Steel | Types, Properties, Limitations, Alloying Elements

What is Steel?

Steel is an alloy of Iron, Carbon and some other elements. These elements are Tungsten, Molybdenum, Chromium, Nickle, manganese, Silicon etc.

Steel is by far the most important engineering material. It has high tensile strength in a low cost, which makes it a suitable choice for the construction of tools, bridges, ships, buildings etc.

Types of Steels

1. Plain Carbon Steel

Plain Carbon Steel is a type of steel which only contains iron and Carbon.

Based on the percentage of carbon in the plain Carbon Steel, it can be further divided into three sub-types.

1. Low Carbon Steel
2. Medium Carbon Steel
3. High Carbon Steel

Low Carbon Steel

Low Carbon steel is also known as Mild Steel. It contains less than 0.3% Carbon.

Properties of Low Carbon Steel

• It is soft
• It is highly machinable
• It is ductile
• It has very high weldability
• Due to low carbon content, it is unresponsive to heat treatment.

Medium Carbon Steel

Medium Carbon Steel is also known as machinery Steel. It contains carbon in the range of 0.3% to 0.5%.

Properties of Medium Carbon Steel

• It has good machinability
• It can be hardened by heat treatment
• It is stronger as compared to Low Carbon Steel
• It is tougher as compared to Low Carbon Steel

High carbon Steel

High Carbon steel is also known as hard Steel or Tool steel. It contains more than 0.5% Carbon.

Properties of High Carbon Steel

• It can be easily hardened by heat treatment
• It has less ductility as compared to Low and Medium Carbon steel
• After heat treatment it gets very high strength and hardness
• It is difficult to weld
• Excessive hardness generally makes it brittle

Limitations of Plain Carbon Steel

• It has low tensile strength as compared to alloy steel (discussed later)
• It is not deep hardenable
• It has low corrosion resistance
• It has poor impact resistance at low temperatures

2. Free Cutting Steel

Free Cutting Steels are generally Carbon Steel or Carbon-Manganese steel, with some amount of Sulphur. Sulphur is added to improve its machinability.

Properties of Free Cutting Steel

• It has very good machinability
• It has less ductility as compared to Plain Carbon steel
• It has less toughness as compared to Plain Carbon steel
• It has poor weldability

3. Alloy Steel

Alloy steels are Plain Carbon Steels added with one or more alloying elements. These alloying elements can be Tungsten, Molybdenum, Chromium, Nickle, manganese, Silicon etc. These alloying elements in sufficient quantities impart desired properties, such as wear resistance, corrosion resistance, magnetic and electric properties etc.

Types of Alloy Steels

1. Low Alloy Steel

2. High Alloy Steel

Low Alloy Steels contain 1% to 4% alloying elements.

High Alloy Steels contain more than 4% alloying elements. Stainless Steels and Heat Resistant Steels are the examples of it.

Properties of Alloy Steel

• It has high strength
• It has high hardness
• It has high toughness
• It is deep hardenable
• It has high hardenability on heat treatment
• It retains strength at elevated temperatures
• It offers high resistance to oxidation and corrosion as compared to Plain Carbon Steel
• It retains hardness at elevated temperatures

Effect of different alloying elements on steel

• Cobalt: Hardness
• Silicon: High elastic limit
• Manganese: High strength in hot rolled or heat-treated condition
• Vanadium: Tensile strength
• Tungsten: hardness at elevated temperatures
• Chromium: Hardness and strength
• Nickel: Strength and toughness
• Molybdenum: Extra tensile strength

Featured Image Source: By Alicia Nijdam – https://www.flickr.com/photos/anijdam/2491713704/in/photostream/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=21954172