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Steel

properties of alloys

What is steel?

Steel is essentially a refined alloy of iron that contains a lower percentage of carbon to improve the mechanical strength of the metal. It is used in different types of manufacturing and construction industries of the world. Steel is making by refining cast iron followed by the addition of alloying chemical elements like carbon, silicon, manganese, cobalt, nickel, chromium, etc. It is the most widely used building material in the world infrastructure and industries, uses for making everything from sewing needles to large useable tools.

Steel, alloy of iron that contains carbon and transition metal to improve strength used in different types of construction

Types, Composition and Uses of Steel

Steel is the principal material or alloy of iron widely used in different types of structural units. Mild steel is used for making most of the machine parts, tools, and utensils. The mechanical and corrosion properties of steel are modified by alloying with silicon and different transition metal like nickel, manganese, chromium, titanium, vanadium, and tungsten, etc.

Types of Steel Composition Uses
Silicon Steel 3.5 percent silicon and very little carbon Used to making electromagnets and transformer.
Stainless Steel 10 to 12 percent Cr Increase resistance, hardness, and corrosion properties, used for making high temperature and high-pressure equipment.
Manganese Steel 12 to 13 percent of Manganese Extremely hard and resistant to wear, used in rails, conveyer, chains, rocks crushers, or grinding machinery and safe.
Nickel Steel 2 to 4 percent of nickel High ductility and tensile strength, used for making cables, propeller shafts, gears, etc.
Cobalt Steel up to 35 percent of cobalt Highly magnetic, used for making permanent magnets.
Chrome-nickel Steel 1 to 4 percent nickel and 0.5 to 2 percent chromium High tensile strength and greater corrosion resistance, used for making motor car and bicycle parts.
Tungsten Steel 10 to 20 percent tungsten and 3 to 8 percent chromium Very hard, corrosion resistance and retain temper at high temperature, used in making high-speed cutting tools.
Molybdenum steel 6 to 7 percent molybdenum Retain temper at high temperature and used for making high-speed cutting tools.

Mechanical Properties of Steel

The iron-carbon and iron silicon phase equilibrium play a vital role in the mechanical and technical strength of alloys of iron or steel. The varying properties and uses of steel by carbon are mainly associated with the formation or distribution of hard carbide (Fe3C) compounds or cementite.

Production Process

The steel production process consists of some general steps. First, we remove the carbon, sulfur, and phosphorus from molten pig iron by oxidation usually by oxygen. In this process, carbon and sulfur oxidize to form carbon dioxide and sulfur dioxide and escape. The impurities like manganese, silicon, and phosphorus also form oxides which combine with lime to form slag.

Next, separate the slag followed by the addition of requisite quantities of deoxidizer and other alloying elements. A reducing agent like petroleum coke together with ferrosilicon and ferromanganese is used as the deoxidizer, they reduce any left FeO. The technique of steel production has undergone vast changes in the last fifty years. At present, it is going through the basic oxygen, open-hearth, and electric arc processes.

Basic oxygen steelmaking process

  • In the basic oxygen process, we used 99 percent pure oxygen for the production of steel from molten pig iron.
  • The pure oxygen is blown onto the surface of molten pig iron at great speed through water cooling pipes. The charge of hot molten pig iron and lime is taken in a converter lined with dolomite and magnesium oxide.
  • The oxygen penetrates into the molten charge creating a large turbulence motion and oxidizes the impurities.
  • The slag is separated by tilting the converter and a rapid analytical check is performed to bring the metal to its desired composition and necessary addition of alloying metals to steelmaking.

Open hearth process for steel production

In this process, the conversion of steel is carried out on the hearth of a furnace heated by burning gas or oil in air or oxygen. The hearth is lined with dolomite, magnesite, cast iron, and scrap iron with 5 to 6 percent of lime and melted. Oxygen is injected into the furnace through a water-cooled pipe to remove the impurities. The process is required a much longer time than the basic oxygen process.

Electric arc process of steel production

Different types of high-grade steels prepared in the electric arc furnace going through the direct arc and indirect arc process. Direct arc furnaces are usually fitted with three electrodes that penetrate the furnace crucible and strike short circuits between their extremities and the molten charge.

In indirect arc furnaces, the arc is struck between two large electrodes, and specific heat from the electrode heated to the charge. In different types of steel-making processes, the charge consists of solid scrap iron, pig iron, coke, and anthracites if enough cast iron is not present. Melting and alloying of steel are also conveniently carried out in induction furnaces which contain a vertical basin around which an induction coil wound.