: austenite phase stainless steel is comprised of nickel and molybdenum as its main alloying elements, and has a face-centered cubic crystal structure. The addition of nickel increases its corrosion resistance and ductility, while molybdenum improves corrosion resistance to acidic conditions. Common grades of austenitic stainless steel are 304 and 316.
: this type of stainless steel is considered heat resistant and is commonly utilized to produce heat exchangers, boilers, and furnaces. Other common applications of austenitic stainless steel are aerospace components, electronics, locomotive parts, and chemical tanks. 200 series stainless steels are also in the austenitic family, but have manganese as an additional alloying element.

 

: the martensite phase stainless steel family uses carbon and chromium as its main alloying elements. At higher carbon concentrations, the material has a body-centered tetragonal crystal structure; lower carbon concentrations form a body-centered crystal structure. Martensite is formed when austenite is quenched rapidly at room temperature. Martensitic stainless steel is commonly known as 400 series of stainless steel, such as grades 410, 420, and 440.

 

: ferrite phase stainless steel is non-magnetic and has a body-centered cubic crystal structure. Its main alloying constituents are chromium, iron (hence the name ferritic), and a low concentration of carbon. Ferritic stainless steel is therefore softer, but also has increased ductility and improved formability - but they cannot be heat treated. Examples of ferritic stainless steel are other 400 series grades, such as 409, 430, and 446.

 

The Origins of Stainless Steel: Groundwork in the 1800s

While some contend that stainless steel was utilized by blacksmiths in ancient India, Sri Lanka, the Arabian peninsula, and China before the Industrial Revolution, those histories weren't well recorded. What we do know is that in the early 1800s, scientists in several countries observed a phenomenon with metal alloys containing chromium and iron - they were able to resist acid corrosion.

A French metallurgist, Pierre Berthier, is credited as the first to recognize in 1821 that a corrosion-resistant steel alloy could be made by adding chromium to iron. While metallurgists drew this conclusion, they didn't yet know the importance of limiting carbon content in those same alloys to prevent intergranular corrosion and fracturing due to brittleness. But, they were off to a good start by gleaning how chromium altered the physical properties of a metal alloy.

In England in 1872, John Clark and John Woods filed the first official patent for what we now consider stainless steel. They called their product a Water Resistant Alloy and it was remarkably similar to modern alloys, but the material still had too much carbon.

The first to officially recognize the carbon issue was a researcher simply known as Brustlein in 1875. His revelation sparked several decades of research to develop the optimal mixture of metals and a production method for developing low-carbon steel. While we have modern processes for creating metal alloys today, the science back then was rudimentary. Finding a way to separate chromium from the carbon it was naturally found with was the main obstacle to producing chromium-based, low-carbon steel.
About two decades after Brustlein's low carbon epiphany, in 1895 Hans Goldschmidt developed the aluminothermic reaction process for producing/refining chromium to be carbon-free. This was the turning point that enabled the development of modern stainless steel. The chemical reaction for what is also known as the aluminum reduction method for producing chromium metal is shown below.

₂O₃ + 2Al → 2Cr + Al₂O₃

It's a process where chromium oxide (and any impurities naturally found with it like silicon, sulphur or carbon) and aluminum are added to a steel container lined with either an organic binder, alumina, or magnesium oxide, and heat. The mixture is ignited with barium peroxide to creat an exothermic reaction - meaning there is a net release of energy (heat) when the chromium and resulting slag are produced.

Today, this process is still used to produce both 'pure' chromium and aluminum oxide for various post-processing applications. The constituent concentrations for the chemical reaction can be modified to produce different purity levels of the resulting metals.

Stainless Steel Advancements in the 1900s

The early 1900s brought about several important discoveries in the advancement of the stainless steel alloy family:

– In 1904 Leon Guilet discovered several 400 series, iron-chromium stainless steel alloys and researched the 300 series, iron-nickel-chromium alloys - though he didn't realize that the metals were corrosion-resistant.

– P. Monnartz and W. Brochure discovered that adding chromium to the metal alloy increased its corrosion resistance, and quantified the minimum chromium concentration as 10.5%. They also worked on detailing and researching the increased corrosion resistance that comes with the addition of molybdenum.

The next major advancement occurred in 1913 when Harry Brearley is reputed to have officially discovered stainless steel in an attempt to create erosion-resistant steel. Now, Harry Brearly is probably the correct answer to trivia question about who invented stainless steel, but the the truth isn't that straightforward. He experimented with adding chromium to steel, but the rest of his story is laced in myth.

In 1908, Kupp Iron Works developed a chromium-nickel alloy for a marine application.

In 1911, Elwood Haynes created a corrosion-resistant steel to replace his rusty razor blades.

From 1911-1914, Becket and Dantsizen created a few ferritic steel alloys.

In 1912, Max Mauermann claimed to have created stainless steel, but he likely only created a viable production method for stainless steel. He filed a patent for the process, but it was disputed at the time, and he wasn't recognized as an inventor of stainless steel until after his death.