Iron Mechanical Properties: Carbon Addition Effects

Adding a small amount of carbon to iron significantly alters its mechanical properties, transforming it into steel. Understanding these changes is crucial.

Statement Analysis

Let's evaluate each statement regarding the effect of adding a small amount of carbon to iron:

• Statement 1: Ductile-Brittle-Transition-Temperature (DBTT) increases.

  This is generally true. Carbon atoms disrupt the iron lattice, hindering dislocation movement and promoting fracture mechanisms that become more dominant at higher temperatures. This raises the temperature below which the material behaves brittlely.

• Statement 2: Hardenability increases.

  This is correct. Carbon is a key element that enhances hardenability, which is the ability of steel to form hard martensite during quenching from the austenitizing temperature.

• Statement 3: Toughness increases.

  This statement is incorrect. While carbon increases strength and hardness, it typically decreases toughness. Higher carbon content leads to microstructures that are harder but more susceptible to fracture, reducing the material's ability to absorb energy before failing.

• Statement 4: Yield point phenomenon occurs.

  This is generally true for low carbon steels. The presence of interstitial carbon atoms can pin dislocations, leading to a distinct upper and lower yield stress, known as the yield point phenomenon.

Therefore, the INCORRECT statement is that toughness increases upon the addition of a small amount of carbon to iron.