Hardness Testing

Brinell Hardness

The brinell  hardness testing method is the first method invented for standardising hardness, from which other hardness measurement methods have been derived. Brinell hardness is the test force F divided by the contact area S (mm2) between the spherical indenter and the specimen calculated on the diameter d (mm) of the impression made when the indenter (a steel ball or cemented carbide ball with a diameter D mm) is pressed into the sample using a test force F and then removed. The symbols HBS is used when the indenter is a steel ball and HBW when the indenter is a cemented carbide ball. K is a constant (1/g=1/9.80665+0.102).

The same loading conditions (F/ ), the brinell hardness test obtained is almost the same for different hardness loads applied. In many countries tesing with small test loads is common as an application of this fact.  Testing with a test load of 2451 N or less can be conducted using the test load of a Vickers or Rockwell tester. For steel F/  = 30. For other softer materials an appropriate value is selected from 15,10,5,2.5,1.25 and 1. In JIS and ISO standards, the test force is 9.807 to 29420N, and the diameter of the spherical indenter is 1 to 10mm.

Vickers hardness

Vickers hardness testing is the most versatile testing method as it can be used with any test force.  More specifically there are many applications of micro hardness below 9.807N. Vickers hardness is the test force F divided by the constant area S (mm”) of the indenter and the sample calculated on the diagonal length d ( the average of 2 directions in mm) of the impression made when a pyramid shaped diamond indenter  (Ѳ = 136 between opposite faces) is pressed into the sample by the test force F (N) and then removed.

Knoop Hardness

Knoop hardness is the test force F divided by the projected area A (mm2) of the impression calculated based on the longer diagonal length d (mm) of indentation made when the pyramid shaped diamond indenter with apical angles of  130o and 172030 rhomboid cross section is pressed against the specimen by the test force F and then removed.  Knoop hardness can be replaced by replacing the indenter on micro hardness testing machines.

Rockwell and Rockwell Superficial

A diamond indenter with an angle of 120o and a tip radius of 0.2mm tip or spherical indenter (steel of cemented carbide) is used. The preliminary test force is applied first, the test force is applied and then the preliminary test force is applied again. Rockwell hardness and Rockwell superficial can be calculated based on the difference in depth H(µm) measured at the first and second application of the initial testforce.

The hardness is called Rockwell hardness when the preliminary test force is 98.07N or Rockwell superficial when it is 29.42N. Unique symbols are assigned to various load and indenter combinations.