2 Marks Question & Answers: Heat Treatment - Engineering Materials and Metallurgy
2 Marks Question & Answers
1. Define the term 'heat treatment'.
Heat treatment may be defined as an operation or combination of operations involving heating and cooling of a metal/alloy in solid state to obtain desirable properties.
2. What are the purposes of the processing heat treatments?
1. To relieve internal stresses.
2. To improve machinability.
3. To refine grain size.
4. To soften the metal.
5. To improve hardness of the metal surface.
6. To improve mechanical properties (like tensile strength, hardness, ductility, shock resistance, etc.,)
3. List the various stages of a heat treatment process.
Stage 1: Heating a metal/alloy beyond the critical temperature.
Stage 2: Holding at that temperature for a sufficient period of time to allow necessary changes to occur.
Stage 3: Cooling the metal/alloy (ie., quenching) at a rate necessary to obtain the desired properties. That is, cooling at a rate necessary to obtain the desired changes in the nature, form, size and distribution of micro-constituents.
4. List some of the important heat treatment operations widely used.
1. Annealing
2. Normalizing
3. Hardening
4. Tempering
5. Austempering
6. Martempering
7. Case hardening
5. What is meant by annealing?
Annealing is defined as a softening process consisting of heating the steel to a temperature at or near the critical point, holding there for a proper time and then allowing it to cool slowly in the furnace itself.
6. What are the purposes of annealing?
(i) To relieve or remove stresses.
(ii) To induce softness.
(iii) To alter ductility, toughness, electrical, magnetic or other properties.
(iv) To refine grain structure.
(v) To remove gases.
(vi) To produce a definite microstructure.
7. List the different types of annealing.
(a) Full annealing,
(b) Process annealing,
(c) Stress relief annealing,
(d) Recrystallisation annealing, and
(e) Spheroidise annealing.
8. What is meant by normalizing?
Normalizing is similar to full annealing, but cooling is established in still air rather than in the furnace.
9. Differentiate between normalizing and full annealing.
10. What is quenching? List some of the quenching medium generally used in industries.
✔ Quenching refers accelerated cooling.
✔ Some of the quenching medium that are used generally in industries are: 5-10% caustic soda, 5-20% brine (NaCl), cold water, warm water, mineral oil (obtained during the refining of crude petroleum), animal oil, vegetable oil (such as linseed, cottonseed, and rapeseed).
11. What are the factors should be considered while selecting a quenching medium?
1. Desired rate of heat removal.
2. Required temperature interval.
3. Boiling point.
4. Viscosity.
5. Flash point (if combustible).
6. Stability under repeated use.
7. Possible reactions with the material being quenched.
8. Cost.
12. What are the three stages for quenching?
Stage 1: Vapour-jacket stage.
Stage 2: Vapur-transport cooling stage.
Stage 3: Liquid cooling stage.
13. Rate the order the effectiveness of the following quench media: oil, brine, water, and molten salt.
Molten salt, brine, water, and oil.
14. What does the term hardening refer? What are the factors affecting the hardness?
✔ Hardening refers to the heat treatment of steel which increases its hardness by quenching.
✔ The hardness obtained from the hardening process depends upon the following factors:
1. Carbon content,
2. Quenching medium,
3. Specimen size, and
4. Other factors.
15. Distinguish the work hardening with the age hardening process.
✔ Work hardening also known as strain hardening, is the process of hardening a metal, while working on it (under cold-working conditions).
✔ Age hardening, also known as precipitation hardening, is the process of hardening a metal when allowed to remain or age after heat treatment.
16. The tempering process usually follows hardening process. Justify.
The martensite which is formed during hardening process is too brittle and lacks good ductility and toughness. Hence, it cannot be used for most applications. Also the internal residual stresses that are introduced during hardening have a weakening effect. The ductility and toughness of martensite can be enhanced and these internal stresses are relieved by a heat treatment process known as tempering.
17. What is the effect of: (a) tempering temperature, and (b) tempering time, on the hardness of steels?
(a) The hardness gradually decreases as the temperature is increased.
(b) The hardness decreases with the increase in tempering time.
18. What do you mean by temper embrittlement?
The tempering of some steels/steel alloys may result in a reduction of toughness (i.e., increase in brittleness). This phenomenon is referred as temper embrittlement.
19. What is a TTT diagram?
The TTT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is a tool used by heat treaters to predict quenching reactions in steels.
20. What is the significance of TTT diagram in the heat treatment of steel?
✔ The TTT diagram is most useful in giving an overall picture of the transformation behaviour of austenite. This enables the metallurgist to interpret the response of a steel to any specified heat treatment.
✔ Using a TTT diagram, one can plan practical heat treatment operations to get desirable microconstituents, to control limited hardening or softening, and the time of soaking.
21. Why are TTT diagrams usually not applicable to industrial engineering practices?
The data for the construction of TTT diagrams are obtained from the isothermal transformation of austenite at differing temperatures. But most industrial heat treatments involve. continuous cooling from the austenitic temperature to room temperature. Thus a TTT diagram may not give a fully accurate representation of the temperatures and times of the transformations occurring.
22. What is a CCT diagram?
The CCT diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to indicate when transformations occur as the initially austenitised material is continuously cooled at a specified rate. In addition, it is also used to predict the final microstructure and mechanical characteristics.
23. Define the tem critical cooling rate. What are the factors affecting it?
✓ The slowest rate of cooling of austenite that will result in 100% martensite transformation is known as the critical cooling rate.
✓ Factors affecting the critical cooling rate are: 1. Chemical composition of steel, 2. Hardening temperature, and 3. Metallurgical nature (i.e., Purity) of steel.
24. What is significance of the critical cooling rate?
The critical cooling rate is most important in hardening. In order to obtain a 100% martensitic structure on hardening, the cooling must be must be much higher than the critical cooling rate.
25. What is meant by hardenability? What are the factors affecting it?
✔ The term hardenability refers to the ease with which hardness may be attained. In other words, hardenability is a measure of ease of forming martensite.
✔ The factors affecting the hardenability are: 1. Composition of the steel, 2. Austenitic grain size, 3. Structure of the steel before quenching, and 4. Quenching medium and the method of quenching.
26. What is the difference between hardness and hardenability?
✓ The term hardness is the property of a material by virtue of which it is able to resist abrasion, indentation and scratching. It is a mechanical property related to strength and is a strong function of the carbon content of a metal.
✓ On the other hand, hardenability is the susceptibility of a material to get hardened. It is affected by the alloying elements in the material and grain size.
27. What is the benefit of the Jominy end-quench test?
For determining the hardenability of a given material.
28. What are hardenability curves? What are the uses of them?
✓ The hardness curves are obtained from the data of Rockwell C hardness readings taken along the length and the distance from the quenched end.
✓ The main practical uses of end-quench hardenability curves are:
1. If the quench rate (i.e., cooling rate) for a given part is known, the Jominy hardenability curves can predict the hardness of that part.
2. If the hardness at any point can be measured, the cooling rate at that point may be obtained from the hardenability curve for that material.
29. What is martempering and austempering?
✓ Martempering, also known as marquenching, is a interrupted cooling procedure used for steels to minimize the stresses, distortion and cracking of steels that may develop during rapid quenching.
✓ The Austempering is an isothermal heat treatment process, usually used to reduce quenching distortion and to make tough and strong steels.
30. What do you mean by the term case hardening?
In many applications, it is desirable that the surface of the components should have high hardness, while the inside or core should be soft. The treatments given to steels to achieve this are called surface heat treatments or surface hardening.
31. List some of the surface-hardening techniques employed for altering surface chemistry.
1. Diffusion methods
(a) Carburizing,
(b) Nitriding,
(c) Cyaniding, and
(d) Carbonitriding.
2. Thermal methods
(a) Flame hardening, and (b) Induction hardening.
32. What are the differences between surface hardening by diffusion methods and thermal methods?
✔ In diffusion surface-hardening method, the hardness of the surface is improved by diffusing interstitial elements like carbon, nitrogen, or both into the surface of steel components.
✔ In thermal method of surface hardening, only the surface of the steel components are heated to temperature above the upper critical temperature and is suddenly quenched to get martensite formation on the surface which gives higher hardness at the surface.
33. Differentiate between pack carburizing and gas carburizing.
✔ In pack carburizing, the components to be treated are packed into steel boxes, along with the carburizing mixture, so that a space of roughly 50 mm exists between them.
✔ Gas carburizing overcomes the drawbacks. difficulties of pack carburizing by replacing the solid carburizing mixture with a carbon-providing gas.
34. In what ways, cyaniding differs from carburizing?
The salt bath composition for cyaniding gives a case high in nitrogen, whereas carburizing gives a case rich in carbon.
35. What is meant by selective hardening technique?
Selective hardening (or heating) technique is a technique by which different properties are obtained simply by varying the thermal histories of the various regions.
36. What are some selective heating techniques employed for surface hardening?
1. Flame hardening, and
2. Induction hardening.
37. In what ways, flame hardening differs from induction hardening?
The mechanism and purpose of induction hardening are the same as for flame hardening. The main difference is that in induction hardening the source of heat input is an induced electric current instead of using flame.
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