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MS3 > Microstructure and Processing II > Flashcards

Microstructure and Processing II Flashcards

1
Q

What are the two main routes to producing steel.

A

Either start with iron ore or recycled scrap.

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2
Q

Is a BF an open or closed system?

A

Closed.

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3
Q

What is the product of a blast furnace?

A

Pig iron (saturated with C)

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4
Q

State the 3 interesting equations in a BF.

A

FeO + CO -> Fe + CO2 (reduction) (low T)
FeO + C -> Fe + CO (reduction) (high T)
CO2 + C -> 2CO (equilibrium)

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5
Q

What happens after pig iron is produced (contains many impurities).

A

A selective oxidation process to oxidise impurities.

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6
Q

What happens in a basic oxygen process (BOF)?

A

O2 gas injected into molten pig iron and it is stirred quickly.

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7
Q

What is the speed of the BOF like?

A

Very quick.

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8
Q

How is recycled scrap steel usually manufactured?

A

In an electric arc furnace.

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9
Q

When are electric arc furnaces typically used?

A

When electricity is cheap (near HEP).

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10
Q

How does an electric arc furnace work?

A

The high current between graphite rods and melt causes it to become molten. Oxygen is added for selective oxidation.

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11
Q

What are cast irons?

A

Very high C conc, graphitic phase forms.

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12
Q

What heat treatments can be applied to a cast iron?

A

Normalising - air cooling to RT giving pearlite with graphite flakes.
Quenching and annealing - graphite flakes and ferrite (use of some Mg results in graphite spheroids).

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13
Q

Which iron-carbon phase diagram is used to understand solid state transformations in steels?

A

The metastable.

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14
Q

Fe-C hypoeutectoid reaction?

A

γ -> γ + α

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15
Q

Fe-C on eutectoid reaction?

A

γ -> α + Fe3C

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16
Q

Fe-C hypereutectoid reaction?

A

γ -> γ + Fe3C

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17
Q

What happens to the strength of steek when the carbon content it raised?

A

More carbide forms so increase in hardness and strength but decrease in ductility.

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18
Q

Describe how eutectoid pearlite forms.

A

One of new phases nucleates at a grain boundary.
Changes carbon conc locally increasing the driving force for the second phase to nucleate.
This causes more of the first phase to nucleate and so on until a colony of alternating lamellae on gb.
Growth happens away from gb.
Cooperative growth removes need form long range diffusion allowing for long range growth.

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19
Q

What is the rate of pearlite growth like?

A

Slow to begin and then accelerates quickly.

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20
Q

What does a TTT diagram show?

A

It shows how much of a transformation has occurred asa function of isothermal annealing time (at constant T).

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21
Q

What are the three parts if a sigmoidal shaped reaction?

A

Nucleation
Growth
Saturation

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22
Q

Mechanical properties of plain carbon steels.

A

Increase C content, a greater fraction of Fe3C forms.
Fe3C is very hard and brittle.
The hardness of alloy increases.
Ductility dramatically decreases.
Tensile strength peaks at eutectoid composition.

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23
Q

What is bainite?

A

Ferrite and fine carbides.

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24
Q

What properties does bainite have?

A

High yield stresses and reasonably high ductility and high fracture toughness.

25
Q

Which has smaller carbide particles, upper or lower bainite?

A

Lower.

26
Q

How is martensite formed?

A

Austenite is quenched, not allowing carbon to diffuse.

Forms in a diffusionless (shear) process.

27
Q

What is the concentration of dislocations like in martensite?

A

High.

28
Q

What are the mechanical properties of martensite like?

A

Very hard but very brittle.

29
Q

How can martensite be toughened?

A

Through tempering, allowing some limited diffusion of carbon which allows the precipitation of carbides.

30
Q

What transformation in structure takes place when martensite forms?

A

fcc austenite to bct martensite.

31
Q

What is the Bain strain?

A

When the c axis is shortened ≈17% and the a and b axes lengthened by ≈12% in the transformation from austenite to martensite.

32
Q

What is the simplest way to modify the TTT curve allowing for martensite to be more easily formed?

A

Add alloying elements to push the nose back.

33
Q

What are the main changes in properties when tempering steel?

A

Reduced hardness but increased toughness.

34
Q

The biggest problem with casting metals.

A

Shrinkage on cooling.

35
Q

Why is the Al-Si system not affected hugely by shrinkage?

A

Si expands on freezing combating the shrinkage from the Al.

36
Q

How can grain size in Al-Si systems be refined?

A

Adding TiB2 allows effective heterogeneous nucleation.

37
Q

What is meant by modified Al-Si and how can it be done?

A

Refines the size of Si flakes and is done by adding Na (0.1at%)

38
Q

What effect does modification have on mechanical properties in Al-Si system?

A

Improves ductility and UTS greatly.

39
Q

Properties of mullite.

A 
High thermal stability
Low thermal expansion coefficient
Low electrical conductivity
High creep resistance
Excellent corrosion resistance
Reasonable strength and fracture toughness
40
Q

What is the key to making mullite have high strength at high temperatures?

A

Production without glassy grain boundaries.

41
Q

Mullite is often produced in what state of reaction?

A

Solid state.

42
Q

Smaller scale mixing changes what about the synthesis of mullite?

A

Lowers the temperature.

43
Q

What is sintering?

A

Solid state reaction between mixed powders.

44
Q

What is fused mullite?

A

Mullite produced from raw materials above 2000°C.

45
Q

What is the typical composition of fused mullite?

A

2:1

46
Q

What is chemical mullite?

A

Mullite produced when Al3+ and Si4+ are mixed on finer scales, before being sintered, using chemical processing techniques such as sol-gel, precipitation and spray pyrolysis.

47
Q

Describe sol-gel processing.

A

Colloidal particles or molecules are in a suspension, a sol, are forced by chemical change (often pH change)to form a continuous network, a gel.

48
Q

What two ways can styrene and butadiene be polymerised?

A

Solution polymerisation

Emulsion polymerisation

49
Q

How can microstructure in polymer blends be controlled?

A

Varying copolymers.

50
Q

What is a block copolymer?

A

A polymer comprising of molecules in which there is a linear arrangement of blocks, a block is defined as a portion of the polymer molecule in which monomeric units have at least one constitutional or configurational feature absent from adjacent portions.

51
Q

What does a compatibiliser do in polymer blends?

A

Reduces the size of the spheres in the microstructure (it is a copolymer).

52
Q

What controls the domain size in a polymer blend with copolymers present?

A

The lengths of the blocks of the copolymers.

53
Q

In the PS-PBD what are the properties of PS and PBD?

A

PS - hard

PBD - soft

54
Q

What is a brush block copolymer?

A

A polymer backbone with different polymer types grafted to it over various blocks.

55
Q

What structure do brush block copolymers take?

A

They self assemble into crystals.

56
Q

Does alternating or random PS-PBD blocks make any difference?

A

Yes, it can change properties (dependent on other factors too).

57
Q

High impact PS has what structure?

A

PS matrix with PS-PBD salami morphology.

58
Q

How does PBD strengthen high impact PS?

A

The soft PBD requires energy to deform, toughening the brittle PS.

59
Q

Adding CNTs to SBR changes what?

A

Depending on the amount added but can increase UTS.

MS3 (5 decks)

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