What’s Stainless Steel?

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What’s Stainless Steel?

Who was the inventor of stainless steel?

This is the question that makes you a million, wins you that holiday of a lifetime, gives you the title of office genius, or gets you that elusive slice of pie on the Trivial Pursuit board.

You know this one…. don’t you?
‘Harry Brearley!! It’s Harry Brearley!! Isn’t it?’ Well, the answer may not be so clear cut…

Harry Brearley

Since the dawn of man colonies have raced against each other to uncover new technologies, to be the first to stamp their names on a discovery, and although we’ve evolved over millions of years (most of us, anyway) the urge to be the first remains at the very core of our nature.

Harry Brearley

This sense of passion and pride can lead some of the more unscrupulous humans to claim others discoveries as their own. Of course many breakthroughs are genuinely made in tandem, or are simultaneously occurring, but unless you can categorically prove that you were the pioneer of these incredible findings then the other party involved will always dispute the fact.

And so we come to stainless steel.

The first point to note is that ‘inventor’ is a very ambiguous term. Is this the first person to think, to document, to patent, or to produce? The second point is that stainless steel wasn’t truly defined until 1911, so are we to cast aside those chromium-iron alloys that don’t quite meet the minimum requirement of 10.5% chromium?

It seems like anyone and everyone has a different claim to being labelled the  ‘inventor’ of stainless steel; from Britain, Germany, France, Poland, the U.S.A, and even Sweden.

The cogs were set in motion by Englishmen Stoddard and Farraday circa 1820 and Frenchman Pierre Berthier in 1821. These scientists, among others, noted that iron-chromium alloys were more resistant to attack by certain acids, but tests were only carried out on low chromium content alloys. Attempts to produce higher chromium alloys failed primarily because of scientists not understanding the importance of low carbon content.

In 1872 another pair of Englishmen, Woods and Clark, filed for patent of an acid and weather resistant iron alloy containing 30-35% chromium and 2% tungsten, effectively the first ever patent on what would now be considered a stainless steel. However, the real development came in 1875 when a Frenchman named Brustlein detailed the importance of low carbon content in successfully making stainless steel. Brustlein pointed out that in order to create an alloy with a high percentage of chromium, the carbon content must remain below around 0.15%

Thus ensued two decades of stagnation for the development of stainless steel, and while many scientists attempted to create a low carbon stainless steel, none succeeded.

 

Hans Goldschmidt

It wasn’t until 1895, when Hans Goldschmidt of Germany developed the

Hans Goldschmidt

aluminothermic reduction process for producing carbon-free chromium, that development of stainless steels became a reality.

In 1904 French Scientist Leon Guillet undertook extensive research on many iron-chromium alloys.

Guillets work included studies on the composition of what would now be known as 410, 420, 442, 446 and 440-C. In 1906 Guillet went on to analyse iron-nickel-chrome alloys, which would now be considered the basics of the 300 series. However, while noting the chemical composition of his alloys, Guillet failed to acknowledge the potential corrosion resistance of his materials.

 

 

Albert Marcel Germain René Portevin

In 1909 Englishman Giesen published an in-depth work regarding chromium-nickel steels, while the French national, Portevin, studied what is now regarded as 430 stainless steel.

Albert Portevin

However, it wasn’t until 1911 that the importance of a minimum chromium content was discovered by Germans P. Monnartz and W. Borchers. Monnartz and Borchers discovered the correlation between chromium content and corrosion resistance, stating that there was a significant boost in corrosion resistance when at least 10.5% chromium was present. The pair also published detailed works on the effects of molybdenum on corrosion resistance.

It is at this point we introduce Harry Brearley, born in Sheffield, England in 1871, he was appointed lead researcher at Brown Firth Laboratories in 1908. In 1912 Brearley was given a task by a small arms manufacturer who wished to prolong the life of their gun barrels which were eroding away too quickly. Brearley set out to create an erosion resistant steel, not a corrosion resistant one, and began experimenting with steel alloys containing chromium. During these experiments Brearley made several variations of his alloys, ranging from 6% to 15% chromium with differing measures of carbon.

On the 13th August 1913 Brearley created a steel with 12.8% chromium and 0.24% carbon, argued to be the first ever stainless steel. The circumstances in which Brearley discovered stainless steel are covered in myth; some enchanted tales of Brearley recite him tossing his steel into the rubbish, only to notice later that the steel hadn’t rusted to the extent of its counterparts, much like Alexander Flemings experience 15 years later.

Other more plausible (but less attractive) accounts claim it was necessary for Brearley to etch his steels with nitric acid and examine them under a microscope in order to analyse their potential resistance to chemical attack. Brearley found that his new steel resisted these chemical attacks and proceeded to test the sample with other agents, including lemon juice and vinegar. Brearley was astounded to find that his alloys were still highly resistant, and immediately recognised the potential for his steel within the cutlery industry.

 

The Half Moon

However, Brearley struggled to win the support of his employers, instead choosing to produce his new steel at local cutler R.F Mosley. Brearley had initially decided to name his invention ‘Rustless Steel’, but cutlery manager of Mosley, Ernest Stuart, dubbed it ‘Stainless Steel’ after testing the material in a vinegar solution, and the name stuck.

And that’s how Harry Brearley discovered stainless steel…. well, not quite…

During the 5 year period between 1908 and Brearleys discovery in 1913 many other scientists and metallurgists have potential claims to Brearleys title.

In 1908 the Germans entered the fray, the Krupp Iron Works in Germany produced a chrome-nickel steel for the hull of the Germania yacht. The Half Moon, as the yacht is now known, has a rich history and currently lies on the seabed off the east coast of Florida. Whether the steel contains the minimum 10.5% chromium content remains inconclusive. Employees of the Krupp works, Eduard Maurer and Benno Strauss, also worked from 1912-1914 on developing austenitic steels using <1% carbon, <20% nickel and 15-40% chromium.

Not happy with Europe hogging the glory, the USA got in on the act. Firstly, Elwood Haynes, after becoming disenchanted at his rusty razor, set out to create a corrosion resistant steel, which he supposedly succeeded in doing during 1911. Two other Americans, Becket and Dantsizen, worked on ferritic stainless steels, containing 14-16% chromium and 0.07-0.15% carbon, in the years 1911-1914.

Elwood Haynes

During 1912 Max Mauermann of Poland is rumoured to have created the first stainless steel,

Elwood Haynes

which he later presented to the public during the Adria exhibition in Vienna, 1913.

Finally, a recently discovered article, which was published in a Swedish hunting and fishing magazine in 1913, discusses a steel used for gun barrels (sound familiar?) which seems to resemble stainless steel. Although this is purely speculation, the Swedes have still made an audacious claim that they were in fact responsible for the first practical application for stainless steel.

That concludes the shambolic discovery of stainless steel! Although there is much mystery and speculation behind the discovery of this wonderful material, there is no question that without the combined effort of all the above scientists and metallurgists (and all the many more that were not mentioned) we would not have such a rich and versatile metal at our fingertips.

Oh, and if we have to give you an answer to that first question? Harry Brearley.

What Is Stainless Steel and Why Is it Stainless?

In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain ‘less’ than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. (Iron, on the other hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather than tightly-packed layer and flakes away.) The passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment.

Types of Stainless Steel

The three main types of stainless steels are austenitic, ferritic, and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase.

Austenitic
Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304, sometimes called T304 or simply 304. Type 304 surgical stainless steel is an austenitic steel containing 18-20% chromium and 8-10% nickel.

Ferritic
Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment.

Martensitic
The characteristic orthorhombic martensite microstructure was first observed by German microscopist Adolf Martens around 1890. Martensitic steels are low carbon steels built around the Type 410 composition of iron, 12% chromium, and 0.12% carbon. They may be tempered and hardened. Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened.

There are also other grades of stainless steels, such as precipitation-hardened, duplex, and cast stainless steels. Stainless steel can be produced in a variety of finishes and textures and can be tinted over a broad spectrum of colors.

Passivation

There is some dispute over whether the corrosion resistance of stainless steel can be enhanced by the process of passivation. Essentially, passivation is the removal of free iron from the surface of the steel. This is performed by immersing the steel in an oxidant, such as nitric acid or citric acid solution. Since the top layer of iron is removed, passivation diminishes surface discoloration. While passivation does not affect the thickness or effectiveness of the passive layer, it is useful in producing a clean surface for a further treatment, such as plating or painting. On the other hand, if the oxidant is incompletely removed from the steel, as sometimes happens in pieces with tight joints or corners, then crevice corrosion may result. Most research indicates that diminishing surface particle corrosion does not reduce susceptibility to pitting corrosion.

 

Additional Reading

  • Citric Acid Passivation of Stainless Steel– Lee V. Kremer’s article discusses the use of citric acid passivation as an alternative to nitric acid passivation of stainless steel.
  • Passivation of Stainless Steel– Dan Englebert’s article defines passivation and describes how it is performed and verified.
  • Phase Diagram Glossary– These are definitions of several relevant terms, including ‘ferritic steel’, ‘microstructure’, and ‘plain carbon steel’.
  • Sheffield Shows Its Metal– The British Canadian Chamber of Trade and Commerce provides this article on Sheffield that includes some history of stainless steel.
  • Stainless Steel Property Database– The Hendrix Group provides this database of material properties, including compositions, corrosion properties, cost data, specifications, and categories of steel.
  • The Stainless Steel Information Center– The Specialty Steel Industry of North America sponsors this resource, which includes general information about stainless steel, industry news, publications, workshops and training, and information for students.
  • What Are Martensites?– This site offers a description and pictures of the martensitic crystal transformation in steels

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