JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III HEAT TREATMENT Previous Question Paper (November/December - 2018 - R16)
Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks.
Part A
Why the hardness of spheroidised structure is smaller than lamellar pearlite of same steels?
Answer
In lamellar pearlite, the cementite exists as thin, continuous plates that act as effective barriers to dislocation motion, providing higher strength and hardness. In spheroidised structures, the cementite is present as discrete, globular particles, which offer less resistance to dislocation movement through the ferrite matrix, resulting in lower hardness and higher ductility.
Distinguish between oxidation and decarburization.
Answer
Oxidation is the chemical reaction of the metal surface with oxygen to form a scale or oxide layer, resulting in material loss and surface degradation. Decarburization is the loss of carbon from the surface of steel due to reaction with gases like hydrogen, water vapor, or carbon dioxide, which reduces the surface hardness and wear resistance without necessarily forming a visible scale.
Distinguish between exothermic and endothermic atmospheres.
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Why aluminium alloy 2017 rivets have to be refrigerated until used? Explain.
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Explain the influence of various alloying elements on grain growth in steels.
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Distinguish between hot and cold wall vacuum furnaces.
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Distinguish between primary cementite and secondary cementite.
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What is bright nitriding?
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Why is it necessary to toughen the core before hardening the case?
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What is ausforming?
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Part B
Describe the structural changes that take place during tempering.
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Is there any fourth stage of tempering? Explain.
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Describe the merits and demerits of the following hardening processes: (i) Direct quenching (ii) interrupted quenching
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Compare and contrast between process annealing and recrystallization annealing.
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Explain the various stages in the tempering process.
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What is the driving force for the tempering process?
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Discuss the effect of the following on the hardenability of a material:
a) Austenite grain size
b) Presence of alloying elements
c) Carbon content.
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Draw a TTT diagram for 0.8% C steel schematically and superimpose the annealing, normalizing and hardening curves for the same steel and explain them fully.
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What is an I-T diagram? Is it different from TTT diagram? Explain.
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What are the limitations on the use of I-T diagrams?
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What are residual stresses? Explain about the presence and nature of residual stresses after carburizing? How they are eliminated/reduced? Explain.
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Which type of steels are best suitable for the following thermomechanical treatments and why? Explain.
a) iso-forming b) ausforming c) Cryoforming
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Describe about liquid nitriding process.
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Explain the advantages and disadvantages of nitriding process.
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Distinguish between HTMT and LTMT. What factors are responsible for increasing strength in HTMT?
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