Heat Treatment of Metals

Question 1

Why do we heat treat materials?

Answer 1

• To relieve stresses caused by dislocations and grain boundaries – e.g
  from cold working or quenching
• To produce specific properties (such as yield strength, hardness,
  ductility and toughness) by manipulating the microstructure
• To use non-equilibrium heating and cooling to create microstructures
  which do not appear on the phase diagram (i.e. meta-stable phases),
  some of which may have highly desirable properties

Question 2

Describe the relationship between temperature, time, and heat treatment.

Answer 2

Temperature and time of heat treatment will affect the
microstructure and material properties

Question 3

Describe austenitize

Answer 3

Heat to a temperature region where
100% Austenite is formed
* Hold to allow complete transformation

Question 4

Describe quenching?

Answer 4

• Use water or oil to rapidly cool
• Form non-equilibrium, hard
  microstructures

Question 5

Describe the tempering of steel

Answer 5

• Reheat to 200 - 550°C
• Decreases hardness, regain some
  ductility

Question 6

What happens during an isothermal process for quenching steel?

Answer 6

once quenched, temperature is held constant
throughout the process (note that in reality, materials are often
continuously cooled)

Question 7

What is bainite?

Answer 7

A non-equilibrium needle-like microstructure with thin needles of cementite in a ferrite matrix.

Question 8

When is bainite formed?

Answer 8

Bainite is formed when cooled
to a low enough temperature
(high cooling rate) and quenched to a temperature between 210-520oC.

Question 9

What is martensite?

Answer 9

hard, brittle microstructure formed by diffusionless rapid cooling

Question 10

When is martensite formed?

Answer 10

If cooled VERY rapidly (such as by
quenching into water), there is no
time for diffusion and martensite
is formed

Question 11

What is the lattice structure for martensite?

Answer 11

Body
Centred Tetragonal (BCT)
structure

Question 12

What material is required to form pearlite, bainite, and martensite?

Answer 12

Austenite

Question 13

What is spheroidite?

Answer 13

Microstructure formed by tempering pearlite

Question 14

How is austenite related to various other microstructures?

Answer 14

The starting material for many microstructures.

Question 15

What is the relationship between the temperature difference between the eutectoid temperature(Te) and the holding temperature (Th), rate of transformation of pearlite and the time required to form pearlite?

Answer 15

Increase in temperature difference of Te and Th = shorter time required to begin transformation into pearlite = greater rate of pearlite transformation

Question 16

What is the rate of transformation of austenite to pearlite dependent on?

Answer 16

Temperature

Question 17

What does cooling to temperatures further from the eutectoid isothermal temperature result in?

Answer 17

Rapid cooling and faster transformation times

Question 18

What is the relationship between the holding temperature and the cooling rate?

Answer 18

The lower the holding temperature you cool to, the faster the cooling rate.

Question 19

What times can you extract from each isothermal transformation diagram?

Answer 19

-Start time
-50% completion time
-End time

Question 20

What is a convenient representation of multiple temperatures for the transformation of austenite to pearlite?

Answer 20

A time-temperature transformation (TTT) diagram

Question 21

What does slow cooling of austenite result in?

Answer 21

A thicker lamellae structure that is coarse pearlite.

Question 22

What does rapid cooling of austenite result in?

Answer 22

A thinner lamellae structure that is fine pearlite.

Question 23

What must diffuse to form distinct pearlite layers?

Answer 23

Carbon

Question 24

What happens if the sample is quenched to a sufficiently low temperature(ie. high cooling rate)?

Answer 24

There is little time for diffusion to occur.

Question 25

Is martensitic transformation unique to steel?

Answer 25

No. Martensitic transformation is not unique to steel and can be formed in other systems.

Question 26

What is the Body Centred Tetragonal Structure (BCT)?

Answer 26

A lattice “stuck” between FCC and BCC.

Question 27

Describe the physical properties of the microstructure martensite

Answer 27

-Hard
-Brittle
-Supersaturated
-Unstable

Question 28

How does martensite compare to other microstructures in steel in terms of hardness, strength, and ductility?

Answer 28

Martensite is the most brittle, hardest, and strongest (high yield strength) microstructure in steel.

Question 29

How is martensite formed?

Answer 29

By a diffusionless process.

Question 30

What is tempering?

Answer 30

Reheating a microstructure to moderate temperatures after quenching to regain ductility and toughness.

Question 31

What is the result of reheating martensite during tempering?

Answer 31

Tempered martensite

Question 32

What type of process forms tempered martensite?

Answer 32

By a diffusion-governed process

Question 33

What is the relationship between yield strength, tensile strength, tempering temperature, and reduction in area?

Answer 33

Decrease in tensile strength = decrease in yield strength = increase in reduction in area = increase in tempering temperature.

Question 34

What phase transformation occurs when martensite is heated from 200-550 °C?

Answer 34

BCT (alpha´) phase transforms to BCC (alpha)
phase

Question 35

What are all of the strengthening mechanisms we have covered so far?

Answer 35

1. Grain Size Reduction
2. Cold Working
3. Solid Solution Hardening
4. Heat treatment
   * Due to Pearlite
   * Due to Martensite (Quenching and Tempering)

Question 36

What is the relationship between % cementite, % wt carbon composition, and hardness of materials (fine pearlite, coarse pearlite, and spheroidite)?

Answer 36

As % carbon wt composition increases and % cementite composition increases, the hardness of these materials increases.

Question 37

What is the relationship between % cementite, % wt carbon composition, and the ductility of these materials (fine pearlite, coarse pearlite, and spheroidite)?

Answer 37

As the % carbon wt composition increases and % cementite composition increases, the ductility of these materials decreases.

Question 38

What is the continuous phase in spheroidite?

Answer 38

alpha ferrite

Question 39

What does spheroidite look similar to?

Answer 39

Tempered martensite

Question 40

What do the cementite layers of spheroidite evolve into?

Answer 40

A dispersion of spheres throughout the metal.

Question 41

What are the following possible transformations of austenite?

Answer 41

Austenite → Pearlite
Austenite → Bainite
Austenite → Martensite
Martensite → Tempered martensite

Question 42

Compare the size of cementite particles in tempered martensite to those in spheroidite.

Answer 42

Cementite particles in tempered martensite are
smaller than those in spheroidite

Question 43

Compare the phase boundary area of tempered martensite to spheroidite

Answer 43

Tempered martensite contains greater phase
boundary area between cementite and ferrite
phases, as compared to spheroidite

Question 44

Compare the strength and hardness of spheroidite to tempered martensite.

Answer 44

Tempered martensite is stronger and harder than spheroidite.

Question 45

Rank the steel microstructures from lowest to highest ductility

Answer 45

1. Martensite (lowest ductility)
2. Tempered martensite
3. Fine pearlite
4. Coarse Pearlite
5. Spheroidite (Highest Ductility)

Question 46

Why does high cooling rate generate finer pearlite microstructure?

Answer 46

Because there is limited time for atomic diffusion to occur, the atoms in the alloy diffuse across small distances, giving rise to thin layers of cementite and ferrite.