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Archaeomaterials > Iron > Flashcards

Iron Flashcards

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?

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

A

used to reveal the extent of the transformation

25
Q

What is the diagram that is valid for specific compositions and isothermal conditions only called?

A

time-temperature- transformation diagrams

26
Q

What is reflected in CCT (Continuous cooling transformation) digrams

A

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
Q

What do CCT time-temperature diagrams show

A

show how microstructures develop as temperature changes over time

determined from study of quenched in microstructures (individual diagrams for individual compositions)

28
Q

In a CCT what happens and the composition changes

A

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
Q

What are CCT diagrams used for?

A

Used to show how the microstructure can change with section thickness as the cooling rate varies due to natural effects

30
Q

Describe how the properties of steel can be manipulated by heat treatment

A
  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
Q

What is annealing?

A

Regime of slow cooling within a furnace

32
Q

What is normalising

A

Removing a product from the furnace so it cools in still air

33
Q

how can faster cooling rates be achieved

A

by quenching

34
Q

list the order of faster quenching methods

A

Slow:

Air quenching (a forced air blast)
oil quenching
water quenching

35
Q

What is the effect of a faster cooling rate

A

greater risk of failure due to residual stresses or excessive distortion

36
Q

what are residual stresses

A

stresses induces by differential contraction within a part

37
Q

what is excessive distortion

A

thermal strain

38
Q

What is one of the commonest microstructual constituents

A

Pearlite, fine alternating lamellae of iron carbide (Fe3C) and ferrite

39
Q

How does pearlite form?

A

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
Q

What wt% C is in Cementite

A

6.67

Austenite is locally denuded by C

41
Q

What scale is the distance between the lamellae in Pearlite

A

Since growth is diffusion -controlled and occurs over short distances, the lamellae are microm apart

42
Q

What happens as cooling rate in pearlite increases?

A

As cooling rate increases, there is less time available for diffusion, and the lamellae form as a finer structure

43
Q

What is Bainite

A

A mixture of ferrite and cementite , but much finer requiring SEM to resolve constituent parts forms at a lower T than pearlite

44
Q

How does Bainite form?

A

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
Q

Which forms are diffusion controlled

A

Formation of pearlite and bainite

46
Q

When does a diffusionlfess, thermal, displace transformation occur

A

if the steel is quenched quickly there is insufficient time for diffusion to occur

47
Q

How does martensite form?

A

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
Q

Is martensite a metastable phase

A

metastable = non equilibrium

yes

it can persist indefinitely at room temperatures and above

49
Q

How can the properties of martensite be improved

A

because product is brittle, properties improves by tempering

50
Q

what is tempering

A

A modest heating to stress relieve the structure

51
Q

What is spheroidisation?

A

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
Q

What is hardenability

A

Steels ability to transform to hard martensite, controlled by its composition as well as cooling rates

53
Q

Why can a low C steel not be hardened

A

Because upon quenching pearlite readily forms

Shown on CCT diagram as nose touching y-axis

54
Q

What happens as the C contents increases on CCT

A

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
Q

What is done to displace the nose even further the the right (CCT steel diagram)

A

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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