This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Interrupted Quenching”.
1. Quenching of austenite steel for martempering is done using __________
b) Still air
c) Cooling tower
d) Hot oil
Explanation: For martempering, the steel is first heated above its critical range to make it austenitic. Then it is quenched in hot oil or molten salt above martensite start temperature.
2. How can temperature differences between center and surface be prevented?
a) Refined grain structure
b) Cooling to room temperature
c) Hot working
d) Addition of silicon
Explanation: Cooling the steel at moderate to room temperature prevents temperature differences between center and surface. This is usually done in air. The resulting microstructure of martempered steel is untempered martensite.
3. Martempering process is usually employed in _________
a) Alloy steels
b) Cast irons
Explanation: Martempering is an interrupted quenching process which eliminates some disadvantages of rapid cooling. It is used to minimize stresses, distortion, and cracking of steels. This process is usually used in alloy steels.
4. Martempering is otherwise known as __________
a) Interrupted quenching
d) Isothermal quenching
Explanation: Interrupted quenching is a technique of eliminating the disadvantages caused by rapid cooling. This consists of two methods, martempering and austempering. Martempering is also known as marquenching, and austempering is also known as isothermal quenching.
5. Austempering forms a _________ structure.
Explanation: Austempering is an isothermal heat treatment process used to reduce distortion caused by quenching and to make tough and strong steels. It forms a bainite structure and is otherwise known as isothermal quenching.
6. It is necessary to carry out _________ after martempering.
c) Surface hardening
Explanation: Untempered martensite structure is transformed into tempered martensite structure by conventional tempering heat treatment processing rapid quenching. This is rarely needed in austempering.
7. Which of the following is not a disadvantage of austempering?
a) Needs to be cold worked
b) Needs special bath
c) Can be used for limited steels
d) Can only be used for small sections
Explanation: Austempering experiences a number of disadvantages compared to quenching and tempering. It needs a special molten salt bath and can be used only for a limited number of steels. Furthermore, only small sections up to 9 mm thickness can be used.
8. Why are bigger sections not used in austempering?
a) Unable to cool
b) Does not fit in the working apparatus
c) Decreased strength
d) Cannot be cut
Explanation: Big sections cannot be used for austempering as they cannot be cooled rapidly to avoid the formation of pearlite. As a result, only small sections up to 9 mm thickness are suitable for this operation.
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