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The Miracle Metal: Steel!

Steel pipes. PC: qswownews.com

BRIEF HISTORY OF STEEL
Steel has been widely regarded as the 'miracle metal', especially of the modern era. This is because of its diversity in property and widespread applications in many different areas. One of the earliest forms of steel, blister steel, began production in Germany and England in the 17th century and was produced by increasing the carbon content in molten pig iron using a process known as cementation. In this process, bars of wrought iron were layered with powdered charcoal in stone boxes and heated.

Steel is all around us. It is used in major constructions such as skyscrapers, stadiums, roads and railways, as well as smaller product designs such as bolts, nails and screws. Steel in also used in vehicles, kitchenware, electronic equipment, and a whole lot more.

WHAT IS STEEL?
Steel is an alloy of Iron (Fe) and Carbon C in which the carbon composition is less than two percent (2%). The 'counter-material' of steel is Cast Iron which has carbon composition of more than 2%. Steel properties can be varied mainly by different heat treatment methods which will be discussed in a later publication. Some of these methods include quenching, tempering, annealing and normalising. Steel gives a unique property when the structure or composition is manipulated.

WHAT ARE THE TWO MAJOR TYPES OF STEEL?
  • Plain Carbon Steel (PCS)
  • Alloy Steels (AS)
Plain Carbon Steep (PCS)
Primarily consists of iron and carbon in which the properties are imparted by the presence of carbon. Other elements may exist in minute quantities but their presence is not significant enough to affect the properties of PCS.
Below is a table of the different types of PCS, their compositions and applications.


Alloy Steels (AS)
Despite its broad usage, steeps have many limitations. Plain carbon steels are liable to ‘mass effect’: large sections cannot be effectively hardened. Drastic water quenching is necessary for full hardening , with consequent risk of cracking and distortion.

Plain carbon steels have poor resistance to corrosion and oxidation at elevated temperatures. In order to overcome these limitations and to meet the specific requirements of engineers, alloy steels have been developed.

The principal alloying elements are nickel, chromium, manganese, molybdenum, silicon, tungsten, vanadium, cobalt and copper.

Each of these elements has particular properties it imparts when used as an alloying agent. Properties bestowed by some of these alloying elements are briefly listed below.

MANGANESE (Mn)
It goes into solid solution, thereby increasing strength and hardness. It also forms hard carbides.

 NICKEL (Ni)
Nickel has a marked strengthening effect on steel, since it goes into solid solution and decreases the carbon content of the eutectoid. It is a graphitising element, but this effect may be countered by the presence of carbide – forming elements such as manganese or chromium. Ni lowers the critical cooling rate, thereby increasing the hardenability of the steel.

CHROMIUM (Cr)
Chromium goes into solid solution in the steel and also forms hard carbides. Cr lowers the carbon content of the eutectoid, and also the critical cooling rate, thereby increasing the hardenability of the steel.

TUNGSTEN (W)
Tungsten is a strong carbide-forming element. Hardened tungsten steel resists tempering up to a relatively high temperature, hence the use of tungsten in high-speed tool steels. Tungsten refines the grain size and decreases the tendency of decarburisation during working. It increases coercive force and a steel containing 1% carbon and 6% tungsten is used for permanent magnets.

MOLYBDENUM (Mo)
It is a strong carbide-forming element. It reduces ‘mass effect and ‘temper-brittleness’; and inhibits grain growth. It is rarely used alone as an alloying element. It improves the mechanical properties at high temperatures. It is used in high speed tools steels, and heat and corrosion-resisting steels.

 VANADIUM (V)
It is also a strong carbide-forming element and is used in high – speed tool steels. It is a grain-refining element and a strong deoxidiser. It ensures a clean steel by acting as a ‘scavenger’ for oxides and other inclusions.

Some Steel Products

Sky scraper frames made of steel. PC: www.thermofisher.com

Stainless steel kitchenware. PC: indiamart.com

Vehicles are largely made of steel. PC: indiatimes.com

HOW IS STEEL PRODUCED?
Steel is produced from pig iron - which is also obtained from iron ore through pyrometallurgical processes in the largest chemical reactor - the Blast Furnace. Some of the mostly commonly used ores include Haematite (Fe2O3) and Magnetite (Fe33O4), with hematite being the chief ore.

The processes involved in pyrometallurgy include one or more of the following: drying, calcination, roasting, smelting and refining. As the iron ore is charged through the blast furnace alongside other materials (coke and flux), various physical and chemical processes take place to finally give molten pig iron. It is interesting to know that parts of the blast furnace are made of steel. Watch an animated video of steel production process here: Steel Production.
Blast Furnace Operation. PC: letslearnnepal.com

 WHY IS STEEL SO WIDELY FOR MANY DIFFERENT PURPOSES?
Durability
One of the main reasons why rsteel is used in so many construction projects is its durability—it has the highest strength-to-weight ratio of any other building material, making it ideal for buildings both large and small.

 Fabrication and Production
Steel can be easily fabricated and produced massively.

Availability of Raw Material
 Some of the most commonly used iron ores from which steel is produced include magnetite (highest content of iron) and hematite (most widely used). Others include limonite and siderite. These are available in abundance and hence makes if easy to obtain pig iron, which is further processed to make steel.

In our next publication about Steel, we shall talk about Stainless Steel! And why it is called stainless! Thank you for reading and stay tuned for more interesting and informative articles.

Credits: dienamics, thebalance
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