To determine the hardness of the material, the invention of the Swedish engineer Brinell is most often used - a method that measures surface properties and gives additional characteristics of polymer metals.
Material grade
Thanks to this discovery, the ways of the most effective use of plastics are now being evaluated. Plastics that are not too hard are tested for elasticity and softness to be used as sealing, sealing, and cushioning materials. The development of Brinell is a method that allows you to determine the strength and hardness of the material that will serve in important designs - in gears and rims, bearings under heavy load, details of threaded joints, etc.
Strength assessment is most precisely given by this method. The value of the parameter, which is denoted by P1B, is difficult to overestimate. The most commonly used Brinell development for this purpose is the method in which a five-millimeter steel ball is pressed into the material. According to the depth of indentation of the ball, GOST is determined.
History
In 1900, Johan August Brinell, an engineer from Sweden, made the method he proposed to world materials science famous. It was not only named after the inventor, but also became the most widely used, standardized.
What is hardness? This is a special property of a material that does not experience plastic deformation from local contact, which most often boils down to the incorporation of an index (harder body) into the material.
Restored and Unrestored Hardness
The Brinell method helps to measure the restored hardness, which is determined by the ratio of the load to the imprint volume, projection area, or surface area. Thus, hardness is volumetric, projective and surface. The latter is determined by the ratio: load to footprint. Volumetric hardness is measured by the ratio of the load to its volume, and projection - the load to the projection area, which left an imprint.
The unreduced hardness according to the Brinell method is determined by the same parameters, only the main measured value becomes the resistance force, the ratio of which to the surface area, volume or projection is shown by the indicator embedded in the material. The volumetric, projection and surface hardness are calculated in the same way: by the ratio of the resistance force either to the surface area of the embedded part of the index, or to the projection area of it, or to the volume.
Hardness test
The ability to resist plastic and elastic deformation when exposed to a harder indicator is a definition of hardness, that is, in fact, it is a test of material for indentation. The Brinell hardness measurement method is to measure how deep an index has penetrated the material. To know the exact value of the hardness of a given material, it is necessary to measure the penetration depth. For this there is the Brinell and Rockwell method, less often the Vickers method is used.
If the Rockwell method directly determines the depth of penetration of the ball into the material, then Vickers and Brinell measure the imprint by its surface area. It turns out that the deeper the index in the material, the larger the imprint. Any material can be tested for hardness: minerals, metals, plastics, and the like, but the hardness of each of them is determined by its own method.
How to find a way
The method of determining Brinell hardness is very good for heterogeneous materials, for alloys that are not too hard. Not only the type of material determines the measurement method, but also the parameters themselves that need to be determined. The hardness of the alloys is measured as if averaged, since they have materials with different characteristics. For example, cast iron. It has a very heterogeneous structure, there are cementite, graphite, perlite, ferrite, and therefore the measured hardness of cast iron is an average value, composed of the hardness of all components.
The Brinell method of measuring the hardness of metals is carried out using a large index, so that an imprint is obtained over a larger area of the sample. Thus, on cast iron, one can obtain in these conditions a value that is an average over many and different phases. This method is very good at measuring the hardness of alloys - cast iron, non-ferrous metals, copper, aluminum and the like. Quite accurately, this method shows the value of the hardness of plastics.
Rockwell method in comparison
It is good for hard and superhard metals, and the obtained value of hardness is also averaged. An indicator is the same steel ball or cone, but in addition to them, a diamond pyramid is also used. The imprint on the material when measured by the Rockwell method also turns out to be large, and the number of hardness for different phases is averaged.
The Brinell and Rockwell methods differ in principle: in the first, the result is presented as a quotient after dividing the indentation force by the surface of the imprint, but Rockwell calculates the ratio of the penetration depth to the unit of the scale of the depth measuring device. That is why Rockwell hardness is practically dimensionless, and according to Brinell it is clearly measured in kilograms per square millimeter.
Vickers method
If the sample is small or it requires measurements of an object smaller than the index fingerprint, which measures Rockwell or Brinell hardness, microhardness methods should be used, among which the most popular is the Vickers method. The diamond pyramid serves as an indicator, and the print is studied and measured by an optical system similar to a microscope. The average value will also be known, but the hardness is calculated over a much smaller area.
If the scale of the measured object is very small, then a microhardness meter is used, which can make an imprint in a separate grain, phase, layer, and the indentation load can be selected independently. Metallurgy allows using these methods to determine both the hardness and microhardness of metals, and material science in the same way determines the microhardness and hardness of non-metallic materials.
Range
There are three ranges for measuring hardness. In the macro range, the load is regulated from 2 N to 30 kN. The microrange limits not only the load on the index, but also the depth of penetration. The first value does not exceed 2 N, and the second - more than 0.2 microns. In the nanoscale, only the depth of penetration of the index is regulated - less than 0.2 microns. The result gives the nanosolidness of the material.
The measurement parameters depend primarily on the load that is applied to the index. This dependence even got a special name - size effect, in English - indentation size effect. The nature of the size effect can be determined by the shape of the index. Spherical - hardness increases with increasing load, therefore, this size effect is the opposite. The Vickers or Berkovich pyramid decreases hardness with increasing load (here the usual or direct size effect). The cone sphere, which is used for the Rockwell method, shows that an increase in the load first leads to an increase in hardness, and then, when the spherical part is introduced, it decreases.
Materials and methods of measurement
The hardest materials to date are two carbon modifications: lonsdaleite, half the hardness of a diamond, and fullerite, which is twice as much as a diamond. The practical application of these materials is just beginning, but for the time being, the hardest one is diamond. It is with its help that the hardness of all metals is established.
The determination methods (the most popular) were listed above, but in order to clarify their features and understand the essence, you need to consider others that can be conditionally divided into dynamic, that is, shock, and static, which have already been considered. The measurement method is otherwise called a scale. It must be recalled that the Brinell scale is still the most popular, where hardness is measured by the diameter of the imprint, which leaves a steel ball pressed into the surface of the material.
Hardness number
The Brinell method (GOST 9012-59) allows you to record the hardness number without units, denoting it HB, where H is hardness and B is Brinell itself. The footprint is measured as part of a sphere, not the area of a circle, as Meyer’s scale does for example. The Rockwell method is distinguished by the fact that by determining the depth of a ball or cone made of diamond that has entered the material, the hardness is dimensionless. It is designated HRA, HRC, HRB or HR. The formula for the calculated hardness is as follows: HR = 100 (130) - kd. Here d is the indentation depth, and k is the coefficient.
Using the Vickers method, hardness can be determined from the imprint left by a tetrahedral pyramid, pressed into the surface of the material, in relation to the load that was applied to the pyramid. The imprint area is not a rhombus, but part of the area of the pyramid. The unit size according to Vickers should be considered kgf per mm 2 , the unit HV is designated. There is also a Shore measurement method (indentation), it is more often used for polymers and has twelve measurement scales. Asker scales corresponding to Shore (Japanese version for soft and elastic materials) are in many respects similar to the previous method, only the parameters of the measuring device are different and other indicators are used. Another method according to Shore - with a rebound - for high modulus, that is, very hard materials. From this we can conclude that all methods that measure the hardness of a material are divided into two categories - dynamic and static.
Instruments and appliances
Hardness testers are called hardness testers, these are instrumental measurements. Testing affects the object in different ways, so methods can be destructive and not destructive. There is no direct relationship between all these scales, since none of the methods reflects the fundamental properties of the material as a whole.
Nevertheless, sufficiently approximate tables have been built where scales and different methods for categories of materials and their individual groups are connected. Creation of these tables became possible after a series of experiments and tests. However, theories that would allow one of the calculation methods to move from one method to another do not yet exist. The specific method by which hardness is determined is usually chosen based on the available equipment, measurement tasks, the conditions for carrying it out, and, of course, on the properties of the material itself.