This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Martensitic Strengthening”.
1. What is the dislocation density of lath martensite?
a) 106-108 cm-2
b) 108-1010 cm-2
c) 1010-1012 cm-2
d) 1012-1014 cm-2
Explanation: Lath martensite have dislocation density of order 1010-1012 cm-2. It is in form of tangled dislocation. It is of the order of dislocation density found in heavily cold worked material.
2. The major strengthening due to martensite transformation is caused by ______
a) Solid solution strengthening
b) Lattice distortion
c) Dislocation hardening
d) Precipitation hardening
Explanation: Lattice distortion causes a major role in the strengthening of martensite. Lattice distortion causes volume expansion. Which causes deformation dipole causing hardening.
3. Martensitic transformation is _____
a) Diffusion less
b) Long range diffusion of atoms
c) Strain less
d) Second order transformation
Explanation: Martensitic transformation is diffusion less transformation. Which means there is no long range diffusion. It causes strain in lattice.
4. With an increase in carbon content, hardness of martensite ____
c) Remains constant
d) First increases then decrease
Explanation: With an increase in carbon content, hardness of martensite increases. More the carbon content, more will be the lattice distortion. Which will increase hardening effect.
5. Which alloying element increases the Ms temperature?
Explanation: Cobalt increases Ms temperature of Steel. Copper, chromium and manganese on the other hand suppress the Ms temperature.
6. Which alloying element enhances the Ms temperature?
Explanation: Cobalt and aluminum alloying elements increase Ms temperature of Steel. While Ni, Mo and Cr decrease it.
7. Ms temperature depends on ______ and Mf temperature depends on ______
a) Cooling rate, composition
b) Composition, cooling rate
c) Cooling rate, austenite grain size
d) Cooling rate, cooling rate
Explanation: Ms temperature depends on steel’s chemical composition. It doesn’t depend on cooling rate. While Mf temperature depends on cooling rate.
8. Amount of retained austenite _______ with an increase in the amount of carbon in martensite.
a) Increases linearly
c) Remains constant
d) Increases nonlinearly
Explanation: The amount of retained austenite increases with increase in the amount of carbon in martensite. It increases nonlinearly. That’s why tempering becomes necessary in high carbon steels.
9. Retained austenite is sometimes desirable because it ______
a) Lowers hardness
b) Is nonmagnetic
c) Provide dimensional stability
d) Control strength
Explanation: Retained austenite in controlled amounts, provide dimensional stability. It is nonmagnetic in nature. It lowers hardness and strength of steel.
10. What is habit plane for medium carbon steel for martensitic transformation?
a) (1 1 1)
b) (1 0 0)
c) (2 2 5)
d) (2 5 9)
Explanation: (1 1 1) is habit plane for low carbon steel for martensitic transformation. (2 2 5) is habit plane for medium carbon steel. While (2 5 9) is habit plane for high carbon steel.
11. Martensitic transformation is irreversible transformation.
Explanation: Martensitic transformation is reversible transformation. Martensite can be converted into austenite on heating above Ms. Here, composition of martensite should remain same.
12. Retained austenite can be eliminated by tempering.
Explanation: Tempering can be used to overcome a problem of retained austenite. In the second stage of tempering, retained austenite transforms into bainite. Bainite gives steel good toughness.
Sanfoundry Global Education & Learning Series – Mechanical Behaviour & Testing of Materials.
To practice all areas of Mechanical Behaviour & Testing of Materials, here is complete set of 1000+ Multiple Choice Questions and Answers.