Material hardness, Types of Hardness & Testing Methods

What is a material hardness? (Definition Hardness)

Material Hardness is the ability of a metal to resist the penetration of another, more solid body into it.

The hardness of a metal is a very important characteristic since it is closely related to such basic characteristics of metals and alloys as strength, wear resistance, etc.

Currently, there are many methods for determining the hardness of metals. Let's consider some of them, the most widely used in the industry.

If we define hardness, material hardness characterizes the resistance that a material opposes to the penetration of a hard body, and this hardness is evaluated in particular according to the Rockwell index.

To establish the hardness tests of a given material, the pressure is applied to it using a diamond pyramid, and its deformation is observed: the harder a material, the less it deforms and, consequently, the higher the Rockwell index will be.

Objectives of the Hardness Testing

• Compliance with safety requirements and also regulatory standards;

• Approximation of mechanical strength and wear resistance;

• Quality assurance and control.

Types of Hardness Testing

Among the most used hardness tests methods we have:

• Brinell hardness test method (HB)

• Vickers method (HV)

• RockWell hardness test method (RC, HRA, HRB, HRC)

The hardness test surface consists of making a mark on the surface of the part, by applying a pressure penetration tip. The measure is given according to the applied load and the characteristics of the impression mark left.

Testing hardness: Which is the most suitable for each material?

How to measure hardness?

The knowledge of the mechanical properties of a material is of great importance to determine its viability in an engineering project. Among the various mechanical hardness testing responsible for the mechanical characterization of brittle materials, hardness testing is one of the most important and applied.

Hardness is a mechanical property whose concept is related to the resistance that material presents to the scratch or to permanent deformation when pressed by another material or by standardized markers.

The meaning of hardness varies according to the context, for example, during a project, hardness is the basis of measurement used to determine mechanical strength, in machining to determine cut resistance, in mechanics to determine penetration resistance, in mineralogy to determine the scratch resistance and metallurgy to determine the resistance to localized plastic deformation and wear resistance.

Hardness is a measure that estimates how long a solid material resists permanent deformation when a force is applied to it.

There are different ways of evaluating the hardness of the materials and with these different scales of hardness have been created.

Rockwell hardness scale (HR)

The hardness scale used in the measurement of steel balls is the Rockwell Scale. This scale is based on the level of penetration of a diamond tip into the material.

The Rockwell scale has 7 variations named by letters: Rockwell A, B, C, D, E, F, G. To measure the hardness of steel balls that have heat treatment, the most used scale is the Rockwell C. of tungsten carbide spheres is measured using the Rockwell A scale.

Brinell test hardness scale (HB)

The Brinell hardness scale is measured through the penetration of a tungsten carbide sphere of different measurements on the surface of the brittle materials to be tested. It was the first hardness scale widely used in the steel market.

In the Brinell hardness test, a hardened steel sphere is slowly compressed onto a flat, polished and clean surface of a metal, by means of a load, for a certain time, producing a spherical cap. Brinell hardness is represented by the letters HB. Brinell hardness (HB) is the relationship between the applied load and the spherical cap area printed on the material tested.

The Brinell test is widely used to assess the hardness of non-ferrous metals, cast iron, steel, steel products in general and non-tempered parts.

Another important factor to note is that it is the only accepted test on metals of uniform internal structure.

However, the Brinell hardness test is limited due to the use of the sphere, which, if it is made of tempered steel, can only measure hardnesses up to 500 HB, since greater hardnesses damage the sphere.

Knowing the hardness number and strength of a metal, steel or coating is very important to choose the correct alternatives for preventing and correcting wear on machine parts.

But hardness is not the only variable used to determine the resistance to wear by abrasion or erosion of a part. In addition to hardness, it is also important to note the wear mechanism and the environment in which that part operates.

For example, a part that works at high temperatures (pieces of equipment from the rolling process in steel mills, for example), needs materials that support this type of environment, such as a chromium carbide coating, for example.

Most importantly, proper knowledge of these hardness relationships, alloy properties and wear mechanisms can make the choice of materials quite assertive and can contribute to improvements in MTBF indicator by more than 500%.

Mohs scale

The Mohs scale is used mainly with minerals, but it is also used to measure the rebound hardness of glass. On this scale of 1 to 10, the number 10 is the diamond, a mineral that can scratch all the others.

Number 9 is sapphire, the mineral that scratches all but diamond and so on. The glass is at level 6 of the Mohs scale.

Vickers scale (HV)

The Vickers scale also uses a diamond tip as a penetrator with a major advantage: a single penetrator measures any material. For this reason, the Vickers scale is applied to metals, ceramics, nano hardness measurements, among others.

The importance of hardness in steel balls is relevant mainly in applications that require high mechanical performance, such as bearings and material grinding.

However, the elasticity and malleability of the material must also be taken into account.

As an example, we can talk about the carbon steel balls that are cemented that have a surface hardness similar to the chrome steel balls. However, chrome steel has greater elasticity and malleability than carbon steel, making it more resistant.

Determination of hardness by the depth of indentation of a diamond cone (Rockwell method)

A 120 ° diamond cone is pressed into the metal with a constant preload of 10 kg, and then with a shelf with a load of 60 or 150 kg.

The diamond cone is mounted in a mandrel 4.

The sample is installed “on the stage 3 and raised using the handwheel 2 to a load of 10 kg.

The handle 1 releases the weights 6 , which create a force to press the cone into the metal. Depth of indentation, i.e. the value of hardness, indicates indicator 5.

The rebound hardness values by this method are determined from the difference in the depth of indentation hardness of the diamond cone under the action of full and preloads.

The harder the metal, the shallower the diamond penetrates when pressed, the higher the hardness number.

The standard load for this method is 150 kg.

The hardness is indicated by H RC . In some cases, for example when measuring the hardness on a thin sample or when measuring the hardness of the surface layer of a metal, the load is applied up to 60 kg.

Hardness testing: Brinell indentation hardness or penetration methods

The hardness testing consists of balling a metal, i.e. driving an extra hard steel ball onto its surface, measuring the imprint left by the ball using a ruler (abacus), and reading its hardness on aboard.

The hardness of a metal, symbolized by the letter H is the resistance it offers to penetration efforts. This test has the advantage of being feasible in the workshops. The device for measuring hardness is a durometer.

Other hardness measurement methods also exist the Vickers hardness method (the ball is replaced by a diamond) used for very hard metals (HB greater than 400) or the Rockwell method (HR).

What makes a material hard or soft?

It all depends on the disposition that its atoms adopt in relation to each other and the links that unite them.

It all depends on the disposition that its atoms adopt in relation to each other and the links that unite them. These bonds are more or less strong according to their nature, which is determined by the conditions under which the material was formed (temperature, pressure…).

Thus we will obtain with carbon atoms a more or less hard material (diamond or graphite).

“These bonds can be compared to more or less stiff springs which connect the atoms”, summarizes Alfonso San Miguel, director of the Laboratory of condensed matter physics and nanostructures of the CNRS in Lyon. If chemists distinguish ten, some are more common.

The hardness tests of material is first and foremost a matter of bonds between atoms

The two strongest are the covalent bond, through which two atoms share an electron, and the metallic bond (iron, aluminum, etc.), which unites atoms by pooling free electrons.

Twenty times weaker than a covalent bond, the hydrogen bond connects molecules containing hydrogen atoms (water, biological tissues, etc.) by electrostatic interaction, a phenomenon comparable to the attraction between two electric charges.

The van der Waals bond, itself still ten times weaker is an electrostatic interaction due to the charge fluctuation of electrons.

Hardness value also depends on the structure of the materials

Since the same material can contain several types of bonds, it will be all the harder the less weak bonds it has. And these bonds will be all the stronger the shorter they are, knowing that the interatomic distance depends on the atoms: it is thus 1.54 angstroms between two carbon atoms in the diamond, against 2.28 between those of the bromine.

But the rigidity also depends on their arrangement: ordered in a compact network of meshes, atoms form solids.

The tubular geometry of a carbon nanotube thus makes this material more rigid in the length direction. Conversely, in soft materials such as polymers (gums, resins, etc.), long chains of atoms are organized like spaghetti in a dish, which makes these materials rather soft, while the atomic bonds are strong.