Steel – Composition, Properties & Types

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Steel is the alloy of iron and carbon. Nowadays, steel has become the backbone of civil engineering construction. Steel may be plain carbon steel or alloy steel. Plain carbon steel is the steel at which carbon only forms an alloy with iron.

About 92% of steel is produced as plain carbon steel. Alloy steel is that at which iron is alloyed with other metals and carbon. About 85% of steel is found as alloy steel.
An increase in the carbon content increases the tensile and compressive strength of the steel. But the brittleness of the steel increases and thus the malleability and ductility decreases.

(A) Composition of steel

Steel is composed of iron, carbon and other alloying metals. Steel containing only iron and carbon is known as plain carbon steel.

Depending upon the carbon content, steel is classified as low carbon steel, medium carbon steel, and high carbon steel.

  • Steel containing less than 0.25% of carbon is known as low carbon steel.
  • If the carbon content lies between 0.25% to 0.7% then the steel is known as medium carbon steel.
  • In this, the carbon content is greater than 0.7% it is referred to as high carbon steel.

If the steel contains other metal also in the form of alloy with iron, steel is known as alloy steel. Stainless steel, nickel steel, vanadium steel, tungsten steel, etc. are examples of alloy steel.

(B) Properties of steel

Mechanical properties of the steel is determined by the carbon content in the steel. In general following are the properties of steel:

  1. Steel possesses high strength than other forms of iron such as cast iron or wrought iron.
  2. It can absorb shock and is elastic.
  3. It is malleable and ductile.
  4. Heat treatment can be done in the steel to improve the grain distribution and the quality of steel.
  5. Some alloy steel has corrosion resistance, but plain carbon steel is affected by corrosion.
  6. It can be welded to plain carbon steel easily.

(C) Types of steels

Depending upon various modes classification of steels can be of various types. However, the classification on the basis of their Chemical Composition is very commonly adopted.

Two major groups of steels recognized on this basis are-
A) Plain carbon steels and
B) Alloy steels

(I) Carbon steel or Plain Carbon Steel

This is the first major group of steels. In them, carbon is the only controlling component besides iron.

They are further sub-divided into three sub-types.

Low carbon steels(C= 0.05 to 0.35%) – These are also termed as soft steels or very commonly as mild steel (MS). The mild steels are tough and ductile. These can be easily welded. These steels are used for making nuts, bolts, rivets, sheets and other parts of common utility.

Medium carbon steels (C=0.35 to 0.55%) – These are also termed as medium steels. These are hard and strong comparatively. Are resistant to wear. They are used commonly for construction purposes as structural members and reinforcement.

High carbon steels(C=0.55 to 1.5%) – These are also termed as hard steels. It posses very great hardness and high compressive strength values. They are resistant to wear. They are used commonly for the manufacturing of ball bearing roller, saw, crushers, and locomotive tyres.

Functions of carbon in steels

Carbon in steel plays a very vital role in controlling their properties, Thus, the tensile strength of steel increases with an increase in the carbon content until its proportion reaches 0.83%. Any increase in carbon content beyond this limit will affect the tensile strength of steel adversely.
The ductility of steels decreases with an increase in carbon content. Also, the hardness of steels increases with the increase in carbon content. The compressive strength is also found to increase with the increase in carbon content.

Usual impurities in steel and their effects

  1. Manganese (0.2 to 1%- has positive influence like increases tensile strength and hardness of steel. But beyond this limit, it increases the brittleness of steel)
  2. Silicon (Kept below 0.4%, since this acts as a deoxidizer. Beyond this limit this increases the brittleness.)
  3. Phosphorus (Kept below 0.05%. This shows a cold-short effect if exceeds this limit.)\
  4. Sulphur (Kept below 0.05%. This shows a red short effect if exceeds this

(II) Alloys Steel (Steel Alloys)

Besides carbon other elements that impart distinctive characteristics to steel are add in iron to produce steel alloy. Formation of steel alloy is made to increase strength, hardness, toughness, resistance against corrosion, etc.

Important steel alloys and function of alloying elements are discussed below.

1. Stainless steel

Stainless Steel is the alloy steel containing more than 12 of the chromium alloyed with the iron Chromium forms a dense and tough layer of oxide around the metal surface and is highly resistive to corrosion.

It is basically sub-categorized into three groups

  1. The first group of stainless steel consists of less than 14% of Chromium and is known as Plan Chromium stainless steel. It contains sass than 0.4 % of carbon with traces of copper, tungsten, Nickel, etc, This Group of stainless steel can be hardened, Welded, machined and are satisfactory corrosion resistant, They are used to prepare steam valves turbine besides surgical instrument scissors knifes gears shafts ball bearing spring, etc.
  2. The second group of stainless steel consists of 14-20% of chromium and o.45 of carbon. They are more brittle and difficult to heat treatment. Also can be forget welded machined and rolled. They are used to prepare wire pipes. They are also used in chemical and food plants.
  3. When the steel contains at least 24 of combined Chromium and Nickel, then it goes under the third category of steel Generally 188 stainless steel –18chromium and nickel alloyed with Iron_ is very common in use. This steel has excellent resistance against corrosion and improved tensile strength. This steel is very tough. It can be welded, forged and rolled. Machining of this steel is difficult. These steels are used in chemical plants, tanks, cooking utensils, preparing pump shaft, screw, nut bolt, etc.

2. Nickel steel

Nickel steel contains about 3.5% of nickel alloyed with iron. It contains 0.5% to 1% of carbon. The presence of nickel imparts hardness, toughness, improves strength and corrosion resistance. This type of alloy steel is used in automatic parts, airplanes, cables, and shafts. Inver, widely used to prepare to measure tape is the alloy of nickel and steel. It consists of 30-40 % of nickel and has a very coefficient of thermal expansion.

3. Tungsten steel

Tungsten steel is known as high-speed steel as it is used in the high-speed cutting tools and drilling machines. It contains 14-20% of tungsten and 3-8% of chromium. It also contains carbon, vanadium, and molybdenum.

4. Vanadium steel

Vanadium steel possesses a high value of elasticity and is capable of resisting shocks. It is very strong and more ductile. Also, it improves fatigue resistance. It is used for making tools. It contains 0.1 to 2% of vanadium.

5. Manganese steel

Manganese steel contains different amounts of manganese ranging from 1-14% depending upon the purpose of the resulting product. Very hard, tough and strong in nature. It improves the plasticity of metal. It is used in preparing machine parts, rails, etc. manganese steel is non-magnetic.

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