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Archaeomaterials > Iron > Flashcards

Iron Flashcards

(55 cards)

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1
Q

When did the iron age start?

A

1000BC

overlap between the use of bronze and iron alloys

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2
Q

What is the most easily accessible source of iron?

A

Telluric/ native iron (largely pure)

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3
Q

Is Cu more reactive than Fe?

A

No, Fe is more reactive, so oxidises more readily –> found less readily

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4
Q

What does Telluric Iron consist of?

Where is it found?

A

Consists of: - Naturally occurring Fe
-up to 3% Ni

Currently found outside Disk Bay Area of Western Greenland

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5
Q

Descrive type 1 telluric Iron

A

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

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6
Q

Describe type 2 telluric Iron

A

Fe and unto 4% Ni, C< 0.7%

Can be cold worked

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7
Q

Where is Iron alloy found/ formed

A

Occurs as grains, few mm in size, formed within basalt

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8
Q

How did the local Inuits exploit the basalt

A

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

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9
Q

What does meteoritic mean?

A

Meteoritic = not formed on earth

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10
Q

Give an example of meteoritic iron and its composition

A

Cape York meteorite, over 50 tonnes of raw material, struck earth about 10,000 yrs ago

Fe-alloy with ~25% Ni

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11
Q

What are the two phases that generally occur in meteoritic iron (and the composition)?

A
  • Kamacite (90-95% Fe & 5-10% Ni)

- Taenite (35-80% Fe & 20-65% Ni)

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12
Q

What is the name of interleaved lamellae of Kamacite and taenite

A

Widmanstatten structure

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13
Q

Why does the Widemanstatten structure occur?

A

Due to slow cooling (as slow as 0.01K/year

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14
Q

When did slow cooling occur?

A

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

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15
Q

How can the Widmanstatten structure be revealed?

A

By etching with nitric acid/ Nital

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16
Q

Where else and how does the Widmanstatten structure form

A

In Steels, as the pro-eutectoid ferrite and cementite form from the austenite phase during cooling

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17
Q

What occurs at low undercooling in steels

A

Ferrite nucleates at austenite grain boundaries to form blocky structures - grain boundary allotriomorphs

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18
Q

What occurs at high undercooling in steels

A

Widmanstatten structure, nucleation occurs on the austenite grain boundaries and ferrite plates grow into the grain

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19
Q

How does proeutectoid cementite behave at high and low undercoolings?

A

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

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20
Q

What does an ISOTHERMAL TRANSFORMATION diagram show?

A

shows what happens when a material is held at a constant Temperature

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21
Q

What does an EQUILIBRIUM PHASE diagram show?

A

shows what happens with unlimited time, when all transformations are allowed to process to completion

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22
Q

How are isothermal transformation diagrams plotted

A

A temperature - time diagrams, show how long a material must be held at a particular T for transformation to be complete

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23
Q

What does an isothermal phase diagram show?

Study These Flashcards
A

If a material is held at a constant temperature for a certain length of time what proportion of the phase transformation will have occured

24
Q

What are quenched in microstructures used to show

Study These Flashcards
A

used to reveal the extent of the transformation

25
What is the diagram that is valid for specific compositions and isothermal conditions only called?
time-temperature- transformation diagrams
26
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
27
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)
28
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
29
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
30
Describe how the properties of steel can be manipulated by heat treatment
1. First step involves heating steel until it completely transforms to austenite 2. 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
31
What is annealing?
Regime of slow cooling within a furnace
32
What is normalising
Removing a product from the furnace so it cools in still air
33
how can faster cooling rates be achieved
by quenching
34
list the order of faster quenching methods
Slow: Air quenching (a forced air blast) oil quenching water quenching
35
What is the effect of a faster cooling rate
greater risk of failure due to residual stresses or excessive distortion
36
what are residual stresses
stresses induces by differential contraction within a part
37
what is excessive distortion
thermal strain
38
What is one of the commonest microstructual constituents
Pearlite, fine alternating lamellae of iron carbide (Fe3C) and ferrite
39
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
40
What wt% C is in Cementite
6.67 Austenite is locally denuded by C
41
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
42
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
43
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
44
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
45
Which forms are diffusion controlled
Formation of pearlite and bainite
46
When does a diffusionlfess, thermal, displace transformation occur
if the steel is quenched quickly there is insufficient time for diffusion to occur
47
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)
48
Is martensite a metastable phase
metastable = non equilibrium yes it can persist indefinitely at room temperatures and above
49
How can the properties of martensite be improved
because product is brittle, properties improves by tempering
50
what is tempering
A modest heating to stress relieve the structure
51
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
52
What is hardenability
Steels ability to transform to hard martensite, controlled by its composition as well as cooling rates
53
Why can a low C steel not be hardened
Because upon quenching pearlite readily forms Shown on CCT diagram as nose touching y-axis
54
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
55
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

Archaeomaterials (9 decks)

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