Steel Flashcards
1
Q
What do Fine ferrite grain sizes with martensite islands leads to?
A
High strength (~800MPa) and good formability
2
Q
Examples of face centred cubic crystal structures:
A
Copper Silver Gold Platinum Nickel Lead Aluminium Cobalt (b) Iron (g)
3
Q
Examples of body centred cubic crystal structures:
A
Chromium Molybdenum Niobium Vanadium Titanium (b) Tungsten Iron (a, d)
4
Q
Examples of Hexagonal-Close-Packed crystal structures:
A
Beryllium Magnesium Zinc Zirconium Titanium (a) Cadmium Cobalt (a)
5
Q
What are atomic scale defects known as in microstructure?
A
Point defects
6
Q
What are microscopic line defects also known as?
A
Dislocations
7
Q
What are some types of line defects?
A
Screw dislocations
Mixed dislocations
Edge dislocations
8
Q
What are micro/macroscopic planar defects also known as?
A
Grain boundaries
9
Q
What are some examples of planar defects?
A
Stacking faults
Twin boundaries
Grain boundaries
10
Q
What are some examples of volume defects?
A
Precipitates
Voids/porosity
Inclusions/particles
11
Q
What scale is metallurgy studied at?
A
10^-9 - 10^-6
12
Q
What is a completely soluble phase?
A
One liquid phase
13
Q
What is partial solubility?
A
Two phases; a liquid and a solid
e.g salt solution
14
Q
What is a compound formation?
A
Two phases; a liquid and a solid
e.g a separate phase
15
Q
What are the insolubility of phases?
A
Two liquid phases
E.g water and oil
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
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.
18
Q
What are unary systems?
A
It contains one component; e.g pure iron (Fe)
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
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
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)
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
23
Q
What are the equations for weight% composition in a binary system?
A
at%A + at%B = 100
wt%A + wt%B = 100
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
25
Liquidus
Defines temperature above which 100% liquid would be present for any given composition
26
Solidus
Defines temperature below which 100% solid would be present for any given composition
27
What does the temperature difference between liquidus and solidus show?
The freezing range of an alloy
28
What is a tie line?
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
29
What does the Lever rule find?
Relative amounts of L and alpha in two phase region
30
What is solidification of a solid solution alloy?
- Nucleation and growth process; nucleation is heterogeneous and requires little undercooling.
- Solidification begins as the liquidus
31
What are the two conditions required for the growth of solid (alpha?
- 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
32
Latent heat
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)
33
What is non-equilibrium solidification?
Occurs when cooling is too rapid for atoms to diffuse (e.g. partition) to yield equilibrium microstructures
34
What is segregation?
Non-uniform composition produced by non-equilibrium solidification is known as segregation
35
What is microsegregation?
- Occurs over short distances (e.g., between dendrite arms)
| - Reduced or eliminated by using a homogenisation heat treatment
36
What is macrosegregation?
- 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.)
37
When does precipitation/ dispersion strengthening occur?
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)
38
What is a interphase boundary?
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
39
What is present in a precipitation/ strengthening phase?
More than one phase is present
40
What is a matrix phase?
Continuous phase which is usually present in larger amounts
41
What is precipitate
Second phase usually present is smaller amounts
42
What does microconstituent mean? And give an example.
Both phases form concurrently with one another, such that
they exist in nearly equal proportions
For example Pearlite (alpha Fe3C in steel)
43
What happens when the tendency for atoms to cluster increases?
The solid solubility becomes more limited, which in turn changes the phase equilibria and finally, the development of a eutectic phase diagram
44
Solubility limit is shown by...
Solvus line
45
What happens during solidification?
- 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
What is the strength of eutectic influenced by?
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
As λ↓, large interfacial area/unit volume, causing...
Increase in strength
48
What is a necessary condition of precipitation strengthening?
Decreasing solid solubility with decreasing temperature is a necessary condition
49
What are the three stages in the design of a heat treatment for precipitation strengthening?
- Solution treatment
- Quenching
- Ageing
50
What happens in a solution treatment?
- 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
What happens in quenching?
- Little time for diffusion results in a supersaturated solid
- Represents a condition of unstable treatment
52
What happens in ageing?
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
What happens to a supersaturated microstructure?
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
Eutectoid reaction
Solid state reaction upon cooling s1 -> s2 + s3
| It forms the basis for heat treatment of steel
55
Definition of steel
Alloys based upon the Fe-C binary phase diagram which contain anywhere from trace amounts of C, up to 1.2wt% C.
56
Ferrite (α)
- 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
Austenite (γ)
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
What is cementite? (Fe3C)
Intermetallic compound of fixed composition (6.67%wt C)
| Orthorhombic crystal structure with 12 Fe and 4C atoms per unit cell
59
What is Ferrite-δ?
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
What are the steel designations?
- First two digits refer to the major alloy
| - Last two or three digits give the C concentration in wt%
61
What part of the phase diagram concerns steel?
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
What is pearlite?
Its not a phase!
| Its the microconstituent of α and Fe3C which forms a lamellar structure.
63
What is the Eutectoid reaction for steel?
γ(0.8%C) --> α(0.02%C) + Fe3C (6.67%C)
64
Image of hypoeutectoid steels
Pearite = white areas
| Proeutectoid Ferrite = Dark areas
65
What are the two types of phase transformations in steel?
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
What do Time-Temperature-Transformation (TTT) diagrams graphically represent
The precipitation kinetics for the transformation products associated with the decomposition of γ
-> Needs to be repeated over a range of temperatures
67
Why is transformation temperature important?
It affects both the kinetics and the microstructure, which in turn influences the final properties
68
What is Martensite?
- 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
What is the Martensitic transformation?
- 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
Dual phase steel (Martensite in as-quenched form):
- 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
What does Intercritical anneal mean?
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
How can sensitisation be minimised?
Cooling the steel rapidly
73
What happens during the cold working of a metal (work hardening)?
The strength and hardness increase and the ductility decreases
74
Ductile Fracture
- Slow Failure
- High energy mode of failure
- dimpled fracture surface
75
Brittle Failure
- Fast rapid failure, with little or no warning
- Low energy mode of failure
- results in cleavage or facetted surface
76
What is the tempering of Martensite?
The process of heating martensitic steel to a temperature below the A1 to make it more soft and ductile
77
What are the microstructural changes during tempering?
- 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
What is the name given to the precipitation of alloy carbides in alloy steels?
Secondary hardening
79
Describe what is meant by Ideal Diameter
the largest bar diameter for a steel composition that can be quenched to produce 50% martensite at the center after ideal quench
80
What is the difference between eutectic and eutectoid reactions?
```
Eutectic = Liquid to two solids L -> S1 + S2
Eutectoid = Solid to two solids S1 -> S2 +S3
```
81
What is ideal diameter a function of?
- Grain size
- Carbon concentration
- alloy elements
82
What is an 'Ideal quench'?
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
Pearlite is...
A two phase lamellar microstructure consisting of ferrite and cementite
84
What is the difference between eutectic and eutectoid reactions?
```
Eutectic = Liquid to two solids L -> S1 + S2
Eutectiod = Soild to two solids S1 -> S2 +S3
```
85
What is a monotectic reaction?
Monotectic = Liquid to a solid and a liquid L1 -> S1 +L2
86
What is the difference between a peritectic and peritectoid reaction?
```
Peritectic = Liquid and a solid to a solid L + S1 -> S2
Peritectoid = Solid and a solid to a solid S1 + S2 -> S3
```
87
What is a cored microstructure :P
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
Transformation kinetics will change with alloy elements by...
Most alloying elements will retard the transformation kinetics (i.e., pushes the C-curve towards longer times)
-Not as pure?
89
Transformation kinetics will change with Austenite grain size by...
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
What is a hardenability test?
Jominy test
| -Quench at bottom; changes in hardness depending on how quickly its cooled down
91
smaller the austenite grain size...
Increase in hardenability
92
Is more carbon an increase or decrease in hardenability when quenching?
Increase
93
Larger section size...
Bigger range of hardenability
94
Describe critical diameter
The diameter for a steel composition and quench that would harden to 50% martensite at the centre
95
Base diameter is a function of...
Carbon content and grain size
96
What is a stainless steel?
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
What are the solutions to grain boundary corrosion in stainless steels?
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
What is the necessary requirement for a Martensitic stainless steel?
At least 12 wt% Cr in solution
99
How does a stainless steel prevent rusting?
(Cr 2O3) Chromia, a transparent layer on the surface meaning oxygen cant react with iron.
