# Mechanical Metallurgy Questions and Answers – Hardness Test – Microhardness Test

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This set of Mechanical Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Hardness Test – Microhardness Test”.

1. The stress concentration and notch sensitivity are the same phenomena in the material.
a) True
b) False

Explanation: The stress concentration is phenomena associated with increase in stress value in the local area due to any factor. But the notch sensitivity leads explicitly with a local area of triaxle state of stress, which fails due to high-stress condition.

2. Which of the following is not a measurement method for hardness?
a) Scratch hardness
b) Indentation hardness
c) Rebound hardness
d) Elongation hardness

Explanation: The scratch hardness is commonly used for minerals, indentation and rebound hardness for metals. But there is no such thing as elongation hardness measurement.

3. The scratch hardness range is ______________
a) 1-10
b) 1-100
c) 10-1000
d) 100-110

Explanation: The scratch hardness is used by mineralogist. It is the ability of one mineral to scratch another. The talc is softest mineral kept at 1 and diamond being hardest is kept at number 10.

4. The hardness value of martensite according to Mohr’s scale is ___________
a) 1
b) 10
c) 3
d) 7

Explanation: 1 is for talc, 10 is for the diamond, 3 is for an annealed copper sample. And the hardness of martensite is around 7 in mohr’s scale.

5. Hardness of most metals fall in the range of __________ in Mohr’s scale.
a) 1-3
b) 3-6
c) 4-8
d) 7-10

Explanation: Hardness of most metals fall in the range of 4-8 in Mohr’s scale. But the disadvantage of using this scale is that the range is widely spaced for metals.

6. The Shore hardness test measurement uses the concept of _____________
a) indentation
b) deformation
c) rebound
d) scratch

Explanation: The Shore hardness tester uses the concept of rebound indentation from the surface of the sample. The impact of energy is used to calculate the hardness of the material.

7. The Brinell Hardness measurement is a concept based on __________
a) indentation
b) shearing
c) rebound
d) scratch

Explanation: The Brinell hardness test is the first standardized and widely used method for hardness testing. The principle is based on the indentation of the metal surface with a hardened steel ball.

8. The diameter of the steel ball used in Brinell hardness is ________
a) 10 mm
b) 10 cm
c) 1 mm
d) 3.6 mm

Explanation: The diameter of the stell ball used for indentation measurement is 10 mm with a load of 3000 kg. For soft material, it will be reduced to 500 kg.

9. For hard materials, the indenter of the Brinell hardness tester is made of __________
a) steel
b) diamond
c) tungsten carbide
d) copper

Explanation: For the hard material, to avoid the distortion of the ball, the tungsten carbide ball is used as the indenter. Generally, the steel ball is used for the measurement.

10. How is Brinell hardness measured?
a) Using a dial gauge showing hardness reading
b) The graph obtained from machine showing hardness
c) Using an indentation profile which will be checked in a microscope for indentation diameter
d) By checking the deformation of the ball

Explanation: The Brinell hardness measurement is done by checking the diameter of indentation in a low magnification microscope. This is divide by the load applied to get the hardness number.

11. The Brinell hardness number is given by:
$$\frac{P}{(\frac{πD}{2})[D-\sqrt{(D^2-d^2)}]}$$
Where P is load applied in kg,
D-diameter of the ball, in mm
d-diameter of the indentation, in mm
a) True
b) false

Explanation: The BHN is calculated by given the formula which can be also written in the form of:
$$\frac{P}{πDt}$$; t – the depth of impression, mm.

12. The unit of Brinell hardness number is ____________
a) kg
b) kg/mm2
c) kg-mm
d) unitless

Explanation: The Brinell hardness number is given as:
$$\frac{P}{πDt}$$
The unit of P is kg and D and t are in mm.
So the unit will become kg/mm2.

13. A major load of 3000 kg is applied on steel, the diameter of ball is 10 mm and width of indentation is 2 mm. Find the Brinell hardness number of the given steel?
a) 350.5
b) 589.25
c) 636.94
d) 452.2

Explanation: The Brinell hardness number is equal to;
$$\frac{P}{(\frac{πD}{2})[D-\sqrt{(D^2-d^2)}]}$$
3000/(3.14*10/2)[10-√(100-4)]
3000/(15.7)(0.3)
636.94.

14. Suppose Brinell hardness measurement is made from a nonstandard ball of diameter 5 mm. If the load applied in 10 mm standard ball is 3000 kg, then what load should be applied on the 5 mm ball to get the consistent reading in hardness measurement?
a) 500 kg
b) 250 kg
c) 750 kg
d) 1500 kg

Explanation: For the consistency in the result the;
P1/D12=P2/D22
To compare the values 3000/100=P2/25
P2=750
So the applied load should be 750 kg.

15. Which of the following hardness is least affected by surface scratched and local inhomogeneity of the material?
a) Brinell hardness
b) Mayer hardness
c) Rockwell hardness
d) Vickers hardness