Hardness

Hardness is the mechanical resistance of a material to the mechanical penetration of a harder test body opposes. Depending on the type of influence, one distinguishes different types of hardness. Thus, hardness is not only the resistance against tougher body, but also against softer, hard body. The definition of different hardness with respect to the strength of which represents the resistance of a material to deformation and separation.

Hardness is a measure of the wear resistance of materials. Hard lenses scratch less, hardened gears use less from. In the selection of cutting tools like milling head or lathe tool, the hardness is of particular importance, hard cutting stay sharp longer, but break down faster.

Hardness and examination are important priorities in the areas of solid state physics, materials science and analysis of materials and in earth sciences for the characterization of rocks and minerals. Hardness belongs to the fracture toughness, strength, ductility, stiffness, density and melting temperature of the material properties.

• 2.4.1 Standardized specification of the Vickers hardness
• 2.4.2 Dental alloys

• 2.6.1 For elastomers
• 2.6.2 For metals

Hardness and strength

The hardness of a material has only partially to do with the strength of the material, even if the strength of the test methods for hardness measurement based on the depth of penetration of various specimens affected. The influence of the strength can be reduced by the measurements on thin films, but can not be completely avoided.

In certain cases, the hardness of a material is, however, in a umwertbaren related to material strength. Can then be replaced by the relatively inexpensive hardness testing a usually much more complex tensile test. Of practical importance is the ability to make a re-evaluation of the Brinell or Vickers hardness on the tensile strength of structural steels. This material mix-ups can be detected, for example, in tests on steel structures.

Most materials have high hardness and high brittleness, they can thus hardly be plastically deformed and break suddenly. This is the basis, among others, the technique of glass- cutting.

For the design of components hardness and toughness must be sensible balanced: The hard, brittle component breaks easily, once a peak load occurs. Tough (ie less hard ) material would survive the harmless or only minor consequences. The latter, however, would wear out quickly. Therefore, it is often desired to provide a large internal area (core) of a tough, solid material having a very hard surface layer. This has two advantages: The resistance to wear increases and cracks can form much worse. The actual loads are taken indoors.

Hardness testing and hardness scales

In materials science, especially in the metals, especially test methods are used which measure the indentation hardness. This standardized test specimens are pressed under specified conditions in the workpiece. Following the surface or depth of the lasting impression is measured. In principle, one distinguishes static and dynamic hardness testing methods. The dynamic test method to bring the burden on the piece to be checked on a sudden; with the static method the load is constant or gradually increasing.

Hardness test Martens ( universal hardness )

The name of universal hardness belies the real use in industrial environments. There, and also in the laboratory, this method is rarely used.

The Martens hardness test method was named after the German physicist Adolf Martens (1850-1914) and is also called instrumented indentation testing. In 2003, the universal hardness was renamed Martens hardness. The method is described in DIN EN ISO 14577 ( Metallic materials - Instrumented indentation test for hardness and materials parameters ) standardized.

In this method, the power and the penetration depth is continuously measured during the loading and unloading phase. The Martens hardness ( HM) is defined as the ratio of the indicated maximum power to the associated contact surface and in newtons per square millimeter.

Unlike the Vickers or Brinell the plastic behavior of the material is not only determined, but it can from the obtained measurement curve, other material parameters such as the indentation modulus ( elastic indentation - EIT), the indentation creep (CIT ) as well as plastic and elastic of plastic deformation can be determined.

As the indenter following forms most commonly used are: the Vickers pyramid (see Vickers method ), a hard metal ball, a spherical diamond indenter and Berkovich indenter. The Berkovich indenter has a tip like a regular tetrahedron with edge angle 65 °. The outline of the impressions is typically approximately triangular.

The translation of the penetration depth to the contact surface must be determined for each Eindringkörperform. The contact area is calculated for Vickers and Berkovich body by the product of the square of the depth of penetration and the constant 26.43.

Rockwell hardness test (HR)

There are several of the American engineer and company founder Stanley Rockwell in 1920 developed hardness testing methods that are specialized for certain areas. The different processes are indicated by the unit and subsequent HR identifier; Examples of a Rockwell designation are HRA, HRB, HRC or HR15N in hardness test on sheets up to a thickness of 0.20 mm HR15T and beyond HR30Tm.

The Rockwell hardness of a material resulting from the penetration depth of a specimen upon application of a specific pre-and test load. Specimen, forces, duration and unit formulas are defined in the standard DIN EN ISO 6508-1 ( formerly DIN EN 10109 ). With a predetermined test load to the test specimen is biased into the surface of the workpiece to be tested. The depth of penetration of the test specimen is used as a pre-load in the reference plane. Thereafter, the penetrator is charged over a period of at least two seconds and a maximum of six seconds with the main load. Subsequently, this is removed, so that only the pre-load is effective. The difference in the penetration depth before and after placement of the main load is a measure of the Rockwell hardness of the material. The Rockwell units are calculated according to a ( different depending on the applied Normskale ) formula from the penetration depth. The penetration depth of the specimen is determined using a dial indicator, which is connected to the probe.

In the method according to the scale C ( unit HRC), a cone- shaped test piece of diamond with an apex angle of 120 ° and a rounded tip with a radius of 0.2 mm is used (the C stands for the English word cone for " cone" ). This test method is primarily used for very hard materials used. As further Rockwelleindringkörper hard metal balls are used having a diameter of 1.5875 millimeters ( HRB, HRF, HRG) or 3.175 mm (HRE, and HRH HRK).

Test procedure:

The Rockwell test is very fast, but places high demands on the clamping of the specimen in the testing device. It is unsuitable for specimens that yield elastically in the test instrument, for example pipes.

Examples of Rockwell hardness:

• A wave in a transmission, for example, have a hardness of 48 HRC,
• A stainless steel knife blade " stainless steel " hardness 53 HRC,
• A knife blade made ​​of Japanese Shiro - Gami steel ( white paper steel ) hardness up to 61 HRC,
• One from Ao - Gami steel ( blue paper steel ) even a hardness up to 65 HRC.

Above 65 HRC end in a rule, the possibilities for machining with geometrically defined cutting edges of surfaces ( turning, drilling, milling); harder surfaces must be sanded (machining with geometrically undefined cutting edges ).

Measuring range: For the method allowable hardness HRC values ​​must lie between 20 and 70.

Brinell hardness test

The method of hardness testing developed by the Swedish engineer Johan August Brinell in 1900 and presented at the World Exhibition in Paris comes in soft to medium hard metals (EN ISO 6506-1 to EN ISO 6506-4 ), such as mild steel, aluminum alloy, with wood (ISO 3350) and in materials with non-uniform structure, such as cast iron, are used. Here, a carbide ball is pressed with a predetermined test force F in the surface of the workpiece to be tested.

Formerly also steel balls were used as in addition to the indentor balls made ​​of hard metal. After the last state of standardization, a steel ball from 2006, however, is no longer permitted. The standard prescribes now available for all substances balls of cemented carbide. The beads used have a diameter of 10 mm, 5 mm, 2.5 mm, 2 mm and 1 mm.

After a loading time of 10 to 15 seconds for steels and cast iron, and 10 to 180 seconds for non-ferrous metals and their alloys, the diameter of the permanent indentation is measured in the workpiece and determines the surface of the impression. D is the diameter to be determined is the average of two mutually perpendicular diameters d1 and d2 of the permanent indentation. In anisotropic deformation of the necessary to calculate the hardness is averaged diameter of the largest and smallest diameter d1 d2.

The Brinell hardness is defined as the ratio of test load to the indentation surface. The test force in Newtons is multiplied by the value of 0.102 (ie, the reciprocal of 9.81 ) to convert the force unit Newton in the older unit kilogram-force. This ensures that hardness measurements using modern units produce the same result as historical values ​​based on today obsolete units.

In the above formula, the force F N, the ball diameter D, and the average indentation diameter d is to be used in mm. The value in the denominator results from the formula for the surface of the round side of a spherical segment, called a spherical cap.

Standardized specification of hardness

According to EN ISO 6506-1 also the method used, the ball diameter and the test load must always be stated next to the hardness value.

Where:

• 345 = hardness value
• HBW = procedure ( W stands for the material of the test sphere: tungsten carbide )
• 10 = ball diameter D in mm
• 3000 = Test force in kp

At a load that takes more than 15, the load time must also be specified. Example: 210 HBW 10/05/60

A variation of Brinell testing is the testing with the Poldi hammer, in which the impression of the sphere is generated by an undefined hammered by hand. Due to the sudden load is a dynamic hardness testing methods. The bullet penetrates the back into a metal rod with a certain hardness. The hardness of the specimen can then be calculated from the ratio of the two indentation diameter. The method has the advantage that any mounted specimens and built components can be checked on the spot with her. Although the hardness values ​​determined in this way do not exactly match the statically determined hardness values ​​match, however, for the claims made ​​in the industry they are sufficient in most cases. The term " Poldi " comes from the same steel plant in the Czech Republic Kladno, where this test method has been developed.

In unalloyed and low-alloy steels can be derived from the Brinell with certain tolerance, the tensile strength (Rm) of the material. Rm ~ 3.5 x HBW

Vickers hardness test ( HV)

Very similar to the Brinell testing is developed in 1925 by Smith and Sandland and named after the British aircraft company Vickers hardness test, which is used for testing of homogeneous materials and is also used for hardness testing of thin-walled or surface- hardened workpieces and marginal zones. It is governed by the standard DIN EN ISO 6507-1:2005 to -4:2005. In contrast to the Rockwell test an equilateral diamond pyramid is pressed with an opening angle of 136 ° at a fixed test load into the workpiece. In established by means of a measuring microscope length of the diagonal of the permanent indentation the indentation surface is calculated. The ratio of test force in newtons to the impression surface (d in millimeters ) yields by a factor of 0.1891 multiplied by the Vickers hardness (HV, Eng. VHN = Vickers Hardness Number). When this hardness test developed, it was still common practice to specify the test load in the unit kilogram-force, and the factor was 1.8544 (= 2 * sin 136 ° / 2).

Is

The number 0.102 is the conversion of Newton in newtons.

The Vickers hardness test is to be divided into three areas:

Standardized specification of the Vickers hardness

In addition to the hardness value and the test methods used and the test load must always be stated.

Where:

• 610 = hardness value
• HV = method
• 10 = test force F in newtons

At a load that does not s lasts 10 to 15, the exposure time must also be specified. Example: 610 HV 10/30

The Vickers test is usually performed on a fixed test unit, which does not wobble, or may be disturbed. For tests on very large and / or solid components, there are also portable hardness testers that are attached magnetically or mechanically or on the test piece.

The Vickers hardness test is extremely versatile and is one of the quasi non-destructive testing, since only a small damage to the component takes place, which can be accepted often. For components that may show no damage after the test, it is considered a destructive test methods, because the component is damaged by the Vickers hardness test.

Common applications for the Vickers hardness, for example, in the descriptor " 45H " with set screws with hexagon socket or " 14H " and " 22H " at grub screws with slot and in dentistry in dental alloys. The strength classes 14H, 22H, 33H and 45H is obtained by dividing the hardness values ​​by 10, so they correspond Vickers hardness HV ( min. ) of 140, 220, 330 and 450

Dental alloys

The hardness of dental metals by Vickers in precious metal alloys with the test load HV5 (5 kgf corresponds to 49.03 N) and measured at non-precious alloys with HV10.

For dental alloys three hardness values ​​can be distinguished:

• W = soft; Hardness of the alloy in the delivery state or after annealing
• A = cured; Hardness of the alloy after a specific heat treatment = " reward "
• G / b = Self- Pay: hardness of the alloy, which can be reached by slow cooling after casting can

In the test performance is to ensure that the holding time of the test load for 10-15 seconds. The sample must be firmly clamped and the test surface be absolutely perpendicular to the test direction. Fouling, etc. must be removed. The test was successful if the edges of the indentation uniform and the top of the pyramid are pressed in the center. In practice, it is recommended to perform several impressions, the max. and min. Value of these measurements to be ignored and to determine the average of the remaining.

Knoop hardness test

A variation of the Vickers hardness test is the Knoop hardness test (DIN EN ISO 4545-1 to -4: Metallic materials - Knoop hardness test ), which was developed by the American physicist and engineer Frederick Knoop 1939. The equilateral in the Vickers test diamond tip in the Knoop test a rhombic shape. The tip angles are 172.5 ° for the long and 130 ° for the short side. It is measured only the long diagonal of the indentation. Knoop test is often used in case of brittle materials such as ceramic or sintered material; in the hardness measurement on multilayer systems, it represents the most accurate method of measurement

Hardness test Shore

For elastomers

The Shore hardness, developed in 1915 by the American Albert Shore, is a material parameter for elastomers and plastics and is defined in the standards DIN EN ISO 868 and DIN ISO 7619-1. The core of the Shore hardness checker consists of a spring -loaded pin of hardened steel. The depth of penetration into the material under test is a measure of the Shore hardness, on a scale of 0 Shore (2.5 mm penetration) to 100 Shore (0 mm penetration depth ) is measured. A high number indicates a high hardness. With a Shore hardness tester is used an additional device which presses the sample to be measured with a force of 12.5 Newtons at Shore A or 50 Shore D to Newton in the measuring table. In determining the Shore hardness, temperature plays a more important role than in the determination of hardness of metallic materials. So here's the set temperature of 23 ° C on the temperature interval of ± 2 K is limited. The material thickness should be at least 6 millimeters. The hardness of the rubber is ( weakly crosslinked = soft rubber, hard rubber = highly crosslinked ) by the crosslinking determined. But even the filler content is paramount to the hardness of a rubber article.

• Shore -A is indicated for soft elastomers, as measured by a needle with a blunt tip. The end face of the truncated cone has a diameter of 0.79 millimeters, the apex angle is 35 °. Base weight: 1 kg, hold time: 15 sec handheld instruments must be read immediately when pressed on the sample most, the displayed value decreases with longer holding time.
• Shore -D is indicated in teeth elastomers as measured by a needle with a tapering angle of 30 ° and a spherical tip having a radius of 0.1 millimeters. Base weight: 5 kg, holding time: 15 s

You can find these, measurements, Shore B Shore and C, but they are rarely used. These test methods each combine the truncated cone of the test procedure Shore A and Shore D with the other test load.

A metrological similar method is to determine the IRHD = " International Rubber Hardness Degree ", also called the German microhardness.

For metals

This method is based on the principle that a falling onto the workpiece ball ( or a shaft with ball tip) more or less bounce, depending on the hardness of the workpiece and the drop height. There is little used because, although it is a very simple method, the precision of both the mass of the workpiece (for small workpieces can easily lead to slip ) as also depends on the perfect vertical axis of the case. The hardness measurement is expressed in Shore points and is only standardized for large ground cylinders.

Hardness test Barcol

The Barcol is a hardness scale for glass fiber reinforced plastics ( GRP). According to the standard DIN EN 59 it is determined as well as the Shore hardness with the aid of a handheld instrument and a truncated cone with a flat top.

Hardness test Buchholz

The Buchholz hardness is used for coatings, and can only be used on smooth, at least (10 microns indentation depth ) thick non- elastic coatings. To determine the Buchholz hardness according to DIN 53 153, ISO 2815 Buchholz hardness tester, consisting of a round, pointed wheel ( = double truncated cone ) and a counter weight is, is placed on the horizontal surface for 30 seconds, then the indentation length with a 20 times magnifying microscope measured. The Buchholz hardness is then obtained using the following formula:

To aid identification, the length of the impression point is illuminated by a lamp at an angle of 30 ° to the plane perpendicular to the impression one place making impression very bright spot stands out from the rest of the paint.

Leeb hardness test

The Leeb hardness test was applied for the first time in 1978 and measures the energy introduced on the rebound.

Mohs hardness test

This hardness value can only be determined by comparison of several materials or material conditions.

Hard materials soft scratch. This insight is the basis of the hardness test Friedrich Mohs (1773-1839), which comes mainly in mineralogy used. Mohs, a geologist, scratched different minerals against each other, thus aligning them according to their hardness. Through the exemplary assigning numerical values ​​for widespread and thus easily accessible minerals an ordinal scale, the Mohs scale, which is in wide use in the mineralogy and geology to this day was born. The differences in hardness between the individual reference minerals are nonlinear. Information on the hardness of minerals always refer to the Mohs scale, unless otherwise specified. Listed for comparison is also referred to as absolute hardness grinding hardness Rosiwal that characterizes the grinding work of the substance and gives a better impression of the actual hardness ratios. Both hardness scales are unitless. In addition, the hardness is indicated by the Vickers method in the table. You are the best reference to the now common hardness measurement method again.

In terms of usability and long-term care of minerals as a gemstone, a somewhat coarser classification is often given. So minerals of the Mohs hardness 1-2 are considered to be soft, 3-5 as medium-hard, and all minerals on the Mohs hardness 6 be designated as hard.

More special hardness testing

In addition, some special hardness testing methods are common:

• The universal hardness testing was renamed Martens hardness test in 2003 and in the standard DIN EN ISO 14577 ( Metallic materials - Instrumented indentation test for hardness and materials parameters ) set
• When ball pressure test according to EN ISO 2039-1 for plastics working with balls of 5.0 mm diameter, an initial load of 9.8 N and test loads of 49.0, 132, 358 or 961 N. The measured penetration depth must be in the range between 0.15 mm and 0.35 mm from each other. This reduced test load and finally the ball indentation hardness HB is in N / mm ² calculated or read from a table.
• For coal the Hardgrove index.
• To investigate nanomechanical properties of materials: nanoindentation

Units of measurement

The Mohs hardness and the absolute hardness are unitless quantities.

The correct from a physical standpoint unit of Brinell and Vickers hardness test would be 1 N / m or 1 N / mm ². However, it must be noted that these test methods were developed in the early 20th century and have since been standardized and internationalized in ever more detailed standards. This has a somewhat abstract for the physicist notation of hardness values ​​and units concerned. As a unit of hardness the abbreviation of the test method and the test conditions specified. Both the Vickers and Knoop Hardness at a conversion in physical quantities is possible, by multiplying by a factor, where H (true) to obtain the true hardness in kg / mm ². The Vickers hardness of this factor is 1.618, with Knoop hardness 1,500.

The following are some examples of common hardness data are listed below:

• Vickers hardness test:

• Brinell hardness test:

• Rockwell hardness test:

• From Newton and kilopond:

Hardening and machining

The hardness of steels can be affected during the production - see also hardship.

Above a hardness of about 60 ( Rockwell hardness HRC ) terminate in a rule, the possibilities for machining with geometrically defined cutting edges of surfaces, ie by turning, drilling or milling. Harder surfaces must be abraded or eroded, one speaks of machining with geometrically undefined cutting or removal.

Revaluation

When dealing with the different hardness testing methods, it is often necessary to overestimate the measured hardness value of a method in a different method or the tensile strength. For this reason, conversion tables ( 18265 ( formerly DIN 50150 ) EN ISO) were determined on the basis of a large number of comparative measurements empirical values ​​, and created in the relevant standard standardized.

Important: for various materials and various heat treatment stages different tables apply. The included grades are also listed in EN ISO 18265.

The following conversion tables can therefore be regarded only as a guide. For a standardized revaluation the appropriate standard is to be applied. If one goes, however, in the design of components to the limits of the possible, the approximations and assumptions made in the normalization are often not enough, in order to ensure correct interpretation and examination.