Ex. No.2 Rockwell Hardness Test.
Aim : To determine the Rockwell hardness for the given metal specimen.
1. Rockwell Hardness Testing Machine.
2. Metal Specimen.
Hardness may be defined as resistance of metal to plastic deformation usually by indentation. However the term may also refer to stiffness or temper or resistance to scratch, abrasion or cutting. There are three general types of hardness measurements depending upon the manner in which the test is conducted.
1. Scratch hardness measurement.
2. Rebound hardness measurement.
3. Indentation Hardness measurement.
In scratch hardness method the materials are rated on their ability to scratch one another and mineralogists use it.In rebound hardness measurement, a standard body is usually dropped on to the material surface and the hardness is measured in terms of the height of its rebound.The general means of judging the hardness is the resistance of a material to indentation.
Indentation hardness may be measured by various hardness test such as Brinell, Rockwell, etc. Rockwell hardness testing differs from Brinell testing. In Rockwell testing, the indenters and loads are smaller and therefore the resulting indentation on the specimen is smaller and shallower.
Rockwell testing is suitable for materials having hardness beyond the scope of Brinell testing. Rockwell testing is faster as compared to Brinell testing , because the diameter of the indentation need not be measured. The Rockwell machine gives arbitrary direct reading, Unlike Brinell testing, Rockwell testing needs no surface preparation (Polishing) of the specimen whose hardness is to be measured.
There are two scales on Rockwell testing specimen. i.e B scale and C scale. B scale uses a steel ball indenter where as a diamond cone penetrate is employed for measuring Hardness on C scale.B scale is for testing materials of medium hardness such as low and medium carbon steels in the annealed condition. The working range of this scale is from 0 to 100. C scale is used for testing materials harder than B-100. C scale is commonly used for testing the hardness of alloy cast irons.
In Rockwell hardness testing, the minor load for all cases is 10 Kg. Where as major loads for scales C and B are 150 Kg and respectively, including minor load.
- The test should be carried out in an ambient temperature of 20 +20 C in temperate climate and 27 +20 C in tropical climates.
- The testing machine shall be protected through out the test from shock and vibrations.
- The test piece shall be placed on a rigid support. The contact surfaces shall be clean and free from foreign matter. ( such as oil and dust)
- The thickness of the test piece shall be at least 8 times the permanent indentation of depth. No deformation shall be visible at the back of the test piece after the test.
- The distance between the centres of the two adjacent indentations shall be at least 4 times the diameters of the indentation and the distance from the centre of any indentation to the edge of the test piece shall be at least 2.5 times the diameter of the indentation unless agreed otherwise.
- Successive impressions should not be superimposed on another nor be made too close together when making hardness determinations.
- Nor should a measurement be made too close to the edge , or on a specimen so thin that the impression comes through the other side.
- Small irregularities, dirt, and scale should be avoided because of the great sensibility of the Rockwell test.
- Test piece is placed upon the machine. The dial may be showing any reading.
- Hand wheel is turned , thereby raising the test piece up against the steel ball indenter till the needle of the small dial is against the red mark. This applies minor load.
- Major load is applied by pressing the crank provided on the right hand side of the machine. Time is given as 30 sec so as to make the load reach specimen fully.
- When the penetration is completed, the crank is turned in the reverse direction thereby with drawing the minor load but the leaving the major load applied.
- The pointer moves further and becomes stand still. This reading is taken as Rockwell Hardness Number – C scale.( HRC)
- Hand wheel is rotated and the test piece is lowered.
Load in KgF
Rockwell Hardness number of the specimen was found for the given material as follows:
- Copper = ----------- HRC
- Brass = ----------- HRC
- Aluminium = ------------ HRC.