Definition
Steel that has been heat-treated to increase its hardness and wear resistance. The process typically involves heating the steel above its critical temperature, then quenching it rapidly in oil, water, or another medium so that the internal structure transforms into a harder form called martensite. Hardened steel is usually tempered afterward to reduce brittleness while retaining most of its hardness.
Plain English
Steel that has been heated and quickly cooled to make it much harder and more resistant to wear, then reheated slightly so it isn't too brittle.
Context Anchor
Seen in aircraft maintenance when discussing tools, pins, gears, bushings, bearings, shafts, and other parts that must resist wear.
Derivation
Harden' comes from the Old English heardian, meaning 'to make hard or firm.' In a metallurgical context, hardening refers specifically to a controlled heat-treatment process, not just 'making something tougher' in a general sense.
Why Pilots Care
Hardened steel parts resist fatigue and wear under the high stresses of flight, reducing the risk of component failure.
Intuition Check
Hardened steel does not just mean “strong steel.” It means steel that has gone through a specific treatment to make it harder and more wear-resistant.
Example Sentence 1
The cam lobes in a piston engine are made of hardened steel to resist wear from the constant pressure of the valve lifters.
Example Sentence 2
Landing gear components are typically made from hardened steel to handle repeated high-impact landings.