Physical Properties of 4140 Steel

By James Croxon

American Iron and Steel Institute grade 4140 steel is one of the most used steels in terms of physical characteristics per dollar. In fact, AISI 4140 is used in industries from construction and engineering to knives and other bladed instruments. What it lacks in corrosion resistance and other specialty characteristics found in high alloy tool steels it makes up for in workability.

AISI 4140 comes in a variety of shapes and sizes.


Much of the versatility of AISI 4140 comes from its simple chemistry. With about .40 percent carbon and .85 percent manganese, it is a tough steel that is able to be heat treated and hardened. It also contains 1 percent chromium, which isn't enough to make it a stainless steel but adds to the overall toughness. It also contains .25 percent molybdenum and small amounts of other elements such as silicon, sulfur and phosphorous.

Strength Characteristics

In terms of strength, AISI 4140 hardened to between 18 and 22 Rockwell hardness has a yield strength of about 417.1 Mpa. It has an elongation constant of 25.7 percent and a tensile strength of 655.0 Mpa. The elastic modulus, measured in Giga Pascals, is between 190 and 210.

Thermal and Electrical Properties

In terms of thermal properties AISI 4104 has a thermal expansion measurement of 12.3 and a specific heat of 473 joules per kilogram Kelvin. It also posses a thermal conductivity of 42.7 W/mK. It has an electric resistivity of 222 10^-9 Wm.

Other Characteristics

Other characteristics include a density of 7.7 to 8.03 kilograms per cubic meter and an average hardness of 197 HB. It has a specific gravity of 7.81. AISI 4140 can be welded with medium carbon steel filler. It is annealed by a 205 degree Celsius (400 degree Fahrenheit) preheat followed by a 260 to 315 degree Celsius (500 to 600 degree Fahrenheit) post heat.