Carbon Steel: Types, Properties, and Applications

Steel is ubiquitous in the modern world, from cars to mobile phones to stadiums and buildings. To differentiate from stainless steels, the term carbon steel refers to a category that encompasses everything from low-strength commercial grades to high performance specialist grades used in aerospace and nuclear applications.

This article will give a run-down of all the types, properties and applications there are of carbon steels.

Types of Carbon Steel

Although there are other types and categorizations, these are the most commonly used types of carbon steel:

• Low carbon steel
• Medium Carbon Steel
• High carbon steel
• Alloy Steel

Low Carbon Steel

Steels with a carbon content between 0.05% and 0.29% are considered low carbon steels. These are typically used in low-stress applications but can also be used on structural steelwork.

Low carbon steels don’t have sufficient carbon content to be hardened, although processes exist that can improve the hardness of the outer surface of a part, namely, case hardening.

Mechanical Properties of Low Carbon Steel

They have excellent weldability and formability, making them an economical choice for general fabrication, bending and welding. Their composition makes them very ductile, meaning complex bending is possible without cracking.

Low carbon steel has a modest tensile strength, lower than that of some of the higher carbon grades, but its higher ductility can mean it is still used in load-bearing applications, albeit with higher safety factors in place.

Low carbon steel can resist wear, but is not as well suited as a higher carbon variant.

Common uses of Low Carbon Steel

Thanks to its ductility, low carbon steel can be formed in a multitude of shapes, meaning it can have many uses:

• Framework and enclosures of appliances
• Bracketry
• Sheet metal panels, including vehicle bodies
• Steel cladding

Medium Carbon Steel

Steels with carbon content ranging from 0.3% to 0.6% are considered medium carbon. These grades typically have higher tensile strength than low carbon steels, but do sometimes need more consideration to utilise in certain manufacturing techniques.

Mechanical Properties of Medium Carbon Steel

The added carbon levels increase both the tensile strength and hardness of the material. In turn, ductility is reduced slightly. This can mean that cracking may be an issue with particularly complex or tight bends.

Medium carbon steel is generally weldable, but certain types and thicknesses may need extra precautions such as pre-heating and post-weld heat treatments. These steps help to avoid cracking and reduce internal stresses in the final part, respectively.

Common uses of Medium Carbon Steel

Thanks to its higher difficulty in welding and forming, medium carbon steels are not as commonly used in fabricated parts, but most medium carbon steels machine very well, meaning they are well suited to parts such as:

• Gears, shafts and couplers
• Rail wheels
• High wear mechanical parts
• Forged parts

High Carbon Steel

Steels with more than 0.6% carbon are considered high carbon steels. These have extremely high hardness and compressive strength, but often lack ductility.

High carbon steels are good for components or tools that need to hold an edge, such as knives and machine tools. The high levels of cementite in their microstructure mean they are extremely hard wearing and resistant to abrasion.

Mechanical Properties of High Carbon Steel

Generally, the higher the carbon content of a steel grade, the less ductile it is. High carbon steels are very poor at resisting tensile loads without cracking or breaking, but in some cases can still have a high yield strength.

This lower ductility and high hardness make for a difficult material to form or weld. However, high carbon steels are often processed in their annealed state, and then heat treated for hardness afterwards, something that the high carbon content enables.

Common uses of High Carbon Steel

Being capable of hardening via heat treatment makes high carbon steels quite versatile:

• Knives and scraper blades
• Machine tools such as drills and taps
• Wear resistant components
• Machined pins and couplers

Alloy Carbon Steel

Alongside the three types listed above, numerous other high performance steels can be grouped into alloy steels.

Whilst this term can be slightly confusing, as any metal that is not pure is technically an alloy, the term alloy steels typically refers to steels with high proportions of other alloying elements, such as cobalt, chromium, molybdenum, vanadium and nickel.

These high performance alloys are used where specific properties are required, such as extremely high temperature resistance, high impact resistance or improved surface hardness.

Mechanical properties of Alloy Steel

The mechanical properties of alloy steels can range widely from one grade to another. For instance:

• Grade 4340 – Nickel-Chromium-Molybdenum Steel has very high fatigue resistance, along with high strength and toughness
• Grade 4140 – Chromium Molybdenum Steel also has high hardness and toughness, but it also has a wide range of heat treating advantages

Common uses of Alloy Steel

Because there exist many different types of alloy steel, you can find them in all sorts of industrial applications, including:

• Pressure vessels
• Nuclear components
• Tools
• High strength supporting or lifting components

Choosing the Right Carbon Steel for Your Project

Which of the four types of steel you need depends entirely on your application, the tools and techniques you have on hand, and, of course your budget.

• Cost: If cost is the main driving force, low carbon steels are generally the most cost effective. If possible increasing thicknesses can mitigate the lower tensile strength, albeit at a weight increase too. Low carbon steel is also very easy to work with, reducing the need for heat treating and special processes.
• Strength: If extremely high strength is essential, one of the numerous high performance alloy steels might be best, offering higher strength, hardness and still retaining manufacturability.
• Hardness: If, for example you are forging a bespoke chef’s knife, a high carbon steel that is hardenable and that will hold an edge would be perfect.