Science Makes Sense-Week 26: Iron and Steel, alloys.

My mother who was born in the early part of the 20th century, was innovative and forward-thinking in many ways. She was the first among her sisters to discard bronze and lead vessels for cooking and start using stainless steel containers on a regular basis. The kitchen used to sparkle with those bright silver-like containers gleaming in the sunshine! I must say my mother loved those new vessels for cooking and storing food. We called them ever-silver, small wonder.
As we already know, iron is a transition metal and we have studied a lot of the characteristics of iron earlier. We cannot forget about the alloys of iron that include the different forms of steel which have revolutionized our lives both in the kitchen and outside. An alloy is basically a mixture of one or more elements added to the main element and melted together. The added metal or metals arrange themselves in between the rows of the main metal.
Stainless steel is an alloy of iron and mainly chromium. Iron alone is prone to oxidation and forms the reddish-brown iron oxide , familiarly known as ‘rust’. When carbon is added, you get stronger iron to use industrially as well as to make cast iron pots and pans. Stainless steel is corrosion resistant and the chromium added gives it a shine.(Ref.1)
Pure iron, like most metals is not particularly strong; the atoms of iron are too free to move around. The addition of other elements allows some of the atoms of iron to be pinned down, but too much addition might make the iron too brittle, too rigid. The trick is to add just the right amount to create a stronger metal. (Ref.2)
The fascinating fact is that this knowledge of forming mixtures with other metals, called alloys has been known to mankind for the longest time. One of the earliest steels made was by Indian metalworkers and it was called ‘wootz”(Ref.2) However, these alloys were made in small quantities and only in the 18th century did we perfect the art of creating alloys on a larger scale.(Ref.2)
Wrought iron was first made in the 18th century using a method called “puddling”: this involved the heating of iron ore and refined iron until impurities like Sulfur, S, and silicon, Si, formed a slag with Oxygen, O. (Wrought iron is a black metal that is used for railings.) After removal of most of the slag by hand, the temperature was raised to let the carbon react with the oxygen and burn. The remaining slag sits among the pure grains of iron to form a material that is stronger and more flexible than the original iron. (Ref.2)
Here are some improvements in the manufacture of alloys of iron in the 20th and 21st centuries:
Basic oxygen process (BOP): The steel is made in a giant egg-shaped container, open at the top, called a basic oxygen furnace. This is like an ordinary blast furnace, only it can rotate to one side to pour off the finished metal. The air draft used in a blast furnace is replaced with an injection of pure oxygen through a pipe called a lance. This furnace is based on the Bessemer process developed by Sir Henry Bessemer in the 1850s. (Ref.3)
Open-hearth process (also called the regenerative open hearth): Reminds one of a giant fireplace in which pig iron, scrap steel, and iron ore are burned with limestone (calcium carbonate) until they fuse together. More pig iron is added, the unwanted carbon combines with oxygen to form the slag that is removed and the iron turns to molten steel. The steel is sampled and the process is continued until the iron has the right carbon content to make the type of steel needed.
Electric-furnace process: The electric furnace, uses electric arcs (effectively giant sparks) to melt pig iron or scrap steel. Since they’re much more controllable, electric furnaces are generally used to make higher-specification alloy, carbon, and tool steels. (Ref.3)
The alloys of iron have become an essential part of our lives; we are beholden to this element,iron for thousands of years. In fact, there is more to talk about this interesting ‘d’ shell element, especially about its magnetic properties. Next time we will delve into this very exciting property of iron.
Activities for Middle School Teachers
Let students compare and contrast the periods of the Bronze Age and the Iron Age. What were the tools created out of bronze and iron mainly used for? What was happening in the world at that period? Consult with Social Studies teachers for this activity.
Nuggets of Information:
The word ‘iron’ comes from a word that means ‘metal from the sky’. Early iron was extracted mostly from meteorites.(Ref.1)
“Puddling” was very popular in the 18th and 19th centuries; one of the famous wrought iron structures built during this period is the Eiffel Tower in Paris, France. (Ref.2)
“Pig iron” is„ raw iron, the immediate product of smelting(a metallurgical process by which any metal is extracted from its ore and heated it to a high enough temperature to melt it(Ref.4)) iron ore with carbon(coke) and calcium carbonate(limestone). It has a high carbon content, around 4-5% and makes it very brittle. (Ref.5)
There are basically two kinds of alloys formed when metals are combined together at high temperatures. Sometimes, the size of the guest metal atom is almost the same as that of the host metal atom and these atoms take the place of the host atom. These alloys are called substitutional alloys like brass where zinc atoms substitute for some copper atoms in the lattice structures.
Then there are interstitial alloys like steel alloys where the manganese, chromium or carbon atoms place themselves in between the spaces around the host atoms, which in this case are iron atoms.(Ref.6)
Carbon steel with the lowest carbon content is typically called wrought iron. The metal is hard, but not brittle and is used for fences, chain links, gates and railings. The low carbon content allows this alloy to be worked into different shapes. The most commonly used carbon steel has a medium carbon content; uses of carbon steel in this category include structural steel to build buildings and bridges. It is also used for making automobile parts and in shipbuilding. High-carbon steel is hard but brittle and less easily worked. This type of carbon steel is used to create springs and high-strength wires. The increased hardness makes this category of steel ideal for cutting tools, punches, dies and industrial knives.
Finally, carbon steel with ultra-high carbon content is commonly referred to as cast iron. This type of cast iron is very hard but highly brittle. It has little to no malleability and cannot be easily welded or tooled. Often, it is used for cast iron pots, hot water radiators and certain types of lamp posts. Industrially, this steel is used for castings.(Ref.7)

References:
1.http://www.chemistryexplained.com/St-Te/Steel.html
2.Trefill, James, A Scientist in the City, p.38 (Doubleday,1994)
3.http://www.explainthatstuff.com/ironsteel.html
4.http://www.thefreedictionary.com/Smelting
5.http://chem.tamu.edu/class/majors/chem470/Steel.pdf
6.http://www.scienceiscool.org/solids/metallic.html
7.http://www.wisegeek.com/what-are-the-uses-of-carbon-steel.htm

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