Learn about the stories behind 10 extraordinary inventions. In this video, you'll learn about stainless steel.
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Male Speaker: 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. 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. Dr. Ralph M. Rowlett: If you get the optimum amount of carbon in there about 0.1% carbon, it still be the highest grade steel. So, that's a pretty good steel right from beginning is just they know when they had it. Male Speaker: When the two are placed in a furnace, the carbon diffuses into the iron and upon cooling, favorably changes the overall crystalline structure. As iron is heated above about 1200 degrees Fahrenheit, 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. 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 f