Steel is in the news at the moment.
For good reason, we owe it a lot..
Steel is mainly iron..
Iron, 26Fe
Iron is the most common element on planet Earth
By weight at least.. just like on most other rocky planets.
Every atom you see was formed in the supernova death of a massive star..
From nuclear fusion of lighter elements.
Iron makes up one third of the Earth’s mass and most of our molten core.
The magnetic field it generates protects our atmosphere..
Stops it being stripped away by the solar wind.
Incredibly, our core has stayed hot, protecting Earth..
Long enough for life (and us!) to evolve.
4.5 BILLION years.
And now each and every one of us contains about 4g of iron in oxygen carrying molecules:
Iron is vital for life.
Thank you iron!
Iron Age
Unfortunately we can't get at Earth's molten core.
In Earth’s crust iron is mostly iron oxide, bound tightly to oxygen.
We had to master temperatures of 1500°C to break those bonds and set iron free. That's some 500°C higher than to smelt copper.
Our ability as humans to generate those temperatures marks a transition from Bronze to Iron age ~3000 years ago.
But still iron wants to bind back to oxygen in our wet atmosphere.
On planet Earth, iron wants to rust.
Iron itself has good mechanical properties.
(strong, stiff but slightly deformable)
Iron oxide rusy is porous and fragile.
Rust does not make for a good hip replacement.
However, add a bit (~11%) of chromium (and a sprinkling of other ingredients) and that iron becomes 'stainless'.
How does that help?
Well, stainless steel forms a microscopic layer of chromium oxide on its surface.
And oxygen (O2) can't get at the rest of the iron.
The iron can NOT rust.
Brilliant!
And rather conveniently, this layer is self-repairing.
Scratch it and more chromium oxide forms.
Again, the oxygen can't get to the iron to form rust. Very convenient indeed.
Late 19th century
The challenge for late 19th century engineers was to ⬇️ carbon content.
Carbon steel is brittle ➡️ Cracks form and propagate quickly.
Again, a sub-optimal property for hip replacement.
Around the turn of the century (that's 19th to 20th) engineers in France, England, Germany and the US worked out how to reduce the carbon to less than 1.2%. And that made..
Stainless steel useful.
V useful indeed.
The worlds of transport, infrastructure, building and healthcare would never be the same again.
The stage was set for hip replacement..
