Definition
An alloy steel containing chromium and molybdenum as its principal alloying elements, valued in aircraft construction for its high strength, toughness, and weldability. Commonly designated by SAE numbers in the 4130 and 4140 series, it is widely used for engine mounts, landing gear components, fuselage tubing in tube-and-fabric aircraft, and other highly stressed structural parts.
Plain English
A type of steel mixed with small amounts of chromium and molybdenum to make it stronger and tougher than ordinary steel, while still being easy to weld. It is a workhorse material for highly stressed aircraft parts.
Context Anchor
Seen in aircraft maintenance, airframe structure descriptions, repair drawings, and material specifications for steel tubing or highly loaded metal parts.
Derivation
Chromium comes from the Greek 'chroma' meaning color, because its compounds are brightly colored. Molybdenum comes from the Greek 'molybdos' meaning lead, since the ore was once confused with lead ore. Adding small amounts of these two metals to steel produces a tougher, stronger alloy than plain carbon steel.
Why Pilots Care
This material forms critical airframe structures whose strength and fatigue resistance directly affect aircraft safety and service life under repeated flight loads.
Intuition Check
Do not confuse chrome molybdenum steel with shiny chrome plating. Here, “chrome” means chromium is mixed into the steel itself to change the steel’s strength and behavior.
Example Sentence 1
The fuselage of the Piper Cub is built from welded chrome molybdenum steel tubing covered with fabric.
Example Sentence 2
The mechanic inspected the chrome molybdenum steel landing gear legs for cracks after the hard landing.