To determine the hardness of a 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 evaluation
It is thanks to this discovery that the most effective use of plastics is now being evaluated. Not too hard plastics are tested for elasticity and softness in order to be used as a sealing, sealing and cushioning material. Brinell's development is a method to determine the strength and hardness of a material that will serve in important applications - in gears and rims, bearings under heavy load, threaded fittings, etc.
This method gives the most accurate assessment of strength. The value of the parameter, which is denoted by P1B, is difficult to overestimate. The most commonly used for this purpose is the development of Brinell, a 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, an engineer from Sweden Johan August Brinell, the method he proposed to the worldmaterials science, made famous. It was not only named after the inventor, but 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 action, which most often comes down to the introduction of an indexer (harder body) into the material.
Recovered and non-recovered hardness
The Brinell method helps to measure the recovered hardness, which is determined by the ratio of the magnitude of the load to the volume of the print, projection area or surface area. Thus, hardness can be volumetric, projection and surface. The latter is determined by the ratio: load to the area of the imprint. Bulk hardness is measured by the ratio of the load to its volume, and the projection hardness is the load to the projection area that the imprint left.
Unrestored hardness by the Brinell method is determined by the same parameters, only the resistance force becomes the main measured value, the ratio of which to the surface area, volume or projection is shown by the index embedded in the material. The volumetric, projection and surface hardness is 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 its projection area, or to the volume.
Determination of hardness
The ability to resist plastic and elastic deformation when exposed to a material harderThe index is a determination of hardness, that is, in fact, this is an indentation test of the material. The Brinell hardness method is a measurement of how deeply an inductor has penetrated a material. To know the exact value of the hardness of a given material, it is necessary to measure the penetration depth. To do this, there is the Brinell and Rockwell method, the Vickers method is less commonly 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 print area. Absolutely any materials 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 Brinell hardness method is very good for inhomogeneous materials, for alloys that are not too hard. Not only the type of material determines the method of measurement, but also the parameters themselves that need to be determined. The hardness of alloys is measured, as it were, on an average, since materials with different characteristics coexist in them. 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, consisting of the hardness of all components.
Measurement of the hardness of metals by the Brinell method is carried out using a large indexer, so that the imprint is obtained on a larger area of the sample. Thus, under these conditions, it is also possible to obtain a value for cast iron, which is an average for many and different phases. This method is very good when measuring the hardness of alloys - cast iron, non-ferrous metals, copper, aluminum and the like. This method accurately indicates the hardness value of plastics.
Rockwell comparison
It is good for hard and super hard metals, and the resulting hardness value is also averaged. The same steel ball or cone serves as an indicator, 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 hardness number for different phases is averaged.
The Brinell and Rockwell methods differ in principle: the first one presents the result as a quotient after dividing the indentation force by the surface of the imprint area, while Rockwell calculates the ratio of the penetration depth to the scale unit of the instrument that measures the depth. 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 too small or it is necessary to measure an object smaller than the size of the indenter imprint, which measures Rockwell or Brinell hardness, microhardness methods should be used, among which the Vickers method is the most popular. The index is a diamond pyramid, and the imprint is examined and measured by an optical system similar to a microscope. The average value will also be known, but the hardness is calculated frommuch smaller area.
If the scale of the measured object is very small, then a microhardness tester is used, which can make an imprint in a separate grain, phase, layer, and the indentation load can be selected independently. Metal science allows using these methods to determine both the hardness and microhardness of metals, and materials 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 indexer, but also the penetration depth. The first value does not exceed 2 N, and the second - more than 0.2 μm. In the nanorange, only the depth of incorporation of the index is regulated - less than 0.2 µm. The result gives the nanohardness of the material.
Measurement parameters depend primarily on the load applied to the index. This dependence even received a special name - the 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 reversed. The Vickers or Berkovich pyramid reduces hardness with increasing load (here, the usual or direct size effect). The cone-sphere, which is used for the Rockwell method, shows that increasing the load first leads to an increase in hardness, and then, when the spherical part is introduced,decreasing.
Materials and measurement methods
The hardest materials currently in existence are two modifications of carbon: lonsdaleite, which is half as hard as diamond, and fullerite, which is twice as hard as diamond. The practical application of these materials is just beginning, but for now, diamond is the hardest of the common ones. It is with its help that the hardness of all metals is established.
Determination methods (the most popular) were listed above, but in order to understand their features and understand the essence, you need to consider others that can be conditionally divided into dynamic, that is, percussion, and static, which have already been considered. The measurement method is otherwise called the scale. It should be recalled that the most popular is still the Brinell scale, where hardness is measured by the diameter of the imprint, which leaves a steel ball pressed into the surface of the material.
Determination of the hardness number
The Brinell method (GOST 9012-59) allows you to write down the hardness number without units of measurement, denoting it HB, where H is hardness (hardness), and B is Brinell itself. The area of an imprint is measured as part of a sphere, not the area of a circle, as the Meyer scale does, for example. The Rockwell method is distinguished by the fact that by determining the depth of a diamond ball or cone that has entered the material, the hardness is dimensionless. It is designated HRA, HRC, HRB or HR. The calculated hardness formula looks like this: HR=100 (130) - kd. Here d is the indentation depth and k is the coefficient.
Vickers hardness can bedetermined by 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 area of the imprint is not a rhombus, but part of the area of the pyramid. Vickers units should be considered as kgf per mm2, denoted by the HV unit. There is also a method of measurement according to Shore (indentation), more often used for polymers and has twelve measurement scales. Shore-corresponding Asker scales (Japanese modification for soft and elastic materials) are in many ways similar to the previous method, only the parameters of the measuring device are different and different indexes 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.
Tools and appliances
Devices for determining hardness are called hardness testers, these are instrumental measurements. Testing affects an object in different ways, so methods can be destructive or non-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 constructed, where scales and different methods are connected for categories of materials and their individual groups. The creation of these tables became possible after a series of experiments and tests. However, theories thatallowed one of the calculation methods to move from one method to another does not yet exist. The specific method by which hardness is determined is usually chosen based on the available equipment, measurement tasks, measurement conditions, and, of course, from the properties of the material itself.
