WK17 - Fe-C | Martensite | Corrosion

Question 1

What are the 5 main phases in the Fe-C phase diagram?

Answer 1

Alpha-ferrite

Gamma-austenite

Delta-ferrite

Fe3C

Fe-C liquid solution (L)

Question 2

What are the main features of the alpha-ferrite phase?

Answer 2

1. Solid solution of C in BCC Fe
   • BCC size < FCC size hence higher C% as u go from BCC to FCC
2. Stable form of iron at room temp
3. Maximum carbon solubility - 0.022 wt.%
4. Transforms to FCC gamma austenite at 912C

Question 3

What are the main features of gamma-austenite phase? (Fe-C)

Answer 3

1. Solid solution of C in FCC Fe
   • more interstitial sites for C
2. Max carbon solubility - 2.14 wt.%
3. Transforms to BCC delta-ferrite @1395C
4. Not stable below eutectoid temperature 727C unless cooled rapidly

Question 4

What are the main features of the delta-ferrite phase? (Fe-C)

Answer 4

1. Solid solution of C in BCC Fe
   • same as alpha ferrite
2. Stable only above 1394C, melts at 1538C

Question 5

What are the main features of the Fe3C phase?

Answer 5

1. Metastable inter-metallic compound (IMC) (iron carbide or cementite)
   • IMC - any class of substance composed of definite proportions of two or more elements
2. Stable at room temp, but decomposes very slowly to alpha-ferrite and graphite at 650-750C

Question 6

What are the main difference between: Iron vs Steels vs Cast Iron?

Answer 6

Iron
• less than 0.008 wt.% carbon
• alpha-ferrite at room temp

Steels
• 0.008-2.14 wt.% C (usually < 1 wt.%)
• Alpha-ferrite + Fe3C at room temp
• 12-20% wt.% Cr for SS
Cast Iron
• 2.14 - 6.7 wt.% C (usually < 4.5 wt.% C)
• used for heavy equipment casting

Question 7

What is Pearlite and how does it form?

Answer 7

A lamellar structure of Fe3C and alpha-ferrite

Thickness of layers depends on rate of change of temperature

Just below eutectoid (temp): thick layers of coarse pearlite form

Rapid cooling (low T): thin layers of fine pearlite form

Question 8

What is bainite? How does it form?

Answer 8

Non-equilibrium transformation that occurs once the temperature fall too low for pearlite to form
• not enough energy for nice strips (pearlite) to form

It’s also a transformation of austenite, but occurs at medium temps below the eutectoid point

Same comp as pearlite (alpha plus Fe3C) but with acicular (needle-like) particle shapes

Question 9

How does spheroidite differ from bainite or pearlite?

Answer 9

Same composition as above however, Fe3C cemenite forms spherical shapes within an alpha-ferrite matrix

Spherical shape due to driving force to reduce the interfacial area between the phases

This is formed by heating pearlite or bainite at temps just below the eutectoid for long periods of time

Question 10

What is martensite? How does it form?

Answer 10

Quenching a Fe-C alloy doesn’t provide enough time for pearlite or bainite formation.

An instantaneous transformation occurs from FCC austenite to a BCT phase

Martensite grains nucleate and grow at an incredibly rapid rate
• appears as a horizontal straight line on the TTT curve at 215C
•dashed lines reveal percentage of material transformed to martensite by that temperature (lines independent of time)

Question 11

What is the structure of martensite?

Answer 11

Forms as laths within the previous austenite structure.
• a lot of residual stress in the system — small grains nor food when u have a lot of residual stress in the system

(Look at slides for picture)

Question 12

Why is tempering used on martensite?

Answer 12

Martensite is much harder (& more brittle) than pearlite

Cracking and internal stresses can form during P —> M transition due to volume change

Tempering improves ductility and toughness whilst annealing out internal stresses
• taken to temp just below eutectoid temp (keep same crystal/grain structure but reduces internal stresses)
• Martensite partially decomposes into separate ferrite and cementite phases
•• unit cell of M is stretched —> tempering allows Carbon to leave unit cell —> cell returns to its original size (removes internal energy)

Question 13

What is corrosion? Why are metals susceptible?

Answer 13

Destructive and unintentional degradation of a material caused by its environment.

Most metals favourable energetic state is in oxide form, apart from gold.

Question 14

What is dry corrosion? What relationships do the constants form?

Answer 14

The corrosion product (typically oxide) forms at the site of the corrosion, and forms uniformly.

Controlled by the rate of movement of ions/electrons through the oxide at the surface.

At low temps, even a small layer of oxide at the surface can provide a protective barrier to further oxidation.

Temperature dependent following the Arrhenius law —> as temps rise oxidation rises exponentially

Question 15

What are the mechanisms behind wet corrosion?

Answer 15

Electrochemical attack by moisture means that wet corrosion is much faster than dry

Moisture enables electron transfer:
• removing electrons from metal surface — oxidation
• these electrons are used to generate ions — reduction

> anodic region — M+ ions are formed
Cathodic region — where OH- ions are formed
• voltage difference between them due to the transfer of electrons

Question 16

What are REDOX potentials?

Answer 16

Potential required to either reduce or oxidise
• important to the rate and mechanism of the corrosion that occurs
•• +ve = reduction
•• -ve = oxidation

More negative a metals potential, the faster it will corrode

Question 17

What is Erosion cracking (EC), Pitting (P) and Crevice?

Answer 17

EC - combined chemical attack and mechanical wear (at pipe elbows)

P - Downward propagation of small pits and holes as a result of localised attack at defects on the surface
• species within the whole get trapped
• harder for fluid to escape
• overtime holes get more aggressive

C - in narrow and confined spaces, the concentration of corrosive species can be higher

Question 18

What are galvanic couples and why are they a problem

Answer 18

A corrosive cell that is developed when two different metals are separated by electrolytes.

It’s the electrochemical action that is produced by the reaction of two dissimilar metals, given there is a path conducive to electrons and electrolytes

Higher resistance metal turns cathodic whilst the other becomes anodic (typically cathode undergoes little corrosion)

Question 19

What is intergranular corrosion?

Answer 19

Microstructure of the grain boundary is more susceptible to attack, and so the corrosion spreads along the edges of the grain.

Question 20

What is stress corrosion cracking?

Answer 20

Cracks started from a corroded region can propagate through a material under stress. The crack continues to corrode into the material, growing and pushing the crack apart.

Question 21

What are the main corrosion prevention techniques (6)

Answer 21

(1) material selection - use metals that are unreadrife in the corrosive environment (Ni great in basic solution, poor in acid)

(2) Passivate metals - metals that form a thin, adhering oxide layer that slows corrosion (much less permeable, Al SS)

(3) Lower temps

(4) physical barriers - films and coatings

(5) sacrificial materials - more cathodic which will corrode first

(6) inhibitors - substances which decrease reactivity, slow oxidation/reduction reaction by removing reactants etc

Question 22

How does chromium in SS protect it from corrosion?

Answer 22

Cr reacts quickly with oxygen to form a CrO surface layer

(Unlike FeO which keeps growing), CrO layer is unstable and doesn’t grow further than a few 100nm

Question 23

What is tribology and how can its effects be overcome?

Answer 23

Tribology - study of wear

When two surfaces are in contact and in motion, the effects of friction can lead to the removal of material (wear)

Typically add lubricant to reduce friction and so create less wear

Question 24

What are the 4 main types of wear?

Answer 24

(1) adhesive wear - two surfaces bond together, and the plastic deformation causes the weaker surface to pull away.

(2) abrasive wear - If one surface is harder, it can sit into the other surface and plough away material.

(3) Fatigue wear - Repeated contact can lead to build up of cracks and damage due to fatigue

(4) corrosive wear - in the presence of corrosive liquid and gases, wear can be enhanced by chemical reactions weakening the surface.

Question 25

Who do things expand as they get hotter?

Answer 25

Amplitude of atomic oscillations increases
• symmetric well - Avg doesn’t change
• Asymmetric well - higher oscillations results in an increase of the average interatomic separation (more common in polymers and metals)

Stronger interatomic bonding —> deeper the well — less this effect matters

Question 26

What does radiation damage do to the material?

Answer 26

Leads to segregation and diffusion of atoms — leading to phase segregation and voids as well as enhancing the effects of SCC

Can cause the expansion of the crystal lattice due to defect creation
• lattice can be completely destroyed creating an amorphous structure