Iron and carbon are the most abundant materials present in steel. Pure iron is not particularly strong or hard on its own, so it is the addition of carbon/other alloying elements that gives steel its increased strength. We will discuss carbon steel types in this article.
Carbon steel is a steel with carbon content from ranging 0.05 up to 2.1 percent by weight.
As the carbon content percentage rises, steel has the ability to become harder and stronger through heat treating; however, it becomes less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. In carbon steels, the higher carbon content lowers the melting point.
The crude iron used to produce steel has a relatively high amount of carbon. Its carbon composition can be as high as 2.1%, which is the greatest amount of carbon a material can contain and still be considered steel.
However, iron can be processed further to reduce carbon. This reduction of carbon content changes several material properties, including:
Strength: The load a material can bear, measured by yield point and tensile strength. Yield point is the point at which a material deforms, but does not break, and tensile strength is the amount of stress needed to actually break a material.
Ductility: The amount a material can be stretched without becoming brittle. Ductility is measured by elongation, which is the percent the length of a material increases before it breaks.
Hardness: The wear resistance of material and machinability of material. This is usually measured on the Rockwell hardness scale or Brinell Hardness scale.
As per carbon present in steel it is divided into three main categories of carbon steel: low (or mild), medium and high carbon steel. Each of these categories contain different levels of carbon, show in the chart below.
| Type of Carbon Steel | Carbon Composition |
| Low Carbon/Mild | 0.05-0.25% |
| Medium Carbon | 0.26-0.60% |
| High Carbon | 0.61-1.25% |
| Ultra Hight Carbon Steel | 1.25 – 2.1 % |
What Is Low Carbon Steel?
Low Carbon Steel also known as mild steel, low carbon steel has lower strength comparative to steel with higher carbon levels. Low carbon steel is the most ductile – or machinable – type of carbon steel as well.
Chemical alloys are also be added to low carbon steel to enhance desired mechanical properties . For example, if low carbon steel requires higher hardness for its application, manganese is generally added to increase hardness. Low carbon steel that contains additional alloys is typically referred to as high strength, low alloy (HSLA) steel.
Grades of Low Carbon Steel
Some of the most common low carbon steel plate grades, include ASTM A36, A572 Grades 42 & 50 and A830-1020. Each of these grades have moderate strength, high ductility and lighter weight due to the low carbon content and addition of other alloys.
Chemical Composition of Common Low Carbon Plate Grades
| Grade | Carbon Max | Manganese | Phosphorus Max | Sulfur Max | Silicon |
| A36 up to 3/4” thick | 0.25% | N/A | 0.030% | 0.030% | 0.40% Max |
| A36 >3/4 – 1 ½” thick | 0.25% | 0.80-1.20% | 0.030% | 0.030% | 0.40% Max |
| A36 > 1 ½ – 2 ½” thick | 0.26% | 0.80-1.20% | 0.030% | 0.030% | 0.15-0.40% |
| A36 >2 ½ – 4” thick | 0.27% | 0.85-1.20% | 0.030% | 0.030% | 0.15-0.40% |
| A36 >4” thick | 0.29% | 0.85-1.20% | 0.030% | 0.030% | 0.15-0.40% |
| A572 Grade 42 | 0.21% | 1.35% | 0.030% | 0.030% | 0.15-0.40% |
| A572 Grade 50 | 0.23% | 1.35% | 0.030% | 0.030% | 0.15-0.40% |
| A830 Grade 1020 | 0.18-0.23% | 0.30-0.60% | 0.030% | 0.030% | N/A |
Mechanical Properties of Common Low Carbon Plate Grades
| Grade | Yield Point | Tensile |
| A36 | 36 ksi | 58-80 ksi |
| A572 Grade 42 | 42 ksi | 60 ksi |
| A572 Grade 50 | 50 ksi | 65 ksi |
Common Low Carbon Steel Applications
Low carbon steel is a widely preferred material for various applications due to its affordability, versatility, and durability. These attributes make it particularly suitable for structural uses, such as in building construction, bridges, and transmission towers, where materials must endure high stress while being easy to shape into structural forms. Beyond structural applications, low carbon steel finds extensive use in industries like construction, automotive, heavy equipment, and oil and gas. In these sectors, it is commonly employed for mechanical tubes, fluid transport pipes, structural tubing, and other similar applications.
What Is Medium Carbon Steel?
Medium carbon steel contains a carbon percentage that falls between that of low and high carbon steel, striking a balance between strength, hardness, and ductility. It offers greater strength and hardness than low carbon steel while retaining more ductility than high carbon steel. Additionally, medium carbon steel often includes other alloying elements, such as manganese, which further enhance its properties.
When applications demand increased toughness and hardness, medium carbon steel plates can undergo heat treatments, such as quenching and tempering, to improve these characteristics without significantly affecting machinability.
Quenching and Tempering Process:
This two-step heat treatment process enhances the mechanical properties of steel:
- Quenching: Steel is heated to a temperature between 1,500 and 1,650 degrees Fahrenheit and rapidly cooled, usually with water or oil, to harden it.
- Tempering: The steel is reheated to a lower temperature, typically between 300 and 700 degrees Fahrenheit, and then air-cooled. This step refines the steel’s crystal grain structure, improving hardness and other mechanical properties while reducing brittleness.
Two common grades of medium carbon steel plate are ASTM A516 Grade 70 and A830-1045. Their moderate carbon content, combined with alloying elements, makes them ideal for a variety of demanding applications and other medium carbon grades – a balance of strength, hardness, ductility and wear resistance.
Chemical Composition of Common Medium Carbon Plate Grades
| Grades | Carbon | Manganese | Phosphorus Max | Sulfur Max | Silicon |
| A516 Grade 70 >2” & ≤4” thick | 0.30% | 0.85-1.20% | 0.025% | 0.025% | 0.15-0.40% |
| A516 Grade 70 >4” thick | 0.31% | 0.85-1.20% | 0.025% | 0.025% | 0.15-0.40% |
| A830-1045 | 0.43-0.50% | 0.60-0.90% | 0.030% | 0.030% | N/A |
Mechanical Properties of Common Medium Carbon Plate Grades
| Grades | Yield Point | Tensile |
| A516 Grade 70 | 38 ksi | 7-90 ksi |
Common Medium Carbon Steel Applications
Medium carbon steel is a highly versatile material used across a wide range of industries due to its strength and durability. It is particularly valued in applications where materials must endure significant stress without breaking or wearing out. Common uses include machine components such as gears, axles, and bolts, as well as pressure vessel tanks and automotive parts. In the construction and manufacturing sectors, medium carbon steel is frequently employed in the fabrication of buildings, bridges, and other heavy-duty equipment.
What is High Carbon Steel?
High carbon steel provides the highest strength and hardness among carbon steel types, surpassing mild and medium carbon steels. However, its reduced ductility makes it more challenging to machine or form compared to steels with lower carbon content.
Similar to medium carbon steel, high carbon steel can undergo heat treatment to further improve its hardness and wear resistance. This makes it particularly suitable for applications where the steel is subjected to extreme stress and heavy wear.
The elevated carbon content in high carbon steel plate grades imparts exceptional strength, hardness, and durability. These properties make it an ideal choice for demanding applications where the steel must consistently withstand intense wear and tear without breaking, such as in cutting tools, chisels, and other heavy-duty equipment.
Ultra-High Carbon Steel
Certain applications demand steel that can withstand forces exceeding the capabilities of high carbon steel. For such scenarios, ultra-high carbon steel, the strongest type of carbon steel, is utilized. Due to its exceptional strength, ultra-high carbon steel is extremely difficult to weld, machine, or shape, making it far less common compared to other carbon steel types. Its specialized nature limits its use to specific high-stress applications where unparalleled durability is essential.
Common High and Ultra-High Carbon Steel Applications
As mentioned earlier, high and ultra-high carbon steels are renowned for their outstanding strength, hardness, and wear resistance, making them ideal for applications requiring extreme durability and toughness. High carbon steel is commonly used in manufacturing cutting tools such as knives, drills, and saw blades, where its high carbon content enables the creation of sharp edges that retain their sharpness over time. In contrast, ultra-high carbon steel is often employed in applications demanding exceptional hardness, including truck springs, metal cutting tools, and other specialized purposes.
How to Choose the Right Carbon Steel Type
Selecting the appropriate type of carbon steel depends on the specific requirements of the project:
- For projects requiring flexibility and ease of machining, low carbon steel is ideal.
- For applications that demand a balance of strength and toughness, medium carbon steel works well.
- When hardness and wear resistance are critical, high carbon steel is the best choice.
- For specialized tools or extreme wear conditions, ultra-high carbon steel should be considered.
Common Applications of Carbon Steel
Carbon steel is ubiquitous in various industries due to its adaptability. Some of its key applications include:
- Construction: Reinforcement bars, structural beams, and building frames.
- Automotive: Vehicle frames, engine components, and suspension systems.
- Manufacturing: Tools, machinery parts, and molds.
- Household: Kitchen knives, cookware, and furniture.
- Infrastructure: Pipelines, bridges, and railway tracks.