ROCKWELL
HARDNESS TEST.
The Rockwell hardness tester is essentially a machine that measures hardness by determining the depth of penetration of a penetrator into the specimen under certain fixed conditions of test. The penetrator may be either a steel ball or a diamond sphero-conical penetrator. The hardness number is related to the depth of indentation and the higher number, the harder the material. A minor load of 10 kg is first applied which causes an initial penetration; the dial is set at zero on the black-figured scale. And the major load is applied.
This major load is customarily 60 kg. Or 100 kg. When a steel ball is used as a penetrator but other loads may be used when found necessary. The ball penetrator is 1/16 inch in diameter normally; but other penetators of larger diameter, such as 1/8 inch, may be employed for soft metals. When a diamond sphero-conical penetrator is employed the load usually is 150 kg. Experience decides the best combination of load and penetrator for use. After the major load is applied and removed, according to standard procedure, the reading is taken while the minor load is still applied.
THE ROCKWELL HARDNESS SCALES
The various Rockwell scales and their applications are shown in the table below.
|
Scale |
Testing Application |
|
A |
For tungsten carbide and other extremely hard materials. Also for thin, hard sheets. |
|
B |
For materials of medium hardness such as low and medium carbon steels in the annealed condition |
|
C |
For materials harder than Rockwell B-100 |
|
D |
Where somewhat lighter load is desired than on C scale, as on case hardened pieces. |
|
E |
For very soft materials such as bearing metals. |
|
F |
Same as E scale but using 1/16 inch ball. |
|
G |
For metals harder than tested on B scale. |
|
H & K |
For softer metals. |
|
15-N; 30-N; 45-N |
Where shallow impression or small area is desired. For hardened steel and hard alloys. |
|
15-T; 30-T; 45-T |
Where shallow impression or small area is desired for materials softer than hardened steel. |
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