Heat Treatment

Question 1

Describe the main features of Austenite

Answer 1

• 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

Question 2

What is the UCT and LCT

Answer 2

910, 723

Question 3

Describe the main features of Ferrite

Answer 3

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

Question 4

How much carbon % does cementite have

Answer 4

6.67% and the rest is iron

Question 5

Properties of Cementite

Answer 5

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

Question 6

What type of structure is Pearlite

Answer 6

• Eutectoid structure in steels

- Lamellar: layers of ferrite and cementite

Question 7

At what temperature does Pearlite form

Answer 7

• LCT, 723 degrees

Question 8

What can process annealing be used for

Answer 8

• Mild steels only (<0.3%C)

Question 9

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

Answer 9

Process Annealing

Question 10

What is the point of Process Annealing

Answer 10

• soften the steel to allow for more cold working

- creates equiaxed (unstressed) ferrite, pearlite stays elongated

Question 11

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

Answer 11

Full Annealing

Question 12

What are the Advantages and Disadvantages of Full Annealing

Answer 12

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

Disadvantages: -Slow process
- Costly

Question 13

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

Answer 13

Full Annealing

Question 14

What temperature do you heat the steel during Normalising

Answer 14

50 degrees above the UCT

Question 15

How is a steel cooled during normalising

Answer 15

cooled in still air (faster than cooling in furnace)

Question 16

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

Answer 16

Normalising, it is also a faster process than annealing

Question 17

What are the results of normalising

Answer 17

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

Question 18

At what temperature do we heat steels during spheroidisation

Answer 18

Just below the LCT

Question 19

What are the results of spheroidising

Answer 19

• 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.

Question 20

How is hardening achieved

Answer 20

rapid cooling (quenching)

Question 21

What is martensite

Answer 21

• 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.

Question 22

Which carbon phase has an acicular structure

Answer 22

Martensite.

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

Question 23

Describe the properties of Martensite

Answer 23

• Strong and hard, but brittle

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

Question 24

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

Answer 24

True

Question 25

Which heat treatment process usually follows hardening?

Answer 25

Tempering

Question 26

Describe the temperature's effect during tempering

Answer 26

- 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

Question 27

What is the result of Tempering

Answer 27

- Trade of the hardness and strength of martensite to gain toughness (less brittle) - Ductility increases

Question 28

What is the process of Austempering?

Answer 28

- 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

Question 29

What is bainite?

Answer 29

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

Question 30

Describe the case hardening process

Answer 30

- 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

Question 31

Describe the flame hardening process

Answer 31

- A steel's surface is heated with a flame and then quenched | - The area that is heated will be hardened

Question 32

Describe the induction hardening process

Answer 32

- 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

Question 33

Describe nitriding

Answer 33

- 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