Mixing steel with various amounts of other elements produces steel alloys with mechanical properties superior to steel alone. SAE 4140 and 4150 steels are standard alloy steels. The major criteria used for comparing alloy steels are chemical composition and tensile strength.
The Society of Automotive Engineers, or SAE, and the American Iron and Steel Institute,or AISI, use a four-digit system to designate the chemical composition of steel. For steel alloys, the first two digits indicate the major alloying elements present and the last two digits give the carbon content in hundredths of a percent. Consequently, 4140 and 4150 steels have common alloying elements but different amounts of carbon.
Alloy steels having “41” as the first two digits are commonly called chromium-molybdenum steels because they contain 0.80 to 1.10 percent chromium and 0.15 to 0.25 percent molybdenum. The presence of chromium and molybdenum make the alloy steels stronger and harder than standard carbon steel.
SAE 4140 and 4150 have carbon contents averaging 0.40 percent and 0.50 percent, respectively. SAE 4140 has a tensile strength of 655 megapascals and SAE 4150 a tensile strength of 729.5 megapascals. Manufacturers use SAE 4140 for making average size automotive parts, such as axle shafts, propeller shafts and steering knuckles. SAE 4150 is used primarily for gears and other parts requiring hardness, strength and toughness.