In the carburising stage of case hardening, what happens?

• A. In carburising, metal is placed in open air at high temp
• B. Carburising reduces carbon content on surface
• C. Carburising involves heating a metal to 100 C
• D. Carburising involves placing metal in a ceramic box packed with carbon and heated to 930-950 degrees allowing carbon to diffuse in

Answer: (D)

Carburising works by driving carbon into the surface of the metal at high temperature to create a high-carbon case while the inner portion stays relatively low in carbon. In pack carburising, the metal is placed in a ceramic box packed with carbon and heated to about 930–950°C, which encourages carbon atoms to diffuse into the surface. This creates a hard, wear-resistant outer layer that can then be quenched to develop hardness while keeping the core tougher.

The other options don’t fit: heating in open air would cause oxidation rather than controlled carbon diffusion; carburising increases, not reduces, surface carbon; and 100°C is far too low for carbon diffusion into steel.