Notes on Hardness Testing Methods

hardness test in many applications components should have not only a high strength but also a high wear resistance this generally applies whenever two or more components are in contact with each other these include for example gears shafts bolts pins and guide rails high wear resistance ultimately means a hard surface so that the surface is not damaged in contact with adjacent components and thus where is kept to a minimum for this reason parameters are required to characterize the hardness of a material before such characteristic values can be obtained the term hardness must first be defined in everyday life a surface or material would be described as hard if it cannot be easily scratched on a soft material on the other hand you would see clear scratch marks and in fact this principle of indentation of a standardized object into a material is used to define the so-called indentation hardness or hardness for short so one can define hardness as follows hardness is the resistance of a material to indentation by a standardized object according to this definition all hardness testing methods are ultimately based on the same principle an indenter for example a spherical a pyramid shape or a cone-shaped object is pressed with a certain force into the material surface to be tested the hardness value is then determined from the indentation left behind depending on the material to be tested and the given boundary conditions different hardness tests have developed whose measured values generally cannot be converted into each other therefore hardness values can only be compared if they have been obtained by identical methods the most important test procedures and their advantages and disadvantages are explained in more detail in this video these testing methods include brunel vickers and rockwell hardness testing specially prepared specimens or real components can be used for hardness testing provided that their functionality is not affected due to the indentation left behind in principle all hardness testing methods are destructive testing methods since the material surface is locally destroyed during the measurement brunel hardness test in brunel hardness testing the material or component to be tested is first placed under the microscope of the testing machine the component surface is then brought into focus by adjusting the table height the surface now has the correct distance to the indenter a small ball made of cemented carbide is used as the indenter the testing machine automatically rotates the mounted indenter and the test procedure begins the cemented carbide ball is placed slowly on the surface the indenter is then pressed into the material surface to be tested with increasing force the force increases to the maximum value set in advance within about 10 seconds the applied test force is now maintained for a further 10 to 25 seconds so that the material can settle during this time this is intended to make the measurement reproducible regardless of the material and to provide comparable measurement results the indentation surface left behind which in this case forms a spherical cap is then evaluated under the microscope the larger the indentation left behind the softer the surface for evaluation the testing machine automatically switches back to microscope mode the evaluation of the spherical cap is done using the diameter of the indentation however the outline of the indentation is usually not an exact circle therefore the diameter is averaged using two diameter values at right angles to each other with the mean diameter the testing machine now calculates the hardness value in this case the surface has a brinnell hardness value hbw of 203 the basics for determining this hardness value based on the indentation diameter are explained in more detail in the following the value for brunel hardness is the ratio of the applied force fkp and the indentation surface s left by the indenter which in this case forms a spherical cap the force fkp was specified in the obsolete unit kilo pond when the brunel hardness test was introduced more than 100 years ago one kilo pound was defined as the force corresponding to a weight of one kilogram on earth one kilo pound thus corresponds to 9.81 newtons nowadays forces are always specified in the unit newton therefore the conversion of a force from the unit newton into the unit kilo pound results from the reciprocal value of the gravitational acceleration this conversion factor is thus 0.102 therefore if the force is used in the unit newton the calculation of the hardness value must be done with the factor 0.102 the indentation surface s forms a spherical cap which is part of the spherical indenter with diameter d the surface area of the spherical segment can be determined from the indentation diameter d according to the shown formula the brunel hardness value hbw which is given without units can therefore be calculated with the given formula note that the force must be used in the unit newton and the diameters must be used in the units millimeter due to the anisotropy in the deformation behavior an exactly round circular indentation is usually not left on the material surface therefore the indentation diameter d is determined from the mean value of two indentation diameters d1 and d2 at right angles to each other to prevent the material from being pushed over the edge of the specimen during testing and therefore pretending a lower hardness value the center of the indentation should be at least as far from the edge as 2.5 times the diameter of the indentation if several hardness tests are carried out on one single specimen care must be taken to ensure that the indentations do not fall below a minimum distance from each other if not the measurement result would be influenced by hardening phenomena occurring around the indentations this distance should not be less than three times the indentation diameter in general the specimen thickness should be at least eight times the indentation depth otherwise there is a risk that the sample will bulge out on the bottom side and falsify the result for comparable results the indentation diameter should also not be less than twenty four percent and not greater than sixty percent of the diameter of the indenter if the indentation diameter is too small compared to the diameter of the indenter the indenter is hardly pressed into the material blurred edges are the result from which it is very difficult to determine the indentation diameter left behind due to the low deformation elastic portions are particularly high so that the indentation diameter decreases relatively strongly when the indenter is lifted the hardness values obtained on indentation diameters that are too small are therefore no longer valid if on the other hand the indentation diameters are too large and lie in the range of the indenter diameter the indenter is pressed too deeply into the material the material deforms very strongly at the edges and forms bulges this can also lead to blurred edges furthermore a stronger penetration of the indenter would hardly lead to a larger indentation diameter due to measurement inaccuracies in the diameter determination this would then lead to almost the same hardness value despite the stronger penetration the results would no longer be reproducible particular care must be taken when testing the hardness of a material with the same force but with different test ball diameters with a smaller diameter the force is distributed over a much smaller area during indentation the material is thus subjected to a much greater load which results in deeper indentation but may not result in a larger indentation surface due to the smaller diameter also strain hardening phenomena then increase significantly which may result in a much higher hardness value therefore hardness values obtained on different indenter diameters are not comparable in order to ensure comparability even with different indenters the test bulb must press on the material surface to be tested with the same contact pressure in all cases in order to stress the material to the same extent the distribution of the force on the circular area of the material projected in the direction of the force is decisive for this contact pressure this area should not be confused with the indentation surface which is used to determine the hardness value in the case shown the large ball is pressed into the material surface with a force of around 375 newtons per square millimeter surface area when the smaller ball is pressed in on the other hand a force of 1040 newtons acts on a square millimeter of surface the smaller ball therefore exerts a significantly greater load on the material which leads to different hardness values to obtain the same contact pressure with the smaller ball and comparable hardness values the test load must be reduced accordingly thus in the present case only with a reduced test force of 662 newtons does one obtain the same surface related force and thus identical loads during the test for comparable hardness values the surface pressure between the indenter and the surface to be tested must therefore be the same in all cases as already explained the area of the spherical indenter projected in the direction of the force is relevant for the contact pressure this area is directly proportional to the square of the indenter diameter thus it is also true that comparable hardness values are only given if the ratio of the force and the square of the diameter of the indenter is identical in all cases this ratio is a measure of the contact pressure and is referred to as load factor b in the original definition of the load factor the kilo pound was used as the unit of force since nowadays forces are specified in the unit newton the conversion factor 0.102 must again be taken into account for the two cases shown the load factor is 30. the load factor is standardized to the values 1 2.5 5 10 15 and 30. depending on the material to be tested and the expected hardness value reference values for the load factor to be used can be found in table books with the help of the given formula the test force in the unit newton to be set can then be determined as a function of the unitless load factor and the selected indenter diameter the diameter of the indenter must be specified in the unit millimeter cemented carbide balls made of tungsten with a standardized diameter of 1 2.5 5 or 10 millimeters are available as indenters for brunel hardness testing small diameters are necessary for thin sheets as indenters that are too large would only bulge out the material on the bottom side of the sheet large indenters are also not suitable for determining the hardness of thin surface layers in such cases there is a risk that the surface layer will only be pressed into the underlying base material however larger indenters are required when testing coarse-grained materials or heterogeneous microstructures such as cast iron due to the large indentation many heterogeneous microstructural components are involved in the deformation resulting in a hardness value that covers the entire microstructure and not just individual areas this testing of heterogeneous microstructures is the special advantage of the brunel hardness test in principle however it is only suitable for soft to medium hard materials depending on the material the brunel method can cover hardness ranges between 10 and a maximum of 650. the standard compliance specification of brinell hardness consists of the hardness value the indenter diameter in the unit millimeter the test force in the unit kilo pond and the time under load in the unit second these values are given without units and separated by slashes the indication of the time under load can be omitted if the test was performed with the standard time of 10 to 15 seconds the abbreviation hbw stands for hardness burnell wolfram which is a synonym for tungsten because in the past hardened steel was also used as the material for the indenter this was then abbreviated as hbs however steel balls are no longer used today for analyte and lowlight steels there is an empirical relationship between the hardness value hbw and the tensile strength this relationship states that the tensile strength in the unit newton per square millimeter corresponds approximately to 3.5 times the brinell hardness value in principle the brunel hardness test is not suitable for very hard materials or hardened surface layers as the indenter does not penetrate sufficiently into the material higher test forces cannot help at this point either as this leads to excessive deformation of the indenter the flattening of the indenter then results in a larger indentation diameter and thus pretends a softer material even very thin sheets cannot be tested due to the aforementioned bulging of the material on the opposite side of the sheet in order to close this gap a further hardness test method was developed by vickers which is explained in the following vickers hardness test the principle of vickers hardness testing is basically the same as for the brunel method the only difference is the shape and material of the indenter a four-sided diamond pyramid with an opening angle of 136 degrees is used in the vickers hardness test instead of a cemented carbide ball the opening angle refers to the angle between two opposing surfaces the angle was chosen so that the vickers hardness values are roughly comparable to the brunel hardness values up to a certain degree this applies up to a hardness value of 400 hbw or 400 hv as the vickers hardness is abbreviated the diamond pyramid is pressed into the material surface with increasing force and maintained for about 10 to 15 seconds when the desired test force is reached the indentation left behind is then determined using two diagonals at right angles to each other the hardness value is now determined from the average length of the measured diagonals in this case the hardness value is 208 hv how exactly does this value come about let's take a closer look at the basics as with the brunel hardness test the ratio of test force fkp and indentation surface s serves as the hardness value for the vickers method when the vickers hardness was introduced the unit kilopond was also used as the unit of force for the definition of the hardness value therefore when using forces in the unit newton the conversion factor 0.102 must be taken into account again the indentation surface s can be determined from the diagonal d using the following formula with a given opening angle of 136 degrees we can now put this formula into the calculation formula for the vickers hardness we then combine both numerical values and finally obtain the given formula here again the diagonal must be specified in the unit millimeter in practice you will not get a symmetrical indentation on the material surface therefore the diagonal d is again determined from the mean value of the two diagonals d1 and d2 if several hardness tests are carried out on one sample a certain minimum distance between the indentations must again be insured furthermore the distance to the sample edge must not fall below a minimum value this minimum distance to the edge is 2.5 times the value of the diagonal the distance of the centers of each further indentation must then be at least three times the value of the diagonal moreover the sheet thickness must not be less than 1.5 times the value of the diagonal with the present diagram depending on the expected hardness the minimum thickness of the sample can be estimated as a function of the standardized test forces the test force is indicated in the diagram in the unit kilo pond for example using a test load of 10 kilo pounds corresponding to 98.1 newtons and an expected vickers hardness of 200 results in a minimum sample thickness of about 0.4 millimeters this example shows that vickers hardness testing can obviously be used to test very thin sheets in contrast to a spherical indenter the pyramid shape provides within certain limits geometrically similar indentations even at different test loads thus for identical material samples twice the force also results in twice the indentation surface area as a ratio of force and indentation surface the hardness value is therefore always identical despite different test loads with the brunel hardness test however this is not the case in the brunel test twice the force on the same indenter would result in a different hardness value since this also increases the load factor with the pyramid-shaped indenter on the other hand the vickers hardness value is almost independent of the test load however this statement only applies to relatively large test loads at low test forces for example elastic deformation accounts for a larger proportion of the total deformation the permanent indentation that is left is therefore smaller and the material sample pretends to have a higher hardness value therefore vickers hardness values should only be compared with each other if they were determined with the same test loads a harder material always requires higher test loads than a softer material depending on the expected hardness of the material different test load ranges are required a distinction is made between three ranges on the one hand the so-called macro test range also referred to as macro hardness with test loads between 5 and 100 kilo pounds in this range the hardness values can be considered to be independent of the test force on the other hand the micro test range is differentiated which is also called micro hardness this range lies between 0.2 and 3 kilo pounds such a load range is used for thin surface layers and sheet metals as well as for finished parts in order not to damage the components too much for special cases the nano test range between 0.01 and 0.1 kilo pounds is used which is also referred to as nano hardness the pyramid tip used here offers another advantage over a spherical indenter since even at low indentation depths the pyramid shaped indentation leaves sharper edges and can therefore be measured more accurately therefore at low indentation depths the accuracy of vickers testing increases compared to brunel hardness testing in contrast to the brunel hardness test the vickers test method is suitable for all hardness ranges from very soft to very hard materials in addition this method can also be used for thin sheets or thin surface layers which makes it a universal hardness testing method the standard compliant specification of the vickers hardness consists of the hardness value the test load in the unit kilo pond and the time under load the latter can again be omitted for the standard time of 10 to 15 seconds the hardness test method based on both brunel and vickers uses the indentation surface as a measure for the hardness of a material in both cases the geometry of the indentation is determined with an optical microscope a glossy surface is usually required for this so that the indentation can be clearly seen it may be necessary to polish the sample before testing therefore these methods are generally not suitable for automated testing for this reason the rockwell hardness testing method was developed this test procedure is described in more detail in the following rockwell hardness test in the rockwell hardness test the indentation depth rather than the indentation surface area is used as the hardness measure as an indenter either a cemented carbide ball or a rounded diamond cone with a tip angle of 120 degrees and a tip radius of 0.2 millimeters is used the deeper the indenter penetrates the material at a given force the softer the material conveniently the indentation depth can be determined directly using the traverse path of the testing machine this eliminates the need for manual evaluation of the indentation under the microscope this process can therefore be automated very well for mass production the measuring procedure of the rockwell test is carried out in three steps first the indenter is placed on the surface to be tested with a so-called preload f0 of 10 kilo pounds which corresponds to 98 newtons in this way the influences of possible setting processes in the sample and any clearance in the measuring instrument can be compensated furthermore this eliminates unevenness on the surface of the material sample which would otherwise have a relatively strong influence on the depth measurement after the preload has been applied for a short time the measuring sensor for the depth measurement is set to zero this level now serves as the basis for the indentation depth now in addition to the preload the actual test load f1 is applied the indenter penetrates the material surface with a total force f equal to f0 plus f1 the test force f1 to be said is taken from table books depending on the indenter used and the material to be tested after the indenter has penetrated the material surface with a given total force the test force f1 is removed finally the material is loaded only by the preload f0 and the indenter is slightly lifted by the elastic deformation of the material sample however contact with the sample remains the remaining indentation depth h under maintaining the preload f0 is then measured and used as a basis for determining the hardness value in the present case an indentation depth of 0.112 millimeters is reached with the diamond cone this corresponds to a rockwell hardness value of 44 hr the basics of how to calculate this hardness value from the measured penetration depth will be discussed in more detail in the following when using diamond cones as indenters the hardness value is determined on the basis of a reference depth of 0.2 millimeters depending on how close the indenter reaches this reference depth a certain hardness value is assigned to the material complete penetration of the indenter to the reference depth would obviously mean that the material is very soft this material would then be assigned a hardness value of zero if on the other hand the diamond cone did not penetrate the material at all it would obviously be an extremely hard material this material would then be assigned the maximum hardness value of 100 the scale has an even subdivision of two micrometers each so that reaching half the reference depth corresponds to half the maximum hardness value in this case a rock wall hardness value of 50. thus when diamond cones are used the rockwell scale is divided into 100 degrees of hardness with the given formula the rockwell hardness value hr can be calculated on the basis of the penetration depth h the indentation depth h must be specified in the unit millimeter the testing method with a diamond cone is particularly suitable for very hard materials such as hardened or tempered steels apart from special procedures the preload is 10 kilo pounds which corresponds to 98 newtons depending on the exact test variant different test loads are used with the so-called test vary in scale c the sample is subjected to a test load of 140 kilo pounds which corresponds to 1 307 newtons however especially when testing thin sheets there is a risk with such high test forces that the material will only bulge out on the opposite side thus falsifying the measurement result for this reason the test variant scale a was introduced for diamond cone testing which uses a reduced test force of 50 kilo pounds or 490 newtons there is also a less common scale d in which the hardness value is determined using a test force of 90 kilo pounds or 883 newtons for test variance c dna the hardness value is calculated using the given formula when testing relatively soft materials a diamond cone would penetrate far too much into the material and the indentation would be deeper than the reference depth of 0.2 millimeters for this reason soft surfaces are tested with cemented carbide balls and the reference depth is extended to 0.26 millimeters the cemented carbide ball has a diameter of 1 16 of an inch which corresponds to 1.5875 millimeters the subdivision of the degrees of hardness in increments of 2 micrometers is still used thus the use of cemented carbide balls results in hardness values in the theoretical range from zero up to a maximum value of one hundred and thirty with the given formula the rockwell hardness value hrb can be calculated on the basis of the penetration depth h whereby the depth must again be used in the unit millimeter in contrast to hardness testing with diamond cones spherical indenters are suitable for softer metals such as structural steels or brass when using a carbide ball for hardness testing a distinction is made between test variants scale b and f in both cases the preload is also 10 kilo pounds or 98 newtons the procedures again differ only in the actual test force with scale b the test force is 90 kilo pounds or 883 newtons while in scale f the test force is 50 kilo pounds or 490 newtons due to the reduced test force scale f is particularly suitable for very soft materials such as copper in the less common scale g the test load is 140 kilo pounds or 1 373 newtons however care must be taken to ensure that the carbide ball does not flatten too much under the high test load and falsify the result the hardness value of test variance f and g is calculated with the same formula as the hardness value of scale b in special scales cemented carbide balls with a diameter of 1 8 of an inch can also be used as indenters the table shows an overview of common rockwell scales the hardness values determined with different scales are generally not comparable with each other furthermore the hardness value determined with a particular scale must lie within a certain range for values outside these ranges the scale must be changed as the indenter has either penetrated too strongly or too weakly into the material for scale a this range is between 20 and 95 hra for scale c hardness values between 20 and 70 hrc are considered permissible when testing with carbide balls of scale b the validity range is between 10 and 100 hrb and for varian f between 60 and 100 hrf a minimum sample thickness must also be insured so that the result is not influenced by the support of the testing machine for test variants with cone-shaped indenters the minimum sample thickness corresponds to 10 times the permanent indentation depth for scales using spherical indenters the minimum sample thickness corresponds to 15 times the permanent indentation depth in all cases the distance of the indentation to the sample edge must be greater than one millimeter the surface of the specimen must also be ground and must not exceed a mean roughness of 1.6 micrometers otherwise the surface roughness would lead to large measurement uncertainties the standard compliant indication of rockwell hardness consists of the hardness value and the used scale when using spherical indenters it is also necessary to specify the material of the ball while nowadays almost exclusively cemented carbide balls made of tungsten are used marked with the letter w for wolfram balls made of steel were also used in the past marked with the letter s the advantage of rockwell hardness testing is the relatively short testing time and the good automation capability since the measured values are determined directly from the indentation depth without optical measurement under a microscope this is why this method is particularly suitable for automated production a disadvantage of the rockwell method is the relatively small depth range even small impurities in the material can lead to significant deviations in the indentation depth and thus in the hardness value especially with very hard materials the measuring accuracy is negatively affected due to only slight differences in the indentation depths in such cases the vickers test method is much more suitable for hard materials however this has disadvantages when it comes to testing heterogeneous materials such as cast iron in this case testing should be carried out using the brunel hardness method the table summarizes the advantages and disadvantages of the three test methods described in this video the diagram shows reference values for the hardness to be expected for unalloyed and low allied steels in the untreated state we hope you enjoyed the video and found it helpful thanks for watching

hardness test in many applications components should have not only a high strength but also a high wear resistance this generally applies whenever two or more components are in contact with each other these include for example gears shafts bolts pins and guide rails high wear resistance ultimately means a hard surface so that the surface is not damaged in contact with adjacent components and thus where is kept to a minimum for this reason parameters are required to characterize the hardness of a material before such characteristic values can be obtained the term hardness must first be defined in everyday life a surface or material would be described as hard if it cannot be easily scratched on a soft material on the other hand you would see clear scratch marks and in fact this principle of indentation of a standardized object into a material is used to define the so-called indentation hardness or hardness for short so one can define hardness as follows hardness is the resistance of a material to indentation by a standardized object according to this definition all hardness testing methods are ultimately based on the same principle an indenter for example a spherical a pyramid shape or a cone-shaped object is pressed with a certain force into the material surface to be tested the hardness value is then determined from the indentation left behind depending on the material to be tested and the given boundary conditions different hardness tests have developed whose measured values generally cannot be converted into each other therefore hardness values can only be compared if they have been obtained by identical methods the most important test procedures and their advantages and disadvantages are explained in more detail in this video these testing methods include brunel vickers and rockwell hardness testing specially prepared specimens or real components can be used for hardness testing provided that their functionality is not affected due to the indentation left behind in principle all hardness testing methods are destructive testing methods since the material surface is locally destroyed during the measurement brunel hardness test in brunel hardness testing the material or component to be tested is first placed under the microscope of the testing machine the component surface is then brought into focus by adjusting the table height the surface now has the correct distance to the indenter a small ball made of cemented carbide is used as the indenter the testing machine automatically rotates the mounted indenter and the test procedure begins the cemented carbide ball is placed slowly on the surface the indenter is then pressed into the material surface to be tested with increasing force the force increases to the maximum value set in advance within about 10 seconds the applied test force is now maintained for a further 10 to 25 seconds so that the material can settle during this time this is intended to make the measurement reproducible regardless of the material and to provide comparable measurement results the indentation surface left behind which in this case forms a spherical cap is then evaluated under the microscope the larger the indentation left behind the softer the surface for evaluation the testing machine automatically switches back to microscope mode the evaluation of the spherical cap is done using the diameter of the indentation however the outline of the indentation is usually not an exact circle therefore the diameter is averaged using two diameter values at right angles to each other with the mean diameter the testing machine now calculates the hardness value in this case the surface has a brinnell hardness value hbw of 203 the basics for determining this hardness value based on the indentation diameter are explained in more detail in the following the value for brunel hardness is the ratio of the applied force fkp and the indentation surface s left by the indenter which in this case forms a spherical cap the force fkp was specified in the obsolete unit kilo pond when the brunel hardness test was introduced more than 100 years ago one kilo pound was defined as the force corresponding to a weight of one kilogram on earth one kilo pound thus corresponds to 9.81 newtons nowadays forces are always specified in the unit newton therefore the conversion of a force from the unit newton into the unit kilo pound results from the reciprocal value of the gravitational acceleration this conversion factor is thus 0.102 therefore if the force is used in the unit newton the calculation of the hardness value must be done with the factor 0.102 the indentation surface s forms a spherical cap which is part of the spherical indenter with diameter d the surface area of the spherical segment can be determined from the indentation diameter d according to the shown formula the brunel hardness value hbw which is given without units can therefore be calculated with the given formula note that the force must be used in the unit newton and the diameters must be used in the units millimeter due to the anisotropy in the deformation behavior an exactly round circular indentation is usually not left on the material surface therefore the indentation diameter d is determined from the mean value of two indentation diameters d1 and d2 at right angles to each other to prevent the material from being pushed over the edge of the specimen during testing and therefore pretending a lower hardness value the center of the indentation should be at least as far from the edge as 2.5 times the diameter of the indentation if several hardness tests are carried out on one single specimen care must be taken to ensure that the indentations do not fall below a minimum distance from each other if not the measurement result would be influenced by hardening phenomena occurring around the indentations this distance should not be less than three times the indentation diameter in general the specimen thickness should be at least eight times the indentation depth otherwise there is a risk that the sample will bulge out on the bottom side and falsify the result for comparable results the indentation diameter should also not be less than twenty four percent and not greater than sixty percent of the diameter of the indenter if the indentation diameter is too small compared to the diameter of the indenter the indenter is hardly pressed into the material blurred edges are the result from which it is very difficult to determine the indentation diameter left behind due to the low deformation elastic portions are particularly high so that the indentation diameter decreases relatively strongly when the indenter is lifted the hardness values obtained on indentation diameters that are too small are therefore no longer valid if on the other hand the indentation diameters are too large and lie in the range of the indenter diameter the indenter is pressed too deeply into the material the material deforms very strongly at the edges and forms bulges this can also lead to blurred edges furthermore a stronger penetration of the indenter would hardly lead to a larger indentation diameter due to measurement inaccuracies in the diameter determination this would then lead to almost the same hardness value despite the stronger penetration the results would no longer be reproducible particular care must be taken when testing the hardness of a material with the same force but with different test ball diameters with a smaller diameter the force is distributed over a much smaller area during indentation the material is thus subjected to a much greater load which results in deeper indentation but may not result in a larger indentation surface due to the smaller diameter also strain hardening phenomena then increase significantly which may result in a much higher hardness value therefore hardness values obtained on different indenter diameters are not comparable in order to ensure comparability even with different indenters the test bulb must press on the material surface to be tested with the same contact pressure in all cases in order to stress the material to the same extent the distribution of the force on the circular area of the material projected in the direction of the force is decisive for this contact pressure this area should not be confused with the indentation surface which is used to determine the hardness value in the case shown the large ball is pressed into the material surface with a force of around 375 newtons per square millimeter surface area when the smaller ball is pressed in on the other hand a force of 1040 newtons acts on a square millimeter of surface the smaller ball therefore exerts a significantly greater load on the material which leads to different hardness values to obtain the same contact pressure with the smaller ball and comparable hardness values the test load must be reduced accordingly thus in the present case only with a reduced test force of 662 newtons does one obtain the same surface related force and thus identical loads during the test for comparable hardness values the surface pressure between the indenter and the surface to be tested must therefore be the same in all cases as already explained the area of the spherical indenter projected in the direction of the force is relevant for the contact pressure this area is directly proportional to the square of the indenter diameter thus it is also true that comparable hardness values are only given if the ratio of the force and the square of the diameter of the indenter is identical in all cases this ratio is a measure of the contact pressure and is referred to as load factor b in the original definition of the load factor the kilo pound was used as the unit of force since nowadays forces are specified in the unit newton the conversion factor 0.102 must again be taken into account for the two cases shown the load factor is 30. the load factor is standardized to the values 1 2.5 5 10 15 and 30. depending on the material to be tested and the expected hardness value reference values for the load factor to be used can be found in table books with the help of the given formula the test force in the unit newton to be set can then be determined as a function of the unitless load factor and the selected indenter diameter the diameter of the indenter must be specified in the unit millimeter cemented carbide balls made of tungsten with a standardized diameter of 1 2.5 5 or 10 millimeters are available as indenters for brunel hardness testing small diameters are necessary for thin sheets as indenters that are too large would only bulge out the material on the bottom side of the sheet large indenters are also not suitable for determining the hardness of thin surface layers in such cases there is a risk that the surface layer will only be pressed into the underlying base material however larger indenters are required when testing coarse-grained materials or heterogeneous microstructures such as cast iron due to the large indentation many heterogeneous microstructural components are involved in the deformation resulting in a hardness value that covers the entire microstructure and not just individual areas this testing of heterogeneous microstructures is the special advantage of the brunel hardness test in principle however it is only suitable for soft to medium hard materials depending on the material the brunel method can cover hardness ranges between 10 and a maximum of 650. the standard compliance specification of brinell hardness consists of the hardness value the indenter diameter in the unit millimeter the test force in the unit kilo pond and the time under load in the unit second these values are given without units and separated by slashes the indication of the time under load can be omitted if the test was performed with the standard time of 10 to 15 seconds the abbreviation hbw stands for hardness burnell wolfram which is a synonym for tungsten because in the past hardened steel was also used as the material for the indenter this was then abbreviated as hbs however steel balls are no longer used today for analyte and lowlight steels there is an empirical relationship between the hardness value hbw and the tensile strength this relationship states that the tensile strength in the unit newton per square millimeter corresponds approximately to 3.5 times the brinell hardness value in principle the brunel hardness test is not suitable for very hard materials or hardened surface layers as the indenter does not penetrate sufficiently into the material higher test forces cannot help at this point either as this leads to excessive deformation of the indenter the flattening of the indenter then results in a larger indentation diameter and thus pretends a softer material even very thin sheets cannot be tested due to the aforementioned bulging of the material on the opposite side of the sheet in order to close this gap a further hardness test method was developed by vickers which is explained in the following vickers hardness test the principle of vickers hardness testing is basically the same as for the brunel method the only difference is the shape and material of the indenter a four-sided diamond pyramid with an opening angle of 136 degrees is used in the vickers hardness test instead of a cemented carbide ball the opening angle refers to the angle between two opposing surfaces the angle was chosen so that the vickers hardness values are roughly comparable to the brunel hardness values up to a certain degree this applies up to a hardness value of 400 hbw or 400 hv as the vickers hardness is abbreviated the diamond pyramid is pressed into the material surface with increasing force and maintained for about 10 to 15 seconds when the desired test force is reached the indentation left behind is then determined using two diagonals at right angles to each other the hardness value is now determined from the average length of the measured diagonals in this case the hardness value is 208 hv how exactly does this value come about let&#39;s take a closer look at the basics as with the brunel hardness test the ratio of test force fkp and indentation surface s serves as the hardness value for the vickers method when the vickers hardness was introduced the unit kilopond was also used as the unit of force for the definition of the hardness value therefore when using forces in the unit newton the conversion factor 0.102 must be taken into account again the indentation surface s can be determined from the diagonal d using the following formula with a given opening angle of 136 degrees we can now put this formula into the calculation formula for the vickers hardness we then combine both numerical values and finally obtain the given formula here again the diagonal must be specified in the unit millimeter in practice you will not get a symmetrical indentation on the material surface therefore the diagonal d is again determined from the mean value of the two diagonals d1 and d2 if several hardness tests are carried out on one sample a certain minimum distance between the indentations must again be insured furthermore the distance to the sample edge must not fall below a minimum value this minimum distance to the edge is 2.5 times the value of the diagonal the distance of the centers of each further indentation must then be at least three times the value of the diagonal moreover the sheet thickness must not be less than 1.5 times the value of the diagonal with the present diagram depending on the expected hardness the minimum thickness of the sample can be estimated as a function of the standardized test forces the test force is indicated in the diagram in the unit kilo pond for example using a test load of 10 kilo pounds corresponding to 98.1 newtons and an expected vickers hardness of 200 results in a minimum sample thickness of about 0.4 millimeters this example shows that vickers hardness testing can obviously be used to test very thin sheets in contrast to a spherical indenter the pyramid shape provides within certain limits geometrically similar indentations even at different test loads thus for identical material samples twice the force also results in twice the indentation surface area as a ratio of force and indentation surface the hardness value is therefore always identical despite different test loads with the brunel hardness test however this is not the case in the brunel test twice the force on the same indenter would result in a different hardness value since this also increases the load factor with the pyramid-shaped indenter on the other hand the vickers hardness value is almost independent of the test load however this statement only applies to relatively large test loads at low test forces for example elastic deformation accounts for a larger proportion of the total deformation the permanent indentation that is left is therefore smaller and the material sample pretends to have a higher hardness value therefore vickers hardness values should only be compared with each other if they were determined with the same test loads a harder material always requires higher test loads than a softer material depending on the expected hardness of the material different test load ranges are required a distinction is made between three ranges on the one hand the so-called macro test range also referred to as macro hardness with test loads between 5 and 100 kilo pounds in this range the hardness values can be considered to be independent of the test force on the other hand the micro test range is differentiated which is also called micro hardness this range lies between 0.2 and 3 kilo pounds such a load range is used for thin surface layers and sheet metals as well as for finished parts in order not to damage the components too much for special cases the nano test range between 0.01 and 0.1 kilo pounds is used which is also referred to as nano hardness the pyramid tip used here offers another advantage over a spherical indenter since even at low indentation depths the pyramid shaped indentation leaves sharper edges and can therefore be measured more accurately therefore at low indentation depths the accuracy of vickers testing increases compared to brunel hardness testing in contrast to the brunel hardness test the vickers test method is suitable for all hardness ranges from very soft to very hard materials in addition this method can also be used for thin sheets or thin surface layers which makes it a universal hardness testing method the standard compliant specification of the vickers hardness consists of the hardness value the test load in the unit kilo pond and the time under load the latter can again be omitted for the standard time of 10 to 15 seconds the hardness test method based on both brunel and vickers uses the indentation surface as a measure for the hardness of a material in both cases the geometry of the indentation is determined with an optical microscope a glossy surface is usually required for this so that the indentation can be clearly seen it may be necessary to polish the sample before testing therefore these methods are generally not suitable for automated testing for this reason the rockwell hardness testing method was developed this test procedure is described in more detail in the following rockwell hardness test in the rockwell hardness test the indentation depth rather than the indentation surface area is used as the hardness measure as an indenter either a cemented carbide ball or a rounded diamond cone with a tip angle of 120 degrees and a tip radius of 0.2 millimeters is used the deeper the indenter penetrates the material at a given force the softer the material conveniently the indentation depth can be determined directly using the traverse path of the testing machine this eliminates the need for manual evaluation of the indentation under the microscope this process can therefore be automated very well for mass production the measuring procedure of the rockwell test is carried out in three steps first the indenter is placed on the surface to be tested with a so-called preload f0 of 10 kilo pounds which corresponds to 98 newtons in this way the influences of possible setting processes in the sample and any clearance in the measuring instrument can be compensated furthermore this eliminates unevenness on the surface of the material sample which would otherwise have a relatively strong influence on the depth measurement after the preload has been applied for a short time the measuring sensor for the depth measurement is set to zero this level now serves as the basis for the indentation depth now in addition to the preload the actual test load f1 is applied the indenter penetrates the material surface with a total force f equal to f0 plus f1 the test force f1 to be said is taken from table books depending on the indenter used and the material to be tested after the indenter has penetrated the material surface with a given total force the test force f1 is removed finally the material is loaded only by the preload f0 and the indenter is slightly lifted by the elastic deformation of the material sample however contact with the sample remains the remaining indentation depth h under maintaining the preload f0 is then measured and used as a basis for determining the hardness value in the present case an indentation depth of 0.112 millimeters is reached with the diamond cone this corresponds to a rockwell hardness value of 44 hr the basics of how to calculate this hardness value from the measured penetration depth will be discussed in more detail in the following when using diamond cones as indenters the hardness value is determined on the basis of a reference depth of 0.2 millimeters depending on how close the indenter reaches this reference depth a certain hardness value is assigned to the material complete penetration of the indenter to the reference depth would obviously mean that the material is very soft this material would then be assigned a hardness value of zero if on the other hand the diamond cone did not penetrate the material at all it would obviously be an extremely hard material this material would then be assigned the maximum hardness value of 100 the scale has an even subdivision of two micrometers each so that reaching half the reference depth corresponds to half the maximum hardness value in this case a rock wall hardness value of 50. thus when diamond cones are used the rockwell scale is divided into 100 degrees of hardness with the given formula the rockwell hardness value hr can be calculated on the basis of the penetration depth h the indentation depth h must be specified in the unit millimeter the testing method with a diamond cone is particularly suitable for very hard materials such as hardened or tempered steels apart from special procedures the preload is 10 kilo pounds which corresponds to 98 newtons depending on the exact test variant different test loads are used with the so-called test vary in scale c the sample is subjected to a test load of 140 kilo pounds which corresponds to 1 307 newtons however especially when testing thin sheets there is a risk with such high test forces that the material will only bulge out on the opposite side thus falsifying the measurement result for this reason the test variant scale a was introduced for diamond cone testing which uses a reduced test force of 50 kilo pounds or 490 newtons there is also a less common scale d in which the hardness value is determined using a test force of 90 kilo pounds or 883 newtons for test variance c dna the hardness value is calculated using the given formula when testing relatively soft materials a diamond cone would penetrate far too much into the material and the indentation would be deeper than the reference depth of 0.2 millimeters for this reason soft surfaces are tested with cemented carbide balls and the reference depth is extended to 0.26 millimeters the cemented carbide ball has a diameter of 1 16 of an inch which corresponds to 1.5875 millimeters the subdivision of the degrees of hardness in increments of 2 micrometers is still used thus the use of cemented carbide balls results in hardness values in the theoretical range from zero up to a maximum value of one hundred and thirty with the given formula the rockwell hardness value hrb can be calculated on the basis of the penetration depth h whereby the depth must again be used in the unit millimeter in contrast to hardness testing with diamond cones spherical indenters are suitable for softer metals such as structural steels or brass when using a carbide ball for hardness testing a distinction is made between test variants scale b and f in both cases the preload is also 10 kilo pounds or 98 newtons the procedures again differ only in the actual test force with scale b the test force is 90 kilo pounds or 883 newtons while in scale f the test force is 50 kilo pounds or 490 newtons due to the reduced test force scale f is particularly suitable for very soft materials such as copper in the less common scale g the test load is 140 kilo pounds or 1 373 newtons however care must be taken to ensure that the carbide ball does not flatten too much under the high test load and falsify the result the hardness value of test variance f and g is calculated with the same formula as the hardness value of scale b in special scales cemented carbide balls with a diameter of 1 8 of an inch can also be used as indenters the table shows an overview of common rockwell scales the hardness values determined with different scales are generally not comparable with each other furthermore the hardness value determined with a particular scale must lie within a certain range for values outside these ranges the scale must be changed as the indenter has either penetrated too strongly or too weakly into the material for scale a this range is between 20 and 95 hra for scale c hardness values between 20 and 70 hrc are considered permissible when testing with carbide balls of scale b the validity range is between 10 and 100 hrb and for varian f between 60 and 100 hrf a minimum sample thickness must also be insured so that the result is not influenced by the support of the testing machine for test variants with cone-shaped indenters the minimum sample thickness corresponds to 10 times the permanent indentation depth for scales using spherical indenters the minimum sample thickness corresponds to 15 times the permanent indentation depth in all cases the distance of the indentation to the sample edge must be greater than one millimeter the surface of the specimen must also be ground and must not exceed a mean roughness of 1.6 micrometers otherwise the surface roughness would lead to large measurement uncertainties the standard compliant indication of rockwell hardness consists of the hardness value and the used scale when using spherical indenters it is also necessary to specify the material of the ball while nowadays almost exclusively cemented carbide balls made of tungsten are used marked with the letter w for wolfram balls made of steel were also used in the past marked with the letter s the advantage of rockwell hardness testing is the relatively short testing time and the good automation capability since the measured values are determined directly from the indentation depth without optical measurement under a microscope this is why this method is particularly suitable for automated production a disadvantage of the rockwell method is the relatively small depth range even small impurities in the material can lead to significant deviations in the indentation depth and thus in the hardness value especially with very hard materials the measuring accuracy is negatively affected due to only slight differences in the indentation depths in such cases the vickers test method is much more suitable for hard materials however this has disadvantages when it comes to testing heterogeneous materials such as cast iron in this case testing should be carried out using the brunel hardness method the table summarizes the advantages and disadvantages of the three test methods described in this video the diagram shows reference values for the hardness to be expected for unalloyed and low allied steels in the untreated state we hope you enjoyed the video and found it helpful thanks for watching

Transcript for:Notes on Hardness Testing Methods

Transcript for:
Notes on Hardness Testing Methods