is the main difference
Steel is a ferrous metal composed mainly of iron atoms. In manufacturing, however, things are not that simple - many different alloys and grades are used in production. To understand them, it's important to distinguish between iron used in everyday products and the scientific element iron (Fe). Elemental iron is a substance found in nature, usually in an oxidized form that requires an intensive process called smelting to extract.
Pure elemental iron is too soft to be used in most applications. When it is alloyed or mixed with carbon it becomes harder and therefore more useful. In fact, the carbon content is the main difference between cast iron and steel. Cast iron typically contains more than 2% carbon, while cast steel typically contains 0.1-0.5% carbon.
Castability
Most people have not encountered iron or steel in their molten state - which is understandable since iron melts at about 2300˚F and steel at 2600˚F and both are higher Pour into molds at temperature. Those who work with liquid steel will quickly discover that there is a huge difference in pourability and shrinkage.
Cast iron is relatively easy to cast because it pours easily and doesn't shrink like steel. This means it can easily fill complex voids in molds and requires less molten material. This fluidity makes cast iron an ideal metal for buildings or ornate ironwork structures such as fences and benches.
Pouring steel is much more difficult. It is less fluid than molten iron and more reactive to mold materials. It also shrinks more as it cools, which means more molten material needs to be poured—usually into excess reservoirs, called risers, from which the casting is drawn as it cools.
However, castings generally do not cool uniformly throughout the internal structure. The outer regions and thinner parts will cool and shrink at a different rate than the inner regions and larger parts - often creating internal tension or stress that can only be relieved by heat treatment. Steel is more susceptible than iron to shrinkage stresses and in some cases these tensions can lead to significant internal and/or external voids and may eventually lead to fracture.
For these reasons, cast steel requires more attention and inspection throughout the casting process, making production more resource-intensive.
Machinability
Depending on the end application, castings may need to be machined to specific tolerances or to achieve a desired finish. At a minimum, objects such as doors and runners need to be cut out and sanded.
Machinability is a measure of how easily a given material can be cut or ground; some materials are more difficult to machine than others. As a rule of thumb, metals that are highly alloyed to improve mechanical properties have lower machinability.
Cast iron is generally easier to machine than steel. The graphite structure in cast iron is easier to shed in a more uniform manner. Harder irons, such as white irons, are more difficult to machine due to their brittleness.
Steel is not easy to cut at the same consistency, it causes more tool wear which leads to higher production costs. Hardened steel or steel with a higher carbon content can also increase tool wear. However, softer steel is not necessarily better - mild steel, although softer, can become viscous and difficult to work with.
Vibration reduction
Damping properties should be considered when selecting casting materials, as insufficient damping capacity can lead to excessive vibration and noise, such as ringing or squealing. Depending on where the material is used, effective damping can lead to stronger, more reliable performance.
The vibration-damping properties make cast iron an ideal choice for engine blocks.
The graphite structure in cast iron, especially the lamellar structure in gray cast iron, is particularly good for shock absorption. This makes cast iron ideal for use in engine blocks, cylinder housings and machine tool beds, as well as other applications that require robustness and precision. Reduced vibration minimizes stress and prevents wear on moving parts.
Compressive strength
Compressive strength is the ability of a material to withstand forces that would reduce the size of an object. This is the opposite of the force that pulls the material apart. Compressive strength is beneficial in mechanical applications where pressure and tightness are factors. In general, cast iron has better compressive strength than steel.
Impact resistance
So far, using cast iron seems to have more advantages than steel, but steel has one significant advantage: impact resistance. Steel is excellent at withstanding sudden impacts without bending, deforming or breaking. This is due to its toughness: it is able to withstand high stress and strain.
Strength without ductility results in a brittle material that fractures very easily, and cast iron is the poster child for strength without ductility. Due to its brittleness, cast iron has limited applications.
At the same time, high ductility, or the ability to deform without failure, is of little use without the strength to withstand significant impacts. For example, a rubber band can undergo significant deformation without breaking, but the forces it can withstand are very limited.
While iron may be easier to machine in most casting applications, steel has the best combination of strength and ductility for many applications, and cast steel is very tough. Steel's impact-resistant properties and all-round load-bearing properties make it ideal for many mechanical and structural applications - which is why steel is the most widely used metal in the world.
Corrosion resistance
Iron has better corrosion resistance than steel. Both metals oxidize when wet, but iron develops a patina that protects the integrity of the metal from deep corrosion.
Another way to prevent corrosion is to use paint or powder coat, or use IronArmor for added protection. Any chips or cracks that expose the underlying metal can lead to corrosion, so regular maintenance is important for coated metals.
If corrosion resistance is an important factor while maintaining the silvery appearance of the original metal, alloy steel may be a better choice—especially stainless steel, which has chromium and other alloys added to prevent oxidation.
wear resistance
Cast iron generally has better resistance to mechanical wear than steel, especially under frictional wear conditions. A certain amount of graphite content in the cast iron matrix produces a graphite dry lubricant that enables solid surfaces to slide against each other without degrading the surface quality and making it more difficult to wear.
Steel wears more easily than iron, but is still resistant to certain types of wear. Adding certain alloys can also improve the wear resistance of steel.
cost
Cast iron is generally less expensive than cast steel because of the lower material costs, energy and labor required to produce the final product. Raw steel is more expensive to purchase and requires more time and effort to cast. However, it is worth considering long-term usage and replacement costs when designing cast products. Parts that cost more to manufacture may end up costing less in the long run.
Steel also comes in many prefabricated forms such as sheet, bar, bar, tube and beam, and can often be machined or assembled to suit specific applications. Depending on the product and the quantity required, fabricating existing steel products may be a cost-effective option.
Different types of cast iron and cast steel
We have compared the qualities of cast iron in its most basic form (grey iron) and cast steel (mild or carbon steel), but the specific composition and phase structure of the steel can greatly affect mechanical properties. For example, the carbon in standard gray iron is in the form of sharp graphite flakes, while ductile iron has a more nodular graphite structure. Flake graphite makes gray cast iron brittle, while round graphite particles in ductile iron increase toughness, making it more suitable for impact-resistant applications.
Alloys can be added to steel to engineer desired properties. For example, manganese increases toughness, while chromium increases corrosion resistance. The varying carbon content is also what differentiates mild, standard carbon and high carbon steels – the higher the carbon content, the harder the material.
Ultimately, the choice between cast iron and cast steel will depend on the type and application of the final installation.