Laboratory 2: Hardness testing Mechanical Metallurgy Laboratory 431303 1 T. Udomphol L L a a b b o o r r a a t t o o r r y y 2 2 Hardness Testing ____________________________________ Objectives • Students are required to understand the principles of hardness testing, i.e., Rockwell, Brinell and Vickers hardness tests. • Students are able to explain variations in hardness properties of selected materials such as aluminium, steel, brass and welded metals and can explain factors that might affects their hardness properties. • Students can select appropriate macro-micro hardness testing techniques for suitable materials-property analysis. • Students are able to analyze the obtained hardness values in relevant to the nature of each material to be measured and use this information as a tool for selecting suitable materials for engineering applications.
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Laboratory 2: Hardness testing
Mechanical Metallurgy Laboratory 431303 1
T. Udomphol
LLaabboorraattoorryy 22
Hardness Testing
____________________________________
Objectives
• Students are required to understand the principles of hardness testing, i.e., Rockwell,
Brinell and Vickers hardness tests.
• Students are able to explain variations in hardness properties of selected materials
such as aluminium, steel, brass and welded metals and can explain factors that might
affects their hardness properties.
• Students can select appropriate macro-micro hardness testing techniques for suitable
materials-property analysis.
• Students are able to analyze the obtained hardness values in relevant to the nature of
each material to be measured and use this information as a tool for selecting suitable
materials for engineering applications.
Laboratory 2: Hardness testing
Mechanical Metallurgy Laboratory 431303 2
T. Udomphol
1. Literature Review
Hardness is one of the most basic mechanical properties of engineering materials.
Hardness test is practical and provide a quick assessment and the result can be used as a good
indicator for material selections. This is for example, the selection of materials suitable for metal-
forming dies or cutting tools. Hardness test is also employed for quality assurance in parts which
require high wear resistance such as gears.
The nomenclature of hardness comes in various terms depending on the techniques used for
hardness testing and also depends on the hardness levels of various types of materials. A scratch
hardness test is generally used for minerals, giving a wide range of hardness values in a Moh.s scale
at minimum and maximum values of 1 and 10 respectively. For example, talcum provides the lowest
value of 1 while diamond gives the highest of 10. The basic principle is that the harder material will
leave a scratch on a softer material. Hardness values of metals generally fall in a range of 4-8 in
Moh.s scale, which is not practical to differentiate hardness properties for engineering applications.
Therefore, indentation hardness measurement is conveniently used for metallic materials. A deeper or
wider indentation indicates a less resistance to plastic deformation of the material being tested,
resulting in a lower hardness value.
The indentation techniques involve Brinell, Rockwell, Vickers and Knoop. Different types
of indenters are applied for each type. The standard test methods according to the American Society
Testing and Materials (ASTM) available are, for instance, ASTM E10-07a (Standard test method for
Brinell hardness of metallic materials), ASTM E18-08 (Standard test method for Rockwell hardness
of metallic materials) and ASTM E92-41 (Standard test method for Vickers hardness of metallic
materials) These hardness testing techniques are selected in relation to specimen dimensions, type of
materials and the required hardness information. Their principles and testing methods are mentioned
as follow.
1.1 Brinell Hardness Test
Brinell hardness test was invented by J.A. Brinell in 1900 using a steel ball indenter with a
10 mm diameter. The steel ball is pressed on a metal surface to provide an impression as
demonstrated in figure 1. This impression should not be distorted and must not be too deep since this
might cause too much of plastic deformation, leading to errors of the hardness values.
Laboratory 2: Hardness testing
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c) Impression on Brinell hardness
test sample
c) Impression on Brinell hardness
test sample
Different levels of material hardness result in impression of various diameters and depths.
Therefore different loads are used for hardness testing of different materials as listed in table 1. Hard
metals such as steels require a 3,000 kgf load while brass and aluminium involve the loads of 2,000
and 1,000 or 500 kgf respectively. For materials with very high hardness, a tungsten carbide ball is
utilized to avoid the distortion of the ball.
Figure 1: (a) Brinell indentation (b) measurement of impression diameter and c) Impression
on Brinell hardness test sample [1].
In practice, pressing of the steel ball on to the metal surface is carried out for 30 second,
followed by measuring two values of impression diameters normal to each other using a low
magnification macroscope. An average value is used for the calculation according to equation 1
Dt
P
dDDD
PBHN
ππ=
−−=
))(2/( 22 ; (1)
where P is the applied load, kg
D is the diameter of the steel ball, mm
d is the diameter of the indentation, mm
t is the depth of impression, mm
Laboratory 2: Hardness testing
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Note: This BHN values has a unit of kgf.mm-2 (1 kgf.mm
-2 = 9.8 MPa) which cannot be
compared to the average mean pressure on the impression.
Generally, the metal surface should be flat without oxide scales or debris because these will
significantly affect the hardness values obtained. A good sampling size due to a large steel ball
diameter is advantageous for materials with highly different microstructures or microstructural
heterogeneity. Scratches or surface roughness have very small effects on the hardness values
measured. However, there are some disadvantages of Brinell hardness test. These are errors arising
from the operator themselves (from diameter measurement) and the limitation in measuring of too
small samples.
Figure 2:Plastic deformation surrounded by elastic material underneath a Brinell indenter
If we considered the plastic zone beneath the Brinell indenter, this plastic region is
surrounded by elastic material which obstructs the plastic flow. This condition is said to be plane
strain compressive where plastic deformation is limited. If the metal is very rigid, the metal flow
upwards surrounding the indenter is possible as illustrated in figure 1 a). However this situation is
rarely seen because the metal displaced by the indenter is accounted for by the reduced volume of
elastic material.
1.2 Rockwell Hardness Test
Rockwell hardness test is commonly used among industrial practices because the Rockwell
testing machine offers a quick and practical operation and can also minimize errors arising from the
operator. The depth of an indentation determines the hardness values. There are two types of
Laboratory 2: Hardness testing
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indenters, Brale and steel ball indenters. The former is a round-tip cone with an included angle of
120o whereas the latter is a hardened steel ball with their sizes ranging from 1.6-12.7 mm. Therefore
different combinations of indenters and loads selected are suitable for hardness testing of various
materials. This is for example; the R scale is employed for soft materials such as polymers while the
A scale is suitable for hardness testing of hard materials such as tool materials according to table 1.
The testing procedure starts with indenting a flatly ground metal surface with a diamond or
hardened steel ball with a minor load of 10 kgf to position the metal surface as shown in figure 3. .
The depth of the impression caused by the minor load will be recorded as H1onto the machine before
applying a major load level according to a standard as shown in table 2 and is recorded as H2. The
difference of the depths (∆H= H1-H2) when applying the minor and the major loads indicates the
hardness value of the material. If the depth difference is small, the deformation resistance of the
metal is high, resulting in a high Rockwell hardness value. The hardness value will be displayed on a
dial or a screen, having 100 divisions and each division represents a depth of 0.002 mm. Therefore
the hardness value can be determined from a relationship as follows
002.0
HMHRX
∆−= ; (2)
Where ∆H is H1-H2 and M is the maximum scale which equals 100 in general for testing
with the diamond indenter (scale A, C and D). The M value equals 130 when testing with a steel ball
for Rockwell scales B, E, M, and R.
Figure 3: Rockwell hardness measurement showing positions to apply the minor and major loads.
Laboratory 2: Hardness testing
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The Rockwell hardness units are in RA, RB and RC (or HRA, HRB, HRC), depending on
material.s hardness. Tables 1 and 2 summarize loads and types of an indenter utilized for each scale.
There are two types of indenters used, Brale indenter and steel ball indenters as mentioned previously.
The applied major loads vary from 60, 100 and 150 kgf, also depending on the Rockwell hardness
scale utilized. For instance, hardened steel is tested on a Rockwell scale C using a Brale indenter and
at a major load of 150 kgf. On the Rockwell scale C, the obtained hardness values range from RC 20 F
RC 70. Metals with lower hardness are tested on a Rockwell scale B using a 1.6 mm diameter steel
ball at a 100 kgf major load, providing RB 0 F RB 100 hardness values. Rockwell scale A offers a
wider range of hardness values which can be used to test materials ranging from annealed brass to
cemented carbide. Due to high accuracy, the Rockwell hardness test is commonly conducted for
measuring hardness of heat-treated steels. Furthermore, the smaller indenter (in comparison to that of
Brinell hardness test) facilitates hardness measurement in small areas. However, this technique
requires good surface preparation since the hardness values obtained is significantly affected by rough
and scratched surfaces.
There are several considerations for Rockwell hardness test
- Require clean and well positioned indenter and anvil
- The test sample should be clean, dry, smooth and oxide-free surface
- The surface should be flat and perpendicular to the indenter
- Low reading of hardness value might be expected in cylindrical surfaces
- Specimen thickness should be 10 times higher than the depth of the indenter
- The spacing between the indentations should be 3 to 5 times of the indentation diameter
- Loading speed should be standardized.
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Mechanical Metallurgy Laboratory 431303 7
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Minor Load Major Load
kgf kgf
A Diamond cone 10 50
B 1/16" steel ball 10 90
C Diamond cone 10 140
D Diamond cone 10 90
E 1/8" steel ball 10 90
F 1/16" steel ball 10 50
G 1/16" steel ball 10 140
H 1/8" steel ball 10 50
K 1/8" steel ball 10 140
L 1/4" steel ball 10 50
M 1/4" steel ball 10 90
P 1/4" steel ball 10 140
R 1/2" steel ball 10 50
S 1/2" steel ball 10 90
V 1/2" steel ball 10 140
Scale Indenter
Table 1: Rockwell hardness scale for various mateirals
Table 2: Applied loads and types of indenter used in Rockwell scale A,B and C hardness testing.
1.3 Vickers Hardness Test
Vickers hardness test requires a diamond pyramid indenter with an included angle of 136o.
This technique is also called a diamond pyramid hardness test (DPH) according to the shape of the
indenter. To carry on the test, the diamond indenter is pressed on to a prepared metal surface to cause
a square-based pyramid indentation as illustrated in figure 4.
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Figure 4: Vickers hardness test (a) Vickers indentation, (b) measurement of impression diagonal.
The Vickers hardness value (VHN) can be calculated from the applied load divided by
areas of indentation, at which the latter is derived from the diagonals of the pyramid as expressed in
the equation below
( )22
854.12/sin2
d
P
d
PVHN ==
θ
;(2)
Where P is the applied load, kg
d is the average length of the diagonals = (d1+d2)/2) , mm
θ is the angle between the opposite faces of the diamond) = 136o
Generally, the applied load should be carefully selected to achieve a perfect square-based
pyramid indentation for accurate hardness values, see figure 5 (a). The pincushion indentation as
shown in figure 5 (b) normally observed in annealed metal results from sinking of metal surrounding
the pyramid faces. The measured diagonals would be too long, thus, giving an under-estimated
hardness value. In figure 5 (c), a barrel-shaped indentation usually achieved from cold-worked metals
provides an indentation with metal pile-up at the pyramid faces. In such a case, the measured
diagonals would be too small and lead to an over-estimated hardness value obtained.
Vickers hardness is widely used in experimental and research areas because the VHN scale
practically offers a wide range of hardness values. For instance, the VHN values range from 5 to
1,500 can be obtained from measuring materials from dead soft to full hard. This method is therefore
more convenient and provides a wider range of the hardness values in comparison to those obtained
c) Impression on Vickers hardness test
sample
www.twi.co.uk
c) Impression on Vickers hardness test
sample
c) Impression on Vickers hardness test
sample
www.twi.co.uk
Laboratory 2: Hardness testing
Mechanical Metallurgy Laboratory 431303 9
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from Rockwell and Brinell hardness tests. The applied loads vary from 1-120 kg, which depends on
the materials being tested. However, Vickers hardness test is incommonly used for company daily
checks. This is due to errors which might occur in the measurement of the diagonals and longer time
required to finish the test.
Figure 5: Vickers hardness indentations a) perfect indentation, b) pincushion and c) barrel-shaped.
1.4 Micro Vickers hardness test
Micro Vickers hardness requires a micro-sized indenter (figure 6), which allows hardness
measurement in very limited areas such as surfaces of fine wires, thin sheets and foils. Moreover
hardness measurements at specific microstructural phases of materials, for instance, hardness
measurment of ferrites and pearlites existing in steels is also possible. This is beneficial for
identifying any hardness variation caused by metallurgical changes such as hardening, quenching,
plating, welding, bonding processes, where the larger indenter used for macro Vickers hardness test
limits its application in this case. The testing procedure of micro Vickers hardness is similar to that of
macro Vickers hardness. However, the prepared surface should be well polished without any fine
scratches in order to minimize errors which might occur when indenting on these scratches.
Another useful type of micro hardness test employs a Knoop indenter as shown in figure 6
(right) in order to accommodate limited testing areas such as on cross-sections of heat-treated
surfaces. The Knoop hardness number (KHN) can be calculated from the applied load divided by the
unrecovered projected area of the indention as follows
2
2.14
l
PKHN = ;(3)
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Where P is the applied load, kg
l is the length of the long diagonals, mm
Figure 6: Micro hardness indentations a) Vickers diamond-pyramid indenter, b) Knoop diamond-
pyramid indenter.
Furthermore, the strength of some metals can be determined from the plastic area under the
stress-strain curve. This is of interest when the strength of the materials can not be measured directly
from the standard tensile test. In this case, the yield strength at 0.2% offset can be determined from
the Vickers hardness number as shown in the expression
n
o
VHN)1.0(
3=σ ;(5)
where σo is the yield strength at 0.2% offset, kgf mm-2 (= 9.8 MPa)
VHN is the Vickers hardness number, VHN
n is the work hardening exponent
In summary, hardness measurements for example Brinell, Rockwell, Vickers and Knoop
are considered to be fast and easy ways to acquire hardness values of materials. Suitable hardness
measurements should be selected depending on the nature of the materials, dimensions, specimen
locations to be measured, metallurgical microstructures or phases of interest, etc. Analysis of the
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hardness data leads to better understanding of materials and further development in advanced
materials. The selection of proper materials to be used in desired applications can be therefore
effectively made. Moreover, prediction of material strength is possible by interpreting the hardness