Laboratory 2: Hardness testing
L L a a b b o o r r a a t t o o r r y y 2 2
Hardness Testing
• 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
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 metalforming 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.
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Laboratory 2: Hardness testing
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
P
BHN =
( π D / 2 )( D − D
2
− d
2
)
P
=
π Dt where
P is the applied load, kg
D is the diameter of the steel ball, mm t d is is the diameter of the indentation, mm the depth of impression, mm
; (1)
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Laboratory 2: Hardness testing
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
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Laboratory 2: Hardness testing indenters, Brale and steel ball indenters. The former is a round-tip cone with an included angle of
120 o 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
H
1 onto the machine before applying a major load level according to a standard as shown in table 2 and is recorded as
H
2
. The difference of the depths ( ∆ H= H
1
-H
2
) 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
HRX = M −
∆ H
0 .
002
; (2)
Where ∆
H
is
H
1
-H
2
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.
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Laboratory 2: Hardness testing
The Rockwell hardness units are in R
A
, R
B
and R
C
(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 R
C
20 F
R
C
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 R
B
0 F R
B
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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Laboratory 2: Hardness testing
Table 1: Rockwell hardness scale for various mateirals
M
P
R
S
F
G
H
K
L
Scale
A
B
C
D
E
Indenter
Diamond cone
1/16" steel ball
Diamond cone
Diamond cone
1/8" steel ball
1/16" steel ball
1/16" steel ball
1/8" steel ball
1/8" steel ball
1/4" steel ball
1/4" steel ball
1/4" steel ball
1/2" steel ball
1/2" steel ball
Minor Load Major Load kgf kgf
10 50
10
10
10
10
90
140
90
90
10
10
10
10
10
10
10
10
10
90
140
50
90
50
140
50
140
50
V 1/2" steel ball 10 140
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 136 o
.
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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Laboratory 2: Hardness testing
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
VHN =
2 P sin
( θ / 2
) d 2
=
1 .
854 P d 2
;(2)
Where P is the applied load, kg d is the average length of the diagonals = ( d
1
+d
2
)/2) , mm
θ is the angle between the opposite faces of the diamond) = 136 o
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
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Laboratory 2: Hardness testing 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
KHN =
14 .
2 P l
2
;(3)
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Laboratory 2: Hardness testing
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 diamondpyramid 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
σ o
=
VHN
3
( 0 .
1 ) n ;(5) where σ o 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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Laboratory 2: Hardness testing 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 values if the work hardening exponent is known.
!"#$% 7: 3456789:;<=>?:@=>?ABCADC83EFG@=HDIJKJLMCกกC<@JOP?3?? Micro Vicker/Knoop 3RS
Rockwell scale C.
!"#$% 8: 3456789:;<=>?:@=>?ABCADC83EFG3RSABCADC83EFG3<GEPG Carbon steel 3RS Alloy steel
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Laboratory 2: Hardness testing
For convenience, the hardness values measured using different methods such as Brinell,
Rockwell or Vickers testing can be converted using the hardness value conversion table as shown in table 3.
Table 3 Hardness value conversion table for Brinell, Rockwell 3RS Vickers hardness values.
Diamond
Brale
Rockwell
1/16" Ball
Superficial Rockwell
"N" Brale Penetrater
80
79
78
77
52
51
50
49
48
47
56
55
54
53
60
59
58
57
64
63
62
61
68
67
66
65
72
71
70
69
76
75
74
73
150 kgf
C Scale
60 kgf
A Scale
100 kgf
D Scale
100 kgf
B Scale
15 kg
Load
15 N
97
96
96
95
95
94
94
93
93
92
86
86
85
85
84
88
87
87
86
90
89
89
88
92
91
91
90
92
92
91
91
77
76
76
75
75
74
79
79
78
77
81
81
80
80
84
83
83
82
86
85
85
84
88
87
87
86
90
90
89
89
87
86
85
84
71
70
69
69
68
67
66
65
65
64
63
62
61
61
77
76
76
75
74
73
73
72
83
83
82
81
80
80
79
78
30 kg
Load
30N
92
92
91
91
70
69
69
68
67
66
74
73
72
71
78
77
76
75
81
80
79
79
85
84
83
82
87
87
86
85
90
89
89
88
45 kg
Load
45N
87
87
86
85
57
56
55
54
53
51
62
61
60
59
67
66
65
63
74
70
69
68
79
75
73
72
80
79
78
77
84
83
82
81
Brinell
2.70
2.75
2.75
2.80
2.55
2.60
2.60
2.65
2.35
2.40
2.45
2.55
2.25
2.30
2.30
2.35
2.85
2.90
2.90
10 mm Ball,
3000 kgf Load
Vickers
Diam. Of Ball
Impression in mm
Hardness
Number
1865
1787
1710
1633
544
528
513
498
484
471
613
595
577
560
697
674
653
633
789
763
746
720
942
894
854
820
1245
1160
1076
1004
1556
1478
1400
1323
514
495
495
477
578
555
555
534
682
653
627
578
745
710
710
682
461
444
444
Tensile strength
Equivalent 1000 lb. Sq.
In.
287
279
269
261
326
315
304
294
254
245
238
232
225
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Laboratory 2: Hardness testing
26
25
24
23
30
29
28
27
22
21
62
61
20
18
61
16*
14*
63
63
62
62
65
65
64
64
34
33
32
31
38
37
36
35
42
41
40
39
46
45
44
43
67
67
66
66
69
69
68
68
72
71
70
70
73
73
73
72
12*
10*
8*
6*
4*
2*
0*
Diamond
Rockwell
1/16" Ball
Brale
150 kgf
C Scale
60 kgf
A Scale
Superficial Rockwell
"N" Brale Penetrater
100 kgf
D Scale
52
52
51
50
49
48
48
47
46
45
45
44
43
42
42
41
100 kgf
B Scale
15 kg
Load
60
59
58
57
56
55
54
53
40
59
55
109
109
108
108
107
106
106
105
104
103
103
102
101
100
99
99
98
97
95
15 N
84
83
83
82
82
81
80
80
79
79
78
78
77
77
76
76
75
75
74
73
73
72
72
71
71
70
69
94
92
90
89
87
85
84
82
81
79
77
74
72
70
46
45
44
43
42
42
50
49
48
47
53
52
51
50
57
56
55
54
60
60
59
58
64
63
62
61
30 kg
Load
30N
65
26
24
23
22
21
20
30
29
28
27
38
34
33
32
40
39
37
36
44
43
42
41
49
48
47
46
45 kg
Load
45N
50
Brinell
4.35
4.40
4.50
4.60
4.65
4.80
4.80
4.90
5.00
5.10
5.20
5.30
10 mm Ball,
3000 kgf Load
Vickers
Diam. Of Ball
Hardness
Impression in
Number mm
3.95
4.00
4.05
4.10
4.15
4.25
3.80
3.80
3.85
3.90
3.60
3.65
3.70
3.75
3.40
3.45
3.50
3.55
3.25
3.30
3.35
3.35
3.10
3.10
3.15
3.20
2.95
3.00
3.00
3.05
235
229
223
217
212
203
255
255
248
241
285
277
269
262
321
311
302
293
352
341
331
331
388
388
375
363
432
415
415
401
248
243
238
230
222
213
272
266
260
254
302
294
286
279
336
327
318
310
372
363
354
345
412
402
392
382
458
446
434
423
192
187
179
170
166
156
156
149
143
137
131
126
204
195
187
180
173
166
160
156
150
143
137
132
Tensile strength
Equivalent 1000 lb. Sq.
In.
83
79
77
92
90
87
74
73
70
67
65
62
116
113
111
107
102
98
126
124
122
118
142
138
134
131
159
154
150
146
176
171
168
163
198
191
185
181
219
211
206
202
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Laboratory 2: Hardness testing
Diamond
Rockwell
1/16" Ball
Brale
150 kgf
C Scale
60 kgf
A Scale
Superficial Rockwell
"N" Brale Penetrater
100 kgf
D Scale
100 kgf
B Scale
68
65
15 kg
Load
15 N
30 kg
Load
30N
45 kg
Load
45N
Brinell
10 mm Ball,
3000 kgf Load
Vickers
Diam. Of Ball
Hardness
Impression in
Number mm
5.40
5.50
5.60
121
116
112
127
122
117
Tensile strength
Equivalent 1000 lb. Sq.
In.
60
58
56
In summary, hardness testing methods for example Brinell, Rockwell, Vickers and Knoops are practical in measuring mechanical properties of metals and other engineering materials. It is essential for engineers to select an appropriate hardness testing method for the desired applications or materials used. This is depending on size and shape of the test pieces, metallurgical phases and their locations to be analysed. The correct hardness values are beneficial for material selection and design together with material development for higher performance. Moreover, the hardness values can be used for estimating other related mechanical properties of the materials, for example, tensile strength or yield strength.
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Laboratory 2: Hardness testing
2. Materials and equipment
2.1 Test specimens
2.2 Brinell hardness machine
2.3 Rockwell hardness machine
2.4 Vickers hardness machine
2.5 Micro Vickers hardness machine
3. Experimental procedure
3.1 Surfaces of aluminium, brass steel and weld samples must be flattened and ground using sand papers. Polishing of the metal surface is required for only Rockwell and Vickers hardness tests while Brinell hardness test requires only flat and ground surfaces.
3.2 Hardness measurement is carried out using Brinell, Rockwell and Vickers hardness testing techniques on the prepared surfaces at 10 positions on each sample.
3.3 Hardness profile testing is conducted across the weld sample at 10 positions and 1 mm intervals using a Vickers hardness testing machine.
3.4 Micro Vickers hardness testing is carried out using the polished samples.
3.5 Summarize the experimental results on the table provided and exhibit the results graphically. Compare and discuss the obtained results in order to relate hardness properties of the metals to their microstructure. Give conclusions.
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Laboratory 2: Hardness testing
4. Results
4.1 Brinell hardness values (BHN)
Position Aluminium Mild steel Brass
Position 1
Position 2
Position 3
Position 4
Position 5
Position 6
Position 7
Position 8
Position 9
Position 10
Mean
Stdev
Table 2: Brinell hardness values of aluminium, mild steel, brass and weld
Figure 4: Graph showing Brinell hardness values of aluminium, mild steel and brass.
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Laboratory 2: Hardness testing
4.2 Rockwell hardness values (HRA, HRB, HRC)
Position Aluminium Mild steel Brass
Position 1
Position 2
Position 3
Position 4
Position 5
Position 6
Position 7
Position 8
Position 9
Position 10
Mean
Stdev
Table 3: Rockwell hardness values of aluminium, mild steel, brass and weld
Figure5: Graph showing Rockwell hardness values of aluminium, mild steel and brass.
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Laboratory 2: Hardness testing
4.3 Vickers hardness values
Position Aluminium Mild steel Brass Weld
Position 1
Position 2
Position 3
Position 4
Position 5
Position 6
Position 7
Position 8
Position 9
Position 10
Mean
Stdev
Table 4: Vickers hardness values of aluminium, mild steel, brass and the weld.
Figure 6: Graph showing Vickers hardness values of aluminium, mild steel and brass.
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Laboratory 2: Hardness testing
4.4 Micro Vickers hardness (VHN)
Position Aluminium Mild steel Brass
Position 1
Position 2
Position 3
Position 4
Position 5
Position 6
Position 7
Position 8
Position 9
Position 10
Mean
Stdev
Table 5: Micro Vickers hardness values of aluminium, mild steel, brass and weld
Figure 7: Graph showing micro Vickers hardness value of aluminium, mild steel and brass.
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Laboratory 2: Hardness testing
4.5 Hardness profile of welded sample in relevant to the weld microstructure
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Laboratory 2: Hardness testing
5. Discussion
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Laboratory 2: Hardness testing
6. Conclusions
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Laboratory 2: Hardness testing
7. Questions
7.1 Which metal does provide the highest hardness values? Why?
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7.2 Explain why the hardness values in the welded area are different from the hardness values obtained in the base metal.
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Mechanical Metallurgy Laboratory 431303
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Laboratory 2: Hardness testing
7.3 Compare Macro Vickers and micro Vickers hardness values obtained from the experimental results.
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7.4
Explain the relationship between hardness and tensile strength values.
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Mechanical Metallurgy Laboratory 431303
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Laboratory 2: Hardness testing
8. References
8.1 Dieter, G.E.,
Mechanical metallurgy
, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-
100406-8.
8.2 Hashemi, S.
Foundations of materials science and engineering
, 2006, 4 th
edition, McGraw-
Hill, ISBN 007-125690-3.
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