Plain Carbon (Unalloyed) Steels

Plain Carbon (Unalloyed) Steels

Plain carbon steels are one of the most basic types of steel. These steels consist only of carbon and iron (Fe) and contain no other alloying elements. For this reason they are also called unalloyed steels. Plain carbon steels are not among the high-strength steels, but thanks to their durability and flexibility they can be used in many applications such as structures, transport vehicles, machine parts, etc.

Plain carbon steels are inexpensive and easy to form. Their mechanical properties vary depending on the carbon content in their structure. Today, close to nine out of ten steels produced by the iron and steel industry are plain carbon steels. Their hardenability is low, and cracking and warping occur in the part after hardening processes. Thick-section parts cannot be hardened to the desired level. They are not resistant to corrosive environments, but flame and induction surface hardening can be applied.

Classification of Plain Carbon Steels

Low-Carbon Steels

Low-carbon steels have a carbon content between 0.05% and 0.3%. They are also known as mild steels.

These steels have superior properties in terms of flexibility, castability, and high boiling point. Low-carbon steels are commonly used in applications such as pipes, sections, sheet metal, sheet plates, sheet metal parts, and section bars. Low-carbon steels also require very little or no hardening. For this reason, low-carbon steels are used very widely.

Low-carbon steels cannot be hardened. It is possible to harden their surfaces using suitable methods (carburizing, nitriding, etc.).

When a steel is weak in carbon content, the alloy is called “wrought iron.” The proportion of alloying substances used in producing a steel alloy allows the resulting steel to be more weldable and more resistant to rusting.

The table above gives the carbon, silicon, manganese, phosphorus, and sulfur ratios for the steels designated by MKE as C1020, C1030, C1040, C1050, and C1060.

Medium-Carbon Steels

Medium-carbon steels have a carbon content between 0.3% and 0.6%. They respond quite well to heat treatment. Their strength is better than that of low-carbon steels.

These steels are generally used for parts requiring high hardness, durability, and strength. For example, parts such as screws, balls, and gears can be made from medium-carbon steel. These steels can also be hardened before machining or used in foundations.

Their weldability is also lower than that of low-carbon steels. Special care must be taken when welding medium-carbon steels.

High-Carbon Steels

High-carbon steels have a carbon content of 0.6% or higher. These steels are used for parts requiring high hardness, durability, and strength. For example, parts such as blades, cutting tools, hammers, and locks can be made from high-carbon steel. High-carbon steels can also be hardened before machining, or are hardened before machining. However, although high-carbon steels have higher strength, they have less flexibility and castability, and therefore are not used in place of the less costly low-carbon steels.

High-carbon steels have low ductility (bending capability). They are difficult to cut and machine. Machinability can be improved through softening (annealing).

Their tensile strength is higher than other steels, and while their hardenability is very good, their weldability is very poor.

Steel alloys with a high carbon content used in modern industry are classified as “cast iron” because they have a lower melting point. Since alloys rich in carbon in this way also have a lower melting temperature, melting and casting can be carried out with less energy, which lowers production cost while increasing production speed.

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