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Hardness and hardness testing in comparison

A wide variety of raw materials and ancillary materials are used in industrial applications. Comprehensive knowledge of their material properties is an absolute prerequisite for using these raw materials for a defined application in technical environments. Hardness tests are a way to determine basic material properties such as hardness/ductility and strength of a material, such as steel. Hardness plays a critical role in the characterization and quality control of materials. The hardness of a steel provides information about its mechanical properties, wear resistance and suitability for specific applications. In this context, various hardness testing methods and scales have been developed to accurately determine and classify the hardness of steel. This article provides an overview of common hardness test methods and describes one possible hardness measurement procedure. A hardness comparison table allows a comparison of the values determined by individual methods.

Various hardness tests in comparison

Hardness testing is critical to the characterization and quality control of steel products. There are various hardness tests, e.g. according to Brinell, Rockwell, Shore and Vickers. The most common method is hardness testing according to Rockwell.

Hardness testing device
Hardness testing device

Brinell hardness

The Brinell hardness test was the first method that was able to directly calculate values. The surface of the material under test is compressed at a specific test force (F) using a spherical hardness test body made of tungsten carbide. The diameter (d) of the impression is then usually measured. The hardness is calculated as follows:

HBW=\frac{2F \times 0,102}{\Pi \times D \times ({D}-\sqrt{D^{2}-d^{2}})}
  • D = Diameter of the ball
  • d = Diameter of the impression
  • F = Test force
  • Conversion factor 0.102 = Previously, the old measurement unit kilopond (kp) was specified for hardnesses. This is no longer used today and the conversion factor is used to determine a value in Newton (1 kp corresponds to 9.81 N)

In practice, however, the hardness is specified in the measuring device and does not have to be calculated. This applies to all procedures.

The hardness test method according to Brinell is standardized in DIN EN ISO 6506-1. The test force to be applied can be read in tables. As a general rule, the impression of the test ball should be as large as possible in order to detect as many constituent components as possible.

The Brinell hardness test is suited for soft to medium-hard materials whose hardness is not above 650 HBW. It is also load independent.

Vickers hardness

The Vickers hardness test method is similar to the Brinell method. It was developed from the fact that the Brinell test method is suited for soft and medium-hard materials, but not for very hard materials. Instead of a carbide ball, a diamond pyramid with a square base surface and an opening angle of 136° between the tips is used here. The angle was chosen to ensure comparisons with Brinell hardnesses.

The shape of the penetrating body allows high forces to be applied (aluminum to steel). A minimum sample thickness of the test specimen Smin is a prerequisite for the procedure. The sample must be at least 1.5 x the mean length of the impression diagonal (1.5 x d) so that the probe does not penetrate through to the sample plate. Here too, standards such as DIN EN ISO 6507-1 provide the minimum distances between the test points or impressions and to the edge of the sample so that the results are not falsified by deformation. For example, Vickers hardness is indicated as follows: 210 HV 40/30 (test force/hold duration of the test). It is calculated using the following formula:

HV=\frac{2F \times sin \frac{136}{2}}{d^{2}} \times 0.102

The value d2 is calculated from the length of the first diagonal d1 and the second diagonal d2, also refer to the highlighted area in the figure below. The value for d must be calculated in a first step:

d=\frac{d_1+d_2}{2}

d2 is then calculated as follows:

d^{2}= d \times d
Vickers hardness test
Vickers hardness test

Rockwell hardness

The Rockwell hardness is for example determined similar to the Brinell method with a steel ball or similar to the Vickers method with a diamond cone with 120° angles. The penetration depth and not the diameter of the impression is taken as the parameter. Depending on the method, the Rockwell hardness is specified in HRA, HRB, HRC or HRF, where HR refers to the Rockwell hardness test and the letter represents the method. The test specimen is placed under load with a force as follows:

  • Rockwell A: Fv= 98.07 N, FZ = 490.3 N
    (Diamond cone, reference depth 0.2 mm - for very hard materials and carbides)
  • Rockwell B: Fv= 98.07 N, FZ = 882.6 N
    (Carbide ball, reference depth 0.2 mm - for medium hardness materials e.g. steel and brass)
  • Rockwell C: Fv= 98.07 N, FZ = 1373 N
    (Diamond cone, reference depth 0.2 mm - for hardened steels)
  • Rockwell F: Fv= 98.07 N, FZ = 490.3 N
    (Carbide ball, reference depth 0.26 mm - for thin sheet metal, soft copper or soft brass)

The test procedure can be as follows:

Rockwell testing procedure
Rockwell testing procedure

In the first step (1), the indenter is loaded with the pre-test force (Fv) and penetrates a little into the sample (reference plane - - - line). In the second step (2), the additional add-on test force (FZ) is additionally exerted, and the total test force now acts on the sample. In the last step (3), the add-on test force is removed again and the permanent penetration depth (h) can now be measured.

The formula for the calculation with diamond cone is as follows:

HRC, HRA = 100 - \frac{h}{S}

S corresponds to the scale classification on the dial indicator (usually 100 scale segments correspond to 0.002 mm).

The formula for calculating with a carbide ball is as follows:

HRB, HRF = 130 - \frac{h}{0.002}

Which Rockwell scale is selected and depending on: Material hardness, workpiece thickness, any hardened surface (such as nitration hardening). The diamond cone is primarily used for tempered or hardened steel and the steel ball is primarily used for softer materials. You will find suitable diamond penetrating bodies in the MISUMI shop.

Shore hardness

Shore hardness testing is mainly used for elastic materials such as rubbers, plastics, gels or foams. Here too, a specialized test specimen is pressed into the material with a defined force. The penetration depth represents the scale. There are six different scales:

  • Shore 00: for very soft materials such as silicone, gels.
  • Shore 0: for soft but slightly more durable materials.
  • Shore A: for medium-hard elastic materials (e.g. natural rubber, synthetic elastomers, flexible plastics, soft rubbers)
  • Shore B: for elastic materials with higher stiffness (e.g. hard rubber composites)
  • Shore C: for harder materials (e.g. thermoplastic elastomers, hard plastics, hardened rubbers)
  • Shore D: for tough elastomers and thermoplastics, such as PPOM (polyoxymethylene), PE (polyethylene), and PA (polyamides)

Shore A hardness and Shore D hardness are relevant for most industrial use cases.

The Shore hardness can for example be measured with a durometer. The latter is pressed by hand against the object under test and then displays the corresponding value. Note that durometers display values of only one Shore scale each, that is, there are Shore A durometers, etc. Durometers are also available in our MISUMI shop.

Example of a durometer
Example of a durometer

Hardness conversion

Which hardness test is used as the standard is not mandated. Different industries and laboratories therefore use different measurement methods. In order to compare the different hardness levels, DIN EN ISO 18265, for example, provides a hardness comparison table for unalloyed and low-alloy steel and cast steel:

Hardness conversion table (SAEJ417) -1983 revised - Approximate conversion of Rockwell hardness C values for steel (*1)
(HRC)
Rockwell hardness scale C		 (HV)
Vickers hardness		 Brinell hardness (HB)
10 mm ball, load 3000 kgf		 Rockwell hardness (*3) Rockwell Hard Diamond Cone Penetrator (Hs)
Shore hardness		 Tensile strength (approximate value)
Mpa
(kgf/mm2)(*2)		 Rockwell hardness
Scale C
(*3)
Standard sphere tungsten carbide ball (HRA)
Scale A,
Load 60 kgf,
Diamond cone
Penetrator		 (HRB)
Scale B,
Load 100 kgf,
Dia. 1.6 mm
(1/16 in.) sphere		 (HRD)
Scale D,
Load 100 kgf,
Diamond cone penetrator		 15-N
scale,
Load 15 kgf		 30-N
scale,
Load 30 kgf		 45-N
scale,
Load 45 kgf
68 940 − − 85.6 − 76.9 93.2 84.4 75.4 97 − 68
67 900 − − 85 − 76.1 92.9 83.6 74.2 95 − 67
66 865 − − 84.5 − 75.4 92.5 82.8 73.3 92 − 66
65 832 − (739) 83.9 − 74.5 92.2 81.9 72 91 − 65
64 800 − (722) 83.4 − 73.8 91.8 81.1 71 88 − 64
63 772 − (705) 82.8 − 73 91.4 80.1 69 9 87 − 63
62 746 − (688) 82.3 − 72.2 91.1 79.3 68.8 85 − 62
61 720 − (670) 81.8 − 71.5 90.7 78.4 67.7 83 − 61
60 697 − (654) 81.2 − 70.7 90.2 77.5 66.6 81 − 60
59 674 − (634) 80.7 − 69.9 89.8 76.6 65.5 80 − 59
58 653 − 615 80.1 − 69.2 89.3 75.7 64.3 78 − 58
57 633 − 595 79.6 − 68.5 88.9 74.8 63.2 76 − 57
56 613 − 577 79 − 67.7 88.3 73.9 62 75 − 56
55 595 − 560 78.5 − 66.9 87.9 73 60.9 74 2075 (212) 55
54 577 − 543 78 − 66.1 87.4 72 59.8 72 2015 (205) 54
53 560 − 525 77.4 − 65.4 86.9 71.2 58.5 71 1950 (199) 53
52 544 (500) 512 76.8 − 64.6 86.4 70.2 57.4 69 1880 (192) 52
51 528 (487) 496 76.3 − 63.8 85.9 69.4 56.1 68 1820 (186) 51
50 513 (475) 481 75.9 − 63.1 85.5 68.5 55 67 1760 (179) 50
49 498 (464) 469 75.2 − 62.1 85 67.6 53.8 66 1695 (173) 49
48 484 451 455 74.7 − 61.4 84.5 66.7 52.5 64 1635 (167) 48
47 471 442 443 74.1 − 60.8 83.9 65.8 51.4 63 1580 (161) 47
46 458 432 432 73.6 − 60 83.5 64.8 50.3 62 1530 (156) 46
45 446 421 421 73.1 − 59.2 83 64 49 60 1480 (151) 45
44 434 409 409 72.5 − 58.5 82.5 63.1 47.8 58 1435 (146) 44
43 423 400 400 72 − 57.7 82 62.2 46.7 57 1385 (141) 43
42 412 390 390 71.5 − 56.9 81.5 61.3 45.5 56 1340 (136) 42
41 402 381 381 70.9 − 56.2 80.9 60.4 44.3 55 1295 (132) 41
40 392 371 371 70.4 − 55.4 80.4 59.5 43.1 54 1250 (127) 40
39 382 362 362 69.9 − 54.6 79.9 58.6 41.9 52 1215 (124) 39
38 372 353 353 69.4 − 53.8 79.4 57.7 40.8 51 1180 (120) 38
37 363 344 344 68.9 − 53.1 78.8 56.8 39.6 50 1160 (118) 37
36 354 336 336 68.4 -109 52.3 78.3 55.9 38.4 49 1115 (114) 36
35 345 327 327 67.9 -108.5 51.5 77.7 55 37.2 48 1080 (110) 35
34 336 319 319 67.4 -108 50.8 77.2 54.2 36.1 47 1055 (108) 34
33 327 311 311 66.8 -107.5 50 76.6 53.3 34.9 46 1025 (105) 33
32 318 301 301 66.3 -107 49.2 76.1 52.1 33.7 44 1000 (102) 32
31 310 294 294 65.8 -106 48.4 75.6 51.3 32.7 43 980 (100) 31
30 302 286 286 65.3 -105.5 47.7 75 50.4 31.3 42 950 (97) 30
29 294 279 279 64.7 -104.5 47 74.5 49.5 30.1 41 930 (95) 29
28 286 271 271 64.3 -104 46.1 73.9 48.6 28.9 41 910 (93) 28
27 279 264 264 63.8 -103 45.2 73.3 47.7 27.8 40 880 (90) 27
26 272 258 258 63.3 -102.5 44.6 72.8 46.8 26.7 38 860 (88) 26
25 266 253 253 62.8 -101.5 43.8 72.2 45.9 25.5 38 840 (86) 25
24 260 247 247 62.4 -101 43.1 71.6 45 24.3 37 825 (84) 24
23 254 243 243 62 100 42.1 71 44 23.1 36 805 (82) 23
22 248 237 237 61.5 99 41.6 70.5 43.2 22 35 785 (80) 22
21 243 231 231 61 98.5 40.9 69.9 42.3 20.7 35 770 (79) 21
20 238 226 226 60.5 97.8 40.1 69.4 41.5 19.6 34 760 (77) 20
(18) 230 219 219 − 96.7 − − − − 33 730 (75) (18)
(16) 222 212 212 − 95.5 − − − − 32 705 (72) (16)
(14) 213 203 203 − 93.9 − − − − 31 675 (69) (14)
(12) 204 194 194 − 92.3 − − − − 29 650 (66) (12)
(10) 196 187 187 − 90.7 − − − − 28 620 (63) (10)
(8) 188 179 179 − 89.5 − − − − 27 600 (61) (8)
(6) 180 171 171 − 87.1 − − − − 26 580 (59) (6)
(4) 173 165 165 − 85.5 − − − − 25 550 (56) (4)
(2) 166 158 158 − 83.5 − − − − 24 530 (54) (2)
(0) 160 152 152 − 81.7 − − − − 24 515 (53) (0)
Note
(*1) Highlighted numbers: Based on ASTM E 140, Table 1 (jointly coordinated by SAE, ASM, and ASTM.)
(*2) The units and numbers shown in parentheses are results of the conversion from psi numbers using the conversion tables from JIS Z 8413 und Z 8438. 1 MPa = 1 N/mm2
(*3) The numbers shown in parentheses are in ranges that are not commonly used. They are for information only.

Hardness measurement procedure

The hardness measurement procedure could be as follows: Before testing, the sample must be prepared. Surface contaminants must be removed by grinding and the sample must be cleaned. The test specimen is then placed on the sample plate and moved until the desired location is reached. It is important not to test too close to the edge since deformation can otherwise lead to falsified results. The inspection optics can be used to detect when the position is correctly adjusted (the image becomes sharp). The test specimen is now clamped parallel to the plane and the test can start. The determined test force is applied slowly but steadily by actuating the lever. Ideally, the final value is reached between 2 and 8 seconds and is then held for a maximum of 15 seconds. The lever is now gently pushed back to remove the pressure.

Classic testing device
Classic testing device

However, such measurements are very impractical in everyday life since the hardness is sometimes also checked on location directly on the material. For this purpose, there are also mobile measuring devices for use directly on location:

Hardness measurement directly on the component
Hardness measurement directly on the component

Applicability of various hardness tests

The following table compares the procedures:

Test procedure for hardness testing
Test procedure (standard) Applicable materials Determination variables Properties Comments
Brinell hardness
(DIN EN ISO 6506-1)		 Soft to medium hard materials

e.g., Nonferrous metals, inhomogeneous materials, soft metals, soft annealed steels		 Test force F in N
Sphere diameter in mm
Impression diameter in mm		 - suitable for inhomogeneous and porous materials such as grey cast iron or forged products, since the indentation is large.
- not suitable for small or thin samples
- not suitable for hard and very hard materials		 JIS Z 2243
Rockwell hardness
(DIN EN ISO 6508-1)		 with testing sphere: Plastics, carbon, and soft to medium hard metals

with diamond cone (HRC):hard to very hard materials		 Test force F in N
Penetration depth of the respective test specimen in mm according to the method
(HRA, HRB, HRC, HRF)		 - the hardness value can be determined quickly.
- suitable for an interim inspection of products already finished
- various types of Rockwell hardness must be considered		 JIS Z 2245
Shore hardness
(DIN ISO 7619-1)		 Elastomers or thermoplastic elastomers

e.g., foams, rubber, soft, medium to hard plastics

Test force F in N
Penetration depth of the respective test specimen according to scale
(Shore 00, Shore 0, Shore A, Shore B, Shore C, Shore D)		 - easy to carry out
- data can be determined quickly
- material to be tested must have a level, smooth surface
- material to be tested must be stored at standardized temperature
- environmental temperature and humidity must be kept constant and wait times must be considered
- the indent is small and suitable for testing of products already finished
- compact and lightweight, portable
- scales use different pressure pins and forces		 JIS Z 2246
Vickers hardness
(DIN EN ISO 6507-1)		 Soft, medium hard to very hard materials (metals and ceramics)

e.g., materials with a layer hardened by induction hardening, carbonization, nitrating, galvanic or ceramic coating, etc.		 Test force F in N
Arithmetic mean of the 2 impression diagonals in mm
- the indenter is made of diamond and can thus test materials of any hardness
- not suitable for porous material
homogeneous microstructure required
 JIS Z 2244

No matter which procedure you choose: The MISUMI shop has a range of hardness gauges.

Hardness of steel

The degree of hardness of steel indicates how resistant the material is to plastic deformation or penetration. It is a measure of the hardness (against a body’s penetration) or strength (against failure or irreversible deformation) of the steel. Different hardness levels of steel can be achieved through purposeful heat treatments. A new structure with the desired properties is creates by relocating, incorporating, or removing material particles:

  • Relocating: annealing, tempering, hardening, tempering, curing
  • Incorporating: Carburizing, nitriding
  • Removing: Decarburizing (Tempering)

The impact of different steel hardnesses

The type of alloy in its finest composition directly influences the capabilities of hardening and hardening processes. Users must always weigh between hardness and ductility. The respective capacity to harden different steel grades has advantages and disadvantages. In order to find the right steel for the intended purpose, these capacities should therefore be weighed carefully. The hardness of steel can have the following influences:

Impact of increasing hardness on ductility and machinability

The hardness of steel affects ductility and machinability. Ductility describes how well a material can withstand stresses (e.g. sudden shock or impact) without failure.

Softer steel is more ductile than harder steel. It is therefore easier to deform and machine. Harder steel, on the other hand, is brittle and breaks faster under high loads. At the same time, however, it is more resistant to abrasion and penetration.

The following figure provides an overview of hardness, ductility and the interaction of both in different steel grades:

Structural steel (steel for steel structures) Hardness - + Toughness Carbon steel (unalloyed cold work steel), machine construction steel, tempered steel Steel (high-alloy), tool steel for cold work, tool steel for hot work HSS (high-speed steel) + -

Interaction of hardness and ductility

While the hardness on the left side decreases towards structural steel, ductility on the right side increases at the same time.

The hardness of steel also influences tool selection. Harder steel leads to faster tool wear. Typical signs of wear are dulling or damage to the blade. In addition, when machining hardened steel, the cutting conditions may need to be adjusted, e.g. reduced cutting speeds. In addition to adjusting cutting speed and cutting conditions, it is necessary to use special-purpose milling and cutting tools depending on the hardness of the steel. For this purpose, the MISUMI shop offers a wide range of tools for machining processes.