Cast Irons — Relative Softness of Gray vs. White Iron Fill in the blank: Gray cast iron is __________ than white cast iron.

Introduction / Context:
Gray and white cast irons differ in the form taken by carbon during solidification. The microstructural form of carbon controls hardness, machinability, wear resistance, and fracture behavior. Recognising these differences is essential for selecting cast irons for machine bases, wear parts, or hard liners.

Given Data / Assumptions:

• Gray iron contains flake graphite in a ferrite/pearlite matrix.
• White iron solidifies as iron carbide (cementite) networks (ledeburite), with little to no free graphite.
• Properties are considered at room temperature.

Concept / Approach:
Flake graphite in gray iron interrupts the metal matrix, lowering hardness and strength but greatly improving machinability and damping. White iron’s cementite-rich structure is very hard and brittle, giving high wear resistance but poor machinability. Therefore, gray iron is characteristically softer than white iron.

Step-by-Step Solution:
Identify carbon morphology: graphite flakes (gray) vs. cementite networks (white).Map morphology to hardness: graphite → softer; cementite → harder.Conclude gray iron is softer than white iron.

Verification / Alternative check:
Hardness measurements: gray iron typically shows lower Brinell hardness than white iron of similar composition.

Why Other Options Are Wrong:
Harder: contradicts the microstructural basis.Stronger in tension: gray iron has relatively low tensile strength due to flake graphite stress concentrators.More brittle: white iron is generally more brittle because of cementite.

Common Pitfalls:
Assuming all cast irons are hard; gray irons are intentionally softer for machinability and damping.

Final Answer:
softer