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Tech Library
Metallurgy
Protanium® Steel: Torque, hardness, and strength
The torque capacity of a hex key is a function of hardness, tensile
strength, and stock size. Hardness determines tensile strength, and
the following formula can be used to determine torque:
Torque = (tensile strength) * .2 * (stock size across flats)3
Example:
A hex key has a hardness of 57 RC, a tensile
strength of 320000 psi, and a stock size of 0.25 inches.
Torque = (320000) * .2 * (0.25)3
Torque = (320000) * .2 * (0.15625)
Torque = (320000) * 0.003125
Torque = 1000 lb.in
The chart below shows the relationship between hardness and tensile
strength.
Hardness
Rockwell
C ~Tensile
StrengthSize across flats in inches
0.078
0.125
0.156
0.25
0.312
5/64 - 2mm
1/8 inch
5/32 - 4mm
1/4 inch
5/16 - 8mm
61
360000
34
141
273
1125
2187
59
340000
32
133
258
1063
2065
57
320000
30
125
243
1000
1944
55
300000
28
117
228
938
1822
53
280000
27
109
213
875
1701
51
260000
25
102
197
813
1579
49
240000
23
94
182
750
1458
46
220000
21
86
167
688
1336
43
200000
19
78
152
625
1215
40
180000
17
70
137
563
1093
37
160000
15
63
121
500
972
All torque values in inch pounds
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