Question

During heat treatment of steel, the hardness of various structures in increasing order is

• A. martensite, fine pearlite, coarse pearlite, spherodite
• B. fine pearlite, coarse pearlite, spherodite, martensite
• C. martensite, coarse pearlite, fine pearlite, spherodite
• D. spherodite, coarse pearlite, fine pearlite, martensite

Hint:

Always remember that Martensite is the hardest phase of steel, while Spheroidite is the softest.
Knowing these two extremes helps you eliminate incorrect options instantly in a multiple-choice format.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the correct order of microstructures formed during the heat treatment of steel, sorted from the lowest hardness to the highest hardness (increasing order).

Step 2: Key Formula or Approach: The hardness of steel microstructures depends directly on the distribution and scale of the hard cementite phase (\(\text{Fe}_{3}\text{C}\)) within the soft, ductile ferrite (\(\alpha\)) matrix.

Step 3: Detailed Explanation:

• Spheroidite (Spherodite): This structure is formed by heating carbon steel at a temperature just below the eutectoid for a long duration. The cementite forms sphere-like particles, which minimizes the interface area between phases. It represents the softest and most ductile steel structure.

• Coarse Pearlite: This forms at slower cooling rates during annealing. It consists of thick, alternating lamellae of ferrite and cementite. The thicker layers provide low resistance to dislocation motion, making it relatively soft, though harder than spheroidite.

• Fine Pearlite: This forms at higher cooling rates (such as normalizing). It consists of thin, closely spaced lamellae of ferrite and cementite. The high concentration of phase boundaries restricts dislocation movement, making it significantly harder than coarse pearlite.

• Martensite: This is a non-equilibrium phase formed via rapid quenching from the austenite region. It has a body-centered tetragonal (BCT) crystal lattice supersaturated with carbon. Due to extreme lattice distortion, dislocation glide is heavily restricted, making martensite the hardest and most brittle phase in steel.

Step 4: Final Answer:
The increasing order of hardness is spherodite, coarse pearlite, fine pearlite, martensite.