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Engineering Materials > Chapter 3 Heat Treatment > Flashcards

Chapter 3 Heat Treatment Flashcards

1
Q

Which of the following processes does not have the same heat treatment temperature for heat treating hypo-eutectoid steels?

A

Process Annealing

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2
Q

What takes place during soaking stage in process annealing?

A

Recrystallization takes place

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3
Q

The microstructure of a normalised 0.8% carbon steel at 500 degrees C is?

A

Pearlite

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4
Q

What is soaking?

A

Holding at temperature

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5
Q

Which of the following microstructures is soft and ductile?

A

Ferrite

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6
Q

What is the appropriate temperature range to carry out full annealing of carbon tool steel?

A

30 to 50 degree celsius above LCT

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7
Q

At which temperature should full annealing of a dead mild steel be carried out?

A

940 degree celsius, which is about 30 degrees celsius above UCT of the dead mild steel

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8
Q

A component made of 0.8%C steel is heated and soaked at 700 degree C for 24 hours and then cooled down in the furnace. This treatment is known as?

A

Spheroidising Annealing

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9
Q

Which microstructure(s) is not found in plain carbon steels under ambient conditions?

A

Austenite

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10
Q

What is ambient conditions?

A

Something like room temperature.

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11
Q

Which heat treatment is carried out to refine the grain structure of the as-cast hypo-eutectoid steels?

A

Normalising & / Full Annealing

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12
Q

The tempering heat treatment is carried out after the hardening heat treatment of steel to improve its?

A

toughness and relief quenching stresses

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13
Q

Which property is not found in freshly hardened plain carbon steels?

A

Toughness

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14
Q

A plain carbon steel with 0.1% carbon is heated to 900 degree C and rapidly quenched in water. What is its property?

A

Ductile

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15
Q

Which of the following media has the fastest rate of quenching?

A

Brine

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16
Q

The wear resistance of a 1.2% carbon steel can be improved by which treatment?

A

Hardening heat treatment

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17
Q

Hardened gears are reheated to 450 degrees C in a furnace followed by slow cooling to room temperature. What is the treatment?

A

Tempering

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18
Q

To avoid distortion and cracking during hardening heat treatment. Which quenching medium is most ideal?

A

Oil

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19
Q

Which temperature would you select to carry out the hardening heat treatment for a 0.8% carbon steel?

A

780 degrees C

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20
Q

Which tempering temperature would you recommend to temper shafts and axels after hardening?

A

400 degrees C

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21
Q

Which tempering temperature would you recommend to temper a newly hardened tool steel file?

A

200 degrees C

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22
Q

What are four main types of heat treatment processes?

A
  1. Annealing
  2. Normalising
  3. Hardening
  4. Tempering
23
Q

What is “heat treatment”?

A

The term that describes the
(1) heating and cooling of a metal or alloy
(2) in the solid state to
(3) obtain certain desired properties.

24
Q

What are the two most important parameters in heat treatment?

A

Temperature and cooling rate

25
Q

What is the general heat treatment cycle?

A
  1. Heating
  2. Hold at temperature (soaking)
  3. Cooling
26
Q

How may purposes are there in the annealing process? What are they?

A

Three.
1. Soften steels so that mechanical operations (e.g. machining, forging, wire drawing, deep drawing and extrusion) can be easily carried out
2. Improve the microstructure of steels
3. Relieve internal stresses induced by cold-working operations

27
Q

What is the brief description of annealing process?

A

The annealing process involves

(1) heating the steel to a certain temperature
(which depends on the types of process and steel) followed by

(2) very slow cooling (furnace cool) to room temperature.

*wdym by depends on the process is it the 3 different proccesses

28
Q

What are the 3 types of annealing process?

A
  1. Process (stress relief) annealing
  2. Spheroidising annealing
  3. Full annealing
29
Q

How many alternate names does process annealing have? What are they?

A

Three.
1. Stress-relief annealing
2. Interstage annealing
3. Sub-critical annealing

30
Q

Which metal is process annealing mainly used for?

A

Steel with carbon content less than 0.4% C

31
Q

Why is process annealing required?

A

When mild steel is cold worked, its grains/crystals become DISTORTED in the working direction.

The distortion in crystal structure will make the steel HARDER due to the effect known as work hardening.

so… then???

31
Q

Why process annealing is required?

A

When mild steel is cold worked, its grains/crystals become distorted in the working direction. The distortion in crystal structure will make the steel harder due to the effect known as work hardening.

As a result, it is more difficult to cold-work the steel further. More power is required & the work-hardened steel may crack.

Therefore, before any further cold-working can be carried out, the work-hardened steel must be softened by a treatment known as process annealing.

32
Q

How is process annealing carried out?

A

Process annealing is carried out by

  1. heating the steel to about 550 - 700 degrees C FOR ONE HOUR
  2. followed by furnace cool to
    induce recrystallisation on the distorted crystals of the cold worked steel & produce stress-free equiaxed crystals,

THEREFORE softening the steel for further cold working operations to be carried out.

33
Q

If no more working is required, should the steel go through process annealing?

A

No. Process annealing should be carried out on the cold worked steel only if further working is required on the steel. This is because steel is best left in the cold worked condition because cold working can impart strength and hardness to the steel components.

34
Q

What is spheroidised annealing applied to?

A

Spheroidised annealing is applied to carbon steels with high carbon contents (with carbon content more than 0.5%) to improve their machinability as well as ductility.

35
Q

Why is machining difficult for high carbon steels?

A

Because the tool bit must cut through the hard cementite layers present in the large amount of pearlite in high carbon steels.

36
Q

How does spheroidising annealing improve the machinability in high carbon steels?

A

Spheroidising annealing improves the machinability by converting these hard plates into round structures so that the tool bit can slide among them.

37
Q

How is spheroidising annealing carried out?

A
  1. hold the steel at a temperature between 650 - 700 degrees C for 24 hours or more.
  2. cool the steel slowly in the furnace
38
Q

How does the cementite become spheroids?

A

At the annealing temperature, the cementite plates break up into smaller plates which gradually become spheroids due to effect of surface tension.

39
Q

What is full annealing used for?

A

Full annealing is used to completely soften steel.

It is also carried out on cast steel to obtain uniform grain structures in as-cast steel as cast structure can cause brittleness.

40
Q

What are the microstructures after full annealing?

A

The microstructures after full annealing depends on the carbon content of the steel treated:

hypoeutectoid steels: ferrite & pearlite
eutectoid steels: pearlite
hypereutectoid steels: pearlite & cementite.

41
Q

What is the grain size after the process is carried out?

A

The grain size is coarser because of the very slow cooling in full annealing.

42
Q

How is full annealing carried out?

A
  1. heat the steel to an appropriate temperature.

what is an appropriate temperature?
for hypoeutectoid steels: 30 to 50 degrees C above UCT

42
Q

How is full annealing carried out?

A
  1. heat the steel to an appropriate temperature.

what is an appropriate temperature?
for hypoeutectoid steels: 30 to 50 degrees C above UCT
for hypereutectoid steels: 30 to 50 degrees C above LCT for hypereutectoid steels

  1. soaking (holding the steel) at that temperature long enough for RECRYSTALLISATION to occur
  2. steel is then cooled slowly in the furnace
43
Q

How many purposes does normalising have? What are they?

A

Four.

  1. To obtain a uniform fine grain structure in steel forgings
  2. To refine the grain structure and reduce segregation in castings.
  3. Before the hardening process in order to

1) obtain a uniform structure and to

2) avoid grain coarsening during hardening which can cause brittleness.

  1. For stress relieving between the rough machining and the finish machining of large components to avoid subsequent “movement” and loss of accuracy.
44
Q

How is the process carried out?

A
  1. Heat the steel to a suitable temperature above the transformation range.
  2. Hold the steel at that temperature for complete transformation to occur.
  3. Taking out and cooling slowly in still air to room temperature.
45
Q

What is the heating (soaking) temperature for normalising?

  • note that heating instead of holding is used here because the holding temperature and the heating temperature is the same.
A

30 to 50 degrees C above the upper critical temperature

46
Q

How is the full annealing process different from the normalising process?

A

For normalising, steel is cooled slightly faster in still air as opposed to furnace cooling in full annealing (therefore less time consuming and less expensive).

47
Q

How is the microstructure after normalising different from the microstructure after full annealing?

A

Microstructures after normalising have finer grains than the microstructures after full annealing because of the slightly faster cooling rate.

48
Q

What is the effect of the finer grains after normalising?

A

There will be an increase of strength and hardness after normalising.

49
Q

What is one key difference between full annealing and normalising?

A

In full annealing, the furnace imposes identical cooling conditions at all locations within the metal, which produces identical properties.

With normalising, the cooling will be different at different locations.
- Properties will vary between surface and interior
- Different thickness regions will also have different properties

** When subsequent processing involved a substantial amount of machining that may be automated, the added cost of a full anneal may be justified, since it produces a product with uniform machinability at all locations.

50
Q

How many purposes are there for the hardening process? What are they?

A
  1. Increase the hardness & wear resistance of steels (cutting tools, dies and moulds)
  2. Increase the strength (and toughness after tempering) of steels (crankshaft, gears, axles)
51
Q

How is the hardening process carried out?

A
  1. heat the steal to the required temperature for changes in structure within the material to occur (austenitic structure)
  2. hold at that temperature for a long enough time for the entire material to reach the required temperature and the structural changes to occur throughout the entire material. [grain size is affected by the length of the holding time]
  3. the metal is then cooled rapidly or quenched in water or oil to bring about the formation of a very hard structure called martensite.
    [temperature range is similar to that for full annealing]
52
Q

What are the principles of hardening? [logic behind hardening???]

A

background info kinda thing:
when a steel is heated to above its UCT, its structure become austenite: F.C.C. iron & can dissolve up to 1.7% carbon
[in contrast to B.C.C. iron which can only dissolve up to 0.006% carbon at room temperature]

Engineering Materials (4 decks)

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