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The Origins of Stainless Steel (Inox)
No-one can dispute the beauty and practicality of stainless steel. It has revolutionized our (or rather our ancestors) lives. In 1913, Harry Brearly, an English metallurgist, accidentally discovered that adding chromium to low carbon steel gave it some degree of resistance to staining. Subsequent refining led to stainless steel as we know it.
Why is it "stainless"
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 to a certain extend. But for the most part, the steel remains low carbon, and therefore prone to rust. However, the added [12% of] chromium in the steel makes it stain 'less' than other types of steel by protecting the underlying carbon-based steel with its own "rust". The chromium in the steel combines with oxygen in the atmosphere (oxidizes) 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.
(Article)
(European Inox Development Association Website)
(Website)
(International Stainles Steel Forum Website)
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