This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Selective Hardening”.
1. Flame hardening can only be performed on steels with a minimum of ______ carbon.
Explanation: Flame hardening is a selective hardening process with a combustible gas flame as the source of heat for austenizing. This process can only be performed with steels with a high carbon content of at least 0.4%. This is in order to allow hardening of the surface.
2. How is the heating of surface done in flame hardening technique?
a) Oxy-acetylene torch
b) Carburizing flame
d) Direct sunlight
Explanation: Flame hardening is a selective hardening process with a combustible gas flame as the source of heat for austenizing. The surface to be hardened is heated to a temperature above its upper critical temperature. This is done using an oxy-acetylene torch.
3. Flame hardening technique is suitable for ____________
a) Plain carbon steels
b) Alloy steels
c) Cast irons
d) Pig iron
Explanation: Flame hardening is a selective hardening process with a combustible gas flame as the source of heat for austenizing. It is more suitable for plain carbon steels having a carbon composition of 0.4-0.95%. It is used to improve wear resistance and surface hardness of gears, wheels, sheaves, shafts, mills, and other components.
4. Which of the following is an application for flame hardening?
a) Distortion occurs
b) Expensive equipment
c) Suitable for larger quantities
d) Overheating causes cracks
Explanation: The advantage of flame hardening lies in the fact that there is no distortion of workpiece since only a small section of the steel is heated. This process is efficient and economical as the cost of equipment is low, thereby making it more suitable for small quantities. It has a disadvantage that overheating of the steel may cause cracks.
5. How does induction hardening differ from flame hardening?
a) Electric current is used
b) Higher quenching time
c) Low-temperature operation
d) Results In fine grain structure
Explanation: The basic mechanism of both induction and flame hardening techniques remains the same. In induction hardening, however, the heat source is derived from the resistance to induced eddy currents.
6. What is the frequency used for depths of 2 to 3 mm in induction hardening?
a) 1200 Hz
b) 3000 Hz
c) 5400 Hz
d) 9600 Hz
Explanation: The depth to which the heating occurs varies inversely as the square root of the frequency in case of induction hardening. This means that as higher frequencies are used, the depths achieved are shallower. The typical frequencies used are 3000 Hz for 3-6 mm depth and 9600 Hz for 2-3 mm depth.
7. Which of the following is true for induction hardening?
a) Requires less time
b) Hardness and depth is difficult to control
c) Only external surfaces can be hardened
d) It gives a scaling effect
Explanation: Induction hardening has a major advantage that it requires very less time, typically of the order of 10 seconds. It is easy to control the surface hardness and depth and can be automated. Here, both external and internal surfaces can be hardened. Since it does not give any scaling effect, the machining is reduced.
8. What is the hardness achieved by flame hardening?
a) HRC 30-40
b) HRC 50-60
c) HRC 80-100
d) HRC 120-140
Explanation: Both flame hardening and induction hardening experience similar results. The hardened layer is about 3 mm thick and has a hardness of about HRC 50-60. Distortion can be often minimized in both processes. Additionally, the surface remains clean in induction hardening.
9. ___________ is a method of obtaining diverse properties by varying thermal histories of various regions.
a) Selective Hardening
Explanation: Selective hardening is a technique by which different properties are obtained by varying thermal histories of various regions. This is otherwise considered as the thermal method of surface hardening. Flame hardening and induction hardening are the two methods of selective hardening.
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