This set of Manufacturing Processes Multiple Choice Questions & Answers (MCQs) focuses on “Hardening – 1”.
1. How many types of hardening techniques are there?
Explanation: Different techniques to improve the hardness of the steels are as follows:
• conventional hardening
2. In a conventional hardening of hypereutectoid steels, the component is heated above _____
Explanation: The first step in hardening involves heating the steel to above 910⁰C for hypoeutectoid steels and above 738⁰C for hypereutectoid steels by approximately 50⁰C.
3. After hardening, the component is cooled ______
a) at the critical cooling rate
b) at the rate just more than the critical cooling rate
c) in still air
d) at rate at which it was heated
Explanation: After austenization, the next step in hardening involves cooling of hot steel components at a rate just exceeding the critical cooling rate of the steel to room temperature or below room temperature.
4. In conventional hardening _______ transforms to _______
a) austenite, martensite
b) cementite, martensite
c) perlite, α-iron
d) austenite, ferrite
Explanation: In this conventional hardening process, the austenite transforms to martensite. The final step in the hardening process is the tempering of the martensite to achieve the desired hardness. This martensite structure improves the hardness of the component.
5. Martempering uses _______ quenching.
Explanation: Martempering process overcomes the limitation of the conventional hardening process. This process follows interrupted quenching operation. In other words, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the centre to cool to the temperature as the surface. Further cooling is continued through the martensite region, followed by the usual tempering.
6. Austempering is used to overcome the limitations of ______
b) conventional hardening
Explanation: This process is also used to overcome the limitation of the conventional hardening process. Here the quench is interrupted at a higher temperature than for martempering to allow the metal at the centre of the part to reach the same temperature as the surface.
7. During austempering, steel is transformed to ______
Explanation: In austempering, both the centre and surface are allowed to transform to bainite and are then cooled to room temperature. Austempering causes less distortion and cracking than that in the case of martempering.
8. Austempering improves ______
a) corrosion resistance
d) tensile strength
Explanation: Austempering improves the impact toughness and the ductility of the metal than that in the case of martempering and conventional hardening. It also avoids the tempering operation.
9. Alloy steels can be hardened by simple air cooling.
Explanation: Alloy steels have less critical cooling rate and hence some of the alloy steels can be hardened by simple air cooling. Proper quenching medium should be used such that the component gets cooled at a rate just exceeding the critical cooling rate of that steel.
10. High carbon steels are hardened by oil quenching.
Explanation: High carbon steels have slightly more critical cooling rate and has to be hardened by oil quenching. Medium carbon steels have still higher critical cooling rates and hence water or brine quenching is necessary.
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