Stainless Steel

La Pyramide Du Louvre, Paris, is one of the many iconic structures created over the last century that has employed stainless steel to great effect, in this case as a sturdy frame to support glass panels. Image credit: photos.com.

Stainless steel – the original wonder material

For most people, stainless steel is not a topic that crosses the mind on a regular basis – nevertheless, stainless steel is always there, quietly improving our everyday lives whether we realise it or not.

It is of course impossible to say how the modern world would have evolved without stainless steel, but certainly a lot of the modern benefits we take for granted - the cars we drive, the buildings we live and work in, even the medical treatment we receive – are irrevocably improved by the presence of stainless steel.

This year stainless steel celebrates its 100th birthday, making this the perfect time to take a holistic look at stainless steel and gain a greater appreciation of what makes this metal such a fundamental building block of modern society.

The iconic 630-foot St. Louis Arch Monument, USA, was constructed in 1965 using a stainless steel exterior and is still the world’s tallest arch. Image credit: photos.com.

What’s so great about stainless steel?

Before we look at why stainless steel is so ubiquitous, we must first outline what we actually mean by stainless steel. Though a broad term, stainless steel can be defined as an alloy (blend of two or more metals) of iron and chromium, with a minimum chromium content of 10.5% by mass.

As well as these two basic elements, various amounts of carbon, silicon, manganese, nickel and molybdenum can be added to impart different qualities to the steel.

The elemental composition is key to the incredible properties of stainless steel. The most obvious of these properties is its resistance to corrosion, staining or rust - hence the familiar name. The chromium content is key to this, as it forms a thin layer (the ‘passive layer’) that prevents oxygen reacting with the iron. As may be expected, the more chromium that is present, the higher the resistance to corrosion.

Apart from this, stainless steel has many other beneficial properties, including low maintenance requirements, high reusability, aesthetically pleasing lustre, and relatively high tensile strength.

There are around 150 stainless steel grades in use today, which are categorised in four main groups: martensitic, ferritic, austenitic and duplex. The main differences between these are shown below:

  • Martensitic – Extremely tough, but not as corrosion resistant as ferritic or austenitic. Magnetic.
  • Ferritic – reduced corrosion resistance compared to Austenitic, but with better engineering properties. Magnetic.
  • Austenitic – the most common stainless steel. Non-magnetic.
  • Duplex – approximately a 50/50 mix of austenite and ferrite.

Where can I find stainless steel?

The applications of stainless steel could fill an entire book on their own, but here is a brief list of some of the areas of application that stainless steel is used in:

  •  Art deco architecture
  •  Street furniture
  •  Cutlery
  •  Razor blades
  •  Surgical instruments
  •  Food processing equipment
  •  Water/sewage treatment
  •  Oil drilling platforms
  •  Bridges
  •  Automotive bodies
  •  Rail cars
  •  Aircraft

The invention of stainless steel

The production of a stainless steel was a long time in the making, but the eventual creation of true stainless steel, like many scientific discoveries, came with a hint of serendipity.

In 1913, after much previous work on steel alloys, Harry Brearly from Sheffield, UK, noticed that gun barrels that had been left in the rain did not rust in the usual way. These steel barrels were found to contain 13% chromium, approximately stainless steel grade 420 by today’s standards. The news was announced in a 1915 addition of The New York Times, and a manufacturing revolution began!

The Art Deco Chrysler Building in New York, at one time the world’s tallest building, boasts an austenitic steel ‘crown’. Image credit: photos.com.

Looking to the future – sustainable stainless steel?

Though there has been much advancement in materials science over the past few decades, stainless steel is certainly not a material of the past – it has an annual production growth rate much higher than other major metals, at around 5.8%. Furthermore, as of 2011, end use consumption of stainless steel reached a record 30.7 million tonnes.

Further innovative uses are being discovered all the time, especially in medical sector, where stainless steel is being used for structural parts in artificial heart valves, and efforts are being made to make it antibacterial.

Stainless steel is also 100% recyclable and does not degrade when reprocessed, meaning it can be used again and again. All this means that, though it has an illustrious past, the future also looks very bright for stainless steel.

The tallest twin building in the world The Petronas Towers, Malaysia, contains 83,500 square metres of stainless steel extrusions. Image credit: photos.com.

References and further reading

 

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