Hardness test

Härteprüfung an einer Walze um 1900
at a roller hardens around 1900

hardness is the mechanical resistance, which a body opposes to the penetration of another, harder body.

The hardness of a body permits conclusions on various characteristics, whereby these depend on the kind of the body. An example is wear characteristics. Hard eyeglass lenses scratch less, hard gear wheels wear out more slowly. With the selection of cutting edges such as milling head or turning tool is the hardness of special importance, because hard cuts remain longer sharply.

Multiple application and depending upon technical emphasis other accent setting finds the termthe hardness in solid-state physics, the material science with the analysis of materials and in the geosciences with the characterisation of rocks and minerals. Both overlap in this connection also with the engineering sciences, whereby the hardness particularly in the engineering geology a larger roleplays. The hardness belongs with the tear tenacity, firmness, ductility, rigidity, density and the fusing temperature to the material properties of a material.


Table of contents

hardness and firmness

the hardness of a material is a surface property and has therefore only causes something alsoto do the firmness of the material, even if the firmness affects the testing methods for hardness measurement, which are based on the penetration depth of different inspection pieces. The influence of the firmness can by the measurement reduced on thin films, but to be not completely avoided.

In certain cases those standsHardens a material however in a revaluable connection to the material firmness. Then a usually much more complex tensile testing can be replaced by the relatively inexpensive hardness test. Is of practical importance the possibility of making a revaluation of the Brinell or Vickers hardness on the tensile strength of structural steels. With examinations onIt makes structural steelworks possible this to prove possible material mistakes.

Most materials of large hardness have also a high brittleness, i.e. they can be deformed hardly plastic, break however suddenly. Whereupon is based among other things the technology of the glass cutting.

hardness test and hardness scales

the hardness leavedetermine themselves only by the comparison of several materials or material conditions.

hardness test after Mohs

hard materials cut yield. This insight is basis of the hardness test after Friedrich Mohs, which is used primarily in the mineralogy. Mohs, a geologist, cut different mineralsagainst each other and it arranged in such a way according to its hardness. From exemplary assigning of numerical values for selected minerals resulted a relative hardness scale, the Mohs scale, which in the mineralogy and geology is in far use. Data to the hardness of minerals always refer to the Mohs scale, if nothing else is indicated. To the comparison also sharpening hardness the called absolute hardness is specified after Rosiwal, which characterizes the sharpening expenditure of the respective material and gives a better impression of actual hardness conditions. Both hardness scales are unitless. In addition is in the tablethe hardness after the Vickersverfahren indicated. It shows the best reference to the today usual hardness measuring procedures.

Mineral hardness (Mohs) absolute hardness Vickers hardness in HV remarks
talc 1 0.03 2.4 with fingernail T/platable
gypsum or Halit 2 1.25 36 with fingernail cutable
Kalzit 3 ,4,5 easily cutable apatite or
manganese 5 ,6.5 ,536 with measurer still cutable
Orthoklas 6 37 795 with steel file cutable quartz 7,120
1120 Fensterglas Topas 8 ,175 cuts 109
with copper coin cutable Fluorit 4 ,5.0
,189 with measurer 1427
corundum 9 1000 2060
diamond 10 140.000 10060 hardest naturally occurring mineral; only of itself cutable

in the materials technology, particularly with the metals, above all testing methods are used, which measure the penetration hardness. Standardized inspection pieces are pressed under fixed conditions into the workpiece. In the connection the surface or depth becomesthe lasting impression measured. In principle one differentiates into static and dynamic hardness testing methods. The dynamic testing methods apply the load of the part which can be examined suddenly; with the static procedures the load is alike remaining or gradually increasing.

hardness test after Brinell

of the SwedishEngineer Johan August Brinell in the year 1900 developed and on the world exhibition into of Paris presented method of the hardness test comes with soft to centralhard metals (DIN EN ISO 6506) like for example unalloyed structural steel or aluminum alloys, with wood (ISO 3350) and with materials with uneven structureas for instance cast irons for application. A steel ball or a tungsten carbide ball with a fixed test load is pressed into the surface of the workpiece which can be examined. After the last conditions of the standardization a steel ball is however no longer permissible. The standard writes now for all materials balls outSintered hardmetal forwards.

After a load period from 10 to 15 seconds for steel or cast irons and 10 to 180 seconds for non-ferrous metals or their alloys the diameter of the lasting impression in the workpiece is measured and from this the surface of the impression is determined. The relationship from test load toMultiplied by the value 0.102 one calls impression surface the BRINELL hardness. The indicated number factor is the reciprocal value of 9,81 and serves the conversion of the unit of force Newton into the older unit kilopond. Thus it is guaranteed that hardness measurements result in the same result using modern unitsas historical values, which are based on today outdated units.

With employment of a tungsten carbide ball the hardness value with HBW, with a steel ball was marked it by HBS. The used balls have diameters of 10 mm, 5 mm, 2.5 mm and 1 mm.

A modification of the Brinell examinationis the examination with the Poldihammer, with which the impression of the ball is produced by hand by an undefined hammer blow. Because of the sudden load it concerns a dynamic hardness testing method. The ball penetrates back into a metal bar with defined hardness. Outthe relationship of the two diameters of impression then the hardness of the test specimen can be computed. The method has the advantage that with it test specimens at will stored and blocked construction units can be examined locally. In this way determined hardness values do not tune accurately with that staticallydetermined hardness values, for the requirements placed in the industry they are however in most cases sufficient. The designation „Poldi “comes from the steel plant of the same name in the Czech Kladno, where this testing method was developed.

With un and low-alloy steel can from the BRINELL hardness with more certainTolerance the tensile strength of the material to be derived.

hardness test after Vickers

of the Brinell examination is very similar in the year 1925 of Smith and sand country developed and after the British aircraft construction company Vickers designated hardness test, which serves for the examination of hard and evenly developed materials, butalso to the hardness test at thin-walled or gas-carburized workpieces and edge zones one uses. It is regulated in the standard according to DIN EN ISO 6507. Contrary to the Rockwell examination an equilateral diamond pyramid with an opening angle is pressed a test load fixed by 136° under into the workpiece. Outthe impression surface is calculated for the length of the diagonals of the lasting impression determined by means of a measuring microscope. The relationship of test load in the unit Newton to the impression surface results in with the factor 0.1891 multiplies the Vickers hardness (HV). One receives the strength classes 14H, 22H, 33H and 45H by divisionby 10, they correspond thus to Vickers hardnesses HV (min.) of 140, 220, 330 and 450.

For examinations locally portable devices are available, which are fastened magnetically or mechanically on the test sample.

Application finds the Vickers hardness for example in the indication „to 45H “with grubscrews with female hexagon or„14H “and „22H “with grubscrews with slot.

hardness test after Knoop

a special form of the Vickers hardness examination is the hardness test designated after the American physicist and engineer Frederick Knoop (DIN EN ISO 4545 - metallic materials - hardness test after Knoop). In the Vickers examination the equilateralDiamond point has a rhombic form in the Knoop examination. The nose angles amount to 172,5° for the long and 130° for the short side. Only the long diagonal of the impression is measured. The Knoop examination applies frequently with brittle materials like for example ceramic(s) or sinter materials,during the hardness measurement of layer systems it represents the most exact measuring method.

exist to hardness test after

Rockwell several by American engineer and firm founder Stanley Rockwell in the year 1920 developed hardness testing methods, which are specialized for certain areas of application. The different procedures become with HRand a following identification characterized; Examples of a Rockwell designation are HRA, HRB, HRC or HR15N.

The Rockwell hardness of a material results from the penetration depth of a conical inspection piece made of diamond. It is fixed and with HRC is shortened in the standard DIN EN ISO 6508;C stands thereby for the English word cone for „cones “. With a fixed test load this cone, which possesses a nose angle of 120°, is preloaded into the surface of the workpiece which can be examined. The penetrated depth of the Eindringkörpers serves here as datum plane. Afterwards the Eindringkörper becomesover one period of at least two seconds and maximally six seconds with the main load loads. Subsequently, this is again removed, so that only read out is effective. The difference of the depths before and after presenting the main load is the measure for the Rockwell hardness of theMaterial. The penetration depth of the diamond cone is determined directly with a dial gauge, which is connected with the test prod. On the scale of the clock one can directly read off the hardness values in Rockwell units (HRC). This testing method is used particularly with very hard materials. As furtherRockwelleindringköper become tungsten carbide balls with a diameter of 1,5875 millimeters (HRB, HRF, HRG) or 3.175 millimeters (of HRE, HRH and HRK) uses.

Test sequence:

1.) Vorkraft give up (with HRA, HRB, HRC etc.: 10 kp; with HRN and HRT: 3 kp)

of 2.) Dial gauge zero

3.) Additionally give up to main strength (e.g.HRB = 90 kp, HRC = 140 kp)

of 4.) Short working period depends on the creep behavior of the material:

2 s - 3 s --> for metals without time-dependent plastic behavior
3 s - 6 s --> for metals with time-dependent plastic behavior

5.) 6 waives

main strength.) 7 reads off hardness value at

the dial gauge.) Vorkraft waive

the Rockwell examination are very fast,places however high requirements against the restraint of the test specimen in the testing set. It is unsuitable for test specimens, those in the testing set flexibly giving way, for example pipes.

hardness test after shore

shore hardness, designated after Albert Shore, is a material parameter for elastomers and plasticsand DIN 7868 is fixed in the standards DIN 53505 and. The shore hardness examiner for rubber and similar materials consists those of a spring-tensioned pin, whose elasticity is with the penetration into the sample a measure for the appropriate shore hardness of the material, on a scale of 0Shore (2.5 millimeters of penetration depth) to 100 shores (0 millimeters of penetration depth) is measured. A high number means a large hardness. With a shore hardness testing set an additional device is necessary, the sample with Kraft of 12,5 Newton with shore A and/or 50 Newton, which can be measured, with shore D upthe measuring table presses in slightly. With the determination of shore hardness the temperature plays a higher role than during the Härtebestimmung of metallic materials. Therefore the target temperature is limited here of 23 °C to the temperature range of ± 2C (Kelvin). The material thickness should within the range of0 to 50 shores at least 9 millimeters, with harder substances of at least 6 millimeters amount to.

  • Shore A indicated with soft elastomers after measurement with a needle with blunted point. The front surface truncated cone has a diameter of 0,79 millimeters, the opening angle amounts to 35°.
  • Shore D indicatedTough elastomers after measurement with a needle, which approaches with a 30° angle and a spherical point with a radius of 0,1 millimeters has.

See also: Coherence

the March LV hardness

the March LV hardness procedure was designated after the German physicist Adolf Martens and becomes alsoinstrumented penetration attempt mentioned. In the year 2003 the universal hardness in March hardness was renamed. The procedure is in the DIN EN ISO 14577 (metallic materials - penetration test instrumented for the determination of the hardness and other material parameters) standardized.

With this procedure becomes during the loads - and discharge phase continuouslyKraft and the penetration depth measured. The March hardness (TC) is indicated defined as the relationship of the maximum force to the pertinent contact area and in the unit Newton per square millimeter.

Differently than with the Vickers or the Brinell procedure the plastic behavior of the material does not only becomedetermined, but also further material parameters can do like for example the penetration module from the won measuring curve, which penetration creeping as well as plastic and flexible works of deformation are determined.

As Eindringkörper the following forms are most common: the Vickerspyramide (see Vickersverfahren), a tungsten carbide ball, a spherical diamond penetration body and the Berkovich Eindringkörper. ThatBerkovich Eindringkörper is a diamond pyramid with an equilateral triangular surface area. The opening angle of the pyramid amounts to 65°.

The conversion of the penetration depth to the contact surface must be intended for each penetration body form. The contact area becomes for the Vickers and Berkovich pyramid by the product from the square of the penetration depth and thatConstant 26.43 calculates.

special hardness testing methods

beside it are usual some special hardness testing methods:

  • the universal hardness test was renamed in the year 2003 in March hardness and in the standard DIN EN ISO 14577 (metallic materials - penetration test instrumented for the determination of the hardness and other material parameters) standardized
  • the ball impression attempt for plastics

units

the Mohs hardness and the absolute hardness are unitless sizes. The hardness values of the procedures after Vickers, Knoop and Brinell are indicated against it in units of the pressure; they actually refer to the unit kp/mm ².In consideration of States of, which the conversion to the decimal system and the use of metric units falls such as N/mm ² heavily, one kept the formulas for the hardness values after the cancellation of the unit kilopond (kp) in kp/mm ², omitted however from now on the unit, around world-wide comparable numerical valuesto receive.

hardnesses and cutting

the hardness steel can be affected during the manufacturing - see also hardnesses.

Above a hardness of approximately 55 (Rockwell hardness HRC) according to 600 (Vickers hardness HV) the possibilities for the cutting treatment end also in all rulegeometrically certain cut of surfaces, thus through tricks, boring or milling. Harder surfaces must become polished, one speak thereby of cutting with geometrically indefinite cut. By use of CBN indexable inserts (cubic boron nitride) also the cutting treatment over 60 HRC is possible.

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