Hardness Testing

HARDNESS TESTING M FACTS
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INTRODUCTION
 Hardness can be defined as the resistance of a material to indentation or abrasion by
another hard body.
 Hardness is resistance of material to plastic deformation caused by indentation.
 Good hardness generally means that the material is resistant to scratching and wear.
Types of quantitative hardness test
 Brinell Hardness Test
 Vickers Hardness Test
 Rockwell Hardness Test
 Meyer Hardness Test
 Micro Hardness Test

 A qualitative Moh’s scale, determined by the ability of a material to scratch another
material: from 1 (softest = talc) to 10 (hardest = diamond)
Diamond 10
Corundum 9
Topaz 8
Quartz 7
Orthoclase (Feldspar) 6
Apatite 5
Fluorite 4
Calcite 3
Gypsum 2
Talc 1

BRINELL HARDNESS TESTING M FACTS
 The Brinell hardness test consists in indenting the metal surface with a 10-mm
diameter steel ball at a load of 3,000 kg.
 For soft metals the load is reduced to 500 kg to avoid too deep an impression.
 For very hard metals Tungsten carbide ball can be used to minimize distortion of
the indenter.
 The load is applied for a standard time, usually 30 sec.
 The surface on which the indentation is made should be relatively smooth and free
from dirt or scale

Brinell Test Method
 The indenter is pressed into the sample by an accurately controlled test force.
 It is important to choose the combination of load and ball size carefully so that the
indentation is free from distortion and suitable for measurement.
 The force is maintained for a specific time (max 30Sec.).
 After this time period, the indenter is removed leaving a round indent in the sample.
 The size of the indent is determined optically by measuring two diagonals of the
round indent using a optical microscope.
 The Brinell hardness number is a function of the test force divided by the curved
surface area of the indent.

BRINELL HARDNESS TESTING
 HB = The Brinell hardness number
 F = Applied load in kg
 D = Diameter of the spherical indenter in
mm
 d = Diameter of the resulting indenter
impression in mm
 UNIT OF BHN (HB)
kilograms per square millimeter.
M FACTS

Advantages of Brinell hardness test
 Different loads are used to cover a wide range of hardness of commercial metals.
 Brinell hardness test is less influenced by surface scratches and roughness than other
hardness tests.
 A wide range of test forces and ball sizes to suit every application.
 Easy to operate the testing equipment.

Limitations of Brinell hardness test
 The test has limitations on small specimens or in critically stressed parts where
indentation could be a possible site of failure.
 Error in Brinell hardness measurements occurs in measuring the diameter of the
impression.
 Indentation develops the residual stresses.
 Surface accuracy is needed for testing the metals.

VICKERS HARDNESS TESTING M FACTS
 The Vickers hardness test uses a square-base diamond pyramid as the indenter.
 This test is preferable to the Brinell test where hard materials are concerned, as it
uses a diamond indenter.
 The included angle between opposite faces of the pyramid is 136°
 The loads ordinarily used with this test range from 1 to 120 kg .
 Standard loads are 5, 10, 20, 30, 50 and 100 kg
 It is necessary to state the load when specifying a Vickers hardness number
For example, if the hardness number is found to be 200 when using a 50 kg load,
then the hardness number is written as HV (50) = 200.

Vickers Hardness method
 The included angle between opposite faces of the pyramid is 136°.
 Because of the shape of the indenter this is frequently called the diamond-
pyramid hardness test.
 The diamond-pyramid hardness number (DPH), or Vickers hardness number
(VHN, or VPH), is defined as the load divided by the surface area of the
indentation,
 This area is calculated from microscopic measurements of the lengths of the
diagonals of the impression.
 A perfect indentation made with a perfect diamond-pyramid indenter would be a
square.

 where F is the load in Kg.
 Average length of diagonals in
mm.
 136° = angle between opposite
faces of diamond

Advantages of Vickers hardness test
 The Vickers method can be used with any and all materials and test specimens, from
soft to hard.
 The procedure covers the entire hardness range.
 There is only one type of indenter, which can be used for all Vickers methods.
 Non-destructive testing is possible, so the test specimen can be used for other
purposes

Limitations of Vickers hardness test
 The test location must be prepared (ground and polished), otherwise precise
evaluation is difficult.
 The process is rather slow (compared with the Rockwell method The test cycle takes
somewhere between 30 and 60 seconds, not including the time taken to prepare the
specimen.
 Allows greater chance for personal error in the determination of the diagonal length

ROCKWELL HARDNESS TESTING M FACTS
 This test utilizes the depth of indentation, under constant load, as a measure of
hardness.
 A 120° diamond cone with a slightly rounded point, called a Brale indenter, and
1/16 and 1/8 inch -diameter steel balls are generally used as indenters.
 Major loads of 60, 100, and 150 kg are used.
 The Rockwell hardness is dependent on the load and indenter, it is necessary to
specify the combination which is used.
 The hardness number is expressed by the symbol HR and the scale designation
 A Rockwell hardness number without the letter prefix is meaningless

Rockwell Scales
 There are 30 different scales. The majority of applications are covered by the
Rockwell C and B scales for testing steel, brass, and other metals.
A Scale diamond penetrator, 60-kg major load, which is usable for materials from
annealed brass to cemented carbides
B Scale 1/16 inch. diameter steel ball and a 100-kg major load, Softer materials are
usually tested .the range of this scale is from RB to RB 100.
C Scale 120° diamond cone indenter and a 150-kg major load. Hardened steel is
tested the useful range for this scale is from about RC20 to RC 70.

Types of the Rockwell hardness Test
 Rockwell: The minor load is 10 kg, the major load is 60, 100, or 150 kg.
 Superficial Rockwell: The minor load is 3 kg and major loads are 15, 30, or 45
kg.
 In both tests, the indenter may be either a diamond cone or steel ball.
 The standard Rockwell test can not be used for very thin sheet and foils
and for these the Rockwell superficial hardness test is used.

Rockwell Hardness Test method
 The indenter moves down into position on the part surface.
 A minor load is applied and a zero reference position is established
 The major load is applied for a specified time period (dwell time) beyond zero
 The major load is released leaving the minor load applied
 The resulting Rockwell number represents the difference in depth from the zero
reference position as a result of the application of the major load

Advantages of Rockwell hardness test
 Ability to distinguish small hardness differences in hardened steel
 Finished heat-treated parts can be tested without damage because of the small
size of the indentation.
 Rockwell test is the only one that allows direct reading of the hardness value
without need of optical reading
 More accurate than other types of hardness testing methods.
 The Rockwell test method is used on all metals.

Limitations of Rockwell hardness test
 The possibility of errors due to shifting of samples under test loads during the
test cycle.
 Rockwell scale suitable for testing of cast iron or of steel sheets having a
thickness lower than 0.15 mm does not exist.
 The tests should not be performed closer to the edge of the specimen.
 Quality of indenter influences the test results more.

MEYER HARDNESS TESTING M FACTS
 The Meyer hardness is a more fundamental measure of indentation hardness.
 Based on the 'projected area of the surface area.
 The principle is that the mean pressure required to test the material is the
measurement of the hardness of the material
 For a cold-worked material the Meyer hardness is essentially constant and
independent of load.
 For an annealed metal the Meyer hardness increases continuously with the load
because of strain hardening produced by the indentation.

MEYER HARDNESS TESTING
 The mean pressure between the surface of the indenter and the indentation is
equal to the load divided by the projected area of the indentation.
 Meyer proposed that this mean pressure should be taken as the measure of
hardness. It is referred to as the Meyer hardness.
 Meyer hardness has units of kilograms per square millimeter.
M FACTS

MICRO HARDNESS TESTING M FACTS
 Many metallurgical problems require the determination of hardness over very
small areas.
 The measurement of the hardness gradient at a carburized surface, the
determination of the hardness of individual constituents of a microstructure, or the
checking of the hardness of a delicate watch gear might be typical problems.
 For this purpose an indentation-hardness test has been found to be more useful.
 The development of the Knoop indenter and the introduction of the Tukon tester
for the controlled application of loads down to 25 g have made micro hardness
testing a routine laboratory procedure.

KNOOP HARDNESS TESTING M FACTS
 The Knoop indenter is a diamond ground to a pyramidal form(172°).
 Produces a diamond-shaped indentation with the long and short diagonals in the
approximate ratio of 7: 1.
 The depth of indentation is about one- thirtieth of the length of the longer
diagonal.
 The long diagonal of the Knoop impression is essentially unaffected by elastic
recovery for loads greater than about 300 g.
 It is usually observed that the Knoop hardness number increases as the load is
decreased below about 300 g.

KNOOP HARDNESS TESTING
 The Knoop hardness number (KHN) is the applied load divided by the
unrecovered projected area of the indentation
Where ;
P = applied load, kg
Ap = unrecovered projected area of indentation, (square millimeter)
L = length of long diagonal, mm
C = a constant for each indenter supplied by manufacturer
M FACTS

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