Carbon Steel Properties: Types, Strength, Density, and Uses

Carbon steel is used in the construction of frames, industrial machinery, shafts, cutting tools, automotive parts, and so on. It's popular because it's a simple mix of strength, availability and cost. But not every carbon steel is the same. The ability of a grade to bend easily during forming might not be appropriate for a component that requires high hardness and wear resistance.

The difference is frequently related to the amount of carbon, heat treatment and the internal structure of the steel. Carbon can be added to enhance strength and hardness, but can also decrease ductility and increase the difficulty of welding.

Carbon steel material properties are important to understand before cutting, forming, machining or finishing a component, as this will help the manufacturer select the appropriate grade of carbon steel.

What is Carbon Steel?

Carbon steel is an alloy of iron, the primary alloying element being carbon, which has a significant effect on the mechanical properties of the material. It may also include manganese, silicon and trace amounts of other elements.

Typical carbon content of steel is from very low to 1% or more depending on the grade. The classification is quite general and applies to about 2.1% carbon by weight, but most ordinary carbon steels have much lower carbon content.

If engineers refer to the carbon wt%, they are talking about the weight of the carbon in the material. For instance, 0.20 wt% carbon in steel means that there are about 0.20 grams of carbon per 100 grams of steel.

That small percentage can make a considerable difference. Carbon affects the response of steel to heat treatment, the hardness, the formability and the mechanical performance of steel.

The properties of carbon steel are thus more than just the name. When choosing material for a manufacturing job, the grade and condition are as important.

What effect does carbon have on the properties of steel?

Carbon alters the internal structure of steel and its reaction to force. The higher the carbon content, the more likely that the steel will be harder and stronger, especially if heat-treated properly.

There's a catch, though. In general, the higher the carbon content, the less ductile the steel will be and the more challenging it will be to weld and form. Depending on the grade and heat treatment, toughness may also decrease.

That is why manufacturers don't just go with the highest carbon content steel for every application.

Ductility is important for a bracket that is to be bent during forming. A cutting tool must be hard and wear resistant. A shaft can be a combination of strength, toughness, machinability and fatigue resistance.

The correct carbon content will be determined by the function the component is required to perform during its service life.

Types of Carbon Steel

Carbon steel is generally classified as low carbon, medium carbon and high carbon. Sometimes ultra-high carbon steel is considered a separate group.

The ranges are approximate, and may differ slightly from standard to standard and supplier to supplier, and should therefore not be interpreted as absolute limits.

Low Carbon Steel

Low-carbon steel (also known as mild steel) is steel with a carbon content of up to about 0.25%, though some classifications go up to 0.30%.

It has a relatively low carbon content, which allows it to be bent, formed and welded. It is easier to machine than hardened high-carbon steel and is readily available in sheets, plates, bars and structural sections.

Typical uses are in the construction of frames, brackets, automotive panels, pipes and other machinery parts.

Low-carbon steels are not typically through hardened as much as higher-carbon steels with standard quenching. Carburizing is one of the processes that can be employed to increase the carbon content of the surface of a component when a hard surface is required.

Medium Carbon Steel

Medium-carbon steels are usually found to have 0.25% to 0.60% carbon content. It provides a good compromise of strength, hardness and toughness. Heat treatment is effective on many grades, and can be used to modify the properties of the metal for specific mechanical uses.

Medium carbon steel is therefore used for shafts, axles, gears, pins, and parts of machines that are subjected to repeated loads.

Generally stronger than low-carbon steel, but less ductile, and harder to weld. Final properties are dependent on the grade, section thickness and heat-treatment condition.

High Carbon Steel

The amount of carbon in high-carbon steel is typically between 0.60% and 1.0% carbon, with some classifications going beyond this.

It has a higher carbon content, and can be made quite hard and wear resistant by appropriate heat treatment. It is useful for springs, blades, cutting tools and components that require an edge to be retained.

The disadvantages are more apparent in the manufacturing process. High carbon steel may be more difficult to machine, weld and crack under poor heat treatment.

Ultra-High Carbon Steel

The term ultra-high-carbon steel is typically used to refer to steels with carbon levels above about 1.0% to 1.25% depending on the classification.

They can be processed to very high hardness, but the process must be carefully controlled. They are used in certain applications where hardness and wear resistance are more significant than ease of forming or welding.

Highest carbon steel is not a specific material, but rather a general term used to describe a range of steels. The maximum carbon content varies with the specific classification, composition and use.

High Carbon Steel vs. Low Carbon Steel

The distinction between low-carbon steel and high-carbon steel isn't just about which is more powerful. Their behavior can be very different in the manufacturing and service.

High carbon steel vs. low carbon steel, take into account the manufacturing process as well as the end use. For parts that require a lot of forming or welding, low-carbon steel is usually the most suitable material. High-carbon steel is better if hardness and wear resistance are the primary considerations.

These are not necessarily the more powerful of the two in every circumstance. The final mechanical properties can vary widely depending on the heat treatment, microstructure and the grade. 

Important Carbon Steel Material Properties

The following are key properties of carbon steel.The following are some of the important properties of carbon steel.

There are a number of physical and mechanical properties that engineers use to decide if carbon steel is appropriate for a specific component.

Density of Carbon Steel

The density of carbon steel is approximately 7.85 g/cm³ or 7,850 kg/m³. A convenient approximate value for estimating the weight of plates, bars, shafts and structural parts.

The density of carbon steel is slightly different depending on its composition and grade, but the density is not significantly affected by the carbon content, as compared to the hardness or strength.

A typical value for the density of carbon steel is 7.85 g/cm³ for preliminary calculations. When weight is a critical factor, the exact number should be verified.

The Yield Strength and Yield Stress are the same.

The stress at which a material starts to undergo permanent deformation is called its yield strength. Below this point, the material will normally regain its original shape when the load is removed, as long as it is not beyond its elastic limits.

The yield stress of carbon steel can vary significantly from one grade to another and from one condition to another. The specified yield strength of a low-carbon structural steel can be approximately 250 MPa, and the yield strength can be increased for cold-worked or heat-treated grades.

Yield strength is important for engineers to consider when designing components that are required to carry a load, as when the yield strength is exceeded, the part will be permanently bent or distorted.

Always use the material specification or certified test data to determine the yield strength of a specific steel, not the carbon content.

Young's Modulus and Carbon Steel Modulus.

Young's modulus (also known as the modulus of elasticity) is the resistance of a material to elastic deformation when it is loaded.

The modulus of carbon steel is approximately 200-210 GPa. This value is fairly consistent for many low, medium and high carbon grades.

The increase in carbon content can significantly influence the hardness and strength, but not to the same extent the Young's modulus.

This is important for engineers. If the geometry and elastic modulus are similar, the stronger the steel, the less it will stretch under the same load.

The melting point of carbon steel is 1370℃.

There is no single melting point for carbon steel. It is composition and other metallurgical dependent.

The melting point of many common carbon steels is approximately 1425°C to 1540°C. Melting does not occur at a single exact temperature as the material does not become completely liquid at one specific temperature.

This is a property that is significant in steelmaking, casting, welding and other high-temperature processes. A steel part can, however, lose strength long before it melts.

Thermal Conductivity of Carbon Steel

Thermal conductivity of carbon steel is the ability to conduct heat.

The representative value of common carbon steels is about 45-60 W/(m·K) at room temperature, but it is different for different grades, temperatures and microstructures.

This property is important in welding, heat treatment and machining. The rate of heat transfer through all grades of steel is not the same, and the manufacturing parameters may require adjustment to avoid overheating or uneven cooling.

Other Important Properties

Carbon steel is magnetic due to its iron content. It also has good strength for its price and can be produced in a variety of shapes.

It is not very corrosion resistant. Ordinary carbon steel is not as rich in chromium as stainless steel, and does not contain enough to create a protective layer of stainless steel. Rust can therefore be caused by exposure to moisture and oxygen.

Depending on the environment, painting, galvanizing, plating, oiling or other protective treatments may be necessary.

Is Carbon Ductile?

The question is a little confusing: "Is carbon ductile?".

Pure carbon is not carbon steel. Carbon is an element that can be found in various forms and ductility is the ability of a material to be plastically deformed before it breaks.

The carbon affects the ductility of the iron-based alloy in steel. Low-carbon steels are typically more ductile and will deform more easily without breaking. Generally, ductility decreases with increasing carbon content and hardness, but the behavior will vary depending on the heat treatment and microstructure.

That is why low-carbon steel is used for bending and sheet metal forming and high-carbon steel is used for hardness and wear resistance.

What is the meaning of High Carbon Metals?

High carbon metals is sometimes used to refer to materials with a high carbon content, but is not an exact description of all metals.

In manufacturing, high-carbon steel is the more correct designation for alloys of iron with relatively high carbon content. The materials are important for their potential hardness and wear resistance.

High-carbon steel grades are employed in cutting edges, industrial blades, springs and some tooling. They are appropriate for the hardness, toughness and repeated loading requirements.

The carbon content is not the only factor that decides the performance of a component. Other factors such as heat treatment, alloying elements, manufacturing quality and operating conditions are also significant.

Applications of Carbon Steel

The uses of Carbon Steel are as follows:The uses of Carbon Steel are given below:

Carbon steel is utilized in construction, automotive manufacturing, agriculture, energy and general engineering.

Low-carbon grades are generally used in structural beams, sheet-metal enclosures, brackets, and welded assemblies. Medium carbon grades are employed in shafts, axles, gears and parts that need a combination of strength and toughness.

High-carbon grades are used for springs, blades, cutting tools and parts that are subjected to repeated wear.

It is also commonly used in the field of CNC machining. Carbon steel can be turned, milled, drilled, and ground by manufacturers to create parts with special dimensions and finishes.

The best grade is determined by the function of the part, how it is produced, the operating environment, and the required service life.

Conclusion

Carbon steel is still a viable material for production due to the ability to tailor its properties to a variety of applications. The strength, hardness, ductility and heat-treatment response are influenced by the carbon content, and the result is influenced by the grade and processing.

Low-carbon steel is typically used for forming and welding, medium-carbon steel is used for a combination of mechanical properties, and high-carbon steel is used when hardness and wear resistance are important.

Knowing the properties of carbon steel before choosing the material can help the manufacturer to avoid unnecessary processing costs, and select the carbon steel grade that can perform well in use.

Frequently Asked Questions 

1. What is the density of carbon steel?

The density of carbon steel is 7.85 g/cm³ or 7,850 kg/m³. The exact value is slightly different for each grade.

2. What is the value of Young's modulus of carbon steel?

The typical range of Young's modulus for common grades of carbon steel is approximately 200–210 GPa.

3. What is the melting point of carbon steel?

The melting range of many carbon steels is approximately 1425°C to 1540°C, depending on the composition of the steel.

4. Does high carbon steel have greater strength than low carbon steel?

High carbon steel can be made harder and stronger, especially by heat treatment. Low carbon steels tend to be more ductile and weldable.

5. How many percent of carbon does carbon steel contain?

The carbon content is dependent upon the grade. The carbon content of low-carbon steel is generally up to 0.25%, medium-carbon steel is generally 0.25%–0.60%, and high-carbon steel is generally 0.60%–1.0%.