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A scale indicating the hardness of materials, first used in 1919, when it was invented by Stanley P. Rockwell. The measurement has no units. The symbol is HR followed by a letter indicating one of a number of possible scales, described in the table below. For example, “HRC 96” means 96 on the Rockwell C scale. Rockwell hardness numbers are usually used to describe the hardness of metals, although they are also used for some plastics.
Like the Brinell hardness scale, the Rockwell scale is based on measuring the depth of the indentation made by pressing a diamond point, or a ball, into the material. Unlike the Brinell test, however, the Rockwell test makes two depth measurements. The ball (say) is brought in contact with the material to be tested. The preliminary force is applied for a set length of time, and then the depth of the indentation is measured. Then the force is increased in a set rate until it reaches the total force. This force is kept constant for a predetermined period, after which the force is reduced to the preliminary force level. After a set period of time, the depth of the indentation is measured for a second time. Usually the entire process is carried out by automatic machinery.
The Rockwell hardness is calculated from the difference between the depth of the indentation after the test material has “bounced back” from the application application of the total force, and its initial depth under the preliminary force. Call this difference, in millimeters, h. Then,
If the test uses a spheroconical diamond:
If the test uses a ball:
| Regular Rockwell Scales | ||||
|---|---|---|---|---|
| Scale symbol | Indenter type (if a ball, diameter) |
Preliminary force | Total force | Typical use |
| HRA | Spheroconical diamond | 98.07 N (10 kgf) | 588.4 N (60 kgf) | Cemented carbides, thin steel, and shallow case hardened steel. |
| HRB | Ball, 1.588 mm (¹⁄₁₆″) | 98.07 N (10 kgf) | 980.7 N (100 kgf) | Copper alloys, soft steels, aluminum alloys, malleable iron, etc. |
| HRC | Spheroconical diamond | 98.07 N (10 kgf) | 1471 N (150 kgf) | Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, and other materials harder than 100 on the Rockwell B scale. |
| HRD | Spheroconical diamond | 98.07 N (10 kgf) | 980.7 N (100 kgf) | Thin steel and medium case hardened steel, and pearlitic malleable iron. |
| HRE | Ball, 3.175 mm (⅛″) | 98.07 N (10 kgf) | 980.7 N (100 kgf) | Cast iron, aluminum and magnesium alloys, and bearing metals. |
| HRF | Ball, 1.588 mm (¹⁄₁₆″) | 98.07 N (10 kgf) | 588.4 N (60 kgf) | Annealed copper alloys, and thin soft sheet metals. |
| HRG | Ball, 1.588 mm (¹⁄₁₆″) | 98.07 N (10 kgf) | 1471 N (150 kgf) | Malleable irons, copper-nickel-zinc and cupronickel alloys. |
| HRH | Ball, 3.175 mm (⅛″) | 98.07 N (10 kgf) | 588.4 N (60 kgf) | Aluminum, zinc, and lead. |
| HRK | Ball, 3.175 mm (⅛″) | 98.07 N (10 kgf) | 1471 N (150 kgf) | Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that does not give anvil effect. |
| HRL | Ball, 6.350 mm (¼′) | 98.07 N (10 kgf) | 588.4 N (60 kgf) | |
| HRM | Ball, 6.350 mm (¼″) | 98.07 N (10 kgf) | 980.7 N (100 kgf) | |
| HRP | Ball, 6.350 mm (¼″) | 98.07 N (10 kgf) | 1471 N (150 kgf) | |
| HRR | Ball, 12.70 mm (½″) | 98.07 N (10 kgf) | 588.4 N (60 kgf) | |
| HRS | Ball, 12.70 mm (½″) | 98.07 N (10 kgf) | 980.7 N (100 kgf) | |
| HRV | Ball, 12.70 mm (½″) | 98.07 N (10 kgf) | 1471 N (150 kgf) | |
| Superficial Rockwell Scales | ||||
|---|---|---|---|---|
| Scale Symbol |
Indenter Type If a ball, diameter in millimeters (inches) |
Preliminary force in newtons (kg-force) |
Total force newtons (kgf) |
Typical Applications |
| 15N | Spheroconical diamond | 29.42 (3) |
147.1 (15) |
Similar to A, C and D scales, but for thinner gage material or case depth. |
| 30N | Spheroconical diamond | 29.42 (3) |
294.2 (30) |
|
| 45N | Spheroconical diamond | 29.42 (3) |
441.3 (45) |
|
| 15T | Ball 1.588 (1/16) |
29.42 (3) |
147.1 (15) |
Similar to B, F and G scales, but for thinner gage material. |
| 30T | Ball 1.588 (1/16) |
29.42 (3) |
294.2 (30) |
|
| 45T | Ball 1.588 (1/16) |
29.42 (3) |
441.3 (45) |
|
| 15W | Ball 3.175 (⅛) |
29.42 (3) |
147.1 (15) |
Very soft material. |
| 30W | Ball 3.175 (⅛) |
29.42 (3) |
294.2 (30) |
|
| 45W | Ball 3.175 (1/8) |
29.42 (3) |
441.3 (45) |
|
| 15X | Ball 6.350 (¼) |
29.42 (3) |
147.1 (15) |
|
| 30X | Ball 6.350 (¼) |
29.42 (3) |
294.2 (30) |
|
| 45X | Ball 6.350 (¼) |
29.42 (3) |
441.3 (45) |
|
| 15Y | Ball 12.70 (½) |
29.42 (3) |
147.1 (15) |
|
| 30Y | Ball 12.70 (½) |
29.42 (3) |
294.2 (30) |
|
| 45Y | Ball 12.70 (½) |
29.42 (3) |
441.3 (45) |
|
The table is adapted from Table 1 of
Samuel R. Low.
Rockwell Hardness Measurement of Metallic Materials.
NIST Recommended Practice Guide.
Special Publication 960-5.
Washington: U.S.G.P.O. 2001.
ASTM E 18-05, Standard Test Methods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials.
ISO 6508-1 Metallic Materials - Rockwell hardness test (scales A, B, C, D, E, F, G, H, K, N, T) - Part 1: Test method, 1999-09-01
ISO 6508-2 Metallic Materials - Rockwell hardness test (scales A, B, C, D, E, F, G, H, K, N, T) - Part 2: Verification of testing machines, 1999-09-01
ISO 6508-3 Metallic Materials - Rockwell hardness test (scales A, B, C, D, E, F, G, H, K, N, T) - Part 3: Calibration of reference blocks, 1999-09-01
http://classes.mst.edu/civeng120/lessons/hardness/equipment/rockwell/index.html
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Last revised: 14 November 2021.