This set of Mechanical Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Relationship Between Hardness and Flow Curve”.
1. If the hardness of the case carburized steel needs to be measured, the possible method to measure the hardness of the case will be _________
Explanation: The Knoop hardness measurement is also known as the microhardness test. So if the deformation needs to be limited only on the surface, a significant load of 25gm is applied with a tiny probe that recreates very localized deformation.
2. The shape of Knoop indenter is of ___________
a) rounded end tip
b) square-based pyramid
c) pyramid with major and minor diagonal
d) pyramid with a rounded end
Explanation: The Knoop hardness measurement is made with pyramid based indenter with major and minor axis in the ratio of 1:7, as shown in the figure. This creates a region of the state of plane strain in the deformed region.
3. The Knoop hardness number(KHN) is defined as the applied load divided by the unrecovered projected area of the indentation.
Explanation: KHN = P/AP;
where the P is applied the load, and the AP is the unrecovered projected area of indentation.
-> P/L2C; Where L is the length of the long diagonal, C is a constant of the indenter.
4. For the same long diagonal length of indenter of Vickers hardness and Knoop hardness, the Knoop hardness is how much percentage of the Vickers indenter?
a) 50 %
b) 15 %
c) 75 %
d) 100 %
Explanation: The following figures show that the primary length of the diagonal is the same.
But the Knoop indenter has minor diameter 1/7 of the primary diameter, so the area will be only 15% of the Vickers indenter.
5. For measuring the hardness of the cementite and ferrite in a pearlite colony individually, the Knoop hardness measurement is the best method.
Explanation: For cementite and ferrite in the pearlite colony, the distance between each respective phase is in order of few microns. So microhardness testing is the best possible way to measure the hardness of these different phases.
6. The hardness value of the metal ______________ with increase in the temperature.
c) remains constant
d) no relationship
Explanation: The hardness of the material decreases with increase in the temperature of the material.
-> The relationship is given as; H=A e-BT
Where H is hardness, A, B is constant, T is the test temperature.
7. The following graph shows the change in hardness with the variation of the temperature of the material. There is an abrupt change in the slope of the line. This cannot be due to ____________
a) change in the crystal structure of the material
b) allotropic transformation
c) change in the deformation mechanism of the material
d) creation of the vacancy in the material
Explanation: Generally when the variation of the hardness is plotted with the variation of the temperature around the half of the melting point, these types of slope change is observed. The main reason for this change is that different types of deformation mechanisms operate at different temperature in a material.
Sanfoundry Global Education & Learning Series – Mechanical Metallurgy.
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