Engineering Materials - Study Mode

[#146] The hardness of steel increases if it contains
Correct Answer

(B) Martensite

Explanation

Solution: Martensite is a hard and brittle microstructure of steel that forms when steel is rapidly cooled from a temperature above the critical transformation temperature. The rapid cooling prevents the formation of other microstructures like pearlite, ferrite, or cementite, resulting in a high hardness. Austenite (Option A) is the high-temperature phase of steel, which can transform into martensite upon rapid cooling. However, it is not inherently hard like martensite. Pearlite (Option C) is a microstructure composed of alternating layers of ferrite and cementite. It does not contribute significantly to the hardness of steel. Cementite (Option D) is a hard and brittle compound of iron and carbon, but it does not directly increase the hardness of steel when it is present in the microstructure. Therefore, Option B: Martensite is the correct choice as it directly correlates with the increased hardness of steel.

[#147] Gamma-iron occurs between the temperature ranges of
Correct Answer

(C) 900°C to 1400°C

Explanation

Solution: As the iron cools further to 1,394°C its crystal structure changes to a face centered cubic (FCC) crystalline structure. In this form it is called gamma iron (γ-Fe) or Austenite. γ-iron can dissolve considerably more carbon (as much as 2.04% by mass at 1,146°C). This γ form of carbon saturation is exhibited in stainless steel.

[#148] Delta-iron occurs between the temperature ranges of
Correct Answer

(D) 1400°C to 1530°C

Explanation

Solution: As molten iron cools down, it solidifies at 1,538°C (2,800°F) into its δ allotrope, which has a body-centered cubic (BCC) crystal structure. δ-iron can dissolve as much as 0.08% of carbon by mass at 1,475°C.

[#149] Cast iron is manufactured in
Correct Answer

(B) Cupola

Explanation

Solution: Cast iron is manufactured in Cupola

[#150] The molecules in a solid move
Correct Answer

(D) Back and forth like tiny pendulums

Explanation

Solution: The molecules in a solid move Back and forth like tiny pendulums