Which term describes steel with a surface that is harder than the interior metal because the surface layer was hardened via special heat treating methods and is used in vehicle construction?

• A. Chassis
• B. Confined space
• C. Cribbing
• D. Case-Hardened Steel

Answer: (D)

The concept here is surface-hardening a metal so the exterior becomes very hard while the inside stays tough. This is done with case hardening processes like carburizing or nitriding, which add hard material to the surface but leave a ductile core. The result is steel that resists wear at the surface but can absorb impact without cracking, which is especially useful in vehicle construction where moving parts endure friction and load. That’s why this term fits: case-hardened steel describes steel with a hardened surface layer and a tougher interior, suitable for components that need both hardness and toughness. The other terms don’t describe material properties or processing: chassis is the vehicle’s frame, confined space is a safety term, and cribbing is stabilization blocks used in rescues.