Objective 03: Heat Treatment of Steels

Question 1

What is the purpose of Heat Treating a metal?

Answer 1

To force a physical and/or chemical transformation in the alloy and then cool it at a rate, and in such a manner, that it retains the desired properties.

Question 2

What is the main application of Heat Treatment for power engineers and inspectors?

Answer 2

Stress relieving

Question 3

Annealing Processses

Answer 3

Heat treatments that produce pearlitic microstructures (ferrite, pearlite, and cementite)

Question 4

What are the effects of Annealing Processes?

Answer 4

1. Homogenize the microstructure
2. Increase ductility
3. Remove residual stresses
4. Improve machinability

Question 5

What is the process of Annealing?

Answer 5

1. Heat the steel to just above the transition temperature required to produce austenite
2. Hold it at that temperature to allow uniform crystal restructuring
3. Cool it very slowly to room temperature at a maximum rate of 38ºC/hour by leaving it in the furnace with the heat shut off or packing it in sand or another material that is a good heat insulator

Question 6

What is the effect of annealing high carbon steel?

Answer 6

It can induce brittleness and reduce toughness and ductility

Question 7

What aspect of the Annealing Proces encourages grain enlargement?

Answer 7

Heat soaking

Question 8

What causes high carbon steels to become brittle in the Annealing Process?

Answer 8

Large grain formation

Question 9

How does Sub-Critical/Process Annealing differ from Full Annealing?

Answer 9

The steel is heated to just below its austenite transformation temperature and then cooled slowly to reduce internal stresses in the metal

Question 10

What is the purpose of Normalizing carbon steel with less than 0.8% carbon?

Answer 10

1. Relieve internal stresses embedded in the metal from welding, machining, or forging
2. Refine grain size and promote uniform composition to increase strength and toughness
3. Improve machinability

Question 11

What is the process of Normalizing?

Answer 11

1. Raise the temperature of the steel to approximately 55ºC above the upper transition temperature into the austenite region
2. The steel is held at that temperature just long enough to ensure even heating throughout
3. It is then allowed to cool in still air at a rate not exceeding 100ºC/hour

Question 12

If a furnace is used to normalize steel, what precaution should be taken?

Answer 12

The furnace should have a reducing atmosphere (no free oxygen present) to prevent oxide scale forming on the surfaces

Question 13

What is the benefit of normalizing as compared to any other heat treatment process?

Answer 13

It produces tougher steel

Question 14

How hard does normalizing make low carbon steels?

Answer 14

Just hard enough to machine freely, leaving the surface free of tears

Question 15

Spheroidizing

Answer 15

Any process of prolonged heating and cooling of steel, similar to anealing, that converts the carbide content of the matrix into a rounded or spheroid structure

Question 16

What characterizes metal that has been spheroidized?

Answer 16

It is the softest and most workable

Question 17

What is the process of Hardening?

Answer 17

1. Mild steel is heated to a temperature above its transformation range (austenizing)
2. Then it’s cooled quickly to increase hardness by the formation of Martensite

Question 18

Martensite

Answer 18

Structure of fine carbide needle-like grains that are extremely hard

Question 19

How is Martensite formed?

Answer 19

They are formed during the transformation from austenite. If the temperature is dropped quickly, the carbon in the austenite does not have time to precipitate as pearlite but instead forms distorted needle-like grains of carbide in the ferrite matrix

Question 20

Critical Cooling Rate

Answer 20

Cooling during the hardening process

Question 21

Is the Critical Cooling Rate the same for all materials?

Answer 21

No

Question 22

What are different mediums that can be used for cooling?

Answer 22

1. Water
2. Brine
3. Oil
4. Air

Question 23

How can cooling be promoted?

Answer 23

Agitation of the liquid or sample

Question 24

Case Hardening

Answer 24

Heat Treatment process that produces Martensite in the outer layer only, leaving the interior to retain a tough ferrite-pearlite composition

Question 25

Which metal parts typically undergo Case Hardening?

Answer 25

1. Bearings 2. Machine Tools 3. Crankshafts 4. Cams 5. Valves 6. Gears 7. Rollers 8. Hand Tools

Question 26

What are 2 important thermochemical case hardening processes for low alloy steels?

Answer 26

1. Carburizing | 2. Nitriding

Question 27

What is the process of Carburizing?

Answer 27

1. The part is heated to its transformation temperature in an atmosphere of carbon monoxide 2. Carbon diffuses into the skin of the metal increasing martensite formation in this area when the part is later quench hardened

Question 28

What is the process of Nitriding?

Answer 28

1. The part is heated in a furnace at a temperature below the transformation range of iron (approximately 500ºC to 600ºC) in an atmosphere of ammonia 2. At this temperature, ammonia dissociates into nitrogen and hydrogen 3. Atomic nitrogen diffuses into the surface layer of the metal forming iron nitrides which are extremely hard

Question 29

What distinguishes Nitriding from Carburizing?

Answer 29

It does not require subsequent quench hardening

Question 30

Carbonitriding

Answer 30

Combination of Carburizing and Nitriding

Question 31

What is the process of Carbonitriding?

Answer 31

1. A source of carbon and nitrogen is introduced into the furnace at a temperature above the transformation range of the steel 2. A less severe quench hardening step than carburizing is then required

Question 32

Which hardening process produces the hardest surface?

Answer 32

Nitriding

Question 33

Quenching

Answer 33

Rapid cooling of a heated metal to obtain the desired transformation products

Question 34

What are the effects of Quenching?

Answer 34

1. Increases strength and hardness | 2. Decreases toughness and ductility

Question 35

How is Quenching typically performed?

Answer 35

1. Steel with carbon content above 0.8% is heated above the upper transformation temperature and held there to allow the formation of austenite 2. The steel part is then quickly cooled by immersing it in a liquid such as water, brine or oil

Question 36

What is the process of Tempering

Answer 36

1. Quenched steel is heated to a specific temperature below their lower transformation ranges 2. The sample is then cooled to room temperature at a rate that prevents martensite reformation

Question 37

What does Tempering do?

Answer 37

It forces the saturated carbon in the martensite to form back into a stable iron carbide (cementite) and ferrite mixture

Question 38

What is the primary purpose of Tempering?

Answer 38

To improve the mechanical properties of steel by increasing ductility and toughness with slightly reduced hardness