Vickers Hardness Test

* Working principle of vickers hardness test is similar to Brinell hardness test 

- In this test vickers indenter is use            which is made of diamond in the form    of square based pyramid 
-The include angle between the       opposite faces of the pyramid indenter    is 136 degree
- 136 degree angle is considered because it correspond to most desirable ratio of indentation diameter to ball diameter 

Test procedure : 
- Test procedure is similar to Brinell hardness test 
- In this as indentation is very small surface need to polish by 0,00,000,etc 
- The vickers hardness tester is semi-automatic in operation 
- After an impression has been made on specimen the diagonals of square impression are measured with the help of special measuring microscope
- To convert measured diagonals into hardness number standard chart are use 

Advantages :
- Because of square impression more accurate results are obtained     as compared to circular impression 
- Due to small size of indenter impression are small which does not affect the life of component 
- As indenter are made of diamond it does not deform while testing of hard material 
- Thin material can be tested easily 
- Life of indenter is more 

Limitation : 
- As indenter size is small test not recommended to measure hardness of heterogeneous materials, casting component, porous power metallurgy component 
- While testing surface must be free from dust an polished properly which is time consuming process 
- More surface finish is required 

Application :
- Use for measuring hardness of soft as well as hard materials, hardened steel, ferrous as well as non ferrous alloy, thin plate, razor blade etc 


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  Durometer 

- Durometer appear like a table clock and are different from the other hardness testing instrument 
- Testing is like durometer is pressed on a specimen surface and it indicates hardness number directly    on a dial
- The use indenter is conical in shape and are made of steel 
- Durometer are use for measuring hardness of plastics and rubbers
- It is small and portable as compared to other hardness testing instrument 
- Result obtained from one type of durometer cannot be correlated with the other type of durameter 



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                                Poldi Hardness Test 

*This technique is use for measuring hardness 

- In some application like large casting in shop floor, it is not possible to use brinell hardness tester so at that time, the another hardness tester which has same working principle like brinell but portable instrument is used.
- This hardness tester is nothing but a poldi hardness tester 
- It is small in size and it consist of hollow steel tube in which plunger is fitted whose top end is out of tube 
- A steel ball of 10 mm is fitted at bottom end of tube 
- A slot is provided above this ball for inserting a steel bar of known hardness which we called standard bar and on that bar  hardness is printed on the bar 

Test procedure :

- In this standard bar of known hardness number is inserted in the slot of the instrument 
- so that standard bar touches the ball indenter as well as lower end of plunger 
- Entire assebly is held with left hand and hammer is given on the top of plunger 
- Impact must be in exact vertical otherwise impression may in elliptical 
- Load is applied manually 
- As indenter is touching to standard bar and specimen simultaneously it create two impression i.e. on the specimen and on the standard bar 
- For measuring this diameter of indenter magniscope is used

Precautions taken during Poldi Hardness Test :

- To avoid elliptical impression, tube should be vertical on the specimen 
- To avoid rebound effect, specimen kept on hard platform 
- Hammer impact should be moderate 

Advantages :

- It is portable 
- Require less time for measuring hardness 
- Low cost 
- Can be use for heterogeneous materials 
- Can be use for soft materials 

Limitation :

- Dependent load 
- Skilled operator is required to conduct test 
- Result obtained from the test are approximate due ti manual error 
- Not use for hard and thin material 


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                  Brinell  Hardness Test 

- This test is nothing but to check how much the testing specimen is hard 
- The machine use for this process is brinell hardness tesing machine 
- This machine work on the principle of measurement of area of indentation for measuring the hardness value 
- Now for that indentation a ball indenter is used which produces circular indentation and on measuring diameter of that indenter and load applied on specimen 

Now hardness number is calculate by-
            BHN=(Load applied in Kg)/(indentation area in mm^2) 

Test Procedure :

- Clean surface from any type of grease or dust 
- Here we measure the area of indentation with help of micrometer microscope so surface should be polish by 0 or 00 paper 
- In machine specimen is placed on anvil and hand wheel is rotated so that specimen along with anvil move up and it make contact with ball indenter 
- The required load is applied by using gear driven screw by lowering handle and ball indents into specimen 
- So as the handle rotate specimen is going up and make contact with indenter for measuring the diameter of indenter 
- Small indenter diameter indicate high hardness number of metal 

Advantages of Brinell Hardness Test :

- More useful for measuring hardness of heterogeneous materials
- Cost of testing is low 
- Skilled operator is not required 
- Use for testing of soft metals 

Limitation/drawbacks of Brinell Hardness Test :

- Due to large size of ball and high load impression are larger,which decreases useful life of component 
- If hard material is test ball indenter will deform 
- Cannot use for material having hardness number more than 350 BHN
- Not use on final component due to large indentation 
- Dependent of load 

Application :

-This test is use for gray cast iron, copper and its alloy aluminium Etc.

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