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Manufacturing Processes II Mcqs

Q:

Quenching medium is decided on the basis of ______

A) weight of the specimen B) material of the specimen
C) thickness of the specimen D) application of the specimen
 
Answer & Explanation Answer: C) thickness of the specimen

Explanation: The thickness of the material to be quenched along with the rate of cooling required helps to choose the quenching medium. The quenching medium has to be chosen carefully.

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Q:

Quenching is the process of _______

A) rapid cooling B) slow cooling
C) rapid heating D) slow heating
 
Answer & Explanation Answer: A) rapid cooling

Explanation: Quenching is the process of rapidly cooling material from high temperature. This rapid cooling is achieved using quenching media. Quenching is done during the hardening process.

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Q:

The reasons for case hardening is to develop compressive residual stress.

A) True B) False
 
Answer & Explanation Answer: A) True

Explanation: One of the main reasons for case hardening is to develop compressive residual stress at the surface of the components that are subjected to fatigue loading. A general thumb rule is that the region that transforms last has a compressive stress. In the case of a carburized steel there is a large difference in the concentration of carbon at the surface and that at the centre. The difference is of large that although the surfaced on quenching cools faster it transforms to martensite later than the core. Since it transforms last it should be under compression. This is the reason why case hardened components have compressive residual stress.

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Q:

The main purpose of this stage is to harden the case consisting of austenite and globules of un?dissolved carbide.

A) True B) False
 
Answer & Explanation Answer: A) True

Explanation: The main purpose of case hardening is to harden the case. Therefore the component after case refining is heated to 30?C-40?C above the lower critical temperature. At this temperature, the case consists of austenite and globules of undissolved carbide. The structure of the core during this stage of heat treatment should have ferrite and austenite.

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Q:

Which of the following is most commonly used as carburizing gas?

A) CH? B) CO
C) N? D) C?H?
 
Answer & Explanation Answer: A) CH?

Explanation: CH? and CO are the most commonly used carburizing gas. It is usually mixed with decarburizing (H2 and CO?) and neutral gases (N?). This helps maintain close control over carbon potential. Gas carburization is done by keeping the samples at the carburizing temperature for a specified time in a furnace having a mixture of carburizing and neutral gas.

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67
Q:

A major limitation of pack carburizing is poor control over_____

A) temperature B) ductility
C) hardness D) strength
 
Answer & Explanation Answer: A) temperature

Explanation: A major limitation of pack carburizing is poor control over temperature & carburization depth. On completion of the process, the steel parts are cooled slowly. Direct quenching is not possible as the job is surrounded by carburizing mixture packed in a sealed box having high thermal mass. This can be overcome by using a gaseous or liquid carburizing medium.

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83
Q:

The hardness of steel depends only on the ______

A) carbon content B) temperature
C) yield strength D) tensile strength
 
Answer & Explanation Answer: A) carbon content

Explanation: The hardness of steel depends only on the concentration of carbon in steel. Therefore it may be enough to have high carbon only at the surface. This can be achieved by increasing the concentration of carbon in a component made of low carbon steel by allowing carbon to diffuse into it.

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68
Q:

Which of following is not the feature of induction hardening?

A) Heat the surface to a temperature above an austenitic region B) Good surface finish
C) Fast heating & short hold time D) Applicable to carbon steels having (0.8– 1% C)
 
Answer & Explanation Answer: D) Applicable to carbon steels having (0.8– 1% C)

Explanation: The salient features of induction hardening are as follows:
• Heat the surface to a temperature above austenitic region
• Core does not get heated: the structure remains unaltered
• Surface converts to martensite on quenching.
• Fast heating & short hold time: needs higher austenization temperature
• Martensite forms in fine inhomogeneous grains of austenite
• Applicable to carbon steels (0.35 – 0.7C)
• Little distortion & good surface finish.

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