Resistance to indentation or penetration of a material is called hardness of a material. The hardness testing is simple and non-destructive in nature. There are three types of hardness test which are commonly used
Brinell hardness test – The sample to be tested is placed on a table. The position is kept such that top position of specimen just touches the ball. The ball is made up of hardened steel ball of 10mm diameter. The ball is pressed into the surface of specimen by applying the load gradually and it is maintained for 10 to 15 seconds. The ball will make an impression on the specimen. The diameter of the specimen is measured and Brinell hardness number (BHN) is calculated as
BHN = Load on ball (in kg) / Area of ball impression (in mm2)
The load applied depends upon the type of material to be tested. It can vary from 500 kg to 3000 kg. For softer material, lower load is applied. For hard material, high load is applied. The hardness of a material is directly proportional to Brinell hardness number.
Rockwell hardness test – The penetrator or indenter used in this test can a ball or a diamond cone, known as ‘Brale’. The indenter is set firmly against the specimen. A small load of 10 kg (minor load) is applied which results in a very small penetration into the surface of specimen. The applied load can be seen on the dial indicator provided on the front of machine. The reading of the indicator is brought to zero. Now the heavy load (major load) is applied to produce a deeper indentation. After that the load is removed but minor load is still there. We can directly get the Rockwell Hardness number from the dial indicator.
Scales in Rockwell hardness test:
|
Scale |
Major load (kgf) |
Represented as |
Indenter used |
Suited for materials |
|
A |
60 |
HRA |
Brale |
Hardened steel, cemented carbides |
|
B |
100 |
HRB |
Ball |
Copper , brass, aluminium |
|
C |
150 |
HRC |
Brale |
Hardened steel |
Rockwell hardness number can also be converted into Brinell hardness number.
Vickers hardness test – The method is same as that of Brinell hardness test. The difference is that the indenter used is cone type and the load applied varies from 5 kg to 120 kg. A square based diamond pyramid, containing 136° angle between opposite faces is used as indenter. The indenter used assures a higher accuracy as compare to Brinell hardness test. The Vickers hardness number can be calculated as
Vickers hardness number (VHN) = Load / Area of the impression
The BHN and VHN has same unit i.e. kg/mm2