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2A2-4: Metallurgy > Objective 03: Heat Treatment of Steels > Flashcards

Objective 03: Heat Treatment of Steels Flashcards

1
Q

What is the purpose of Heat Treating a metal?

A

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.

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2
Q

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

A

Stress relieving

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3
Q

Annealing Processses

A

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

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4
Q

What are the effects of Annealing Processes?

A
  1. Homogenize the microstructure
  2. Increase ductility
  3. Remove residual stresses
  4. Improve machinability
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5
Q

What is the process of Annealing?

A
  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
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6
Q

What is the effect of annealing high carbon steel?

A

It can induce brittleness and reduce toughness and ductility

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7
Q

What aspect of the Annealing Proces encourages grain enlargement?

A

Heat soaking

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8
Q

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

A

Large grain formation

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9
Q

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

A

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

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10
Q

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

A
  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
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11
Q

What is the process of Normalizing?

A
  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
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12
Q

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

A

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

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13
Q

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

A

It produces tougher steel

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14
Q

How hard does normalizing make low carbon steels?

A

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

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15
Q

Spheroidizing

A

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

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16
Q

What characterizes metal that has been spheroidized?

A

It is the softest and most workable

17
Q

What is the process of Hardening?

A
  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
18
Q

Martensite

A

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

19
Q

How is Martensite formed?

A

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

20
Q

Critical Cooling Rate

A

Cooling during the hardening process

21
Q

Is the Critical Cooling Rate the same for all materials?

A

No

22
Q

What are different mediums that can be used for cooling?

A
  1. Water
  2. Brine
  3. Oil
  4. Air
23
Q

How can cooling be promoted?

A

Agitation of the liquid or sample

24
Q

Case Hardening

A

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

25
Q

Which metal parts typically undergo Case Hardening?

A
  1. Bearings
  2. Machine Tools
  3. Crankshafts
  4. Cams
  5. Valves
  6. Gears
  7. Rollers
  8. Hand Tools
26
Q

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

A
  1. Carburizing

2. Nitriding

27
Q

What is the process of Carburizing?

A
  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
28
Q

What is the process of Nitriding?

A
  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
29
Q

What distinguishes Nitriding from Carburizing?

A

It does not require subsequent quench hardening

30
Q

Carbonitriding

A

Combination of Carburizing and Nitriding

31
Q

What is the process of Carbonitriding?

A
  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
32
Q

Which hardening process produces the hardest surface?

A

Nitriding

33
Q

Quenching

A

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

34
Q

What are the effects of Quenching?

A
  1. Increases strength and hardness

2. Decreases toughness and ductility

35
Q

How is Quenching typically performed?

A
  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
36
Q

What is the process of Tempering

A
  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
37
Q

What does Tempering do?

A

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

38
Q

What is the primary purpose of Tempering?

A

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

2A2-4: Metallurgy (7 decks)

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