This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Hardening and Hardenability”.
1. For hardening of steel by quenching, the steel is cooled in __________
b) Still air
c) Oil bath
d) Cooling tower
Explanation: After heating and soaking of the steel, it must be properly cooled. The steel is quenched to room temperature in a water or oil bath.
2. The cooling rate must be _________ the critical cooling rate for hardening of steel by quenching.
a) Higher than
b) Lower than
c) Equal to
d) Half of
Explanation: Post heating and soaking, the steels must be cooled. The cooling rate must be higher than the critical cooling rate to get the completely martensitic structure. The steel is quenched to room temperature with the help of a water bath or oil bath.
3. Phase transformation during hardening transforms _________
a) BCC to FCC
b) FCC to BCT
c) BCT to HCP
d) FCC to HCP
Explanation: Due to rapid cooking, austenite is supercooled by nearly 500oC. The large force helps convert the FCC into BCT structure. The resulting structure is called martensite.
4. The slip does not occur in martensite due to the presence of _______ in the lattice.
Explanation: Martensite is a super-saturated solution of carbon in α-iron. Due to the presence of carbon in the lattice, slip does not occur. As a result, martensite is strong, hard, and brittle.
5. The hardening process is carried out on ________ steel.
a) No carbon
b) Low carbon
c) Medium carbon
d) High carbon
Explanation: As the carbon content increases, the hardness also increases. Due to this, the hardening process is carried out on high carbon steels containing 0.35-0.50% C.
6. How does the rate of cooling affect the hardness of steel?
a) Faster cooling results in low hardness
b) Slow cooling results in high hardness
c) Fast cooling results in high hardness
d) No change is found
Explanation: We know that hardness depends on the nature and properties of the quenching medium. It was found that faster cooling resulted in greater hardness of the steel, and slow cooling lowers the hardness.
7. How does the size of the specimen affect the hardness of steel?
a) Smaller size results are high hardness
b) Smaller size results in low hardness
c) Larger size results in high hardness
d) No change is found
Explanation: The size of the specimen also greatly affects the hardness of the steel during the hardening process. As the size of the specimen increases, its hardness decreases. For example, hardness with a 50 mm diameter steel bar will be higher than a similar one of 100 mm diameter.
8. ___________ is defined as the ease of forming martensite.
Explanation: Hardenability is defined as a measure or ease with which hardness is achieved. It can also be said that hardenability is the ease with which martensite is formed. Hardness, on the other hand, is the ability of a metal to resist abrasion, indentation, and scratching.
9. Hardenability of a material can be measured using __________ test.
a) Jominy end-quench
Explanation: Hardenability is the measure of inclination of a material to achieve hardness. It is affected by the alloying elements in the material and grain sizes. The hardenability of the material can be measured using the Jominy end-quench test method. Charpy and Izod impact testing methods, whereas Rockwell is a hardness testing scale.
10. Which of the following factors affect the hardenability of a material?
a) Composition of steel
b) Grain size
c) Temperature of specimen
d) Quenching medium
Explanation: The hardenability of the material can be measured using the Jominy end-quench test method. The composition of steel, austenitic grain size, structure before quenching, and quenching medium and method affect the hardenability of the steel.
11. Quenching of the sample in Jominy end-quench method is done at _______
Explanation: The given sample of material is heated above the upper critical temperature and dropped into position in the frame of the apparatus. It is then quenched on one end at 25oC to achieve different rates of cooling along the length of the sample.
12. Hardness readings are taken every ________ after quenching in Jominy end-quench test.
a) 0.01 mm
b) 0.1 mm
c) 1.6 mm
d) 2.5 mm
Explanation: The given sample of material is heated above the upper critical temperature. It is then quenched at one end at 25oC. After cooling, a flat is ground along the length of the bar. Rockwell C hardness readings are then taken for every 1.6 mm along the length from the quenched end, which are then plotted in the form of a graph.
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