History of Stainless Steel
While Brearley is given credit by most for the invention of stainless steel, French scientist Leon Gillet had documented the constitution of stainless steel in 1904. While Gillet noted the composition and properties of his alloy mix, he never recognized the corrosive resistance of the material. In 1911 Philip Monnartz of Germany published the first detailed work on the corrosion resistance of stainless steel. In 1912, two German's at the Krupp Iron Works, Eduard Maurer and Benno Strauss, patented the first austenitic stainless steel of a 21% chromium and 7% nickel combination. Brearley patented the first martensitic stainless in 1913. While Brearley is generally given credit for the discovery of stainless, many historians feel this is disputable.
Harry Brearley was born on Feb 18, 1871 and by 1907 was in charge of the Brown-Firth Research Labratory in Sheffield, England. The lab was investigating ways to eliminate rust in gun barrels, when by accident, Mr Byerly noticed a discarded steel sample from an earlier test was not rusting, while other samples were. The result was a chrome alloy steel, much more rust resistant than seen before. The date was June 4, 1912. Two months later, on August 20, 1912, stainless steel was cast for the first time.
Brearley immediately set out to market his new invention. He called his new metal "rustless steel". Sheffield, known as a city of cutlery manufacturers, and the new material , a forerunner of today's 420, seemed to be a perfect replacement for silver or nickel plated steel. But manufacturers were hesitant, so Brearley had one make him some knives. One manufacturer, Ernest Stuart, upon testing the material in vinegar suggested a more marketable name of "stainless steel". By 1914, the George Ibberson & Co, using stainless manufactured by Thomas Firth & Sons, began producing stainless steel knives. The product was not an immediate success, and Brealey soon earned the reputation of being the inventor of the "knife that would not cut".
Brearley left Firth, over an ownership dispute of the stainless steel invention, and W. H. Hatfield became his successor. In 1924, Hatfield patented the 18-8 stainless steel, 18% chromium and 8% nickel. This austentic stainless would soon rise to become the most popular and widely used type of stainless steel. Adding titanium to the 18-8, Hatfield is also credited with the invention of 321 stainless. In the earlier years, German scientists, such as the Krupp Research Institues, were the quickest to realize the potential of austentic stainless, inventing 316, among others.
As PBS reported in its special "The Streamliners"... "Stainless steel, with its sleek, shiny surface and tremendous strength, is a marvel of technology. It has revolutionized most modern industries, including food, medicine, and transportation. The non-corrosive and rust-resistant properties of stainless steel have made it essential in the preparation, delivery and storage of food. Stainless steel is a standard in modern restaurant kitchens since it can be easily cleaned and dried. The surface of stainless steel resists oxidation at high temperatures, making the sterilization of medical instruments possible. Its light weight and durability allowed the development of streamlining in transportation. The streamlined design of new trains, planes, and automobiles allowed for less wind resistance, and trains such as the Zephyr helped spark a new design movement. Everything from toasters to vacuum cleaners emulated the new vehicles. Stainless steel paved the way for modern technology and continues to influence our lives every day."
Families of Stainless Steel
There are five families of stainless steel: ferritic, martensitic, austenitic, duplex, and precipitation hardening. These names are derived from the crystal structure of the steels, which determines their metallurgical behavior.
Ferritic
Ferritic stainless steels are plain chromium stainless steels with a chromium content varying between 12% and 18% and low carbon content. They are magnetic and not hardenable by heat treatment.
Martensitic
Martensitic stainless steels were the first stainless steels that were commercially developed (as cutlery) and have a relatively high carbon content (0.1% - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12% and 18% chromium. Alloy 410 is the basic, general purpose, and magnetic grade that is hardenable by quenching and tempering. These stainless steels can be heat treated to obtain high strength with good ductility.
300 Series Austenitic
300 series austenitic stainless steels are non-magnetic. When nickel is added to stainless steel in sufficient amounts, the crystal structure changes to "austenite." The basic composition of 300 series austenitic stainless steels is 18% chromium and 8% nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. Austenitic grades are the most commonly used stainless steels, accounting for more than 70% of production. (Alloy 304/304L followed by Alloy 316L are the most commonly specified grades by far.) They are not hardenable by heat treatment.
Duplex
These are stainless steels containing relatively high chromium levels (between 18% and 28%) and moderate amounts of nickel (between 4.5% and 8%). The high corrosion resistance and the excellent mechanical properties of duplex stainless steels can be explained by their chemical composition and balanced ("duplex") microstructure of approximately equivalent volume fractions of ferrite and austenite. Alloys 2304 and 2205 are the most common grades. "Super" duplex grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due to the further additions of chromium, molybdenum, and nitrogen to these grades. Alloy 2507 is the most common grade.
Precipitation Hardening
Precipitation hardening stainless steels, like the martensitic types, can be strengthened (i.e., hardened) by heat treatment. The mechanism is metallurgically different from the process in the martensitic types. This means that either martensitic or austenitic precipitation hardening structures can be produced. These stainless steels combine high strength and hardness with corrosion resistance which is superior to that of the martensitic chromium stainless steels.
