Different categories of carbon steel plates: low carbon steel, medium carbon steel, high carbon steel

What is Carbon Steel?

Carbon steel is primarily composed of iron and carbon, which are the two most abundant materials in steel. On its own, pure iron lacks significant strength and hardness, but the addition of carbon significantly enhances these properties, making steel a versatile and robust material.

Carbon Steel Plates

The crude iron used to create steel typically has a high carbon content, up to 2.1%, which is the maximum carbon content that a material can have and still be classified as steel. However, the carbon content can be reduced through processing, which alters several of the material's properties, including:

  • Strength: This is measured by the yield point (the stress at which a material begins to deform) and tensile strength (the maximum stress a material can withstand while being stretched or pulled before breaking).
  • Ductility: This refers to the material's ability to be stretched without breaking and is measured by elongation, the percentage increase in length before rupture.
  • Hardness: This is the resistance to wear and indentation, often measured using the Rockwell or Brinell hardness scales.

Carbon steel can be classified into three main categories based on its carbon content: low (or mild), medium, and high carbon steel. The table below details these categories:

Type of Carbon SteelCarbon Composition
Low Carbon/Mild0.05-0.25%
Medium Carbon0.26-0.60%
High Carbon0.61-1.50%

Low Carbon Steel

Low carbon steel, also known as mild steel, contains the lowest carbon content of the three categories. This makes it the most ductile and easily machinable form of carbon steel. To enhance specific properties without adding significant weight, chemical alloys can be incorporated into low carbon steel. For instance, manganese can increase hardness without adding extra weight. When these additional alloys are included, the material is typically referred to as high strength, low alloy (HSLA) steel.

Common low carbon steel grades stocked by suppliers like Steel Friend include ASTM A36, A572 Grades 42 and 50, and A830-1020. These grades are noted for their moderate strength, high ductility, and relatively lighter weight, making them suitable for structural applications such as building construction, bridges, and transmission towers.

Chemical Composition of Common Low Carbon Plate Grades

GradeCarbon MaxManganesePhosphorus MaxSulfur MaxSilicon
A36 (up to 3/4” thick)0.25%N/A0.030%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 420.21%1.35%0.030%0.030%0.15-0.40%
A572 Grade 500.23%1.35%0.030%0.030%0.15-0.40%
A830 Grade 10200.18-0.23%0.30-0.60%0.030%0.030%N/A

Mechanical Properties of Common Low Carbon Plate Grades

GradeYield PointTensile
A3636 KSI58-80 KSI
A572 Grade 4242 KSI60 KSI
A572 Grade 5050 KSI65 KSI

Medium Carbon Steel

Medium carbon steel strikes a balance between low and high carbon steel. It offers greater strength and hardness than low carbon steel while maintaining more ductility than high carbon steel. This type of steel typically contains additional alloys, such as manganese, which further enhance its properties.

Medium carbon steel can undergo heat treatments like quenching and tempering to improve its toughness and hardness without compromising machinability. The quenching process involves heating the steel to 1,500-1,650°F and then rapidly cooling it with water. The tempering process involves reheating the steel to 300-700°F and then air cooling it. These steps modify the steel’s crystal grain structure, enhancing its mechanical properties.

Common medium carbon steel grades include ASTM A516 Grade 70 and A830-1045, which are known for their balanced strength, hardness, ductility, and wear resistance. These properties make medium carbon steel ideal for applications requiring materials that withstand strong forces without breaking or wearing out, such as machine parts, pressure vessels, and automotive components.

Chemical Composition of Common Medium Carbon Plate Grades

GradeCarbonManganesePhosphorus MaxSulfur MaxSilicon
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%
A830-10450.43-0.50%0.60-0.90%0.030%0.030%N/A

Mechanical Properties of Common Medium Carbon Plate Grades

GradeYield PointTensile
A516 Grade 7038 KSI70-90 KSI

High Carbon Steel

High carbon steel has the highest carbon content among the three types, giving it the greatest strength and hardness. However, it is less ductile than lower carbon steels, making it harder to machine or form. Like medium carbon steel, high carbon steel can also be heat-treated to further enhance its hardness and wear resistance, making it suitable for applications where steel is subjected to high stress levels.

Very high carbon steel, a subset with even higher carbon content, offers extreme strength but is almost impossible to weld, machine, or shape. This type of steel is used in specialized applications requiring exceptional durability, such as cutting and chiseling tools.

Sourcing Carbon Steel Plate

When sourcing carbon steel plates, it is essential to consider suppliers who offer a comprehensive inventory of mild, medium, and high carbon steel grades from both domestic and international mills. A robust quality management system, such as ISO certification, ensures high-quality plates that meet specific requirements. Additionally, a wide geographic footprint near delivery locations can help control freight costs, providing better value for your project needs.

 


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