Hardness of Metals

the resistance of metals to indentation. Hardness is not a physical constant but rather a complex property that depends on the strength and plasticity of the metal, as well as on the method of measurement. The property is expressed quantitatively by a hardness number.

The indentation test is the most common method for measuring hardness. Here, the hardness is either equal to the load relative to the area of indentation or inversely proportional to the depth of indentation for a specified load. The indentation is usually made with a hardened steel ball (Brinell test, Rockwell test), a diamond cone (Rockwell test), or a diamond pyramid (Vickers test, microhardness test). Less common are tests involving impact loading, where hardness is measured by the rebound height of a small steel ball dropped onto the surface of the metal (Shore test), or measurements of the time of the dampening of the swings of a pendulum resting on the metal (method developed by Kuznetsov, Herbert, and Rebinder). A method of testing hardness with ultrasonic vibrations is coming into wider use; with this method, the response of an oscillating system (change in natural frequency) to the hardness of the metal is measured.

The hardness number is expressed in units of HB (Brinell test), HV (Vickers test), or HR (Rockwell test), with H standing for hardness. Since the Rockwell test uses both a steel ball and a diamond cone, the letters “B” (ball) “C” (cone), and “A” (also cone but with different load) are appended. Special tables and diagrams are available for converting a number obtained from, say, the Rockwell test into a number corresponding to another test. The choice of a hardness test depends on such factors as the material being tested and the dimensions and shape of the specimen or article.

Hardness is very sensitive to changes in the metal structure. Since the hardness of metals and alloys changes in the same way as does the yield point with a change in temperature or after various types of thermal and mechanical treatment, variations in the mechanical properties of metals after treatment are often monitored through the hardness, which can be readily measured. Microhardness measurements are used in studying the mechanical properties of individual grains, as well as the structural constituents of complex alloys.

In making a relative determination of a metal’s heat resistance, the long-term hardness or microhardness of the metal, measured at elevated temperatures over long periods (minutes, hours), is sometimes used.