What causes brittleness in cast iron?

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Discussion Overview

The discussion centers on the factors contributing to brittleness in cast iron, particularly focusing on the structural and material properties that lead to this characteristic. Participants explore both theoretical and practical aspects of brittleness, as well as related properties such as vibration absorption.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses a desire for a clear explanation of what specifically causes brittleness in cast iron, indicating dissatisfaction with existing literature.
  • Another participant suggests that the nature of covalent bonds contributes to brittleness, although they do not elaborate on this point.
  • A participant explains that in flake graphite cast iron, the presence of graphite flakes contributes to brittleness, as these flakes act like microcracks due to low cohesion between atomic layers, leading to stress concentration and rapid crack propagation.
  • One participant poses an additional question regarding how cast iron can effectively absorb vibrations despite its brittleness, inquiring about its material damping ratio.
  • A participant notes that grey cast iron has a relatively low elastic modulus compared to low carbon steel, suggesting a relationship between this property and its behavior under stress.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specific causes of brittleness in cast iron, and multiple viewpoints regarding the contributing factors remain present in the discussion.

Contextual Notes

The discussion does not fully address the assumptions underlying the claims made about the material properties of cast iron, nor does it resolve the relationship between brittleness and vibration absorption.

pukb
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I want to know, what exactly gives brittleness to cast iron. Have read many articles on it but people beat around bush and don't answer the question what exactly makes it very brittle.

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

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