How should we learn about different hardness testers?

Hardness is the resistance of a material to being scratched or penetrated, which is why we are measuring the cohesion between the atoms of the material. This, therefore, is related to resistance to deformation and breakage, and the harder a material is, the stronger it will also be.

With metals, the Martens method is used, which consists of measuring the groove left by a pyramid-shaped diamond point that moves on the surface. However, it is more common to speak of other types of tests in which the mark left on a material is measured when another piece of defined shapes, called a penetrator, is nailed.

The method consists of applying a force, which is usually proportional to the square of the indenter size (F = k · D 2) and measuring the footprint that remains. Depending on the shape of the penetrator and the way of applying force, we have several hardness tests, among which three stand out: the Brinell method, the Vickers method and the Rockwell methods.

This method consists of applying a force to a steel ball and calculating the quotient between the force and the surface of the footprint, which is given by the expression S = Π · D · f

The diameter d of the footprint is easily measured with a microscope, but the depth f is not straightforward. Therefore, the mathematical relationship between the values is sought:

Now the force is divided by the surface of the footprint and the Brinell hardness value is obtained, which is expressed through a normalized expression, in which the hardness value is written in kp / mm², the letters HB (Hardness Brinell), the diameter in mm of the ball, the force in kp and the time that the force is applied in seconds:

100 HB 5 250 30

Hardness tests make it possible to quickly determine the resistance of a material to being damaged, scratched or penetrated by a piece made of a different material and its state of hardening, being able, for example, to estimate its state of heat treatment. In this way we can know that the harder a material is, the more resistant it will be to deformation, wear and tear.

Types of Hardness Tests

There are different hardness test methods to determine the resistance of materials:

Vickers hardness testing

This method is considered the best alternative to the Brinell hardness test when vickers hardness tester is used for testing materials of smaller thickness and very high hardness. It is also an indentation method; in this case the indenter is a diamond pyramid with a square base and an angle at the vertex of 136 °. The possibility of using loads from a few grams (microhardness) to 120 Kgf allows a wide range of utility. It is the method used to make hardness maps in welded joints.

Brinell

The Brinell hardness method was proposed by the Swedish engineer Johan August Brinell in 1900, being the oldest hardness method. It is a scale of measurement by means of the indentation method, the penetrator used is a tungsten sphere, applying different loads for a certain time the hardness is obtained as a function of the average diameter of the mark. It can be used in a wide range of materials and sizes.

Rockwell hardness testing

This hardness calculation test consists of measuring the penetration depth of an indenter or penetrator in the material under the action of a predetermined load. It is the most used method to measure hardness because it is the durometer, and the method is simpler. By varying the penetrators (tungsten balls or diamond cone at 120º) and loads, materials in wide ranges of hardness and thicknesses can be tested.

leeb hardness testing

According to Leeb’s dynamic principle, the hardness value can be derived from the loss of energy from a defined impact after hitting a metal sample, similar to the principle employed by Shore. The indentations left by this method are smaller (around 0.5mm in diameter in steel samples, compared to the 3mm that a Brinell test would produce).

Depending on the specimen and the indenter, which varies in geometry, size, weight, material and spring force, there are several impact devices and hardness units. The hardness results obtained in Leeb hardness testing or HL hardness can be converted to the traditional HRC, HB and HV hardness scales, in order to have a greater appreciation of the tested samples.