Effect of Carbon Addition on Mechanical Properties of Iron

Ductile-Brittle Transition Temperature (DBTT)
- The addition of a small amount of carbon to iron can increase the Ductile-Brittle Transition Temperature (DBTT).
- DBTT is the temperature at which a material changes from being ductile (able to undergo plastic deformation) to brittle (fracturing without significant plastic deformation).
- With the addition of carbon, the lattice structure of iron changes, leading to an increase in the DBTT.
- This means that the material becomes more brittle at lower temperatures, which can have negative implications for its mechanical properties.

Hardenability
- The addition of carbon increases the hardenability of iron.
- Hardenability refers to the ability of a material to be hardened by heat treatment, typically through quenching and tempering.
- Carbon atoms act as a hardening agent by forming carbide precipitates within the iron matrix.
- These carbides increase the strength and hardness of the material, making it more resistant to deformation.

Toughness
- The statement that toughness increases with a small amount of carbon addition is incorrect.
- Toughness refers to the ability of a material to withstand both plastic deformation and fracture.
- The addition of carbon can actually reduce the toughness of iron.
- The presence of carbon atoms can lead to the formation of brittle phases, such as cementite, which can reduce the material's ability to absorb energy before fracture.
- Therefore, the toughness of iron generally decreases with increasing carbon content.

Yield Point Phenomenon
- The addition of carbon to iron can result in the occurrence of a yield point phenomenon.
- In pure iron, deformation occurs uniformly, without any noticeable yield point.
- However, with the addition of carbon, the material can exhibit a distinct yield point where plastic deformation begins.
- This yield point phenomenon is a result of the interaction between carbon atoms and dislocations, which impedes their motion and leads to the observed yield behavior.

Conclusion
- The incorrect statement is "c) Toughness increases."
- The addition of a small amount of carbon to iron does not increase toughness; instead, it tends to decrease it.
- The other statements, a) DBTT increases, b) Hardenability increases, and d) Yield point phenomenon occurs, are correct and supported by the effects of carbon addition on the mechanical properties of iron.