Which statement describes the second effect of aluminum in steel, contributing to a finer grain structure?

• A. Up to 0.20% aluminum is used as a deoxidizer
• B. Increases hardenability
• C. Inhibits grain growth by forming dispersed oxides or nitrides, yielding a finer grain structure
• D. Raises carbon content

Answer: (D)

Aluminum in steel can refine the grain by pinning grain boundaries during heating and processing. The aluminum forms tiny dispersed oxide or nitride particles (like Al2O3 or AlN) that sit at or near grain boundaries and stop them from moving as the steel is heat-treated. That restriction on grain boundary migration keeps the grains smaller, producing a finer grain structure. This grain-refining action is the second effect of adding aluminum, after its role as a deoxidizer.

So, the statement that best describes this second effect is that aluminum inhibits grain growth by forming dispersed oxides or nitrides, yielding a finer grain structure. The idea that aluminum raises carbon content isn’t correct, and while aluminum does deoxidize steel, that’s considered a different primary role, not the grain-refining effect described here.