Hardness comparison table

Brinell HB
(10 mm Ball, 3000 kg load) Vickers HV
(120 kg) Rockwell C HRC
(120 degree cone 150 kg) Rockwell B HRB
(1/16" ball 100 kg) Leeb HLD[1]
800 - 72 - 857
780 1220 71 - 850
760 1170 70 - 843
745 1114 68 - 837
725 1060 67 - 829
712 1021 66 - 824
682 940 65 - 812
668 905 64 - 806
652 867 63 - 799
626 803 62 - 787
614 775 61 - 782
601 746 60 - 776
590 727 59 - 770
576 694 57 - 763
552 649 56 - 751
545 639 55 - 748
529 606 54 - 739
514 587 53 120 731
502 565 52 119 724
495 551 51 119 719
477 534 49 118 709
461 502 48 117 699
451 489 47 117 693
444 474 46 116 688
427 460 45 115 677
415 435 44 115 669
401 423 43 114 660
388 401 42 114 650
375 390 41 113 640
370 385 40 112 635
362 380 39 111 630
351 361 38 111 621
346 352 37 110 617
341 344 37 110 613
331 335 36 109 605
323 320 35 109 599
311 312 34 108 588
301 305 33 107 579
293 291 32 106 572
285 285 31 105 565
276 278 30 105 557
269 272 29 104 550
261 261 28 103 542
258 258 27 102 539
249 250 25 101 530
245 246 24 100 526
240 240 23 99 521
237 235 23 99 518
229 226 22 98 510
224 221 21 97 505
217 217 20 96 497
211 213 19 95 491
206 209 18 94 485
203 201 17 94 482
200 199 16 93 478
196 197 15 92 474
191 190 14 92 468
187 186 13 91 463
185 184 12 91 461
183 183 11 90 459
180 177 10 89 455
175 174 9 88 449
170 191 7 87 443
167 168 6 87 439
165 165 5 86 437
163 162 4 85 434
160 159 3 84 430
156 154 2 83 425
154 152 1 82 423
152 150 - 82 420
150 149 - 81 417
147 147 - 80 413
145 146 - 79 411
143 144 - 79 408
141 142 - 78 405
140 141 - 77 404
135 135 - 75 397
130 130 - 72 390
114 120 - 67 365
105 110 - 62 350
95 100 - 56 331
90 95 - 52 321
81 85 - 41 300
76 80 - 37 287

Knoop hardness test

The Knoop hardness test (pronounced kuh-nūp) is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. The Knoop hardness HK or KHN is then given by the formula:

where:

L = length of indentation along its long axis

C_p = correction factor related to the shape of the indenter, ideally 0.070279

P = load

HK values are typically in the range from 100 to 1000, when specified in the conventional units of kg_f·mm^-2. The SI unit, pascals, are sometimes used instead: 1 kg_f·mm^-2 = 9.80665 MPa.

The test was developed by Frederick Knoop and colleagues at the National Bureau of Standards (now NIST) of the USA in 1939, and is defined by the ASTM D1474 standard.

The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

Sample values

Material HK

Gold Foil 69

Quartz 820

Silicon Carbide 2480

Diamond 8000

Brinell hardness Tester

Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The oldest of the hardness test methods in common use today, the Brinell test is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force independent as long as the ball size/test force relationship is the same.

In the USA, Brinell testing is typically done on iron and steel castings using a 3000Kg test force and a 10mm diameter carbide ball. Aluminum and other softer alloys are frequently tested using a 500Kg test force and a 10 or 5mm carbide ball. Therefore the typical range of Brinell testing in this country is 500 to 3000kg with 5 or 10mm carbide balls. In Europe Brinell testing is done using a much wider range of forces and ball sizes. It's common in Europe to perform Brinell tests on small parts using a 1mm carbide ball and a test force as low as 1kg. These low load tests are commonly referred to as baby Brinell tests.

Standards

Brinell Test methods are defined in the following standards:

• ASTM E10
• ISO 6506

Brinell Test Method

All Brinell tests use a carbide ball indenter. The test procedure is as follows:

• The indenter is pressed into the sample by an accurately controlled test force.
• The force is maintained for a specific dwell time, normally 10 - 15 seconds.
• After the dwell time is complete, the indenter is removed leaving a round indent in the sample.
• The size of the indent is determined optically by measuring two diagonals of the round indent using either a portable microscope or one that is integrated with the load application device.
• The Brinell hardness number is a function of the test force divided by the curved surface area of the indent. The indentation is considered to be spherical with a radius equal to half the diameter of the ball. The average of the two diagonals is used in the following formula to calculate the Brinell hardness.

The Brinell number, which normally ranges from HB 50 to HB 750 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple. A typical Brinell hardness is specified as follows:

356HBW

Where 356 is the calculated hardness and the W indicates that a carbide ball was used. Note- Previous standards allowed a steel ball and had an S designation. Steel balls are no longer allowed.

Applications

Because of the wide test force range the Brinell test can be used on almost any metallic material. The part size is only limited by the testing instrument's capacity.

Strengths

1. One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same.
2. A wide range of test forces and ball sizes to suit every application.
3. Nondestructive, sample can normally be reused.

Weaknesses

1. The main drawback of the Brinell test is the need to optically measure the indent size. This requires that the test point be finished well enough to make an accurate measurement.
2. Slow. Testing can take 30 seconds not counting the sample preparation time.

Rockwell Hardness Test

Rockwell hardness testing system is the workhorse of hardness testing - designed to meet the toughest challenges: clamp large parts, operate in poor environments, and test high volumes - all with less service problems and less operator skill requirements.

Vickers hardness test

The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with a pressure, which also has units of pascals. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not a pressure.

The hardness number is not really a true property of the material and is an empirical value that should be seen in conjunction with the experimental methods and hardness scale used. When doing the hardness tests the distance between indentations must be more than 2.5 indentation diameters apart to avoid interaction between the work-hardened regions.

The yield strength of the material can be approximated as:

.

where c is a constant determined by geometrical factors usually ranging between 2 and 4.

Types of Hardness Test

Four types of hardness :

1. Brinell
Indenter : 10 mm sphere of steel or tungsten carbide

2. Vickers
Indenter : Diamond pyramid

3. Knoop microhardness
Indenter : Diamond pyramid

4. Rockwell
A, C, D Indenter : Diamond cone
B, F, G Indenter : 1/16 in diameter steel sphere
E Indenter : 1/8 in diameter steel sphere

Definition of Hardness Tester

The hardness test measures the resistance of a material to an indenter or cutting tool. The indenter is usually a ball, pyramid, or cone made of a material much harder than that being tested. A load is applied by slowly pressing the indenter at right angles to the surface being tested for a given period of time. An empirical hardness number may be calculated from knowledge of the load and the cross-sectional area or depth of the impression.
Tests are never taken near the edge of a sample nor any closer to an existing impression than three times the diameter of that impression. The thickness of the specimen should be at least ten and one-half times the depth of the impression.