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Stainless Steel

What is Stainless Steel?

First and foremost stainless steel's great claim to fame is its ability to resist corrosion.

Stainless Steel is an alloy of iron with at least 10.5% chromium by weight. It can have a lot more, as much as 30%.

Information about Chromium from the US Geological Survey.

In order to modify various properties such as ability to machine, brittleness, strength, resistance to corrosion, other substances are added to the alloy. Varying quantities of Nickel, manganese and carbon are often present. Specialty alloys also contain molybdenum, tungsten, nitrogen, titanium, niobium, zirconium, cerium, sulphur, silicon and copper.

Each of these other substances bring qualities to the stainless steel at the expense of other characteristics.

We think of Stainless Steel as being able to resist rusting and tarnishing, but many alloys of SS have been developed to resist highly corrosive environments such as combustion chambers or chemical holding containers, handling equipment and pipes, often in high temperature environment.

How does Stainless steel resist corrosion?

When chromium is exposed to oxygen, an invisible layer of chromium oxide (Cr2O3) is formed on the surface of the metal. This is known as the "oxide skin". This oxide skin protects the metal against further rusting/oxidation. If oxide skin gets damaged, a new layer of oxide gets formed when the chromium is exposed to air (which contains oxygen).

What do the colours often painted on the end of metals stock including Stainless steel mean?

Nothing looks more like a piece of stainless steel than another piece of stainless steel. In order to be able to identify visually a specific alloy, most metal suppliers will paint the end of the stock and sometimes also the middle with a distinctive colour. Since there is no standard for colour coding, if you are looking to identify a specific alloy you need to know who the supplier is and look up the colour in their supplied charts.

What are the numbers used after the name STAINLESS STEEL

These numbers indicate specific alloys which are classified according to their crystal structure, and which share similar characteristics. More further down.

Some history

The development of Stainless steel is a relatively recent event. It was recognized at the beginning of the 1800 and was subject to a great deal of research and development particularly in the early 1900's. By the 1930's it was being manufactured in relatively large amounts.

In the early 1900's the research and development was fast and furious and there was quite a lot of competition. There are many claims of who "discovered" stainless steel first. It looks more like it was developped and improved gradually by several researchers, in several countries notably France, England and Germany, then in the US, over a period of about 100 years.

The link is to a book by Carl Zapffre on Stainless steel and has a good historical background section.

What are Martensitic, Ferritic and Austenitic Stainless Steel Alloys?

Besides the basic Iron-Chromium, there are different elements added Stainless steel and each bring specific properties. The Crystal Structure is another factor that helps determine the properties of the specific alloy.

Stainless steels can be classified into 3 main types of crystal structures. These are Austenitic, Ferritic and Martensitic.

Austenitic Stainless Steel

These are the most common types of stainless that are generally available.

Stainless steel kitchen

Uses for Austenitic Steel

There is some overlap for use between the different alloys. These are only some of the uses.

  • Most commonly used Stainless steels.
  • 304 304L: Cookware, cutlery, sinks, food and beverage containers, industrial equipment including storage tanks, auto trim, architectural elements, heat echangers and for contact with milder chemicals.
  • 309 310: Furnace and kilns, components for catalytic converters.
  • 318 316L: Chemical Storage Tanks, pressure vessels and piping. 316 contains molybdinum which increases resistance to pitting. Pulp and paper industry and other more corrosive environments.
  • 317: contains higher level of molybdinum and are thus more corrosion resistant. Article from the International Molybdenum Association on Stainless steel buildings in New York City
  • 321 347: Superheaters, afterburners, expansin bellows. Corrosion resistance for intermittent exposure to temperatures above 800F. Used primarily in aircraft industry.
  • 200 series: Diswashers and washing machines, cutlery and cookware, water tanks, indoor and non structural architectural elements, food and beverage, decorative auto parts.
  • Ferritic Stainless Steel

    Uses for Ferritic Stainless Steel

    Martensitic Stainless Steel

    Martensitic (named for a german scientist, Adolf Martens) is one of the crystalline structures of Stainless Steel. Because it contains more carbon than other grades, .1% to as much as 1% it can have a wide range of properties. Although it has a lower chromium content than most other Stainless alloys (at least 11.5% Chromium), and shows only moderate corrosion resistance, it is a true Stainless Steel.

    It has a body centered tetragonal structure. Martensitic Stainless steels share properties with carbon steels. They can be hardened and strengthened by heat treatment. They are "HARD" ferro-magnetic alloys.

    When annealed, they can be machined and are ductile. After heat treatment, they become harder but more brittle and loose ductility and toughness.

    Uses for Martensitic Steel

    Duplex Stainless Steel

    It is possible to produce stainless steel by controlling the cooling, that has a combination of 2 crystal structures. In Duplex Stainless steel both ferritic and austenitic grains can be found.

    Link to a guide to high-performance alloys published by the Moly Review.

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