Carbon Steel: Properties, Production, Examples and Applications

Carbon steel is an iron-carbon alloy, which contains up to 2.1 wt.% carbon. For carbon steels, there is no minimum specified content of other alloying elements, however, they often contain manganese. The maximum manganese, silicon and copper content should be less than 1.65 wt.%, 0.6 wt.% and 0.6 wt.%, respectively.

Types of carbon steel and their properties

Carbon steel can be classified into three categories according to its carbon content: low-carbon steel (or mild-carbon steel), medium-carbon steel and high-carbon steel [1]. Their carbon content, microstructure and properties compare as follows:

 

Carbon content (wt.%)

Microstructure

Properties

Low-carbon steel

< 0.25

Ferrite, pearlite

Low hardness and cost.

High ductility, toughness, machinability and weldability

Medium-carbon steel

0.25 – 0.60

Martensite

Low hardenability, medium strength, ductility and toughness

High-carbon steel

0.60 – 1.40

Pearlite

High hardness, strength, low ductility

Low-carbon steel

Low-carbon steel is the most widely used form of carbon steel. These steels usually have a carbon content of less than 0.25 wt.%. They cannot be hardened by heat treatment (to form martensite) so this is usually achieved by cold work.

Carbon steels are usually relatively soft and have low strength. They do, however, have high ductility, making them excellent for machining, welding and low cost.

High-strength, low-alloy steels (HSLA) are also often classified as low-carbon steels, however, also contain other elements such as copper, nickel, vanadium and molybdenum. Combined, these comprise up to 10 wt.% of the steel content. High-strength, low-alloy steels, as the name suggests, have higher strengths, which is achieved by heat treatment. They also retain ductility, making them easily formable and machinable. HSLA are more resistant to corrosion than plain low-carbon steels.

Medium-carbon steel

Medium-carbon steel has a carbon content of 0.25 – 0.60 wt.% and a manganese content of 0.60 – 1.65 wt.%. The mechanical properties of this steel are improved via heat treatment involving autenitising followed by quenching and tempering, giving them a martensitic microstructure.

Heat treatment can only be performed on very thin sections, however, additional alloying elements, such as chromium, molybdenum and nickel, can be added to improve the steels ability to be heat treated and, thus, hardened.

Hardened medium-carbon steels have greater strength than low-carbon steels, however, this comes at the expense of ductility and toughness.

High-carbon steel

High-carbon steel has a carbon content of 0.60– 1.40 wt.% and a manganese content of 0.30 – 0.90 wt.%. It has the highest hardness and toughness of the carbon steels and the lowest ductility. High-carbon steels are very wear-resistant as a result of the fact that they are almost always hardened and tempered.

Tool steels and die steels are types of high-carbon steels, which contain additional alloying elements including chromium, vanadium, molybdenum and tungsten. The addition of these elements results in the very hard wear-resistant steel, which is a result of the formation of carbide compounds such as tungsten carbide (WC).

Production and processing

Carbon steel can be produced from recycled steel, virgin steel or a combination of both.

Virgin steel is made by combining iron ore, coke (produced by heating coal in the absence of air) and lime in a blast furnace at around 1650 °C. The molten iron extracted from the iron ore is enriched with carbon from the burning coke. The remaining impurities combine with the lime to form slag, which floats on top of the molten metal where it can be extracted.

The resulting molten steel contains roughly 4 wt.% carbon. This carbon content is then reduced to the desired amount in a process called decarburisation. This is achieved by passing oxygen through the melt, which oxidises the carbon in the steel, producing carbon monoxide and carbon dioxide.

Applications & Examples

Low-carbon steel

Low carbon steels are often used in automobile body components, structural shapes (I-beams, channel and angle iron), pipes, construction and bridge components, and food cans.

Medium-carbon steel

As a result of their high strength, resistance to wear and toughness, medium-carbon steels are often used for railway tracks, train wheels, crankshafts, and gears and machinery parts requiring this combination of properties.

High-carbon steel

Due to their high wear-resistance and hardness, high-carbon steels are used in cutting tools, springs high strength wire and dies.

Examples, properties, and applications of the various carbon steels are compared in the following table.

Type

AISI/ASTM name

Carbon content (wt.%)

Tensile strength (MPa)

Yield strength (MPa)

Ductility (% elongation in 50 mm)

Applications

Low

1010

0.10

325

180

28

Automobile panels, nails, wire

Low

1020

0.20

380

205

25

Pipes, structural steel, sheet steel

Low

A36

0.29

400

220

23

Structural

Low

A516 Grade 70

0.31

485

260

21

Low-temperature pressure vessels

Medium

1030

0.27 – 0.34

460

325

12

Machinery parts, gears, shifts, axles, bolts

Medium

1040

0.37 – 0.44

620

415

25

Crankshafts, couplings, cold headed parts.

High

1080

0.75 – 0.88

924

440

12

Music wire

High

1095

0.90 – 1.04

665

380

10

Springs, cutting tools

Gallery

Lightbox

Sources

  • W. D. Callister, Jr., Materials Science and Engineering: An Introduction 7th Edition. Wiley, 2007.
  • "Classification of Carbon and Low-Alloy Steels," Key to Metals AG. [Online]. [Accessed: Nov. 6, 2018].
  • "AISI Carbon Steel Mechanical Characteristics - Yield, Tensile, Hardness," Engineers Edge. [Online]. [Accessed: Nov. 6, 2018].
  • "Materials: Carbon Steel," Coburn-Myers Fastening Systems Incorporated. [Online]. [Accessed: Nov. 6, 2018].
  • B. Index, "Carbon Steel and Mild Steel: The Basics," Reliance Foundry. [Online]. [Accessed: Nov. 6, 2018].