What Are the Differences Between Cast Iron and Steel?

Cast iron and steel are both ferrous metals comprised primarily of iron atoms (labeled as Fe on the periodic table). Elemental iron is abundantly found on Earth, but it typically exists in an oxidized form and requires intensive processing, called smelting, to extract.

Pure iron, with its high magnetic properties, is often employed in fabricating the cores of electromagnetic equipment. However, its soft texture and tendency to deform limit its direct use in industrial manufacturing. Therefore, it's typically alloyed with other elements, notably carbon, to enhance its properties. Cast iron and steel, for instance, are widely utilized metal materials derived from this improved iron alloy.

While cast iron and steel have a similar appearance, each possesses distinct properties which are crucial for specific applications. This article provides an in-depth comparison of these two metals to help you choose the right metal for your project.

What Is Cast Iron?

Cast iron is a type of iron-carbon alloy, with relatively high carbon content between 2% and 4%. It's made by first smelting iron ore in a blast furnace to produce pig iron. Then, the pig iron is melted along with quantities of scrap iron, and the molten metal is poured into molds to solidify. The name "cast iron" comes from this casting process used to shape it.

In addition to its carbon content, typically contains 1% to 3% silicon, along with trace amounts of manganese, phosphorus, and sulfur. Carbon and silicon are key elements that influence the structure and properties of cast iron. The form and distribution of graphite, which originate from the carbon content, are crucial in determining the type and characteristics of cast iron. Based on the form of carbon and the microstructure, cast iron can be further classified into:

• Gray Cast Iron: Graphite appears in flake form, providing good castability and vibration damping properties, but it is quite brittle.
• White Cast Iron: Carbon is present as iron carbide (Fe3C), making it hard and brittle with high wear resistance.
• Ductile (Nodular) Cast Iron: Graphite is in the form of spherical nodules, giving it high strength and toughness.
• Malleable Cast Iron: Through annealing, carbon is present as clumpy graphite, which provides good toughness and machinability.

What Is Steel?

Steel is an alloy of iron, with a carbon content that varies but typically includes less than 2%. Steel is made by refining the molten pig iron in a basic oxygen furnace (BOF) or electric arc furnace (EAF) to reduce carbon content and remove impurities, and then pouring it into molds and cooling to form solid steel.

Additional elements are added and the carbon content is adjusted during the process to produce different grades of steel with specific properties. For example, steel is generally prone to corrosion and rust, but stainless steel becomes an exception, exhibiting remarkable resistance to corrosion and rust because of its low carbon content and a minimum of 10.5% chromium.

According to the difference of compositions and uses, steel can be divided into many types. The following are some common types of steel:

• Carbon steel: A type of steel containing carbon as the main alloying element, which can be further divided into low carbon steel, medium carbon steel and high carbon steel.
• Alloy steel: Besides iron and carbon, it also contains other alloying elements, such as chromium, nickel, molybdenum, vanadium. Common alloy steels are stainless steel, tool steel, etc.

The Differences between Cast Iron and Steel

To differentiate the two metals, let's start by examining the table as below to make a preliminary comparison.

Above table provides a general comparison between cast iron and steel. You may pay attention to the major items listed at the first column. Now, we’ll take you to obtain an in-depth understanding of them one by one.

Carbon Content & Chemical Composition

The main difference between the two metals is their carbon content. Cast iron typically contains more than 2% carbon, while steel contains less than 2% carbon. Typically, higher carbon content results in greater hardness and brittleness.

Additionally, the silicon content in cast iron significantly influences its properties, providing better fluidity and reduced shrinkage during casting. In contrast, steel often contains other alloying elements such as chromium, nickel, and molybdenum, which enhance its strength, toughness, and corrosion resistance.

Melting Point

The melting point of cast iron typically ranges from 1150°C to 1300°C (2100°F to 2370°F), while the melting point of steel is usually between 1370°C and 1510°C (2500°F to 2800°F).

The lower melting point of cast iron enables it to have better fluidity in a molten state, making it easier to fill complex mold shapes and form dense castings. Moreover, its lower melting point leads to a relatively slower cooling rate, which grants more time for the liquid metal within the casting to fill any voids resulting from shrinkage. Consequently, this minimizes the formation of pores and cracks during the casting process, resulting in a casting with increased density. Conversely, steel's higher melting point enables it to maintain strength and toughness during mechanical processing and heat treatment at high temperatures.

Castability

As mentioned above, cast iron has a lower melting point and contains a certain silicon; it exhibits excellent fluidity and less shrinkage during cooling—as graphite precipitates during solidification, partially compensating for volume contraction. Therefore, compared to steel, cast iron possesses superior castability.

In contrast, steel is less fluid, more reactive to mold materials, shrinks more during cooling, and requires more attention and inspection throughout the entire casting process.

Strength

Both cast iron and steel are highly regarded for their durability, yet they differ significantly in their various strength aspects. Let's explore the differences of their tensile, compressive, and impact strengths.

Tensile Strength: Cast iron possesses a relatively low tensile strength. This is due to the graphite flakes or nodular graphite present in its structure, which tend to become stress concentration points, making it susceptible to fracture under tensile forces. Conversely, steel's microstructure is more uniform, and its tensile strength can be greatly enhanced through alloying and heat treatment.

Compressive Strength: Generally speaking, cast iron possesses superior compressive strength compared to steel. Under compressive stress, the graphite structure in cast iron does not contribute to stress concentration, but instead aids in dispersing the stress, thus producing a high compressive strength. Conversely, steel exhibits comparable performance in both compressive and tensile strengths, with a wider range of compressive strengths.

Impact Strength: Both of them exhibit good impact strength, but steel typically outperforms cast iron, especially in its capacity to withstand sudden shocks and dynamic loading. This is attributed to steel's ductility and uniform microstructure, enabling it to absorb a significant amount of energy without fracturing under impact loads. Furthermore, steel undergoes work hardening during impact loading, which further enhances its strength. On the other hand, cast iron’ brittleness makes it prone to fracture under impact. However, ductile iron, a variant of cast iron, boasts a higher impact strength that significantly surpasses gray iron. Even so, ductile iron cannot match the impact strength of steel.

Wear Resistance

Cast iron generally shows better resistance to mechanical wear than steel (carbon steel). This is attributed to its hardness and unique graphite structure, which acts as a natural lubricant. However, cast iron's brittleness limits its application in dynamic wear environments. As a result, it is best suited for manufacturing static and low-impact wear-resistant components, such as mechanical bed frames, sliding bearings, and friction linings.

Steel, with its high toughness, maintains good wear resistance in dynamic wear environments and resists fracturing. It is suitable for manufacturing dynamic and high-impact wear-resistant components, such as gears, cutting tools, and high-wear parts.

Corrosion Resistance

Both metals remain susceptible to corrosion or rusting when exposed to oxygen and moisture. However, iron develops a patina that helps prevent deep corrosion, preserving the metal’s integrity.

To prevent corrosion, one approach is by applying paint or powder coating for added protection. Alternatively, alloying elements such as chromium, nickel, and molybdenum can be incorporated into steel to enhance the corrosion resistance of steel and adapt it to different corrosive environments.

Machinability

Cast iron possesses good machinability. The presence of graphite serves as a natural lubricant, reducing tool wear and extending tool life. Additionally, the brittleness of cast iron (especially gray cast iron) allows the chips to fracture easily into small pieces during machining. This prevents them from adhering to the cutting tool, resulting in a smoother machining process.

In contrast, steel's machinability varies greatly depending on its composition. Hardened steel or high-carbon steel pose challenges as they are more resistant to machining and can accelerate tool wear. On the other hand, low-carbon steel, while softer, can become adhesive during machining, making it challenging to handle. Therefore, careful consideration of machining techniques and tooling selection is imperative for successful steel machining.

Weldability

Steel exhibits significantly better weldability compared to cast iron. Due to its high carbon content and brittleness, cast iron is prone to cracking during welding. Moreover, molten cast iron readily absorbs nitrogen and oxygen from the air, leading to the formation of pores.

The low carbon content of steel makes it easy to control during the welding process and is suitable for a variety of welding methods, including arc welding (SMAW), gas shielded welding (GMAW, TIG), laser welding, etc.

Cost

Cast iron tends to be cheaper than steel primarily because its raw materials, like iron ore, are more readily available and cheaper compared to the higher-grade iron and alloys used in steel production. Additionally, the production process for cast iron is simpler and less energy-intensive than that for steel.

Cast Iron vs. Steel: Which One to Choose?

Based on the detailed introduction and comparison above, you should now have a comprehensive understanding of the differences between these two metals. To help you choose the best metal for your needs, you can quickly reference the table below to see their unique advantages, limitations, and primary applications.

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