Definition of Mechanical Properties

Mechanical properties of steel are defined as the reaction of the material to certain types of external forces. Mechanical properties include:

Tensile strength
The maximum force that a material can withstand before fracturing. Also called Ultimate Strength. This is usually reported in terms of force per unit of area: lbs. per square inch or newtons per millimeter squared.

Yield strength
The force that a material can withstand before permanent deformation occurs. Also reported as force per unit of area.

Ductility
The ability of a material to deform without fracturing. Generally reported as elongation and reduction of area in a cross section that has been purposely fractured.

Tensile, Yield, and Ductility of steel are determined by performing a Tension Test in which a standard sample of the material is subjected to a pulling force that increases gradually until the material deforms, stretches, and fractures.

Hardness
The resistance of the material to penetration. Not to be confused with hardenability, which is a measure of the ability of a certain steel to respond to heat treatment. Hardness is measured by applying a standard force to the surface of the steel through a small, hardened ball point, and then measuring the diameter of the resulting impression. Hardness usually is reported as a value on one of two industry standard scales, Brinell or Rockwell. Brinell hardness is usually more accurate for measuring hardness of plate products.

Impact strength
The ability of a material to withstand a high velocity impact. Impact strength is measured by subjecting a standard notched sample to a swinging weight. As it is often important to know how the steel will perform in colder environments, this test is often done at sub-zero temperatures. Known as a notch test or Charpy test, the testing requires three standard samples of a defined grain orientation. The results are reported as foot-lbs or joules, showing the average of the three specimens and the lowest value of the three, at the testing temperature.

Physical properties of steel pertain to the physics of the material, such as density, electrical conductivity, and coefficient of thermal expansion. The term “physical properties” is often used erroneously to refer to mechanical properties.