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Engineering > Heat Treatment > Flashcards

Heat Treatment Flashcards

1
Q

Describe the main features of Austenite

A
  • FCC iron with carbon dissolved in it
  • Not stable below the UCT
  • Can exist in room temperature with other elements added to steel such as large amounts of manganese
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2
Q

What is the UCT and LCT

A

910, 723

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3
Q

Describe the main features of Ferrite

A
  • BCC iron with tiny amounts of carbon dissolved in it
  • Soft, ductile and tough
  • The more carbon content there is, the less ferrite present
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4
Q

How much carbon % does cementite have

A

6.67% and the rest is iron

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5
Q

Properties of Cementite

A
  • intermetallic compound
  • hard and brittle
  • the properties of steel depend on how much cementite there is
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6
Q

What type of structure is Pearlite

A
  • Eutectoid structure in steels

- Lamellar: layers of ferrite and cementite

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7
Q

At what temperature does Pearlite form

A
  • LCT, 723 degrees
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8
Q

What can process annealing be used for

A
  • Mild steels only (<0.3%C)
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9
Q

Which heat treatment process provides stress relief by removing cold working effects

A

Process Annealing

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10
Q

What is the point of Process Annealing

A
  • soften the steel to allow for more cold working

- creates equiaxed (unstressed) ferrite, pearlite stays elongated

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11
Q

Which heat treatment process involves heating steel to well above UCT before slowly cooled in a furnace

A

Full Annealing

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12
Q

What are the Advantages and Disadvantages of Full Annealing

A

Advantages: - Creates equiaxed structure with large ferrite grains and coarse pearlite
- Produces the SOFTEST steel

Disadvantages: -Slow process
- Costly

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13
Q

Which heat treatment process would be used to remove undesirable microstructural effects after casting

A

Full Annealing

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14
Q

What temperature do you heat the steel during Normalising

A

50 degrees above the UCT

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15
Q

How is a steel cooled during normalising

A

cooled in still air (faster than cooling in furnace)

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16
Q

If you want to produce a stronger steel, would you choose annealing or normalising

A

Normalising, it is also a faster process than annealing

17
Q

What are the results of normalising

A
  • equiaxed grain structure with small ferrite grains and fine pearlite
  • often done to refine grain structure
18
Q

At what temperature do we heat steels during spheroidisation

A

Just below the LCT

19
Q

What are the results of spheroidising

A
  • Causes the cementite layers in the pearlite to break up and form rough spheres
  • This is done to make steel easier to machine as the long cementite band will no longer exist.
20
Q

How is hardening achieved

A

rapid cooling (quenching)

21
Q

What is martensite

A
  • A distorted Body Centred Tetragonal (BCT) structure
  • This is caused by trapping carbon atoms within an FCC interstitial site during quenching and distorting the normal BCC structure.
22
Q

Which carbon phase has an acicular structure

A

Martensite.

Acicular structure is a microstructure of ferrite which is conveyed by needle-shaped crystallites or grains when viewed in 2D

23
Q

Describe the properties of Martensite

A
  • Strong and hard, but brittle

- Great surface finish but not desirable for the structure as a whole

24
Q

Martensite can only be formed if there is >0.3% carbon. True or False.

A

True

25
Q

Which heat treatment process usually follows hardening?

A

Tempering

26
Q

Describe the temperature’s effect during tempering

A
  • Tempering is usually done from 200-600 degrees Celcius
  • This heat allows the martensite created from hardening to break down into ferrite and cementite
  • The greater the temperature, the more martensite breaks down
27
Q

What is the result of Tempering

A
  • Trade of the hardness and strength of martensite to gain toughness (less brittle)
  • Ductility increases
28
Q

What is the process of Austempering?

A
  • A heat treatment process that involves quenching to an elevated temperature
  • Steel is heated to above the UCT then quenched to 400 degrees celsius in a salt bath
  • It is held at 400 degrees celsius to allow austenite to change into bainite
29
Q

What is bainite?

A
  • Cementite plates or particles in a ferrite matrix
  • Similar properties to tempered martensite
  • Cannot be formed by regular cooling processes
30
Q

Describe the case hardening process

A
  • Involves heating and soaking steel in a carbon-rich atmosphere
  • Carbon will diffuse into the surface of the steel which increases the carbon content, hence increasing hardness and tensile strength
31
Q

Describe the flame hardening process

A
  • A steel’s surface is heated with a flame and then quenched

- The area that is heated will be hardened

32
Q

Describe the induction hardening process

A
  • Similar to flame hardening but induction coil is used instead of flame.
  • An electric current is turned on to increase the temperature of the steel’s surface to the desired range before quenching to harden it
33
Q

Describe nitriding

A
  • Surface hardening process where special alloy steel is heated in a furnace with gaseous nitrogen present
  • Offers high hardness in the core, Clean finish, Corrosion-resistant surface
  • If the steel is heated above 500 degrees Celcius after nitriding, the hardness will be permanently lost

Engineering (5 decks)

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