Rock Well hardness Testing
Rockwell hardness test is based on the indentation of a hard tip brale or indenter into test piece under the action of two consecutively applied loads, Minor [initial] and major [final]. In order to eliminate zero error and possible surface effect due to roughness or scale, the initial or minor load is first applied and produces and initial indentation.
A conical shaped diamond called Brale with 120o apex angle and 0.2 mm radius is used as the indentor or penetrator in the Rockwell test for hard materials. For soft material a hardened steel ball 1.2mm in diameter is generally used.
Rock well testing is faster as compared to Brinell testing because diameter of indentation need not be measured. The Rockwell machines give arbitrary direct reading
Procedure for measuring Rock well Hardness:-
- Test piece is placed upon the machine the machine dial is showing any reading.(Fig a)
- Hand wheel is turned thereby rising the test piece up against the steel ball indenter till the needle onthe dial reads zero. This applies minor load.(Fig b)
- Major load is applied by pressing the handle of the ram provided on the right hand side of the machine for a definite period. . (Fig c)
- Handle is turned to the other direction, thereby withdrawing major load but leaving minor load applied.(Fig d)
- Hand wheel is rotated and test piece is lowered. At this stage the hardness of the test piece material
- can be directly read from the dial scale. (Fig e)
There are two scales on a rock well testing machines i.e. B-scale and C-scale.
B scale uses a steel ball indenter where a diamond cone penetrate is employed for measuring hardness on C-scale. B scale is for testing materials of medium hardness such as medium carbon steel in the annealed condition.
C scale is commonly used for the testing the hardness of alloys of cast iron. The minor load for all cases is 10 kg where as the major load for scale C & B are 150 kg and 100 kg respectively (including minor load).
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