Cast iron and brittleness

pukb

I want to know, what exactly gives brittleness to cast iron. Have read many articles on it but people beat around bush and dont answer the question what exactly makes it very brittle.

Thanks

Simon Bridge

It breaks easily instead of bending.
It boils down to the nature of covalent bonds.
You'll have seen these answers in your searches - what is it that is unsatisfactory about them?

Matcon

With flake graphite cast iron, it's the graphite flakes that make it brittle. Cohesion between layers of atoms in graphite is very low in comparison to iron. This means that graphite flakes are effectively microcracks. Seen in a polished section under moderate magnification, they look like cracks.

With high aspect ratio, there is high stress concentration at the tip of the flakes when a tensile stress is applied normal to the plane of the flake. Since there are large numbers flakes in the structure, a crack starting at the tip of one flake does not have to travel far before it encounters another so the total crack length increases very quickly until it reaches a critical length.

pukb

additional question : how does cast iron, then help in vibration absorption. How is its material damping ratio high?

Matcon

It has a relatively low elastic modulus. A grey cast iron would have a typical elastic modulus of around 70GPa while low carbon steel would be around 200GPa.