Steel Flashcards
What do Fine ferrite grain sizes with martensite islands leads to?
High strength (~800MPa) and good formability
Examples of face centred cubic crystal structures:
Copper Silver Gold Platinum Nickel Lead Aluminium Cobalt (b) Iron (g)
Examples of body centred cubic crystal structures:
Chromium Molybdenum Niobium Vanadium Titanium (b) Tungsten Iron (a, d)
Examples of Hexagonal-Close-Packed crystal structures:
Beryllium Magnesium Zinc Zirconium Titanium (a) Cadmium Cobalt (a)
What are atomic scale defects known as in microstructure?
Point defects
What are microscopic line defects also known as?
Dislocations
What are some types of line defects?
Screw dislocations
Mixed dislocations
Edge dislocations
What are micro/macroscopic planar defects also known as?
Grain boundaries
What are some examples of planar defects?
Stacking faults
Twin boundaries
Grain boundaries
What are some examples of volume defects?
Precipitates
Voids/porosity
Inclusions/particles
What scale is metallurgy studied at?
10^-9 - 10^-6
What is a completely soluble phase?
One liquid phase
What is partial solubility?
Two phases; a liquid and a solid
e.g salt solution
What is a compound formation?
Two phases; a liquid and a solid
e.g a separate phase
What are the insolubility of phases?
Two liquid phases
E.g water and oil
How are phases distinguished?
-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
Whats is the definition of a phase transformation?
Any re-arrangement within the assembly of atoms or molecules which carries the system from one configuration to a lower order of energy.
What are unary systems?
It contains one component; e.g pure iron (Fe)
When does unlimited insolubility occur?
- When only one solid phase will be produced
regardless of the ratio between solvent and solute - Represented by a binary isomorphous system
When does limited solubility occur?
- 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
What is solid solution strengthening?
-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)
What are the effects on the properties to add a strengthening solute?
- YS, TS and hardness increase
- Ductility (%RA, %elong) decrease
- Resistivity increases
- Creep resistance increases
What are the equations for weight% composition in a binary system?
at%A + at%B = 100
wt%A + wt%B = 100
What do isomorphous phases diagrams show?
- 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
Liquidus
Defines temperature above which 100% liquid would be present for any given composition
Solidus
Defines temperature below which 100% solid would be present for any given composition
What does the temperature difference between liquidus and solidus show?
The freezing range of an alloy
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
What does the Lever rule find?
Relative amounts of L and alpha in two phase region
What is solidification of a solid solution alloy?
- Nucleation and growth process; nucleation is heterogeneous and requires little undercooling.
- Solidification begins as the liquidus
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
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)
What is non-equilibrium solidification?
Occurs when cooling is too rapid for atoms to diffuse (e.g. partition) to yield equilibrium microstructures
What is segregation?
Non-uniform composition produced by non-equilibrium solidification is known as segregation
What is microsegregation?
- Occurs over short distances (e.g., between dendrite arms)
- Reduced or eliminated by using a homogenisation heat treatment
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.)
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)
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
What is present in a precipitation/ strengthening phase?
More than one phase is present
What is a matrix phase?
Continuous phase which is usually present in larger amounts
What is precipitate
Second phase usually present is smaller amounts
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)
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
Solubility limit is shown by…
Solvus line
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
What is the strength of eutectic influenced by?
- Amount of eutectic phase
- Maximum strength observed for 100% eutectic (other factors being equal - Microstructure of eutectic phase
- Influenced primarily through cooling rate
- As cooling rate increases, the interlamellar spacing (λ) of the eutectic decreases
As λ↓, large interfacial area/unit volume, causing…
Increase in strength
What is a necessary condition of precipitation strengthening?
Decreasing solid solubility with decreasing temperature is a necessary condition
What are the three stages in the design of a heat treatment for precipitation strengthening?
- Solution treatment
- Quenching
- Ageing
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.
What happens in quenching?
- Little time for diffusion results in a supersaturated solid
- Represents a condition of unstable treatment
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
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)
Eutectoid reaction
Solid state reaction upon cooling s1 -> s2 + s3
It forms the basis for heat treatment of steel
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.
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
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
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
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
What are the steel designations?
- First two digits refer to the major alloy
- Last two or three digits give the C concentration in wt%
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%.
What is pearlite?
Its not a phase!
Its the microconstituent of α and Fe3C which forms a lamellar structure.
What is the Eutectoid reaction for steel?
γ(0.8%C) –> α(0.02%C) + Fe3C (6.67%C)
Image of hypoeutectoid steels
Pearite = white areas
Proeutectoid Ferrite = Dark areas
What are the two types of phase transformations in steel?
- Reactions near equilibrium conditions
- Nucleations and growth processes i.e. requires diffusion - Reactions removed from equilibrium conditions
- Equilibrium phase diagram is not adequate to describe these reactions
- Requires the use of TTT diagrams
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
Why is transformation temperature important?
It affects both the kinetics and the microstructure, which in turn influences the final properties
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.
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)
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
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
How can sensitisation be minimised?
Cooling the steel rapidly
What happens during the cold working of a metal (work hardening)?
The strength and hardness increase and the ductility decreases
Ductile Fracture
- Slow Failure
- High energy mode of failure
- dimpled fracture surface
Brittle Failure
- Fast rapid failure, with little or no warning
- Low energy mode of failure
- results in cleavage or facetted surface
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
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
What is the name given to the precipitation of alloy carbides in alloy steels?
Secondary hardening
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
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
What is ideal diameter a function of?
- Grain size
- Carbon concentration
- alloy elements
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.
Pearlite is…
A two phase lamellar microstructure consisting of ferrite and cementite
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
What is a monotectic reaction?
Monotectic = Liquid to a solid and a liquid L1 -> S1 +L2
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
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.
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?
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
What is a hardenability test?
Jominy test
-Quench at bottom; changes in hardness depending on how quickly its cooled down
smaller the austenite grain size…
Increase in hardenability
Is more carbon an increase or decrease in hardenability when quenching?
Increase
Larger section size…
Bigger range of hardenability
Describe critical diameter
The diameter for a steel composition and quench that would harden to 50% martensite at the centre
Base diameter is a function of…
Carbon content and grain size
What is a stainless steel?
- Resistant to attack by strong acid (H N O3)
- Resistant to atmosphere corrosion
- Resistant to aqueous corrosion
- Resistant to high temperature oxidation
What are the solutions to grain boundary corrosion in stainless steels?
- Cool faster, avoid Cr23 C6
- Minimize the carbon content (however this is already pretty low)
- Add another allow element with a stronger affinity for carbon than chromium
What is the necessary requirement for a Martensitic stainless steel?
At least 12 wt% Cr in solution
How does a stainless steel prevent rusting?
(Cr 2O3) Chromia, a transparent layer on the surface meaning oxygen cant react with iron.
