Quality Control and Hardness: Brinell test

What is hardness and how it could be checked

We can easily say that hardness is the steel resistance to an indenter, under the action of a static load, without changing intensity and positioning. In simpler words, the hardness of a material is the resistance exhibited to a load that pricks the material, without changing in either intensity or position over time.

We would try to give in some articles some information of all of the different ways to measure hardness.

Hardness Tests

When we want to measure the hardness of a material, we subject it to a so-called hardness test. Going into detail, we can say that there is not just one hardness test, but several tests that basically change according to three parameters:

• The material I want to test
• The material of the indenter
• The shape of the indenter

The Brinell test (Hb)

The Brinell test consists of penetrating a very hard steel ball of a certain diameter (Db) into the material under test by applying a load (F) to the indenter. The diameter (Di) of the indentation left by the indenter on the surface of the workpiece is then measured.

It is reasonable to assume that the Brinell hardness exhibited by a certain material decreases as the diameter (Di) increases. In fact, the larger the indentation left by the indenter, the more easily the material has been deformed and is therefore less hard.

Critical Issues

This kind of test has some boundary conditions:

• The indenter has to be taken with proper dimension, in order to reduce the ball damage against the surface.
• The difficulty of comparing the results of several tests carried out on the same material. For the results of several tests to be comparable, it's necessary that the sinking of the ball into the material is always the same. This, however, is made difficult by constraining reactions. When the indenter first sinks into the material, the layers of material close to those directly involved in the deformation react to the state of tension by cracking. If I then perform a second test on a neighbouring layer of material, the material, which has previously been work-hardened, will not be in the normal test condition.

Conditions for successful testing

As in all hardness tests, there are conditions for successful testing:

• The indenter must act on a smooth and flat surface, free of oxides and surface impurities, in order to easily measure the indentation left by the indenter.
• The minimum thickness of the workpiece must be at least 8 times greater than the depth of the indentation, as the test is carried out on the finished product and must not in any way compromise the quality or performance of the workpiece.
• Do not apply the test to materials with a hardness greater than 450 Hb, to avoid damaging the indenter due to the excessive hardness of the material.
• Subsequent tests should not be carried out too close together, due to the work hardening phenomenon described in the previous paragraph.

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The other hardness tests

Compared to the Brinell test, there are two other tests that follow the same philosophy but essentially change for the indenter:

• The Vickers test uses a straight pyramid with a square base and a vertex angle of 136° in diamond as the indenter and is therefore able to test materials that are significantly harder.
• The Knoop test, in which the indenter is a pyramid with a length/width ratio of approximately 7:1, is called the micro-hardness test because the applied loads are very light.

There is also a third test, the Rockwell test, which we will discuss in the next article, analysing it in detail and highlighting similarities and differences to the above-mentioned tests.

Need advice?

At Lorenzon, every product manufactured and sold is rigorously checked before it's shipped to the customer. Dimensional, roughness and hardness checks are carried out with the utmost accuracy, to which non-destructive tests such as liquid penetrant and ultrasound are added on request.

We are at your disposal for any doubts or curiosity and to evaluate, together, which is the best knives according to its application and the material to be cut.

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