Carbon steels are steels in which the main alloying additive is carbon. Alloy steels are alloyed with other metals or materials, in addition to carbon, to improve properties.

Carbon steels are classified based on the amount of carbon content in the steel. The four main classes of carbon steels are mild and low carbon steel, medium carbon steel, high carbon steel, and ultra-high carbon steel.

Mild and low carbon steels contain 0.16–0.29% carbon. They are the most common form of steel as they come at a relatively low cost and provide material properties that are acceptable for many applications. They are neither brittle nor ductile, but are malleable. The surface hardness can be increased through carburizing.

Medium carbon steels contain approximately 0.30–0.59% carbon. They balance ductility and strength and have good wear resistance. They are used in forging and for large industrial and automotive components.

High carbon steels contain approximately 0.6–0.99% carbon. They are very strong and are used for springs and high-strength wires.

Ultra-high carbon steels contain approximately 1–2% carbon. These steels can be tempered to great hardness and are used for specialized products such as knives, axles, or punches. Steels above a carbon content of 1.2% are generally formed through powder metallurgy. Steels with carbon content above 2% are considered to be cast iron.

Alloy steels contain varying amounts of different metals and materials to specialize their properties. Some of the most common materials added include chromium, molybdenum, nickel, and silicon.

Chromium is added in smaller amounts (0.5-2%) to increase hardenability and larger amounts (4-18%) to increase corrosion resistance.

Molybdenum is added in amounts of 0.25-0.40% to increase the toughness of the steel.

Nickel is added in smaller amounts (2-5%) to increase toughness and in larger amounts (12-20%) to increase corrosion resistance.

Silicon is added to steel in smaller amounts (0.2-0.7%) to increase strength and in larger amounts (>2%) to improve its magnetic properties.

Specifications

Selecting metal alloys requires an analysis of the desired dimensions and specifications. Dimensions to consider include outer diameter (OD), inner diameter (ID), overall length, and overall thickness.

Other specifications of importance (based on application) include:

• Product shape
• Tensile strength
• Yield strength
• Melting point
• Conductivity
• Corrosion resistance
• Ductility
• Malleability

These properties differ based on the forming method and alloy composition.

Applications

Carbon steels are the base metals widely used in manufacturing today around the world in nearly every industry, including aerospace, aircraft, automotive, chemical, and defense. Alloy steels’ varying properties can be applied in numerous areas including in construction for gears, pipes, supports, and other components of infrastructure.

Standards

AISC 360 - Specification for structural steel buildings.

AIST PB-334 - Making, shaping & treating of steel.

ASTM F541 - Standard specification for alloy steel eyebolts.

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Photo by Jatinsanghvi / CC BY-SA 3.0 | Photo by Bill Abbott / CC BY-SA 2.0