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MAT2600 > Steel > Flashcards

Steel Flashcards

1
Q

What do Fine ferrite grain sizes with martensite islands leads to?

A

High strength (~800MPa) and good formability

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2
Q

Examples of face centred cubic crystal structures:

A 
Copper
Silver
Gold
Platinum
Nickel
Lead
Aluminium
Cobalt (b)
Iron (g)
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3
Q

Examples of body centred cubic crystal structures:

A 
Chromium
Molybdenum
Niobium
Vanadium
Titanium (b)
Tungsten
Iron (a, d)
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4
Q

Examples of Hexagonal-Close-Packed crystal structures:

A 
Beryllium
Magnesium
Zinc
Zirconium
Titanium (a)
Cadmium
Cobalt (a)
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5
Q

What are atomic scale defects known as in microstructure?

A

Point defects

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6
Q

What are microscopic line defects also known as?

A

Dislocations

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7
Q

What are some types of line defects?

A

Screw dislocations
Mixed dislocations
Edge dislocations

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8
Q

What are micro/macroscopic planar defects also known as?

A

Grain boundaries

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9
Q

What are some examples of planar defects?

A

Stacking faults
Twin boundaries
Grain boundaries

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10
Q

What are some examples of volume defects?

A

Precipitates
Voids/porosity
Inclusions/particles

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11
Q

What scale is metallurgy studied at?

A

10^-9 - 10^-6

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12
Q

What is a completely soluble phase?

A

One liquid phase

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13
Q

What is partial solubility?

A

Two phases; a liquid and a solid

e.g salt solution

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14
Q

What is a compound formation?

A

Two phases; a liquid and a solid

e.g a separate phase

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15
Q

What are the insolubility of phases?

A

Two liquid phases

E.g water and oil

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16
Q

How are phases distinguished?

A

-by their state of aggregation, crystal structure, composition, and/or degree of order
(i.e., atomic, magnetic)
-Phases are delineated in space by a definite boundary
(interface or interphase), and are in principle, mechanically
separable from the whole

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17
Q

Whats is the definition of a phase transformation?

A

Any re-arrangement within the assembly of atoms or molecules which carries the system from one configuration to a lower order of energy.

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18
Q

What are unary systems?

A

It contains one component; e.g pure iron (Fe)

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19
Q

When does unlimited insolubility occur?

A
  • When only one solid phase will be produced
    regardless of the ratio between solvent and solute
  • Represented by a binary isomorphous system
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20
Q

When does limited solubility occur?

A
  • When one phase can only tolerate some of the additional solute.
    -Describes the extent to which one phase can tolerate the
    addition of a solute
    -In general, the solid solubility will increase with temperature
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21
Q

What is solid solution strengthening?

A

-The addition of solute elements to a phase (solvent) gives rise to a solid solution
-In doing so, local dislocations are introduced to the lattice
-Localised stress fields which interact with dislocations
-Strengthening effect
(Extent of strengthening will be a function of the size of solute and the amount of the solute)

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22
Q

What are the effects on the properties to add a strengthening solute?

A
  • YS, TS and hardness increase
  • Ductility (%RA, %elong) decrease
  • Resistivity increases
  • Creep resistance increases
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23
Q

What are the equations for weight% composition in a binary system?

A

at%A + at%B = 100

wt%A + wt%B = 100

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24
Q

What do isomorphous phases diagrams show?

A
  • Simplest type of binary phase equilibria
  • Shows phases (and their compositions) at any combination of temperature and composition
  • In an isomorphous system, only one solid phase will be present
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25
Q

Liquidus

A

Defines temperature above which 100% liquid would be present for any given composition

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26
Q

Solidus

A

Defines temperature below which 100% solid would be present for any given composition

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27
Q

What does the temperature difference between liquidus and solidus show?

A

The freezing range of an alloy

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28
Q

What is a tie line?

A

Tie line is a horizontal line at the temperature of interest in a two phase region; The ends of the tie line will give the
composition of both liquid and solid phases

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29
Q

What does the Lever rule find?

A

Relative amounts of L and alpha in two phase region

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30
Q

What is solidification of a solid solution alloy?

A
  • Nucleation and growth process; nucleation is heterogeneous and requires little undercooling.
  • Solidification begins as the liquidus
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31
Q

What are the two conditions required for the growth of solid (alpha?

A
  • Latent heat of fusion must be removed from the solid/liquid interface
  • Diffusion must occur so that the compositions of the solid and liquid phases follow the solidus and liquidus during cooling
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32
Q

Latent heat

A

Latent heat of fusion is removed over a range of temperatures, which is why the cooling curve exhibits a
change in slope rather than a flat plateau (as is the case for a pure metal)

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33
Q

What is non-equilibrium solidification?

A

Occurs when cooling is too rapid for atoms to diffuse (e.g. partition) to yield equilibrium microstructures

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34
Q

What is segregation?

A

Non-uniform composition produced by non-equilibrium solidification is known as segregation

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35
Q

What is microsegregation?

A
  • Occurs over short distances (e.g., between dendrite arms)

- Reduced or eliminated by using a homogenisation heat treatment

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36
Q

What is macrosegregation?

A
  • Occurs over large distances (e.g., surface and centre of casting)
  • Since distances are too great, cannot be reduced by homogenisation
  • Can, however, be reduced during processing (e.g., hot forging, rolling, extrusion, etc.)
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37
Q

When does precipitation/ dispersion strengthening occur?

A

When the solubility of one material is exceeded by the addition of too much of a given alloy element

  • > Occurs in systems of limited solubility
  • > Gives rise to the formation of a second phase (and the creation of an interphase boundary)
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38
Q

What is a interphase boundary?

A

interphase boundary separates two different phases which may have different composition, crystal structure and/or lattice parameter
Interphase boundary interferes with the movement of dislocations → Strengthening mechanism

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39
Q

What is present in a precipitation/ strengthening phase?

A

More than one phase is present

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40
Q

What is a matrix phase?

A

Continuous phase which is usually present in larger amounts

41
Q

What is precipitate

A

Second phase usually present is smaller amounts

42
Q

What does microconstituent mean? And give an example.

A

Both phases form concurrently with one another, such that
they exist in nearly equal proportions
For example Pearlite (alpha Fe3C in steel)

43
Q

What happens when the tendency for atoms to cluster increases?

A

The solid solubility becomes more limited, which in turn changes the phase equilibria and finally, the development of a eutectic phase diagram

44
Q

Solubility limit is shown by…

A

Solvus line

45
Q

What happens during solidification?

A
  • During solidification, the proeutectic constituent (α or β) is nucleated once we cross the liquidus
  • Once eutectic isotherm is crossed, lamellar (α+β) forms from the remaining liquid
46
Q

What is the strength of eutectic influenced by?

A
  1. Amount of eutectic phase
    - Maximum strength observed for 100% eutectic (other factors being equal
  2. Microstructure of eutectic phase
    - Influenced primarily through cooling rate
    - As cooling rate increases, the interlamellar spacing (λ) of the eutectic decreases
47
Q

As λ↓, large interfacial area/unit volume, causing…

A

Increase in strength

48
Q

What is a necessary condition of precipitation strengthening?

A

Decreasing solid solubility with decreasing temperature is a necessary condition

49
Q

What are the three stages in the design of a heat treatment for precipitation strengthening?

A
  • Solution treatment
  • Quenching
  • Ageing
50
Q

What happens in a solution treatment?

A
  • Alloy is heated above solvus temperature and held until a homogeneous solid solution (single phase) is produced
  • This step ensures that alloy elements are in the solution.
51
Q

What happens in quenching?

A
  • Little time for diffusion results in a supersaturated solid
  • Represents a condition of unstable treatment
52
Q

What happens in ageing?

A

The supersaturated solid solution is heated to and held at a given temperature below the solvus.
Natural ageing; hold at room temperature
Artificial ageing; hold at some temperature larger than room temperature but lower than solvus temp

53
Q

What happens to a supersaturated microstructure?

A

Supersaturated microstructure is unstable and decomposes (e.g.precipitates) as a new phase
• Precipitates initially nucleate at defects, and often nucleate with coherent interfaces (low γ)

54
Q

Eutectoid reaction

A

Solid state reaction upon cooling s1 -> s2 + s3

It forms the basis for heat treatment of steel

55
Q

Definition of steel

A

Alloys based upon the Fe-C binary phase diagram which contain anywhere from trace amounts of C, up to 1.2wt% C.

56
Q

Ferrite (α)

A
  • Solid solution of C in α-Fe
  • BCC crystal structure
  • Maximum solubility of C in α is 0.02wt% at 727
  • Solubility of C in α decreases with decreasing temp (0.008wt% at 0 degrees)
  • Curie temp at 768
  • Above 768 is Paramagnetic
  • Below 768 is Ferromagnetic
57
Q

Austenite (γ)

A

Solid solution of C in γ-Fe
FCC crystal structure
Maximum solubility of C in γ is 2.1wt% at 1147
This is 2 orders in magnitude greater than α and forms the basis for strengthening steel

58
Q

What is cementite? (Fe3C)

A

Intermetallic compound of fixed composition (6.67%wt C)

Orthorhombic crystal structure with 12 Fe and 4C atoms per unit cell

59
Q

What is Ferrite-δ?

A

Solid solution of C in δ-Fe
BCC crystal structure (different lattice parameter then α-Fe)
Maximum solubility of C in δ is 0.09wt% at 1495

60
Q

What are the steel designations?

A
  • First two digits refer to the major alloy

- Last two or three digits give the C concentration in wt%

61
Q

What part of the phase diagram concerns steel?

A

The Eutectoid Reaction

- Alloys based upon the Fe-Fe3C binary phase diagram which contain anywhere from trace amounts of C up to 1.2wt%.

62
Q

What is pearlite?

A

Its not a phase!

Its the microconstituent of α and Fe3C which forms a lamellar structure.

63
Q

What is the Eutectoid reaction for steel?

A

γ(0.8%C) –> α(0.02%C) + Fe3C (6.67%C)

64
Q

Image of hypoeutectoid steels

A

Pearite = white areas

Proeutectoid Ferrite = Dark areas

65
Q

What are the two types of phase transformations in steel?

A
  1. Reactions near equilibrium conditions
    - Nucleations and growth processes i.e. requires diffusion
  2. Reactions removed from equilibrium conditions
    - Equilibrium phase diagram is not adequate to describe these reactions
    - Requires the use of TTT diagrams
66
Q

What do Time-Temperature-Transformation (TTT) diagrams graphically represent

A

The precipitation kinetics for the transformation products associated with the decomposition of γ
-> Needs to be repeated over a range of temperatures

67
Q

Why is transformation temperature important?

A

It affects both the kinetics and the microstructure, which in turn influences the final properties

68
Q

What is Martensite?

A
  • Martensite is a phase that forms as a result of a diffusionless solid state transformation
  • The transformation is said to be athermal, as it is only a function of temperature
  • Transformation occurs at or near the speed of sound
  • Martensite is very hard and brittle, and is therefore not very useful as an engineering material in the as-quenched form.
69
Q

What is the Martensitic transformation?

A
  • Martensitic transformations occur in both ferrous alloys and some non-ferrous alloys (e.g,Co, Ti- Ni, shapes memory alloys)
  • Martensitic transformation is a non-equilibrium transformation
  • α’ is a non-equilibrium structure (metastable phase)
70
Q

Dual phase steel (Martensite in as-quenched form):

A
  • Microstructure consisting of islands of martensite (hard & brittle) in a ferrite matrix (soft & ductile)
  • This results in composite-type behavior and gives rise to high rates of work hardening & excellent formability
71
Q

What does Intercritical anneal mean?

A

The dual phase steels are heat treated in the two phase (α +γ) region

  • The temperature within the (α +γ) phase field fixes the amount of α formed (from the lever rule) and the composition of both α and γ (from ends of tie line)
  • Fraction of γ which remains can thus form α’ upon quenching to room temperature
72
Q

How can sensitisation be minimised?

A

Cooling the steel rapidly

73
Q

What happens during the cold working of a metal (work hardening)?

A

The strength and hardness increase and the ductility decreases

74
Q

Ductile Fracture

A
  • Slow Failure
  • High energy mode of failure
  • dimpled fracture surface
75
Q

Brittle Failure

A
  • Fast rapid failure, with little or no warning
  • Low energy mode of failure
  • results in cleavage or facetted surface
76
Q

What is the tempering of Martensite?

A

The process of heating martensitic steel to a temperature below the A1 to make it more soft and ductile

77
Q

What are the microstructural changes during tempering?

A
  • Segregation of C atoms
  • Precipitation of carbides (Fe3C)
  • Decomposition of retained γ to bainite
  • Recovery and recrystallisation of matrix
  • Can also have the precipitation of alloy carbides (e.g., Mo, Cr, Ti, Nb, V) in alloy steels
78
Q

What is the name given to the precipitation of alloy carbides in alloy steels?

A

Secondary hardening

79
Q

Describe what is meant by Ideal Diameter

A

the largest bar diameter for a steel composition that can be quenched to produce 50% martensite at the center after ideal quench

80
Q

What is the difference between eutectic and eutectoid reactions?

A 
Eutectic = Liquid to two solids L -> S1 + S2
Eutectoid = Solid to two solids S1 -> S2 +S3
81
Q

What is ideal diameter a function of?

A
  • Grain size
  • Carbon concentration
  • alloy elements
82
Q

What is an ‘Ideal quench’?

A

A quench for which there is no resistance to heat transfer from the bar to the quenching medium, so the surface comes immediately from the temperature of the bath.

83
Q

Pearlite is…

A

A two phase lamellar microstructure consisting of ferrite and cementite

84
Q

What is the difference between eutectic and eutectoid reactions?

A 
Eutectic = Liquid to two solids L -> S1 + S2
Eutectiod = Soild to two solids S1 -> S2 +S3
85
Q

What is a monotectic reaction?

A

Monotectic = Liquid to a solid and a liquid L1 -> S1 +L2

86
Q

What is the difference between a peritectic and peritectoid reaction?

A 
Peritectic = Liquid and a solid to a solid L + S1 -> S2
Peritectoid = Solid and a solid to a solid S1 + S2 -> S3
87
Q

What is a cored microstructure :P

A

Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions. This causes the exterior of the material to solidify before the interior. … Coring is predominantly observed in alloys having a marked difference between liquidus and solidus temperatures.

88
Q

Transformation kinetics will change with alloy elements by…

A

Most alloying elements will retard the transformation kinetics (i.e., pushes the C-curve towards longer times)
-Not as pure?

89
Q

Transformation kinetics will change with Austenite grain size by…

A

As Dγ increases, the transformation kinetics become slower (i.e., pushes C-curve towards longer times)
• Less grain boundary area/unit volume
• Fewer nucleation sites

90
Q

What is a hardenability test?

A

Jominy test

-Quench at bottom; changes in hardness depending on how quickly its cooled down

91
Q

smaller the austenite grain size…

A

Increase in hardenability

92
Q

Is more carbon an increase or decrease in hardenability when quenching?

A

Increase

93
Q

Larger section size…

A

Bigger range of hardenability

94
Q

Describe critical diameter

A

The diameter for a steel composition and quench that would harden to 50% martensite at the centre

95
Q

Base diameter is a function of…

A

Carbon content and grain size

96
Q

What is a stainless steel?

A
  1. Resistant to attack by strong acid (H N O3)
  2. Resistant to atmosphere corrosion
  3. Resistant to aqueous corrosion
  4. Resistant to high temperature oxidation
97
Q

What are the solutions to grain boundary corrosion in stainless steels?

A
  1. Cool faster, avoid Cr23 C6
  2. Minimize the carbon content (however this is already pretty low)
  3. Add another allow element with a stronger affinity for carbon than chromium
98
Q

What is the necessary requirement for a Martensitic stainless steel?

A

At least 12 wt% Cr in solution

99
Q

How does a stainless steel prevent rusting?

A

(Cr 2O3) Chromia, a transparent layer on the surface meaning oxygen cant react with iron.

MAT2600 (5 decks)

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