Rockwell hardness scales

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 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:

Two equations. 
Regular Rockwell hardness equals 130 minus the fraction h over two thousandths of a millimeter. Superficial Rockwell hardness
 equals one hundred minus the fraction h over one thousandth of a millimeter

If the test uses a ball:

Two equations. 
Regular Rockwell hardness equals 100 minus the fraction h over two thousandths of a millimeter. Superficial Rockwell hardness
 equals one hundred minus the fraction h over one thousandth of a millimeter

 

Regular Rockwell
Scales
Scale Symbol Indenter Type.
If a ball,
diameter in millimeters
(diameter in inches)
Preliminary
force in newtons (kg-force)
Total force
newtons (kgf)
Typical Applications
A Spheroconical diamond 98.07
(10)
588.4
(60)
Cemented carbides, thin steel, and shallow case hardened steel.
B Ball
1.588
(1/16)
98.07
(10)
980.7
(100)
Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
C Spheroconical diamond 98.07
(10)
1471
(150)
Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel, and other materials harder than 100 on the Rockwell B scale.
D Spheroconical diamond 98.07
(10)
980.7
(100)
Thin steel and medium case hardened steel, and pearlitic malleable iron.
E Ball
3.175
(1/8)
98.07
(10)
980.7
(100)
Cast iron, aluminum and magnesium alloys, and bearing metals.
F Ball
1.588
(1/16)
98.07
(10)
588.4
(60)
Annealed copper alloys, and thin soft sheet metals.
G Ball
1.588
(1/16)
98.07
(10)
1471
(150)
Malleable irons, copper-nickel-zinc and cupronickel alloys.
H Ball
3.175
(1/8)
98.07
(10)
588.4
(60)
Aluminum, zinc, and lead.
K Ball
3.175
(1/8)
98.07
(10)
1471
(150)
Bearing metals and other very soft or thin materials. Use smallest ball and heaviest load that does not give anvil effect.
L Ball
6.350
(1/4)
98.07
(10)
588.4
(60)
M Ball
6.350
(1/4)
98.07
(10)
980.7
(100)
P Ball
 6.350 
(1/4)
98.07
(10)
1471
(150)
R Ball
 12.70 
(1/2)
98.07
(10)
588.4
(60)
S Ball
12.70
(1/2)
98.07
(10)
980.7
(100)
V Ball
12.70
(1/2)
98.07
(10)
1471
(150)
Superficial Rockwell
Scales
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
(1/8)
29.42
(3)
147.1
(15)
Very soft material.
30W Ball
3.175
(1/8)
29.42
(3)
294.2
(30)
45W Ball
 3.175
(1/8)
29.42
(3)
441.3
(45)
15X Ball
6.350
(1/4)
29.42
(3)
147.1
(15)
30X Ball
6.350
(1/4)
29.42
(3)
294.2
(30)
45X Ball
6.350
(1/4)
29.42
(3)
441.3
(45)
15Y Ball
12.70
(1/2)
29.42
(3)
147.1
(15)
30Y Ball
12.70
(1/2)
29.42
(3)
294.2 
(30)
45Y Ball
12.70
(1/2)
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.

Standards

ASTM E 18 - 2000, 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

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