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How to Choose Rockwell Hardness Scale?

In this blog post, you’ll read:The ASTM specifies different Rockwell hardness scales for various combinations of test loads and indenter types. Rockwell scales allow you to follow the standard scale tables and rules to get the accurate values of the hardness of your test material.

Table of Contents

There are numerous appropriate procedures to measure the load that a material resists under certain conditions. Among the various scales such as the Vickers scale, Brinell scale; Rockwell hardness scale stands out, whose usefulness, after almost a century, continues to be required. That is the reason why Rockwell is the most commonly used scale. The procedure is simple: establish the force of an element, subjected to penetration, and calculate the depth of the mark it leaves on its surface, but it has some limitations too.

Below, we will discuss what is the Rockwell scale, its differences, and factors which are affecting the results? Let’s get started!

I. Why Different Scales are Used in Rockwell Hardness Testing?

Different scales can be used which come from using different combinations of indenters and test force, allowing virtually any metal or alloy to be tested. Different Rockwell hardness scales are used for different sample objects since it is not the same to analyze such as copper as steel or another element. Various scales are used so as not to damage the machine or the indenter, which is often very expensive.

II. Which Scale is Used in Rockwell Scale?

The Rockwell hardness scale consists of applying a load on the object under measurement mediated by an indenter (it can be a spheroconical diamond indenter or a steel or hard metal sphere), and it is measured how much that indenter penetrates the object to be measured. For normal scales, each unit of Rockwell hardness is equivalent to 0.002 mm in depth, and for Superficial Rockwell hardness scales, each unit of hardness is equivalent to 0.001 mm in depth.

III. What are B Scale and C Scale in Rockwell Hardness?

There are 11 basic scales for determining hardness (A, B, C, D, E, F, G, H, K, N, T), which are designed to work in various combinations of “intended – load”. But the most common scales are

  • HRB – with a ball Ø 1.588 and a total force on the measuring head of 100 kgf. It is used for unhardened steel and non-ferrous metals.
  • HRC – with a cone and a total force on the measuring head of 150 kgf. It is used for hardened steel.

IV. How do you Choose a Rockwell Hardness Scale?

  • Types of Test Material

While performing the Rockwell hardness test, the type of testing material should be first known. Different types of materials have different testing scales for reliable outcomes. You can choose a scale from standard tables of  Rockwell hardness scale which is given in the literature. Mostly, softer material has bigger scal value.  If you do not take care of this factor, then it is not possible to apply an accurate Rockwell hardness procedure and get reliable results. 

 

ROCKWELL HARDNESS SCALE

 

  • Test Material Area

Deformation of the material not only flows below indentation but also flows outside of the material. This not only makes the thickness of the material crucial but also limits the width of the material also.  Keep in mind to place the test material in the middle of the testing surface. If you process the Rockwell testing at the end or corner of the testing surface, then the material can push out of the testing area at the time of deformation around the indentation. This can provide less hardness value than actual. This is more common when measuring soft objects.

Generally, according to the Rockwell ISO standard, the distance should be more than 1mm.

Many Rockwell test standards also stated that while processing the test, there should be a distance of more than 2.5 times the diameter of the indentation between mid of an indentation and the periphery of the material.

  • Thickness of Test Material 

In the Rockwell hardness test, indentation depth depends on the thickness of the material. In the process of the test, material present near indentation is deformed in plastic. This deformation is actually creating depth of indentation. If the thickness of the material is thin for the specific test, then the whole material will be deformed and will flow on the test surface with the hanging anvil, and indentation depth will not be created. This affects the procedure of deformation in a negative way which can lead to flawed hardness outcomes.

To avoid this and to get Rockwell test outcomes, the testing material should have an adequate thickness. You can choose a Rockwell hardness scale with suitable test forces (load) and indenter size according to the material thickness that must be used in the procedure.

The thickness can also be selected from the Rockwell scale table.

First, find the estimated value of the hardness of the test material. The approximate Rockwell measurement of the thickness can be checked from graphs or tables (which can be found in the literature on the Rockwell hardness test). Some of the examples of this data are available in ASTM standards.

  • Test Material Homogeneity

Material size and its location should show homogeneity while processing the hardness test. Also these Features should keep in mind while choosing the scale in performing the Rockwell hardness. But if these two features are not homogeneous then alternatively you can adopt other measures for reliable outcomes of results. 

If the size of  testing material is not large enough  then you can choose a scale that can create an appropriately large indentation to cover the whole test surface. Surrounding area of the Rockwell indentation should be adjusted in a way that testing material should come in the middle to avoid deformation of the material out of the testing area. 

  • Scale Limitations

It is theoretically defined that Rockwell has a wide range of different scales starting from 0 to 100 which covers specific hardness values, and all the scales can be used for testing.

But there are the practical limitations in material testing with low hardness, these limitations can cause the material to reach the place of the indenter cap for the intender ball. This is due to the deep penetration of the indenter in the material

V. Other Important Factors to Consider

Scales are employed to test the hardness of metals with various methods for the static application of the loads, No matter whether you are measuring depth caused by penetration or the size of the indentation. 

Different methods with static hardness are: 

  • Depth measurement methods

Scales are applied while measuring the value of  residual depth of indentation caused by the indenter used in the test. The Rockwell method has the advantage to be the only standardized method by the ASTM 18 and ISO 6508. There are two not standardized methods for depth measurement called the Brinell(HBT) and the Vickers(HVT).

  • Optical measurement methods

These types of methods are also used to measure the value of the residual size of indentation caused by the indenter in the test. All three common methods are standardized in this case of optical measurement by ISO and ASTM.  

  • Leeb rebound method

This is the alternate method which involves application of dynamic force. In this method the value of the height of a ball rebound is calculated to analyze the hardness of the testing material.

VI. Rockwell Hardness Scale Chart

Hardness Conversion for Rockwell B scale:

B

A

E

15T

30T

45T

HV

HK

HB

HB

30 kgf

60 kgf

100 kgf

15 kgf

30 kgf

45 kgf

Vickers

Knoop

Brinell

Brinell

1/16 Ball

Diamond

1/8 Ball

1/16 Ball

1/16 Ball

1/16 Ball

Scale

>500 gf

10/3000

10/500

100

61.5

93.1

83.1

72.9

240

251

240

201

99

60.9

92.8

82.5

71.9

234

246

234

195

98

60.2

92.5

81.8

70.9

228

241

228

189

97

59.5

92.1

81.1

69.9

222

236

222

184

96

58.9

91.8

80.4

68.9

216

231

216

179

95

58.3

91.5

79.8

67.9

210

226

210

175

94

57.6

91.2

79.1

66.9

205

221

205

171

93

57

90.8

78.4

65.9

200

216

200

167

92

56.4

90.5

77.8

64.8

195

211

195

163

91

55.8

90.2

77.1

63.8

190

206

190

160

90

55.2

89.9

76.4

62.8

185

201

185

157

89

54.6

89.5

75.8

61.8

180

196

180

154

88

54

89.2

75.1

60.8

176

192

176

151

87

53.4

88.9

74.4

59.8

172

188

172

148

86

52.8

88.6

73.8

58.8

169

184

169

145

85

52.3

88.2

73.1

57.8

165

180

165

142

84

51.7

87.9

72.4

56.8

162

176

162

140

83

51.1

87.6

71.8

55.8

159

173

159

137

82

50.6

87.3

71.1

54.8

156

170

156

135

81

50

86.9

70.4

53.8

153

167

153

133

80

49.5

86.6

69.7

52.8

150

164

150

130

79

48.9

86.3

69.1

51.8

147

161

147

128

78

48.4

86

68.4

50.8

144

158

144

126

77

47.9

85.6

67.7

49.8

141

155

141

124

76

47.3

85.3

67.1

48.8

139

152

139

122

75

46.8

85

66.4

47.8

137

150

137

120

74

46.3

84.7

65.7

46.8

135

147

135

118

73

45.8

84.3

65.1

45.8

132

145

132

116

72

45.3

84

64.4

44.8

130

143

130

114

71

44.8

100

83.7

63.7

43.8

127

141

127

112

70

44.3

99.5

83.4

63.1

42.8

125

139

125

110

69

43.8

99

83

62.4

41.8

123

137

123

109

68

43.3

98

82.7

61.7

40.8

121

135

121

108

67

42.8

97.5

82.4

61

39.8

119

133

119

106

66

42.3

97

82.1

60.4

38.7

117

131

117

104

65

41.8

96

81.8

59.7

37.7

116

129

116

102

64

41.4

95.5

81.4

59

36.7

114

127

114

100

63

40.9

95.1

81.1

58.4

35.7

112

125

112

99

62

40.4

94.5

80.8

57.7

34.7

110

124

110

98

61

40

93.5

80.5

57

33.7

108

122

108

96

60

39.5

93

80.1

56.4

32.7

107

120

107

95

59

39

92.5

79.8

55.7

31.7

106

117

106

94

58

38.6

92

79.5

55

30.7

104

117

104

92

57

38.1

91

79.2

54.4

29.7

103

115

103

91

56

37.7

90.5

78.8

53.7

28.7

101

114

101

90

55

37.2

90

78.5

53

27.7

100

112

100

89

54

36.8

89.5

78.2

52.4

26.7

111

87

53

36.3

89

77.9

51.7

25.7

110

86

52

35.9

88

77.5

51

24.7

109

85

51

35.5

87.5

77.2

50.3

23.7

108

84

50

35

87

76.9

49.7

22.7

107

83

49

34.6

86.5

76.6

49

21.7

106

82

48

34.1

85.5

76.2

48.3

20.7

105

81

47

33.7

85

75.9

47.7

19.7

104

80

46

33.3

84.5

75.6

47

18.7

103

80

45

32.9

84

75.3

46.3

17.7

102

79

44

32.4

83.5

74.9

45.7

16.7

101

78

43

32

82.5

74.6

45

15.7

100

77

42

31.6

82

74.3

44.3

14.7

99

76

41

31.2

81.5

74

43.7

13.6

98

75

40

30.7

81

73.6

43

12.6

97

75

39

30.3

80

73.3

42.3

11.6

96

74

38

29.9

79.5

73

41.6

10.6

95

73

37

29.5

79

72.7

41

9.6

94

72

36

29.1

78.5

72.3

40.3

8.6

93

72

35

28.7

78

72

39.6

7.6

92

71

34

28.2

77

71.7

39

6.6

91

70

33

27.8

76.5

71.4

38.3

5.6

90

69

32

27.4

76

71

37.6

4.6

89

69

31

27

75.5

70.7

37

3.6

88

68

30

26.6

75

70.4

36.3

2.6

87

67

Hardness Conversion for Rockwell C scale:

Rockwell

Superficial Rockwell

Brinell

Vickers

Knoop

Tensile Strength

 

Diamond Penetrator

 

Diamond Penetrator

10 mm

Carbide Ball

Diamond Pyramid

Diamond Penetrator

Steel

C
Scale
150
kgf

D
Scale
100
kgf

A
Scale
60
kgf

15N
Scale
15
kgf

30N
Scale
30
kgf

45N
Scale
45
kgf

3000
kgf
Hardness

 

10 kgf

500

gf & over

Equil.
1000 lb sq in

68

77

86

93

84

75

940

920

67

76

85

93

84

74

900

895

66

76

85

93

83

73

865

870

65

75

84

92

82

72

832

846

64

74

84

92

81

71

800

822

63

73

83

91

80

70

772

799

62

73

83

91

79

69

746

776

61

72

82

91

79

68

720

754

60

71

81

90

78

67

697

732

59

70

81

90

77

66

634

674

710

58

69

80

89

76

64

615

653

690

57

69

80

89

75

63

595

633

670

56

68

79

88

74

62

577

613

650

55

67

79

88

73

61

560

595

630

54

66

78

87

72

60

543

577

612

53

65

77

87

71

59

525

560

594

52

65

77

86

70

57

512

544

576

51

64

76

86

69

56

496

528

558

50

63

76

86

69

55

481

513

542

49

62

75

85

68

54

469

498

526

48

61

75

85

67

53

455

484

510

47

61

74

84

66

51

443

471

495

46

60

73

84

65

50

432

458

480

45

59

73

83

64

49

421

446

466

44

59

73

83

63

48

409

434

452

43

58

72

82

62

47

400

423

438

42

57

72

82

61

46

390

412

426

41

56

71

81

60

44

381

402

414

40

55

70

80

60

43

371

392

402

39

55

70

80

59

42

362

382

391

38

54

69

79

58

41

353

372

380

37

53

69

79

57

40

344

363

370

36

52

68

78

56

38

336

354

360

35

52

68

78

55

37

327

345

351

34

51

67

77

54

36

319

336

342

33

50

67

77

53

38

311

327

334

32

49

66

76

52

34

301

318

326

31

48

66

76

51

33

294

310

318

30

48

65

75

50

31

286

302

311

29

47

65

75

50

30

279

294

304

28

46

64

74

49

29

271

286

297

27

45

64

73

48

28

264

279

290

26

45

63

73

47

27

258

272

284

25

44

63

72

46

26

253

266

278

24

43

62

72

45

24

247

260

272

23

42

62

71

44

23

243

254

266

22

42

62

71

43

22

237

248

261

21

41

61

70

42

21

231

243

256

20

40

61

69

42

20

226

238

251

Conclusion

No wonder, Rockwell hardness testing scales are globally used and are the most popular ones.  It is used for static hardness testing of the material. We have described thoroughly in this article the scales of the hardness test. Just follow the standard scale tables and rules to get the accurate values of the hardness of your test material. Want to learn more about the scales of different hardness testing or other factors of hardness testing by visiting our website hardnessgauge.com.