Heat Treatment | Material Science Interview Question and Answers

Heat Treatment | Material Science Interview , Viva , Oral Question and 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, 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 (i.e., 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. Mar tempering and
7. Case hardening.

5. What is meant by annealing?

Annealing is defined as a softening process consisting of heating the steal 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?

1. To relieve or remove stresses.
2. To induce softness.
3. To refine grain structure,
4. To alter ductility, toughness, electrical, magnetic or other properties.
5. To remove gases.
6. 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. 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).

10. 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.

11. What are the three stages for quenching?

Stage 1: Vapour-jacket stage.
Stage 2: Vapour-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 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 more 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.

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 embritlement.

19. What is 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 five a fully accurate representation of the temperatures and times of the transformations occurring.

22. What is 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. 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.

24. 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.

25. What is the benefit of the Jominy end-quench test?

For determining the hardenability of a given material.

26. What is martempering and austempering?

Martempering, also known as marquenching, is a interrupted cooling procedure used for steels to minimize 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.

27. 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.

28. 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.

29. 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.

30. 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.

31. What are some selective heating techniques employed for surface hardening?

1. Flame hardening, and 2. Induction hardening.

32. 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.