UNIVERSITY OF JORDAN MECHANICAL ENGINEERING DEPARTMENT STRENGTH OF MATERIALS LAB .
HARDNESS TEST
HARDNESS The Metals Handbook defines hardness as "Resistance of metal to plastic deformation, usually by indentation .However, the term may also refer to stiffness or temper or to resistance to scratching, abrasion, or cutting .It is the property of a metal, which gives the ability to resist being permanently, deformed )bent, broken, or have its shape changed(, when a load is applied .The greater the hardness of the metal, the greater resistance it has to deformation. In materials science, there are three principal operational operational definitions of hardness :Scratch hardness :Resistance to fracture or plastic )permanent (deformation due to friction from a sharp object Indentation hardness :Resistance to plastic )permanent (deformation due to a constant load from a sharp object Rebound hardness :Height of the bounce of an object dropped on the material, related to elasticity. The equation based definition of hardness is the pressure applied over the projected contact area between the indenter and the material being tested .As a result hardness values are typically reported in units of pressure
Material
Science
Hardness is a characteristic of a solid material expressing its resistance to permanent deformation .Hardness can be measured on the Mohr·s scale or various other scales .Some of the other scales used for indentation hardness in engineering³Rockwell, Vickers, and Brinell³can be compared using practical conversion conversion tables.
Scratch
Hardness
In mineralogy, hardness commonly refers to a material's ability to penetrate softer materials .An object made of a hard material will scratch an object made of a softer material .Scratch hardness is usually measured on the Mohr·s scale of mineral hardness . One tool to make this measurement is the sclerometer . Pure diamond is the hardest readily-available natural mineral substance and will scratch any other natural material .Diamond is therefore used to cut other diamonds; in particular, higher-grade diamonds are used to cut lower-grade diamonds.
Indentation
Hardness
Indentation hardness tests are primarily used in engineering and metallurgy fields .The tests work on the basic premise of measuring the critical dimensions of an indentation left by a specifically dimensioned and loaded indenter.
OBJECT IVES IVES: This experiment objective is to perform the Rockwell test on three different metals using the correct indenter, minor and major loads, and scale, to determine the Rockwell number for these metals .Also to perform Brinel test on the same materials, read the impression with a Brinel microscope, then to calculate the hardness number .
S AN D C ALC UL AT IO ION : RESULT
ES EST : ROCKWELL T MAT ERIAL
ST EE EEL
B RA RASS
AL UMIN UM
HR
94
80
61.7
HR
93
81.6
61.6
B ELL T ES EST : RIN MAT ERIAL d1 )mm(
d2)mm(
d )mm(
D
ST EE EEL
2
2
2
BRASS
2.8
2.4
UMIN UM AL
2
2.5
)mm(
F )Kgf(
HB N UMBER )Kgf/mm(
10
3000
495.28
2.6
10
750
138.83
2.25
10
500
124.13
sample of calculation: When
d =2.00 mm, D =10 mm, F =3000 kgf, then HB number equals: 2 * 3000
Hb= T
*10 * (10 10
2
2
2.00 )
=945.2829
kgf/mm2
Significance of
Rockwell hardness
test
The Rockwell hardness test is an empirical indentation hardness test .Its worldwide adoption has likely resulted from the many advantages provided by the test method .The test is fast, inexpensive, and relatively non-destructive, leaving only a small indentation in the material .The simplicity in the operation of a Rockwell hardness machine has provided the added advantage that Rockwell hardness testing usually does not require a highly skilled operator. By way of correlation with other material properties, the Rockwell hardness test can provide important information about metallic materials, such as the tensile strength, wear resistance, and ductility .The test is generally useful for material selection, for process and quality control, and for acceptance testing of commercial products .Consequently, in today·s manufact ma nufacturing uring facilities, Rockwell hardness machines can be found in use in almost every testing Environment, from the hot, oily surroundings of some manufacturing facilities, to environmentally environmentally controlled metallographic metallographic and calibration laboratories.
Rockwell
Scales ]Major and Minor loads:[
Rockwell
hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter indenter and the minor and major loads .The hardness number is expressed by the symbol HR and the scale designation.
The minor load is used to increase the accuracy of the measurement .The minor load eliminates any backlash effects in the measuring system and breaks through any slight surface roughness . The Rockwell hardness test is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy total )major (load beyond the depth resulting from a previously applied preliminary )minor (load .Initially a minor load is applied, and a zero datum position is established .The major load is then applied for a specified period and removed, leaving the minor load applied .The resulting Rockwell number represents the difference in depth from the zero datum position as a result of the application of the major load .The entire procedure requires as little as a few seconds up to 15 for plastics .In the Rockwell test, results are quickly and directly obtained without the need for a secondary, dimensional measurement requirement .
T est est
Material
Thickness
As a Rockwell hardness measurement is being made, the material surrounding the indentation is plastically deformed with the deformation extending well below the indentation depth .If the deformation extends completely through the thickness of thin test material, then the deformed material will flow at the interface with the supporting anvil .This will influence the deformation process likely causing the test to give erroneous hardness results .Thus, the test material must have a sufficient thickness in order to obtain a valid Rockwell test value .Similarly, for products that are manufactured to a specific thickness, a Rockwell scale having the appropriate combination of test forces and indenter size must be chosen based on that thickness . When
the approximate hardness of the test material is known, the minimum thickness needed to obtain valid Rockwell measurements may be estimated from data tables and graphs available in the literature, such as in the ASTM standard .In general, the zone of deformation extends no more than 10 times the depth of indentation for a diamond
indenter test and 1 5 times the depth of indentation for a ball indenter .As a rule, there should be no deformation on the support side of the test material following a Rockwell test, although such markings are not always indicative of a bad test .
T est est
Material
Area )or
Width(
In the same way that the deformation extends below an indentation, thus limiting the minimum material thickness, the deformation also extends outward through the material width .If a Rockwell measurement is made near the edge of the test material, the deformation surrounding the indentation may extend to the edge and push out the material, thus lowering the measured hardness value .This effect is more significant for softer materials .The general rule as specified by the test method standards is that the distance between the center of an indentation and the edge of the material must be at least 2½ times the diameter of the indentation .The ISO test method standard)3 (also specifies that the distance must not be less than 1 mm .Therefore, in cases where Rockwell hardness testing is to be made on narrow width material or material having a small area size, a Rockwell scale must be chosen that produces indentations small enough to prevent this edge interaction.
T est est
Material
Homogeneity
The size and location of metallurgical features in the test material should be considered when choosing the Rockwell scale .For materials that are not homogeneous, an appropriate Rockwell scale should be chosen that would produce a sufficiently large indentation to obtain a hardness value representative of the material as a whole .Also keep in mind that the area surrounding a Rockwell indentation also affects the test result .If the deformation zone surrounding a Rockwell indentation extends into adjacent regions of a differing hardness, such as the heat affected zone of a weld, the test measurement may be influenced .In such cases, a Rockwell scale should be chosen that uses test forces and indenters that produce a small enough indentation to avoid the influence of these areas.
T est est
reparation Surface P reparation
An important feature of the Rockwell hardness test procedure is the use ofthe preliminary force as part of the testing cycle .Application of the preliminaryforce acts to push the indenter indenter through minor surface imperfections and tocrush residual foreign particles present on the test surface .By establishing areference beneath the surface prior to making the first depth measurement, itallows testing of materials with slight surface flaws while maintaining much ofthe test accuracy .Still, as a general rule, the better a test surface is prepared,the more likely the measurement will represent the true Rockwell hardnessvalue of a material. For the best results, the test surface and the surface in contact c ontact with thesupport anvil should be smooth, flat, and free of oxides, foreign matter, andlubricants .The test surface should be prepared in a manner that will not alterthe properties pr operties of the test material such as by overheating or cold-working. The test surface should be representative of the material under test .For thatreason, surface effects, such as carburization or decarburization, decarburization, should beremoved beremoved prior to testing, unless the purpose of the test is to measure thesesurface features .Similarly, other types of coatings, such as paint, galvanizing,etc ., should also be removed prior to testing. The degree of surface roughness that can be tolerated depends on theforce levels to be applied .A finish ground surface is usually sufficient for theRockwell C scale and for the Rockwell ball scales that apply a force of at least980.7 N )100 kgf .(In general, lighter test forces require better surface finishes.For the superficial scales that use a total force of 147.1 N )15 kgf(, a polishedsurface polishedsurface is usually required.
G ood ood P ra ractice Recommendation Not all Rockwell hardness machines are equal .All machines may be capableof performing a r equirementsspecified ntsspecified in test method Rockwell hardness test in accordance with the requireme standards, but some may be more suitable for yourspecific needs .When choosing a Rockwell hardness machine, consider factorssuch as :the accuracy and measurement repeatability that is required; whetherversatility whetherversatility in the testing cycle may be required; the required speed of testing;the Rockwell scales that will be used; the required resolution of
the hardnessnumber; hardnessnumber; the size of material normally tested; and the accessories a ccessories that may beneeded .
estin esting P re recautions]Those may cause Rockwell T Errors:[ When
using devices that employ measurement methods other than theRockwell indentation hardness principle, the type of measurement device thatwas used should be reported with the correlated Rockwell numbers .Thisinformation provides the user of the measurement data a better understandingof how the data was obtained . Testing of too thin material can damage a steel anvil by marring the surface or producing a small indentation .In either case, further testing should not continue with the damaged anvil. Stacking one or more additional layers of metallic material together cannot make up for an insufficient material thickness .The material flow between the layers will produce inaccurate measurements. If the objective of the Rockwell test is to measure the hardness of a surface feature such as a case-hardened surface, the scale chosen should be based on the thickness of this surface feature . The anvil must present the material test surface perpendicular to theindentation direction of the indenter .If the test surface is tested at an anglewith respect to the indentation direction, the measurement will be adverselyaffected, usually lowering the measured value from the true hardness. If an indenter is dropped or hit h it with the test piece or anvil, it is imperativethat before using it further, it should be thoroughly inspected for damage andverified for performance for each Rockwell scale that is used .Performanceverification Performanceverification is necessary because the measuring ability of an indenter,particularly a diamond indenter, can change significantly without anyoutward visible signs of damage.
A steel ball can be flattened quickly if a test is mistakenly made on amaterial above the appropriate hardness range )over 100 HRB (or if theindenter is hit by the anvil or is used to test too thin material. When testing very soft materials, it is important to ensure that the designof the indenter cap allows adequate protrusion of the ball .Otherwise, thecap may contact the test material, preventing full penetration into the testmaterial, testmaterial, and result in an erroneously high hardness value .Be aware thatit is possible for the cap to contact c ontact the test material without any physicalindication on the surface of the test material .
est ELL Hardness T est BRIN The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg .For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation .The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals .The diameter of the indentation left in the test material is measured with a low powered microscope .The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation .Brinell testing is typically done on iron and steel castings using a 3000Kg test force and a 10mm diameter carbide ball .Aluminum and other softer alloys are frequently tested using a 500Kg test force and a 10 or 5mm carbide ball .Therefore the typical range of Brinell testing is 500 to 3000kg with 5 or 10mm carbide balls.
Strength One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same .A wide range of test forces and ball sizes to suit every application .Nondestructive, sample can normally be reused .Compared to the other hardness test methods, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structures and any irregularities in the uniformity of the material.
Weaknesses The main drawback of the Brinell test is the need to optically measure the indent size . This requires that the test point be finished well enough to make an accurate measurement .Slow testing can take 30 seconds, not counting the sample preparation time . Because of the relatively large indentations, the work piece may not be usable after testing
L imitations However, because of the large ball diameter the test cannot be used to determine the hardness variations in a welded joint for which the Vickers test is preferred .Very hard metals, over 450BHN may also cause the ball to deform resulting in an inaccurate reading . To overcome this limitation a tungsten carbide ball is used instead of the hardened steel ball but there is also a hardness limit of 600BHN with this indenter .The oxide layer should be remove from the sample by using sand paper or grinders otherwise it would cause variation in hardness number .We cannot perform this operation on a very thin sheet of metal usually having thickness less than 10mm.For this we need to pile few sheets accurately without any impurity or oxide layer.
Errors in Brinell hardness
T estin esting
There are many factors that can affect the accuracy of the hardness test .Some of these such as flatness and surface finish have already been mentioned above but it is worth re emphasizing the point that flatness is most important -a maximum angle of approximately ± 1° would be regarded as acceptable .To achieve the required flatness tolerance and surface finish surface grinding or machining may be necessary .The correct load must be applied and to achieve this there must be no friction in the loading system otherwise the impression will be smaller than expected -regular maintenance and calibration of the machine is therefore essential .The Brinell ball will deform over a period of time and inaccurate readings will result .This deterioration will be accelerated if a large proportion of the work is on hard materials .The length of time that the load is applied is important and must be controlled .The specimen dimensions are important -if the test piece is too thin the hardness of the specimen table will affect the result .As a rule of thumb the
specimen thickness should be ten times the depth of the impression for the Brinell test and twice that of the Vickers diagonal .Similarly, if the impression is too close to the specimen edge then low hardness values will be recorded -again as a rule the impression should be some 4 to 5 times the impression diameter from any free edge .The specimen table should be rigidly supported and must be in good condition -burrs or raised edges beneath the sample will give low readings .Impact loading must be avoided .It is very easy to force the indenter into the specimen surface when raising the table into position .This can strain the equipment equipment and damage the indenter .Operator training is crucial and regular validation or calibration is essential if hardness rest results are to be accurate and reproducible.
References
1 . ASTM E10 -08 Standard Test Method for BrinellHardness of Metallic Materials Materials ^
2 . ISO 6506-1:2005 Metallic materials -Brinell hardness test -Part 1 :Test method ^
3 .http//:en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness 4 .http//:en.wikipedia.org/wiki/Brinell_hardness_test 5 .http//:www.scribd.com/doc/18004150/Hardness-Report
6.National Institute of Standards and Technology Special Publication 960-5
