Iron Flashcards
When did the iron age start?
1000BC
overlap between the use of bronze and iron alloys
What is the most easily accessible source of iron?
Telluric/ native iron (largely pure)
Is Cu more reactive than Fe?
No, Fe is more reactive, so oxidises more readily –> found less readily
What does Telluric Iron consist of?
Where is it found?
Consists of: - Naturally occurring Fe
-up to 3% Ni
Currently found outside Disk Bay Area of Western Greenland
Descrive type 1 telluric Iron
Up to ~ 4% C and Ni
A Ni-bearing cast iron with C locked up as cementite crystals or pearlite
Brittle and cannot be cold worked
Describe type 2 telluric Iron
Fe and unto 4% Ni, C< 0.7%
Can be cold worked
Where is Iron alloy found/ formed
Occurs as grains, few mm in size, formed within basalt
How did the local Inuits exploit the basalt
They crushed the basalt to release the telluric iron grains - then hammered into discs, inserted into a bone haft to make a tool similar to a serrated knife
What does meteoritic mean?
Meteoritic = not formed on earth
Give an example of meteoritic iron and its composition
Cape York meteorite, over 50 tonnes of raw material, struck earth about 10,000 yrs ago
Fe-alloy with ~25% Ni
What are the two phases that generally occur in meteoritic iron (and the composition)?
- Kamacite (90-95% Fe & 5-10% Ni)
- Taenite (35-80% Fe & 20-65% Ni)
What is the name of interleaved lamellae of Kamacite and taenite
Widmanstatten structure
Why does the Widemanstatten structure occur?
Due to slow cooling (as slow as 0.01K/year
When did slow cooling occur?
During the formation of a photo-planet early in the Solar Systems’ History
Fe-Ni core formed, was destroyed by collision with another protoplanet, scattering fragments of Fe-Ni alloy core which landed on the Earth
How can the Widmanstatten structure be revealed?
By etching with nitric acid/ Nital
Where else and how does the Widmanstatten structure form
In Steels, as the pro-eutectoid ferrite and cementite form from the austenite phase during cooling
What occurs at low undercooling in steels
Ferrite nucleates at austenite grain boundaries to form blocky structures - grain boundary allotriomorphs
What occurs at high undercooling in steels
Widmanstatten structure, nucleation occurs on the austenite grain boundaries and ferrite plates grow into the grain
How does proeutectoid cementite behave at high and low undercoolings?
High: plates of cementite nucleate on the grain boundaries and grow into the grain to form a Widmanstatten structure
Low: grain boundaries are suitable nucleation sites
What does an ISOTHERMAL TRANSFORMATION diagram show?
shows what happens when a material is held at a constant Temperature
What does an EQUILIBRIUM PHASE diagram show?
shows what happens with unlimited time, when all transformations are allowed to process to completion
How are isothermal transformation diagrams plotted
A temperature - time diagrams, show how long a material must be held at a particular T for transformation to be complete
What does an isothermal phase diagram show?
If a material is held at a constant temperature for a certain length of time what proportion of the phase transformation will have occured
What are quenched in microstructures used to show
used to reveal the extent of the transformation
What is the diagram that is valid for specific compositions and isothermal conditions only called?
time-temperature- transformation diagrams
What is reflected in CCT (Continuous cooling transformation) digrams
heat treatments consist of continuous cooling regimes: different parts of the material experience different cooling rates, depending upon local conditions, but conditions are not isothermal
What do CCT time-temperature diagrams show
show how microstructures develop as temperature changes over time
determined from study of quenched in microstructures (individual diagrams for individual compositions)
In a CCT what happens and the composition changes
As composition changes, the RATE and EXTENT of transformation can change
Diagrams used by superimposing known or desired cooling rates, to predict what microstructures will occur
What are CCT diagrams used for?
Used to show how the microstructure can change with section thickness as the cooling rate varies due to natural effects
Describe how the properties of steel can be manipulated by heat treatment
- First step involves heating steel until it completely transforms to austenite
- Then by applying different cooling rates, the austenite can be partially/ completely transformed into other products with varying properties
The composition of the steel can be modified to assist or delay in these transformations
What is annealing?
Regime of slow cooling within a furnace
What is normalising
Removing a product from the furnace so it cools in still air
how can faster cooling rates be achieved
by quenching
list the order of faster quenching methods
Slow:
Air quenching (a forced air blast)
oil quenching
water quenching
What is the effect of a faster cooling rate
greater risk of failure due to residual stresses or excessive distortion
what are residual stresses
stresses induces by differential contraction within a part
what is excessive distortion
thermal strain
What is one of the commonest microstructual constituents
Pearlite, fine alternating lamellae of iron carbide (Fe3C) and ferrite
How does pearlite form?
forms by nucleation and growth of the cementite phase, at the austenite grain boundary.
Nucleation of adjacent lamellae of ferrite
2 phases then grow as lamellae into austenite grain, with C diffusing between the phases
What wt% C is in Cementite
6.67
Austenite is locally denuded by C
What scale is the distance between the lamellae in Pearlite
Since growth is diffusion -controlled and occurs over short distances, the lamellae are microm apart
What happens as cooling rate in pearlite increases?
As cooling rate increases, there is less time available for diffusion, and the lamellae form as a finer structure
What is Bainite
A mixture of ferrite and cementite , but much finer requiring SEM to resolve constituent parts forms at a lower T than pearlite
How does Bainite form?
Forms by nucleation of ferrite within the austenite grains, causing rejection of C into the austenite to form a supersaturated solution –> precipitation of an adjacent cementite phase
Which forms are diffusion controlled
Formation of pearlite and bainite
When does a diffusionlfess, thermal, displace transformation occur
if the steel is quenched quickly there is insufficient time for diffusion to occur
How does martensite form?
C atoms shift their positions slightly to form a BODY-CENTERED TETRAGONAL PHASE = martensite
forms as hard brittle plates, with the transformation spreading at the speed of sound (not confined to steels)
Is martensite a metastable phase
metastable = non equilibrium
yes
it can persist indefinitely at room temperatures and above
How can the properties of martensite be improved
because product is brittle, properties improves by tempering
what is tempering
A modest heating to stress relieve the structure
What is spheroidisation?
Heat treatment where the steel is held below the A3 transformation temperature (not heated above austenitisation T)
In this case the Fe3C particles will gradually form sphere –> increase in strength and ductility
What is hardenability
Steels ability to transform to hard martensite, controlled by its composition as well as cooling rates
Why can a low C steel not be hardened
Because upon quenching pearlite readily forms
Shown on CCT diagram as nose touching y-axis
What happens as the C contents increases on CCT
As C content increases pearlite nose is shifted to the right i.e. pearlite requires a longer time for nucleation to occur, and very fast cooling rates will avoid the nucleation of pearlite –> formation of martensite and a harder steel
What is done to displace the nose even further the the right (CCT steel diagram)
Alloying elements are added in small amounts such as Mn, Mo, Cr
Longer times = makes martensite formation easier and more extensive –> harder tougher steel
