Cast Steel vs Cast Iron: What are the Differences?

Cast steel vs cast iron is a common topic in manufacturing. Both materials start as iron-carbon alloys and take their shape by pouring liquid metal into molds. But even with these shared beginnings, there are clear ways they are different. Understanding the cast iron and cast steel difference is quite important when selecting materials for various parts. This article will look closely at cast iron and cast steel, explaining how they vary in their makeup, how they behave, and where they are typically used.

Before we begin, I’d like to introduce our company and services. Taiyuan SIMIS Investment Casting Co., Ltd is a professional casting manufacturer in China. At SIMIS, we offer various metal casting services, including investment casting, sand casting, shell mold casting, and lost foam casting. We use these methods to produce a wide range of cast steel and cast iron parts, which are widely used in an array of industries.

We are equipped with advanced casting equipment and facilities, dedicated to producing both steel castings and iron castings. Besides, we offer one-stop services, including precision machining, heat treatments, surface treatments, and all kinds of testing and inspection. Now, back to the article.

Basic Definitions and Composition

The main difference between cast iron and cast steel starts with their chemical makeup, especially how much carbon they contain. This difference in carbon content leads to many other variations in their properties. For a deeper understanding of the overall difference between iron and steel, you can read our ‘Difference Between Steel and Iron’ article.

Cast Iron

Cast iron is an iron alloy with a carbon content usually higher than 2 percent, often going up to 4 percent or more. In cast iron, much of this carbon is present as graphite, which is pure carbon in a specific form. This graphite can appear as flakes, as seen in gray iron castings, or as small, round structures called nodules, which you find in ductile iron castings. Another type, white iron castings, has carbon mostly in a hard compound called iron carbide. There are also ADI castings, which get special treatment. These various forms of carbon inside iron castings affect how the material behaves.

Cast Steel

Cast steel, on the other hand, is an iron alloy that contains less carbon, typically below 2 percent and often much lower, around 0.05 percent to 1.5 percent. In cast steel, the carbon does not usually exist as separate graphite. Instead, it is mostly combined with iron in forms like cementite or dissolved within the iron metal itself, creating structures such as pearlite. Beyond carbon, cast steel often has other elements added to it.

For instance, carbon steel castings primarily contain iron and carbon, while alloy steel castings have other elements like nickel or chromium for specific strengths. Stainless steel castings contain chromium for rust resistance, and manganese steel castings are known for their toughness. These added elements help give cast steel a wide range of properties.

Appearance Differences

When comparing cast steel vs cast iron, their looks can offer some clues, though appearance alone is not enough to tell them apart for sure. The difference between cast iron and cast steel often shows even before their mechanical tests.

Cast iron typically has a duller, darker, and sometimes more textured surface straight from the mold. If a piece of gray iron castings breaks, the broken surface often looks dull and gray, almost like pencil lead, because of the graphite flakes inside. White iron castings, however, have a very bright, silvery-white fracture surface due to the hard iron carbides present. The general appearance of iron castings often has a more rugged, less refined look.

Cast steel, on the other hand, generally has a brighter, somewhat shinier appearance. Its surface can often be smoother than cast iron. When a piece of cast steel breaks, the fracture surface usually looks more fibrous or crystalline and has a brighter, metallic sheen. Different types of cast steel can also have slightly different looks. For instance, stainless steel castings will have a distinct bright, silvery look, which resists rust and stains, setting them apart from plain carbon steel castings. Manganese steel castings might develop a specific dark, work-hardened surface over time.

While these visual differences can give an initial idea, it is important to remember that surface finish treatments or coatings can change a material’s outside look. So, while appearance might offer a hint about cast iron and cast steel, it is not a reliable way to know what a part is truly made of.

Microstructure Differences

Looking closely at cast steel vs cast iron under a microscope shows the deepest differences between them. This internal structure, called microstructure, comes from their carbon content and how they cool. It truly sets cast iron and cast steel apart.

In cast iron, the microstructure is shaped by its high carbon content. Much of the carbon separates out as pure graphite. In gray iron castings, this graphite forms as flat, thin flakes. These flakes act like tiny internal cuts in the metal, which explains why gray iron is good at dampening vibrations but can be brittle.

In ductile iron castings, the carbon forms into small, round balls or nodules of graphite. These round shapes disturb the metal less, allowing ductile iron to bend and stretch more than gray iron. ADI castings take this further with special heat treatment that changes the metal around these nodules for even better strength. White iron castings have a different microstructure; their carbon stays combined with iron as a very hard compound called iron carbide, giving them great wear resistance but also making them very brittle. These distinct forms of carbon within iron castings are a key part of the difference between cast iron and cast steel.

Cast steel has a different microstructure because it has less carbon and the carbon behaves differently. In cast steel, there are usually no free graphite flakes or nodules. Instead, the carbon is either dissolved within the iron metal or forms compounds like cementite, which then combine with pure iron to create structures such as pearlite.

The specific mix of these structures in carbon steel castings or alloy steel castings can be controlled by heat treatment. For instance, different cooling rates can change the size and arrangement of these internal components, changing the steel’s properties. Other elements, like those in stainless steel castings or manganese steel castings, also change the internal structure, giving these steels their special properties, but always without the visible graphite common in many iron castings.

Mechanical Property Differences

The way cast steel and cast iron behave under force is one of the most important ways they differ. Their distinct microstructures lead to very different mechanical properties. This is where the practical difference between cast iron and cast steel becomes very clear.

Tensile and Yield Strength

When considering the strength difference between cast iron and cast steel, cast steel generally has higher tensile strength and yield strength. This means it can handle more pulling force before it stretches permanently or breaks. Carbon steel castings, for example, are known for their strength in various applications. Iron castings, on the other hand, usually have lower tensile strength. However, gray iron castings show good compressive strength, meaning they can handle a lot of squeezing force without breaking.

Ductility and Toughness

Another difference between cast steel and cast iron is ductility and toughness, cast steel generally has a clear advantage over cast iron. Ductility refers to a material’s ability to stretch or deform without breaking. Toughness is about soaking up energy from a sudden hit.

Cast steel, including alloy steel castings and stainless steel castings, can deform quite a bit before fracturing, making them good for parts that might bend or face shocks. Most iron castings, like white iron castings and even common gray iron castings, are generally brittle. This means they tend to break suddenly with little warning or deformation.

However, ductile iron castings are a notable exception, as their round graphite shapes give them much better ductility and toughness than other cast irons. ADI castings, through special processing, show even higher strength and toughness among iron types.

Impact Strength

Regarding impact strength, cast steel shows much better resistance to sudden blows. A part made from manganese steel castings, for instance, can deform significantly when hit, absorbing the impact energy. In contrast, most cast irons have lower resistance to sudden impacts and can break easily if hit sharply. For some applications, impact strength can be a deciding factor when choosing between cast steel vs cast iron.

Hardness and Wear Resistance

hardness and wear resistance is another significant difference between cast iron and cast steel, some cast irons can be very hard. White iron castings are extremely hard and offer good wear resistance due to their hard iron carbide structure. Gray iron castings also provide good wear resistance because the graphite acts as a natural lubricant. Cast steel can also be made very hard through heat treatment, which is common for carbon steel castings and alloy steel castings when wear resistance is needed.

Damping Capacity

Lastly, let’s talk about the damping capacity difference between cast iron and cast steel. Cast iron, especially gray iron castings, has a good damping capacity. This means it can absorb vibrations well, which is helpful in machine bases to reduce noise and shaking. Cast steel has a lower damping capacity compared to cast iron.

Casting Characteristics Differences

The way cast steel and cast iron behave when melted and poured into molds also shows clear differences. These characteristics affect how easily they can be cast into various shapes and sizes. This is an important part of understanding the difference between cast iron and cast steel.

Castability

Castability refers to how easily a molten metal can be shaped into a sound, defect-free part through the casting process. When comparing cast steel vs cast iron, castability difference is an important consideration. Cast iron generally has better castability compared to cast steel. Its lower melting point and good fluidity mean it fills molds well and adapts to intricate designs. This is why various iron castings are often chosen for complex shapes. Cast steel, however, is considered less castable. Its higher melting point and lower fluidity mean it needs more control and design to prevent casting problems, especially for intricate carbon steel castings or alloy steel castings.

Melting Point and Fluidity

Cast iron generally has a lower melting point compared to cast steel. This means it needs less heat to become liquid, which can save energy in the foundry. Iron castings also show higher fluidity. This means the molten metal flows very easily and can fill thin sections and complex details within a mold without much trouble. For example, high fluidity makes cast iron a good choice for intricate parts made through sand casting.

Cast steel, on the other hand, has a higher melting point. This needs more heat and can affect the casting process. Cast steel also has lower fluidity. It does not flow into all the small corners and thin walls of a mold as easily as cast iron. This means that designing molds for cast steel parts can be more challenging, especially for very complex shapes, regardless of whether they are carbon steel castings or alloy steel castings. Investment casting (lost wax casting) is a versatile casting method, capable of casting various types and grades of steel components.

Shrinkage During Cooling

Another important difference is how much each material shrinks as it cools and solidifies. Cast steel experiences greater shrinkage during this process. As the molten metal turns solid, it contracts more significantly. This higher shrinkage means that mold designs for cast steel need special features, like larger risers (reservoirs of molten metal that feed the solidifying part) and carefully planned gates, a common consideration in sand casting dueences due to the larger mold volumes typically involved.

Cast iron typically shows less shrinkage as it cools. This property makes it easier to cast parts with consistent dimensions and fewer shrinkage-related flaws. This lower shrinkage is a benefit when making various iron castings, including gray iron castings and ductile iron castings, as it simplifies some aspects of mold design and production.

Tendency for Defects

The distinct casting characteristics of cast iron and cast steel can also influence the types of defects that are more likely to appear. Because cast iron has good fluidity and lower shrinkage, it can sometimes be less prone to certain types of internal shrinkage defects if poured correctly. However, issues like cold shuts can occur if the metal streams do not merge properly. Cast steel’s higher melting point and greater shrinkage mean foundries must take more steps to prevent shrinkage cavities and hot tearing (cracks that form as the metal cools and contracts). For example, careful feeding systems are very important for steel castings when using shell mold casting process to avoid these issues.

Cost Differences

When selecting materials for a part, the overall cost is a very important point, and there is a clear difference between cast iron and cast steel in this regard. Generally, cast iron tends to be a more budget-friendly option than cast steel.

Raw Material Costs

The initial cost of raw materials for iron castings is typically lower than for cast steel. This is mainly because the base ingredients for common types like gray iron castings are less expensive. When you consider alloy steel castings or stainless steel castings, the cost of the raw materials goes up significantly due to the added elements that give them special properties.

Melting and Processing Costs

The casting process itself also contributes to the cost difference between cast iron and cast steel. Cast iron has a lower melting temperature, which means foundries use less energy to melt it. Its high fluidity also simplifies the pouring process, sometimes allowing for faster mold filling. Cast steel, with its higher melting point, needs more energy for melting. Also, its greater shrinkage means that molds might need more complex designs, like larger feeding systems, to avoid defects, which can add to the processing costs for carbon steel castings or manganese steel castings.

Machining Costs

After casting, parts often need machining to reach their final shape and precise dimensions. Here, the machinability of the material plays a part in the overall cost. Gray iron castings and ductile iron castings are generally easier to machine. The graphite in their structure acts like a natural lubricant and helps break chips, leading to less tool wear and faster machining times, which keeps costs down. Cast steel, on the other hand, can be harder to machine. Depending on the specific alloy and its hardness (especially if it has been heat treated), machining cast steel can lead to more tool wear and longer machining times, potentially increasing the total cost of the finished part.

Overall Cost Considerations

Considering all these aspects, the total manufacturing cost of a finished part often shows cast iron as the more economical choice. This comes from its lower raw material price, simpler melting process, and easier machining. The difference between cast iron and cast steel in terms of overall expense is noticeable. While cast steel typically carries a higher total cost, this higher expense is often accepted when the application truly needs its greater strength, toughness, or specific heat treatment capabilities. ADI castings, for example, might be more costly than standard gray iron but offer properties that justify their price for certain applications.

Application Differences

The distinct properties of cast steel and cast iron mean they find their place in very different kinds of products and industries. The difference between cast iron and cast steel applications directly reflects their strengths and weaknesses.

Where Cast Iron is Used

Iron castings are widely used where their cost-effectiveness and specific properties are a good fit. Gray iron castings are a common choice for machine bases, engine blocks, and various housings because of their good vibration damping ability and ease of machining. Their ability to handle compressive loads also makes them suitable for counterweights and some pipe fittings.

For parts that need some bending or toughness, like automotive suspension components or certain agricultural machinery parts, ductile iron castings are often used. If a part needs extreme hardness and wear resistance, such as grinding mill liners or specific rollers, white iron castings might be chosen. In general, iron castings are popular for parts with complex shapes where good molten fluidity helps fill the mold accurately.

Where Cast Steel is Used

Cast steel is chosen for applications that demand higher strength, ductility, and toughness, especially where parts might face sudden impacts or heavy loads. Carbon steel castings and alloy steel castings are frequently found in heavy machinery frames, gears, and railway components, where their ability to withstand pulling forces and shocks is important.

For parts that need to resist corrosion while still being strong, stainless steel castings are a common option in the chemical, food processing, and marine industries. When extreme impact resistance combined with hardness is a requirement, such as for rock crushers or wear plates, manganese steel castings are often selected. Cast steel’s weldability also makes it suitable for large structures that need to be assembled from several cast pieces. The overall cast steel vs cast iron selection for an application often comes down to these specific demands.

Advantages and Disadvantages of Each

Both cast steel and cast iron have their own set of strengths and weaknesses. The difference between cast iron and cast steel becomes clear when we look at where each material shines and where it has limitations. Understanding these points helps in choosing the right material for a specific job.

Advantages of Cast Iron

Cast iron generally offers a more economical solution. Its lower raw material costs and lower melting temperature mean less energy is needed during production. Iron castings also have very good fluidity when molten, which lets them fill intricate mold shapes easily. This is a big plus for parts with complex designs. Additionally, types like gray iron castings have good damping capacity, meaning they can soak up vibrations, making them good for machine bases. Many iron castings are also easier to machine than steel, which can save time and money during manufacturing. Some varieties, like white iron castings, have very good wear resistance.

Disadvantages of Cast Iron

Despite its benefits, cast iron has some notable downsides. Its main drawback is generally being brittle. This means most cast irons, like gray iron castings and white iron castings, tend to break suddenly under tension or impact with little deformation. They do not bend much before failing. This limits their use in applications where parts might experience sudden shocks or bending forces. Welding cast iron can also be difficult due to its nature, making repairs or joining parts harder. While ductile iron castings and ADI castings offer improved toughness and ductility, they still do not reach the levels typically seen in cast steel.

Advantages of Cast Steel

Cast steel is known for its superior mechanical properties. It has higher tensile strength and yield strength, meaning it can handle stronger pulling and bending forces before failing. Cast steel also has much better ductility and toughness compared to most cast irons. This ability to deform without breaking makes it suitable for parts that might face shocks or bending, like those made from manganese steel castings or alloy steel castings. Its good weldability allows for easier repairs and assembly. Furthermore, carbon steel castings and stainless steel castings can be heat treated to achieve a wide range of properties, making them very versatile.

Disadvantages of Cast Steel

Cast steel generally comes with a higher cost, both for raw materials and for the energy needed to melt it due to its higher melting point. Its lower fluidity means it doesn’t fill complex molds as easily as cast iron, sometimes requiring more complex mold designs and casting methods. Cast steel also experiences greater shrinkage during cooling, which means careful attention to mold design is needed to avoid internal defects. Machining cast steel can also be more difficult and time-consuming than machining cast iron, potentially adding to overall production costs.

Choosing Between Cast Steel and Cast Iron

Deciding between cast steel and cast iron for a specific part comes down to matching the material’s properties with the job’s demands. There is no single “better” material; instead, the choice depends on what the finished part needs to do and the overall cost. Understanding the core difference between cast iron and cast steel here is key.

When to Choose Cast Iron

Choosing iron castings often relates to cost savings and specific property needs. If the part has a very complex shape, cast iron’s high fluidity makes it easier to produce with fewer casting problems, especially for standard gray iron castings. It’s also a good choice for parts that need to dampen vibrations, like machine bases, or for those that face mostly squeezing forces rather than pulling or bending. Its easier machining can also save money. For specific wear needs, white iron castings might be the answer.

When to Choose Cast Steel

Cast steel is usually the go-to when strength, the ability to bend without breaking, or resistance to sudden hard hits are most important. For parts that will experience high pulling forces, heavy impacts, or bending stress, cast steel offers the necessary toughness. This includes many carbon steel castings, alloy steel castings, and manganese steel castings. When a part needs to be welded into a larger structure, cast steel is also typically a better choice. For environments where rust is a concern, stainless steel castings provide strength along with corrosion resistance. Even among iron types, if higher strength and some ductility are needed but steel is too costly, ductile iron castings or ADI castings can be a good middle ground.

Overall Decision Factors

Ultimately, the decision to use cast steel vs cast iron depends on a careful look at the application’s specific requirements. This includes how much force the part will face, what temperatures it will operate in, how it needs to look, and the total cost. Understanding these factors helps make the right material selection.

Conclusion

The cast steel vs cast iron comparison reveals two distinct materials, each with unique strengths. The main difference between cast iron and cast steel comes down to their carbon content and how that carbon shows up in their microstructure. This leads to varied mechanical properties, casting behaviors, and costs.

While cast iron offers advantages like lower cost, good fluidity for complex shapes, and excellent vibration damping (seen in gray iron castings), it typically lacks the strength and ductility of steel. Cast steel, including types like carbon steel castings and alloy steel castings, stands out for its higher strength, toughness, and ability to handle impacts (like manganese steel castings), along with better weldability. Even within iron types, ductile iron castings and ADI castings offer a step up in performance from traditional iron castings.

Ultimately, choosing between cast iron and cast steel depends on the job. No single material is always better; the right selection matches the part’s specific needs with the material’s properties and the overall cost. As mentioned at the beginning of this article, Taiyuan SIMIS Investment Casting Co., Ltd is an expert casting manufacturer. We understand all metal casting-related topics and can help you choose the best material and processes for your projects. Contact us today for more information.