Page 35 - CITS - Fitter Trade Practical - Volume -2

FITTER - CITS

hardness tester has been zeroed. After the penetrator has stopped moving into the specimen, the final position of
the dial pointer indicates the Rockwell hardness number that is related to the depth of penetration.

Test Location
If indentation is placed too close to the edge of specimen, the workpiece edge will bulge, and the hardness
number will decrease accordingly. To ensure an accurate test, the distance from the center of the indentation to
the edge of the specimen must be at least two and one-half diameters.
An indentation hardness test cold works the surrounding material. If another indentation is placed within this
cold worked area, the reading usually will be higher than the real value. Generally, the softer the material, the
more critical the spacing of indentations becomes. However, a distance three diameters from the center of one
indentation to another is sufficient for most materials.
Thickness of Specimen

The depth of material affected has been found by extensive experimentation to be on the order of 10 times the
depth of the indentation. Therefore, unless the thickness of the material being tested is at least 10 times the depth
of the indentation, an accurate Rockwell test cannot be expected. This ‘minimum thickness’ ratio of 10:1 should
be regarded only as an approximation.

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CITS : C G & M - Fitter - Exercise 61