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- Stainless Steel
Man has been experimenting with iron alloys for thousands of years. But, it wasn't until 1913 that an English metallurgist stumbled upon one of the world's most useful iron alloys: stainless steel.
Iron, as found in nature, is known as iron ore and is the fifth most abundant element on earth. It is second only to aluminum among the metals and constitutes nearly 5 percent of the earth's crust.
From a structural engineering perspective, iron ore is not very useful because it is relatively soft, not much harder than copper.
- The Hittite civilization was almost unknown until excavations in the 19th century revealed the extent and importance of culture.
- "High-carbon" steels contain a mere 0.5% carbon.
- Nearly 1.5 million pounds of steel are present in the Empire State building.
- There are over 4300 different steel alloys in use today.
- Machine guns used during World War I required several soldiers to operate and were prone to quickly overheating. During the heat of battle, urine would be used when the water supply was exhausted to cool the machines.
Sometime around 1300 BC in what is now Turkey, the Hittites began to derive iron from its ore, beginning a rapid export of iron metallurgy techniques across Europe and Asia and ushering in the Iron Age.
Previous metalworking techniques consisted of heating two metals until they became liquid and then pouring the mixture into a mould while it cools.
This technique, if applied to iron, results in a rigid, brittle metal, undesirable for tools and weapons.
The Hittites discovered that Iron ore can be strengthened and hardened by heating it over a bed of charcoal.
When the two are placed in a furnace, the carbon diffuses into the iron and upon cooling, favorably changes the overall crystalline structure.
Carbon + Iron = Steel
As iron is heated above about 1200Â¼F, its crystalline form changes from a tightly-packed cubic structure, to one with roughly 25% more space.
At these high temperatures, a small carbon atom can fit into the tiny spaces between the iron atoms. If this mixture is then quickly cooled, the iron atoms will attempt to reconfigure themselves into the tightly packed structure. The presence of the carbon atoms, however, creates a distortion, which makes it much harder to bend or shape the steel.
Creating steels from iron ore resulted in stronger weapons and tools. Over the next several thousands of years, the processes of refining became more advanced.
Steels were used for more applications such as railroads and buildings. The only problem was that traditional steels could not resist corrosion.
In sufficiently humid air, steel would rust.
Harry Brearley, Metallurgist
In 1908, Harry Brearley, a 36-year old English metallurgist, was given the opportunity to head up the Brown Firth Research Laboratory in Sheffield, England.
Brearley left school at the age of 12. Through self-study and night school, he became an expert in the analysis and production of steel. In 1912, Brearley's laboratory was commissioned to research ways to eliminate erosion in gun barrels.
When a bullet is fired by a gun, it is actually propelled down the barrel by expanding gas detonated in the chamber. The friction between the bullet and grooves in the barrel called "rifling" causes the bullet to spin, increasing accuracy.
The friction, however, causes wear on the barrel. Eventually the barrel will be too big for the bullet, causing inaccuracy. What was needed was a harder metal that could resist higher temperatures.
Brearly spent months experimenting with steel alloys, varying the mixture of steel and other metals. None of his alloys had much success in reducing barrel erosion, and Brearley simply tossed the failed samples into a junk pile.
Months later Brearley noticed one of the samples in the pile retained its bright luster while the others had rusted. This sample had particularly high levels of chromium, approximately 12%, which accounted for its corrosion resistance.
Chromium, which accounts for stainless steel's resistance properties, has a high affinity for oxygen, and when present in sufficient quantities in iron, forms a protective layer on the surface of the steel.
This layer is incredibly thin, roughly one ten-thousandths of a centimeter thick. This is roughly the equivalent of a 30 foot tall building being protected by a film the thickness of a piece of copier paper. Yet, this microscopic layer clings tightly to the steel.
If it is damaged or forcibly removed, more chromium will be exposed to the air and more of the protective chromium oxide will be formed.
This self-renewing feature makes stainless steel an ideal material for thousands of applications. It has a life-span of nearly 100 years and requires no upkeep.
Brearley called his new invention "rustless steel". Because the city of Sheffield was known for its cutlery, he had a knifesmith make him a batch of knives.
Unfortunately, Brearley's rustless steel wasn't particularly well suited to cutlery: it couldn't hold an edge. Poorly received by the public, Brearly soon earned a reputation for making knives that would not cut. It would take years of refinement and research to create a stainless steel that also held a cutting edge.
The name "stainless steel" is attributed to a manufacturer who noticed that the steel did not stain when dipped into vinegar. Its low weight and resistance to oxidation at high temperatures has had an effect on almost every modern industry, from food preparation and storage, to transportation, to medicine.
You can find it in battleships and in sewing needles. It is in your home in the form of utensils, your kitchen sink, and vacuum cleaner. It is truly a marvel of technology.