A pinch of carbon changes everything
What Makes Steel Stronger Than Iron?
Add a tiny amount of carbon to iron and it becomes several times stronger. Add a little more and it turns brittle.
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Step 1 of 5
Pure iron bends too easily.
In pure iron the atoms sit in neat layers that slide over one another when pushed. Sliding layers means bending — useful for shaping it, useless for holding a bridge up.
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Step 2 of 5
Carbon gets in the way.
Mix in a small amount of carbon and those atoms lodge between the layers. Now the layers cannot slide past each other easily, so the same push barely moves it. That is steel.
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Step 3 of 5
The amount matters enormously.
About one part in a hundred is the sweet spot. Much less and it is soft iron; much more and the carbon makes it hard but brittle, so it snaps instead of bending — that is cast iron.
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Step 4 of 5
Heating and cooling changes it again.
Heat steel red hot and plunge it into water and it becomes very hard and brittle. Warm it gently afterwards and some of the toughness returns — which is how a blade gets both an edge and some give.
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Step 5 of 5
Other metals give it new tricks.
Add chromium and the surface forms an invisible skin that stops rust — that is stainless steel. Other additions make it springy, or magnetic, or able to keep an edge when hot.
Steel is endlessly recyclable. A magnet pulls it out of a pile of mixed rubbish, which is why so much of it actually does get used again.
The short version
Steel is iron with about one percent carbon, which stops the metal's layers sliding and makes it far stronger.
Try it yourself
Bend a paperclip back and forth. It gets harder to bend at the fold — you are jamming the layers, the same way carbon does.
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