Laboratory 2 Hardness testing - Suranaree University of...

Laboratory 2: Hardness testing

Mechanical Metallurgy Laboratory 431303 1

T. Udomphol

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Hardness Testing

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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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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.



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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



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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



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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.



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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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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

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sample


sample

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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

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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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.

Rockwell Superficial Rockwell Brinell

Diamond

Brale 1/16" Ball "N" Brale Penetrater

10 mm Ball,

3000 kgf Load

Tensile strength

150 kgf

C Scale

60 kgf

A Scale

100 kgf

D Scale

100 kgf

B Scale

15 kg

Load

15 N

30 kg

Load

30N

45 kg

Load

45N

Diam. Of Ball

Impression in

mm

Hardness

Number

Vickers

Equivalent 1000 lb. Sq.

In.

80 92 87 97 92 87 1865

79 92 86 92 87 1787

78 91 85 96 91 86 1710

77 91 84 91 85 1633

76 90 83 96 90 84 1556

75 90 83 89 83 1478

74 89 82 95 89 82 1400

73 89 81 88 81 1323

72 88 80 95 87 80 1245

71 87 80 87 79 1160

70 87 79 94 86 78 1076

69 86 78 94 85 77 1004

68 86 77 85 79 942

67 85 76 93 84 75 894

66 85 76 93 83 73 854

65 84 75 92 82 72 2.25 745 820

64 84 74 81 74 2.30 710 789

63 83 73 92 80 70 2.30 710 763

62 83 73 91 79 69 2.35 682 746

61 82 72 91 79 68 2.35 682 720

60 81 71 90 78 67 2.40 653 697

59 81 70 90 77 66 2.45 627 674 326

58 80 69 89 76 65 2.55 578 653 315

57 80 69 89 75 63 2.55 578 633 304

56 79 68 88 74 62 2.60 555 613 294

55 79 67 88 73 61 2.60 555 595 287

54 78 66 87 72 60 2.65 534 577 279

53 77 65 87 71 59 2.70 514 560 269

52 77 65 86 70 57 2.75 495 544 261

51 76 64 86 69 56 2.75 495 528 254

50 76 63 86 69 55 2.80 477 513 245

49 75 62 85 68 54 2.85 461 498 238

48 75 61 85 67 53 2.90 444 484 232

47 74 61 84 66 51 2.90 444 471 225



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Diamond


10 mm Ball,

3000 kgf Load

Tensile strength

150 kgf

C Scale

60 kgf

A Scale

100 kgf

D Scale

100 kgf

B Scale

15 kg

Load

15 N

30 kg

Load

30N

45 kg

Load

45N

Diam. Of Ball

Impression in

mm

Hardness

Number

Vickers


In.

46 73 60 84 65 50 2.95 432 458 219

45 73 59 83 64 49 3.00 415 446 211

44 73 59 83 63 48 3.00 415 434 206

43 72 58 82 62 47 3.05 401 423 202

42 72 57 82 61 46 3.10 388 412 198

41 71 56 81 60 44 3.10 388 402 191

40 70 55 80 60 43 3.15 375 392 185

39 70 55 80 59 42 3.20 363 382 181

38 69 54 79 58 41 3.25 352 372 176

37 69 53 109 79 57 40 3.30 341 363 171

36 68 52 109 78 56 39 3.35 331 354 168

35 68 52 108 78 55 37 3.35 331 345 163

34 67 51 108 77 54 36 3.40 321 336 159

33 67 50 107 77 53 38 3.45 311 327 154

32 66 49 106 76 52 34 3.50 302 318 150

31 66 48 106 76 51 33 3.55 293 310 146

30 65 48 105 75 50 32 3.60 285 302 142

29 65 47 104 75 50 30 3.65 277 294 138

28 64 46 103 74 49 29 3.70 269 286 134

27 64 45 103 73 48 28 3.75 262 279 131

26 63 45 102 73 47 27 3.80 255 272 126

25 63 44 101 72 46 26 3.80 255 266 124

24 62 43 100 72 45 24 3.85 248 260 122

23 62 42 99 71 44 23 3.90 241 254 118

22 62 42 99 71 43 22 3.95 235 248 116

21 61 41 98 70 42 21 4.00 229 243 113

20 61 40 97 69 42 20 4.05 223 238 111

18 95 4.10 217 230 107

16* 94 4.15 212 222 102

14* 92 4.25 203 213 98

12* 90 4.35 192 204 92

10* 89 4.40 187 195 90

8* 87 4.50 179 187 87

6* 85 4.60 170 180 83

4* 84 4.65 166 173 79

2* 82 4.80 156 166 77

0* 81 4.80 156 160 74

79 4.90 149 156 73

77 5.00 143 150 70

74 5.10 137 143 67

72 5.20 131 137 65

70 5.30 126 132 62



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Diamond


10 mm Ball,

3000 kgf Load

Tensile strength

150 kgf

C Scale

60 kgf

A Scale

100 kgf

D Scale

100 kgf

B Scale

15 kg

Load

15 N

30 kg

Load

30N

45 kg

Load

45N

Diam. Of Ball

Impression in

mm

Hardness

Number

Vickers


In.

68 5.40 121 127 60

65 5.50 116 122 58

5.60 112 117 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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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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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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4.2 Rockwell hardness values (HRA, HRB, HRC)


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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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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4.4 Micro Vickers hardness (VHN)


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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4.5 Hardness profile of welded sample in relevant to the weld microstructure



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5. Discussion

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6. Conclusions

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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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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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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, 4th edition, McGraw-

Hill, ISBN 007-125690-3.

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