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MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 1 of 12
Department of Materials and Metallurgical Engineering
Bangladesh University of Engineering and Technology, Dhaka
MME222 Materials Testing Sessional 1.50 Credits
January 2017 Term
Laboratory 2
Hardness Test of Materials
1. Objective
Hardness is usually defined as resistance to cause localise permanent deformation in material. Hardness,
however, is not a fundamental property of material, because the resistance to indentation depends on the
shape of the indenter and on the load applied. This laboratory experiment is designed to introduce the
principles of different methods of hardness testing, emphasizing the limitations and significance of the
results in each method.
After completion of this experiment, students should be able to
1.1 understand the principles of a different methods of hardness testing and gain their practices on
operating the hardness testing machines to achieve the required hardness value following a specific
standard,
1.2 estimate the tensile properties of materials from their hardness values.
2. Materials and Equipment
2.1 Hardness specimens
2.2 Automatic Rockwell hardness tester
2.3 Automatic Brinell hardness tester and Brinell microscope
2.4 Automatic Vickers (Microhardness) hardness tester
2.5 Different indenters
3. Experimental Procedure
3.1 Rockwell hardness testing
3.1.1 Keep the loading and unloading lever at the unloading position.
3.1.2 Select the suitable indenter and weights according to the scale.
3.1.3 Fix the indenter in the hardness tester and switch ON the power supply of the machine. Place the
specimen on testing table anvil.
3.1.4 Turn the hand wheel clockwise to raise a job until it makes contact with indenter and continue
turning till the dial gauge reads between 290 and 299 and the SET indicator becomes lighted. This
indicates that the ‘minor load’ (usually 10 kg) is now applied to the indenter.
3.1.5 Press the button START to apply the ‘major load’ (usually 60, 100 or 150 kg depending on the scale
selected). The dial readings will continue changing and, after about 15 second intervals, the reading
will become stationary and OK indicator will be lighted.
3.1.6 Note down the reading.
3.1.7 Turn back the hand wheel anti-clockwise and remove the job.
3.1.8 Similarly repeat the step from 3.1.1-3.1.8 for different trials and for different samples.
3.1.9 Complete Data Sheet 2.1.
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 2 of 12
3.2 Brinell hardness testing
3.2.1 Keep the loading and unloading lever at the unloading position.
3.2.2 Fix the indenter in the hardness tester and switch ON the power supply of the machine. Select the
suitable load and dwell time according to the scale.
3.2.3 Place the specimen on testing table anvil.
3.2.4 Turn the hand wheel clockwise to raise a job until it makes contact with indenter and continue
turning till the load indicator becomes green.
3.2.5 The load will now be applied on to the sample by the machine and when the full load is applied,
time will start counting.
3.2.6 After the application of load for the required amount of time, the orange indicator will lid indicating
unloading. Turn back the hand wheel anti-clockwise to remove the job.
3.2.7 Read the diameter of the indentation using Brinell microscope and using appropriate formula
calculate BHN.
3.2.8 Similarly repeat the step from 3.2.1-3.2.8 for different trials and for different samples.
3.2.9 Complete Data Sheet 2.2.
3.3 Vickers hardness testing
3.3.1 Clean and polish the surface of the specimen.
3.3.2 Select the load using manual handle (from 1 to 50 kg). Switch ON the power supply.
3.3.3 Place the specimen with cleaned surface facing the indenter on the anvil at the work table.
3.3.4 Focus the work piece surface for clean visibility by rotating the hand wheel at the work table
upwards and downwards. Once focused, remove the lens and move the indicator at the top of the
sample.
3.3.5 Set the dwell time and press RESET to erase previous history of data from the machine memory.
Press START to apply load. After the pre-set dwell time, the indicator will move automatically and
the lens will move at the top of the sample.
3.3.6 For the first reading, D1, of the indentation, adjust the display at the indentation made by the
indenter to coincide with the micrometer on the display screen. Press READ to store the first
reading of the indentation. For the second reading, D2, of the indentation, rotate the micrometer
head to 90 degrees and press READ again to store the second reading of the indentation.
3.3.7 The measurement is to be made for two opposite corners of the diagonal indentation. Using
appropriate formula calculate VHN.
3.3.8 Similarly repeat the step from 3.3.1-3.3.8 for different trials and for different samples.
3.3.9 Complete Data Sheet 2.3.
4. Results
4.1 List hardness values and determine the average hardness number of each specimen. Compare your
result with those published in reference book.
4.2 Predict the tensile strength of steel sample from its hardness value.
5. Discussion
5.1 Answer the following questions:
(a) How do the Rockwell and Brinell tests actually measure hardness? Give any appropriate sketches
and formulae. Are there any units involved? Explain the reason for the pre-load applied in the
procedure for the Rockwell test.
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 3 of 12
(b) What is the limitation on the thickness of specimens for a hardness test? Explain. Calculate the
minimum thickness for one specimen for the Rockwell test and one for the Brinell test.
(c) What are the limitations for distance from specimen edge to indentation and distance between
indentations? Explain why these limitations exist in both cases.
(d) What surface condition is necessary for Brinell, Rockwell and Vickers?
(e) What are the advantages of Vickers test against Brinell test? Of Brinell test against Rockwell test?
(f) Would you suggest conducting Rockwell-Type hardness tests on ceramic or polymeric materials?
Why or why not?
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 4 of 12
Table 2.1: Data Sheet for Rockwell Hardness Number
Material Type of Indenter Rockwell Hardness
Number
Average Rockwell
Hardness Number
1
2
3
4
5
Table 2.2: Data Sheet for Brinell Hardness Number
Material Diameter of
Indenter, D
Applied
Load, F
Diameter of
Indentation, d
Average Diameter
of Indentation
Average Brinell
Hardness Number
mm kgf mm mm BHN
1
2
3
4
5
Table 2.3: Data Sheet for Vickers Hardness Number
Material Applied
Load, F
Length of Indentation Vickers Hardness
Number
Average Vickers
Hardness Number D1 D2 Average D
kgf mm mm mm VHN VHN
1
2
3
4
5
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 5 of 12
6. Theoretical Background
6.1 Introduction
Hardness is a measure of a material’s resistance to localized plastic deformation (by scratching or
indentation). Methods to characterize hardness can be divided into three primary categories:
1. Scratch Tests. These are the simplest form of hardness tests. In this test, various materials are rated
on their ability to scratch one another. If two materials are compared, the harder one is capable of scratching the softer one, but not vice versa. Mohs hardness test is of this type. This test is used mainly
in mineralogy.
2. Dynamic Hardness or Rebound Tests. Here an object of standard mass and dimensions is bounced
back from the surface of the test specimen after falling by its own weight. The hardness number is proportional to the height of rebound of the standard mass. Shore hardness is measured by this method.
3. Static Indentation Tests. Test tests are based on the relation of indentation of the specimen by a
penetrator under a given load. The relationship of total test force to the area or depth of indentation
provides a measure of hardness; the softer the material, the larger and deeper the indentation, and the
lower the hardness index number. The Rockwell, Brinell, Knoop, Vickers, and ultrasonic hardness
tests are of this type. For engineering purposes, static indentation tests are mostly used, Fig. 6.1.
Figure 6.1: General characteristics of hardness-testing methods and formulas for calculating hardness.
Indentation tests actually produce a permanent impression in the surface of the material. The force and size
of the impression can be related to a quantity (hardness) which can be objectively related to the resistance
of the material to permanent penetration. Because the hardness is a function of the force and size of the
impression, the pressure (and hence stress) used to create the impression can be related to both the yield
and ultimate strengths of materials. For materials that undergo plastic deformation primarily via slip (e.g.,
metals for which dislocation motion requires shear stress), it has been demonstrated empirically and by
“slip line theory” that:
𝐻 ~ 3 𝜎𝑦 (6.1)
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 6 of 12
Thus, as hardness increases so does the yield strength and the ultimate tensile strength. For this reason,
specifications often require the results of hardness tests rather than tensile tests.
Several different types of hardness tests have evolved over the years. These include macro hardness test
such as Brinell, Vickers, and Rockwell and micro hardness tests such as Knoop and Tukon. Rockwell and
Brinell testing is the most commonly applied materials test in industry due to several factors:
1. Simple to perform and does not require highly skilled operators;
2. Through the use of different loads and indenters, hardness testing can be used for determining the
hardness and approximate strength of most metals and alloys including soft bearing materials and
high strength steels;
3. Hardness readings can be taken in a few seconds with minimal preparation; and
4. For Rockwell hardness testing, no optical measurements are required; all readings are direct.
Factors influencing hardness include microstructure, grain size, strain hardening, etc. Measured hardnesses
are only relative (rather than absolute) thus care must be taken when comparing values determined by
different techniques.
6.2 Brinell Hardness Test
Introduced by J. A. Brinell in 1900, this test involves pressing a hardened steel or tungsten-carbide ball 10
mm in diameter against a flat surface of a workpiece, with a load of 500, 1500, or 3000 kg. Brinell hardness
is evaluated by taking the mean diameter of the indentation (two readings at right angles to each other) and
calculating the Brinell hardness number (HB) by dividing the applied load by the curved surface area of the
indentation according to the expression:
𝐻𝐵 = 2 𝑃
𝜋𝐷 (𝐷 − √𝐷2 − 𝑑2) (6.2)
where P is the load in kilograms, D is the diameter of the ball in millimetres, and d is the average diameter
of the indentation in millimeters. Calculations have already been made and are available in tabular form
for various combinations of diameters of impressions and load.
As shown in Eq. (6.2), hardness has units of [N/m2], but is not physically equivalent to stress because the
measured projected area of the indentation is not the actual area normal to the applied load of the 3-
dimensional indenter.
The Brinell hardness number followed by the symbol HB without any suffix numbers denotes standard test
conditions using a ball of 10 mm diameter and a load of 3000 kg applied for 10 to 15 s. For other
conditions, the hardness number and symbol HB are supplemented by numbers indicating the test
conditions in the following order: diameter of ball, load, and duration of loading. For example, 75 HB
10/500/30 indicates a Brinell hardness of 75 measured with a ball of 10 mm diameter and a load of 500 kg
applied for 30s.
The 500-kilogram load is usually used for testing nonferrous metals such as copper and aluminium alloys,
whereas the 3000-kilogram load is most often used for testing harder metals such as steels and cast irons.
The load is held for a specified time (10 to 15 seconds for iron or steel and about 30 seconds for softer
metals), after which the diameter of the recovered indentation is measured in millimeters. This time period
is required to ensure that plastic flow of the work metal has stopped.
The harder the material to be tested, the smaller the impression; hence, a 1500-kg or 3000-kg load is usually
recommended in order to obtain impressions sufficiently large for accurate measurement. Depending on
the condition of the material, one of two types of impression develops on the surface after the performance
of this test (Fig. 6.2) or of any of the other tests described in this section. The impressions in annealed
metals generally have a rounded profile (Fig. 6.2a); in cold-worked metals, they usually have a sharp
profile (Fig. 6.2b). The correct method of measuring the indentation diameter, d, is shown in the figure.
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 7 of 12
Figure 6.2: Indentation geometry in Brinell
hardness testing: (a) annealed metal; (b) work-
hardened metal. Note that the depth of the
permanently deformed zone is about one order
of magnitude larger than the depth of
indentation. For a hardness test to be valid, this
zone should be fully developed in the material.
Highly hardened steel (or other very hard metals) cannot be tested by a hardened steel ball by the Brinell
method because the ball would plastically deform and flatten during penetration. One method for
minimizing this effect is to use tungsten carbide balls because of their higher modulus of elasticity, they
distort less than steel balls do. Tungsten carbide balls are recommended for Brinell testing materials of
hardness from 444 HB up to 627 HB (indentation of 2.45 mm in diameter). However, higher Brinell values
will result when using carbide balls instead of steel balls because of the difference in elastic properties. The
degree of accuracy that can be attained by the Brinell hardness test is greatly influenced by the surface
smoothness and therefore the test surface should be filed, ground, machined or polished with emery paper
such that the indentation diameter is clearly enough defined to permit its’ accurate measurement.
Typical advantages of Brinell hardness measurements are:
1. This measurement uses one scale for virtually all materials.
2. Brinell hardness is good for averaging heterogeneities over a relatively large area, thus lessening the
influence of scratches or surface roughness, or presence of small defects.
3. It can be used to estimate yield strength of steel:
y (psi) ≈ HB * 515 (for HB < 175) (6.3)
y (psi) ≈ HB * 490 (for HB > 175) (6.4)
General precautions of Brinell hardness testing include the following:
1. Indentations should not be made on a curved surface having a radius of less than 25 mm.
2. The load should be applied in such a way that the direction of loading and the test surface are
perpendicular to each other within 2°.
3. The thickness of the workpiece being tested should be such that no bulge or mark showing the effect of
the load appears on the side of the workpiece opposite the indentation. In any event, the thickness of
the specimen shall be at least 10 times the depth of the indentation
The Brinell hardness test has the following principal limitations:
1. Size and shape of the workpiece must be capable of accommodating the relatively large indentations.
2. Because of the relatively large indentations, the workpiece may not be usable after testing.
3. Because of the spherical shape of the indenter ball, the BHN for the same material will not be the
same for different loads if the same size ball is used. Thus, geometric similitude must be imposed by
maintaining the ratio of the indentation load and indenter diameter (P1/D1 = P2/D2 = P3/D3 etc.).
3. The limit of hardness range -- about 11 HB with the 500-kg load to 627 with the 3000-kg load -- is
generally considered the practical range.
6.3 Rockwell Hardness Testing
This hardness test uses a direct reading instrument based on the principle of differential depth
measurement. Rockwell hardness differs from Brinell hardness testing in that the indentation size is
measured by the diameter of the indentation in Brinell testing while Rockwell hardness is determined based
on the inverse relationship to the difference in depths of indentation produced by minor and major load.
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 8 of 12
For thin test samples or samples for which the relatively large Brinell or Rockwell indentations must be
avoided, the Superficial Rockwell hardness test is often employed. Superficial Rockwell hardness testing
employs lower loads to the indenter in order to minimize penetration.
The Rockwell Hardness Test consists of measuring the additional depth to which an indenter is forced by a
heavy (major) load beyond the depth of a previously applied light (minor) load as illustrated in Fig. 6.3.
Application of the minor load eliminates backlash in the load train and causes the indenter to break
through slight surface roughness and to crush particles of foreign matter, thus contributing much greater
accuracy in the test.
The Rockwell hardness test method consists of indenting the test material with a diamond cone or
hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load F0
(Fig. 3A) usually 10 kgf. When equilibrium has been reached, an indicating device, which follows the
movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a
datum position. While the preliminary minor load is still applied an additional major load is applied with
resulting increase in penetration (Fig. 1B). When equilibrium has again been reach, the additional major
load is removed but the preliminary minor load is still maintained. Removal of the additional major load
allows a partial recovery, so reducing the depth of penetration (Fig. 1C). The permanent increase in depth
of penetration, resulting from the application and removal of the additional major load is used to calculate
the Rockwell hardness number.
Figure 6.3: Principle of Rockwell testing.
The minor load is applied first, and a reference or “set” position is established on the measuring device or
the Rockwell hardness tester. Then the major load is applied at a prescribed, controlled rate. Without
moving the workpiece being tested, the major load is removed and the Rockwell hardness number is
indicated on the dial gage. The entire operation takes from 5 to 10 seconds. In Rockwell testing, the minor
load is 10 kgf, and the major load is 50, 90 or 140 kgf. In superficial Rockwell testing, the minor load is 3
kgf, and major loads are 12, 27 or 42 kgf. In both tests, the indenter may be either a diamond cone or steel
ball, depending principally on the characteristics of the material being tested.
The 120° sphero-conical diamond indenter is used mainly for testing hard materials such as hardened steels
and cemented carbides. Hardened steel ball indenters with diameters 1/16, 1/8, 1/4, 1/2 in. are used for
testing softer materials such as fully annealed steels, softer grades of cast irons, and a wide variety of
nonferrous metals.
There are 30 different Rockwell scales (regular and superficial), defined by the combination of the indenter
and minor and major loads, Table 6.1. The suitable scale is determined due to the type of the material to be
tested. The majority of applications are covered by the Rockwell C and B scales for testing steel, brass, and
other materials.
Hardness values taken for round samples need to be corrected because the material flows more easily to the
edge and, in effect, creates a lower hardness value. Rockwell Round Correction Charts (Table 6.2) can be
used for such correction.
F0 = preliminary minor load in kgf
F1 = additional major load in kgf
F = total load in kgf
e = permanent increase in depth of penetration due to major load F1 measured in units of 0.002 mm
E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenter
HR = Rockwell hardness number = E – e
D = diameter of steel ball E
e
Stage A
F0 applied
Stage B
F0 + F1 = F applied
Stage C
F0 applied (F1 Withdrawn)
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 9 of 12
Table 6.1: Rockwell hardness scales.
Scale Indenter Minor
Load, F0
Major
Load, F1
Total
Load, F
Typical Applications
kgf kgf kgf
A Diamond cone 10 50 60 Cemented carbides, thin steel and
shallow case hardened steel
B 1/16" steel ball 10 90 100 Copper alloys, soft steels,
aluminium alloys, malleable irons
C Diamond cone 10 140 150
Steel, hard cast irons, pearlitic
malleable iron, titanium, deep case
hardened steel and other materials
harder than 100 HRB
D Diamond cone 10 90 100
Thin steel and medium case
hardened steel and pearlitic
malleable iron
E 1/8" steel ball 10 90 100 Cast iron, aluminium and
magnesium alloys, bearing metals
F 1/16" steel ball 10 50 60 Annealed copper alloys, thin soft
sheet metals
G 1/16" steel ball 10 140 150
Phosphor bronze, beryllium copper,
malleable irons. Upper limit is
HRG 92, to avoid flattening of ball.
H 1/8" steel ball 10 50 60 Aluminium, zinc, lead
K 1/8" steel ball 10 140 150
Bearing metals, plastics and other
very soft or thin materials
L 1/4" steel ball 10 50 60
M 1/4" steel ball 10 90 100
P 1/4" steel ball 10 140 150
R 1/2" steel ball 10 50 60
S 1/2" steel ball 10 90 100
V 1/2" steel ball 10 140 150
No Rockwell hardness value is expressed by a number alone. A letter has been assigned to each
combination of load and indenter, as indicated in Table 6.1. Each number is suffixed first by the letter H
(for hardness), then the letter R (for Rockwell), and finally the letter that indicates the scale used. For
example, a value of 60 in the Rockwell C scale is expressed as 60 HRC, etc. One Rockwell number
represents an indentation of 0.002 mm (0.00008 in.). Therefore a reading of 60 HRC indicates an
indentation from minor to major load of (100 - 60) x 0.002 mm = 0.080 mm or 0.0032 in. A reading of 80
HRB indicates an indentation of (130 - 80) x 0.002 mm = 0.100 mm.
The metal immediately surrounding the indentation from a Rockwell hardness test is cold worked thus
multiple readings cannot be taken at the same point on a material’s surface. If multiple tests are conducted
on a single part the indentations should each be a minimum of 3 indentation diameters apart. The depth of
material affected during testing is on the order of ten times the depth of the indentations. Therefore, unless
the thickness of the metal being tested is at least ten times the depth of the indentation, an accurate
Rockwell hardness test cannot be expected. In addition to the limitation of indentation depth for a
workpiece of given thickness and hardness, there is a limiting factor on the minimum material width. If the
indentation is placed too close to the edge of a workpiece, the edge will deform outward and the Rockwell
hardness number will be decreased accordingly. Experience has shown that the distance from the centre of
the indentation to the edge of the workpiece must be at least 3 times the diameter of the indentation to
ensure an accurate test.
A fundamental requirement of the Rockwell hardness test is that the surface of the workpiece being tested
be approximately normal to the indenter and that the workpiece must not move or slip in the slightest
degree as the major load is applied. As one point of hardness represents a depth of only 0.0008 inches, a
movement of only 0.001 inches could cause an error of over 10 Rockwell numbers. The support must be of
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 10 of 12
sufficient rigidity to prevent its permanent deformation in use. Indenters are not calibrated below values of
20 or above values of 95, thus if readings are outside of this range then a different indenter must be
selected. Minimum thickness requirements, conversions between various Rockwell scales and round work
corrections
Table 6.2: Rockwell Round Correction Chart
ROCKWELL C, A, D SCALES
Diameter .25" .375" 0.5" .625" .75" .875" 1" 1.25" 1.5"
Result 6.4 mm 10 mm 13 mm 16 mm 19 mm 22 mm 25 mm 32 mm 38 mm
20 6.0 4.5 3.5 2.5 2.0 1.5 1.5 1.0 1.0
25 5.5 4.0 3.0 2.5 2.0 1.5 1.0 1.0 1.0
30 5.0 3.5 2.5 2.0 1.5 1.5 1.0 1.0 0.5
35 4.0 3.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5
40 3.5 2.5 2.0 1.5 1.0 1.0 1.0 0.5 0.5
45 3.0 2.0 1.5 1.0 1.0 1.0 0.5 0.5 0.5
50 2.5 2.0 1.5 1.0 1.0 0.5 0.5 0.5 0.5
55 2.0 1.5 1.0 1.0 0.5 0.5 0.5 0.5 --
60 1.5 1.0 1.0 0.5 0.5 0.5 0.5 -- --
65 1.5 1.0 1.0 0.5 0.5 0.5 0.5 -- --
70 1.0 1.0 0.5 0.5 0.5 0.5 0.5 -- --
75 1.0 0.5 0.5 0.5 0.5 0.5 -- -- --
80 0.5 0.5 0.5 0.5 0.5 -- -- -- --
85 0.5 0.5 0.5 -- -- -- -- -- --
90 0.5 -- -- -- -- -- -- -- --
ROCKWELL B, F, G SCALES
Diameter .25" .375" 0.5" .625" .75" .875" 1"
Result 6.4 mm 10 mm 13 mm 16 mm 19 mm 22 mm 25 mm
0 12.5 8.5 6.5 5.5 4.5 3.5 3.0
10 12.0 8.0 6.0 5.0 4.0 3.5 3.0
20 11.0 7.5 5.5 4.5 4.0 3.5 3.0
30 10.0 6.5 5.0 4.5 3.5 3.0 2.5
40 9.0 6.0 4.5 4.0 3.0 2.5 2.5
50 8.0 5.5 4.0 3.5 3.0 2.5 2.0
60 7.0 5.0 3.5 3.0 2.5 2.0 2.0
70 6.0 4.0 3.0 2.5 2.0 2.0 1.5
80 5.0 3.5 2.5 2.0 1.5 1.5 1.5
90 4.0 3.0 2.0 1.5 1.5 1.5 1.0
100 3.5 2.5 1.5 1.5 1.0 1.0 0.5
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 11 of 12
6.3 Vickers Hardness Test
The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the
form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to
a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the
indentation left in the surface of the material after removal of the load are measured using a microscope
and their average calculated. The area of the sloping surface of the indentation is calculated. The HV
number is then determined by the ratio F/A, where F is the force applied to the diamond in kilograms-
force and A is the surface area of the resulting indentation in square millimeters, Fig. 6.4.
Figure 6.4: Hardness impression on to the sample during Vickers hardness test.
A can be determined by the formula
𝐴 = 𝑑2
2 sin(136/2) =
𝑑2
1.8544
(6.5)
where d is the average length of the diagonal left by the indenter. Hence
𝐻𝑉 = 𝐹
𝐴 = 1.8544 (𝐹
𝑑2⁄ ) kgf/mm2 (6.6)
Here F is in kg, and d is in mm.
When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated
from the formula, but is more convenient to use conversion tables. The Vickers hardness should be reported
like 800 HV/10, which means a Vickers hardness of 800, was obtained using a 10 kgf force. Several
different loading settings give practically identical hardness numbers on uniform material, which is much
better than the arbitrary changing of scale with the other hardness testing methods. The advantages of the
Vickers hardness test are that extremely accurate readings can be taken, and just one type of indenter is
used for all types of metals and surface treatments. Although thoroughly adaptable and very precise for
testing the softest and hardest of materials, under varying loads, the Vickers machine is a floor standing
unit that is more expensive than the Brinell or Rockwell machines.
There is now a trend towards reporting Vickers hardness in SI units (MPa or GPa) particularly in academic
papers. To convert the Vickers hardness number to SI units the force applied needs converting from kgf to
newtons to give results in MPa (or N/mm2) using the formula above and furthermore divided by 1000 to
get the hardness in GPa. For example, 700 HV/30 changes into HV/294 N = 6.87 GPa.
MME222/Jan 17 Term/Expt. 02: Hardness Testing of Materials Page 12 of 12
6.4 Hardness Conversion
The facility to convert the hardness measured on one scale to that of another is most desirable. However,
because hardness is not a well-defined material property, and because of the experimental dissimilarities
among the various techniques, a comprehensive conversion scheme has not been devised. Hardness
conversion data have been determined experimentally and found to be dependent on material type and
characteristics. The most reliable conversion data exist for steels, some of which are presented in Figure 6.5
for Knoop, Brinell, and two Rockwell scales; the Mohs scale is also included.
Figure 6.5: Comparison of several hardness scales.