Definition
A heat-treatment process that produces a hard, wear-resistant outer layer (the case) on a steel part while leaving the inner core soft and tough. Carbon, nitrogen, or both are diffused into the surface of the metal at high temperature, then the part is hardened so the outside resists wear and the inside resists shock and shattering.
Plain English
A way of treating a steel part so the outside becomes hard and tough to wear, while the inside stays softer and able to absorb shock without cracking.
Context Anchor
Seen in aircraft maintenance when discussing steel parts such as gears, shafts, pins, and other parts that need a hard wearing surface.
Derivation
‘Case’ here is the old metalworking sense of an outer covering or shell — the same idea as a case wrapping around something. So ‘case hardening’ literally means hardening the outer shell of the part, not the whole thing.
Why Pilots Care
Case-hardened parts last longer in high-load areas without becoming brittle enough to fracture in flight.
Analogy
Think of a part with a hard outer shell and a tougher center. The shell resists wear, while the center helps the part avoid cracking under load.
Intuition Check
Case hardening does not mean hardening a box or cover. Here, case means the outside surface layer of the metal part.
Example Sentence 1
The camshaft lobes are case hardened so they resist wear from the lifters while the shaft itself stays tough enough to handle engine loads.
Example Sentence 2
Case hardening allows the crankshaft to handle repeated high torque without surface damage or internal failure.