Alloy Steel 4140 / 4340: Properties & Heat Treatment

Low-alloy chromium-molybdenum steels such as 4140 and 4340 are the backbone of demanding mechanical parts. 4140 is versatile (quench & temper to ~28-32 HRC), while 4340 adds nickel for higher toughness in aerospace and heavy forgings.

Typical heat treatment: normalize, then quench and temper to the required hardness. Always control cooling rate and tempering temperature to hit the target strength-toughness balance.