Heat Treatment of Metals Flashcards
Why do we heat treat materials?
- 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
Describe the relationship between temperature, time, and heat treatment.
Temperature and time of heat treatment will affect the
microstructure and material properties
Describe austenitize
Heat to a temperature region where
100% Austenite is formed
* Hold to allow complete transformation
Describe quenching?
- Use water or oil to rapidly cool
- Form non-equilibrium, hard
microstructures
Describe the tempering of steel
- Reheat to 200 - 550°C
- Decreases hardness, regain some
ductility
What happens during an isothermal process for quenching steel?
once quenched, temperature is held constant
throughout the process (note that in reality, materials are often
continuously cooled)
What is bainite?
A non-equilibrium needle-like microstructure with thin needles of cementite in a ferrite matrix.
When is bainite formed?
Bainite is formed when cooled
to a low enough temperature
(high cooling rate) and quenched to a temperature between 210-520oC.
What is martensite?
hard, brittle microstructure formed by diffusionless rapid cooling
When is martensite formed?
If cooled VERY rapidly (such as by
quenching into water), there is no
time for diffusion and martensite
is formed
What is the lattice structure for martensite?
Body
Centred Tetragonal (BCT)
structure
What material is required to form pearlite, bainite, and martensite?
Austenite
What is spheroidite?
Microstructure formed by tempering pearlite
How is austenite related to various other microstructures?
The starting material for many microstructures.
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?
Increase in temperature difference of Te and Th = shorter time required to begin transformation into pearlite = greater rate of pearlite transformation
What is the rate of transformation of austenite to pearlite dependent on?
Temperature
What does cooling to temperatures further from the eutectoid isothermal temperature result in?
Rapid cooling and faster transformation times
What is the relationship between the holding temperature and the cooling rate?
The lower the holding temperature you cool to, the faster the cooling rate.
What times can you extract from each isothermal transformation diagram?
-Start time
-50% completion time
-End time
What is a convenient representation of multiple temperatures for the transformation of austenite to pearlite?
A time-temperature transformation (TTT) diagram
What does slow cooling of austenite result in?
A thicker lamellae structure that is coarse pearlite.
What does rapid cooling of austenite result in?
A thinner lamellae structure that is fine pearlite.
What must diffuse to form distinct pearlite layers?
Carbon
What happens if the sample is quenched to a sufficiently low temperature(ie. high cooling rate)?
There is little time for diffusion to occur.
Is martensitic transformation unique to steel?
No. Martensitic transformation is not unique to steel and can be formed in other systems.
What is the Body Centred Tetragonal Structure (BCT)?
A lattice “stuck” between FCC and BCC.
Describe the physical properties of the microstructure martensite
-Hard
-Brittle
-Supersaturated
-Unstable
How does martensite compare to other microstructures in steel in terms of hardness, strength, and ductility?
Martensite is the most brittle, hardest, and strongest (high yield strength) microstructure in steel.
How is martensite formed?
By a diffusionless process.
What is tempering?
Reheating a microstructure to moderate temperatures after quenching to regain ductility and toughness.
What is the result of reheating martensite during tempering?
Tempered martensite
What type of process forms tempered martensite?
By a diffusion-governed process
What is the relationship between yield strength, tensile strength, tempering temperature, and reduction in area?
Decrease in tensile strength = decrease in yield strength = increase in reduction in area = increase in tempering temperature.
What phase transformation occurs when martensite is heated from 200-550 °C?
BCT (alpha´) phase transforms to BCC (alpha)
phase
What are all of the strengthening mechanisms we have covered so far?
- Grain Size Reduction
- Cold Working
- Solid Solution Hardening
- Heat treatment
* Due to Pearlite
* Due to Martensite (Quenching and Tempering)
What is the relationship between % cementite, % wt carbon composition, and hardness of materials (fine pearlite, coarse pearlite, and spheroidite)?
As % carbon wt composition increases and % cementite composition increases, the hardness of these materials increases.
What is the relationship between % cementite, % wt carbon composition, and the ductility of these materials (fine pearlite, coarse pearlite, and spheroidite)?
As the % carbon wt composition increases and % cementite composition increases, the ductility of these materials decreases.
What is the continuous phase in spheroidite?
alpha ferrite
What does spheroidite look similar to?
Tempered martensite
What do the cementite layers of spheroidite evolve into?
A dispersion of spheres throughout the metal.
What are the following possible transformations of austenite?
Austenite → Pearlite
Austenite → Bainite
Austenite → Martensite
Martensite → Tempered martensite
Compare the size of cementite particles in tempered martensite to those in spheroidite.
Cementite particles in tempered martensite are
smaller than those in spheroidite
Compare the phase boundary area of tempered martensite to spheroidite
Tempered martensite contains greater phase
boundary area between cementite and ferrite
phases, as compared to spheroidite
Compare the strength and hardness of spheroidite to tempered martensite.
Tempered martensite is stronger and harder than spheroidite.
Rank the steel microstructures from lowest to highest ductility
- Martensite (lowest ductility)
- Tempered martensite
- Fine pearlite
- Coarse Pearlite
- Spheroidite (Highest Ductility)
Why does high cooling rate generate finer pearlite microstructure?
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.
