Which ferrous metal is characterized by having a hard outer shell and brittle inside?

Which ferrous metal is characterized by having a hard outer shell and brittle inside?

• A. Low carbon steel
• B. Cast Iron
• C. Stainless steel
• D. Medium carbon steel

Answer: (B)

The idea being tested is how carbon content and microstructure in ferrous metals affect hardness and brittleness. Cast iron has a higher carbon content than steels, typically around 2–4%, and its structure (with graphite and cementite in the iron matrix) makes it very hard on the surface while giving little ability to deform plastically. That combination leads to a material that can resist indentation but tends to crack or break rather than bend, which aligns with the description of having a hard outer character and brittle interior behavior. The other options—low carbon steel, medium carbon steel, and stainless steel—are generally more ductile and tougher, so they don’t fit that hard-on-surface, brittle-forced-failure idea as well.

The idea being tested is how carbon content and microstructure in ferrous metals affect hardness and brittleness. Cast iron has a higher carbon content than steels, typically around 2–4%, and its structure (with graphite and cementite in the iron matrix) makes it very hard on the surface while giving little ability to deform plastically. That combination leads to a material that can resist indentation but tends to crack or break rather than bend, which aligns with the description of having a hard outer character and brittle interior behavior. The other options—low carbon steel, medium carbon steel, and stainless steel—are generally more ductile and tougher, so they don’t fit that hard-on-surface, brittle-forced-failure idea as well.