Calculation of the Rockwell Hardness Number

During a Rockwell test, the force on the indenter is increased from a preliminary test force to a total test force, and then returned to the preliminary test force. The difference in the two indentation depth measurements, while under the preliminary test force, is measured as h (see Fig. 1).

The unit measurement for his mm. From the value of h, the Rockwell hardness number is derived. The Rockwell hardness number is calculated as:

For scales using a diamond conical indenter (see Tables 1 and 2):
Rockwell Hardness = 100 - h/0.002
Rockwell Superficial Hardness = 100 - h/0.001
where h is in mm.

For scales using a ball indenter (see Tables 1 and 2):
Rockwell Hardness = 130 – h/0.002
Rockwell Superficial Hardness = 100 – h/0.001
where h is in mm.

The Rockwell hardness number is an arbitrary number, which, by method of calculation, results in a higher number for harder material. Rockwell hardness values shall not be designated by a number alone because it is necessary to indicate which indenter and forces have been employed in making the test (see Tables 1 and 2). Rockwell hardness numbers shall be quoted with a scale symbol representing the indenter and forces used. The hardness number is followed by the symbol HR and the scale designation. When a ball indenter is used, the scale designation is followed by the letter “W” to indicate the use of a tungsten carbide ball or the letter “S” to indicate the use of a steel ball.

Examples:
52 HRC = Rockwell hardness number of 52 on Rockwell C scale
79 HR30N = Rockwell superficial hardness number of 79 on the Rockwell 30N scale
89 HRBW = Rockwell hardness number of 89 on the Rockwell B scale using a tungsten carbide ball indenter

TABLE 1 Rockwell Hardness Scales

Scale Symbol	Indenter	Total Test Force,kgf	Dial Figures	Typical Applications of Scales
B	1/16-in.(1.588-mm)ball	100	red	Copper alloys,soft steels,aluminum alloys,malleable iron,etc.
C	diamond	150	black	Steel,hard cast irons,pearlitic malleable iron,titanium,deep case hardened steel,and other materials harder than B100.
A	diamond	60	black	Cemented carbides,thin steel,and shallow case-hardened steel.
D	diamond	100	black	Thin steel and medium case hardened steel,and pearlitic malleable iron.
E	1/8-in.(3.175-mm)ball	100	red	Cast iron,aluminum and magnesium alloys,bearing metals.
F	1/16-in.(1.588-mm)ball	60	red	Annealed copper alloys,thin soft sheet metals.
G	1/16-in.(1.588-mm)ball	150	red	Malleable irons,copper-nicker-zinc and cupro-nickel alloys.Upper limitG92 to avoid possible flattening of ball.
H	1/8-in.(3.175-mm)ball	60	red	Aluminum,zinc,lead.
K	1/8-in.(3.175-mm)ball	150	red	bearing metals and other very soft or thin materials.Use smallest ball and haeviset load that does not give anvil effect.
L	1/4-in.(6.350-mm)ball	60	red
M	1/4-in.(6.350-mm)ball	100	red
P	1/4-in.(6.350-mm)ball	150	red
R	1/2-in.(12.70-mm)ball	60	red
S	1/2-in.(12.70-mm)ball	100	red
V	1/2-in.(12.70-mm)ball	150	red

TABLE 2 Rockwell Superficial Hardness Scales

Total Test Force,kgf(N)	Scales Symbols
	N Scale,Diamond Indenter	T Scale,1/16-in.(1.588-mm)Ball	W Scale,1/6-in.(3.175-mm)Ball	X Scale,1/4-in.(6.350-mm)Ball	T Scale,1/2-in.(12.70-mm)Ball
15(147)	15N	15T	15W	15X	15Y
30(294)	30N	30T	30W	30X	30Y
45(441)	45N	45T	45W	45X	45Y

A reported Rockwell hardness number or the average value of Rockwell hardness measurements shall be rounded in accordance with Practice E 29 with a resolution no greater than the resolution of the hardness value display of the testing machine. Typically, the resolution of a Rockwell hardness number should not be greater than 0.1 Rockwell units.