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Materials module 2 > Material development: iron & steel > Flashcards

Material development: iron & steel Flashcards

1
Q

What kind of alloys are steel and cast iron?

A

iron carbon alloys

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2
Q

When is iron-iron carbide considered useful when seeing a phase diagram?

A

Only up to 6.7% is considered (practically useful)

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3
Q

When is steel formed in a iron-iron carbide phase diagram?

A

• Steels : 0.008 ‐ 2.14 wt % C (usually < 1 wt % ) α‐ferrite + Fe3C at room T (Chapter 12)

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4
Q

When is iron formed in a iron-iron carbide phase diagram?

A

• Iron: less than 0.008 wt % C in α−ferrite at room T

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5
Q

When is cast iron formed in a iron-iron carbide phase diagram?

A

• Cast iron: 2.14 ‐ 6.7 wt % (usually < 4.5 wt %)

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6
Q

What are 5 phases in Fe-Fe3C equilibrium diagram?

A
  • α‐ferrite – solid solution of C in BCC Fe
  • γ‐austenite – solid solution of C in FCC Fe
  • δ‐ferrite ‐ solid solution of C in BCC Fe
  • cementite (iron carbide or Fe3C )
  • Fe‐C liquid solution
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7
Q

What is the α‐ferrite phase? temperature, structure, type of alloying.

A

• α‐ferrite – solid solution of C in BCC Fe
low temperature BCC structure of carbon interstitially dissolved in iron

– Stable form of iron at room temperature.
– The maximum solubility of C is 0.022 wt%
– Transforms to FCC γ‐austenite at 912 °C

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8
Q

What is the γ‐austenite phase? temperature, structure, type of alloying.

A

• γ‐austenite – solid solution of C in FCC Fe
elevated temperature FCC structure of carbon interstitially dissolve in iron

– The maximum solubility of C is 2.14 wt %.
– Transforms to BCC δ‐ferrite at 1394°C
– Is not stable below the eutectic temperature (727 °
C) unless cooled rapidly (Chapter 10)

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9
Q

What is δ‐ferrite phase? temperature, structure, type of alloying.

A

δ‐ferrite ‐ solid solution of C in BCC Fe
elevated temp BCC structure of carbon interstitially dissolve in iron

– The same structure as α‐ferrite
– Stable only at high T, above 1394 °C
– Melts at 1538 °C

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10
Q

What is cementite phase? temperature, structure, type of alloying.

A

cementite (iron carbide or Fe3C )
iron-carbon inter-metallic compound that occurs with 6.67% carbon content.

– An intermetallic compound of iron and carbon with the chemical formula Fe3C. C content is around 6.67%. It is metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into α‐Fe and C (graphite) at 650 ‐ 700 °C. It is hard and brittle

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11
Q

How is ferrite created?

A

A BCC lattice structure of iron, in which carbon is interstitially dissolved.

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12
Q

How is austenite created?

A

A FCC lattice structure of iron, in which carbon is interstitially dissolved.

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13
Q

How is pearlite created?

A

A eutectoid composition consisting of a fine plate like or lamella structure of ferrite and cementite

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14
Q

How is cementite formed?

A

An intermetallic compound of iron and carbon with the chemical formula Fe3C‐C content of around 6.67%. A hard and brittle compound (hardest on the diagram) with an orthorhombic structure

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15
Q

What is C in Fe?

A

C is an interstitial impurity in Fe. It forms a solid solution with α, γ, δ phases of iron

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16
Q

What is the maximum solubility in BCC α‐ferrite?

A

Maximum solubility in BCC α‐ferrite is limited (max. 0.022 wt% at 727 °C) ‐ BCC has relatively small interstitial positions

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17
Q

What is the maximum solubility in FCC austenite?

A

Maximum solubility in FCC austenite is 2.14 wt% at 1147°C ‐ FCC has larger interstitial positions

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18
Q

What are the mechanical properties of ferrite and cementite?

A

Mechanical properties: Ferrite is soft. Cementite is very hard and brittle ‐ can strengthen steels. Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.

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19
Q

When is α ‐ferrite and austenite magnetic or non magnetic?

A

Magnetic properties: α ‐ferrite is magnetic below 768 °C, austenite is non‐magnetic

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20
Q

What will happen if the Fe-C system is left over a very long period of time at room temp?

A

The compound iron‐ carbide will decompose into iron and carbon (graphite) over a very long period of time at room temperature.

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21
Q

What is the percentage of elements in a typical carbon steel? (5 elements)

A

Carbon 0.6% , Silicon 0.2%, Manganese 0.7% , Sulphur 0.03% , Phosphorus 0.03%

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22
Q

What are 5 elements that are impurities in plain carbon steel?

A 
Manganese 
silicon
phosphorus
sulphur
manganese
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23
Q

What does manganese do in plain carbon steel?

A

Manganese ‐ is highly soluble in austenite and ferrite, it also forms a compound Mn3C (manganese carbide). It improves the tensile strength and toughness of the steel but should not exceed 0.5% in high carbon since it has the tendency to induce cracking after heat treatment quenching.

24
Q

What does silicon do in plain carbon steel?

A

Silicon ‐ should be kept to less than 0.2% since it has a tendency to decompose cementite into graphite.

25
Q

What does phosphorus do in plain carbon steel?

A

Phosphorus ‐ every endeavour is made to keep the phosphorus content to less than 0.05%. It forms a brittle compound Fe3P which may form planes of weakness at the expense of pearlite; requires strict control.

26
Q

What does sulphur do in plain carbon steel?

A

Sulphur ‐ does not dissolve in the solid steel to any marked extent, solid solubility falling to an equivalent 0.03%. It does exist as either manganese sulphide (MnS) or ferrous sulphide (FeS) that form brittle films at the crystal grain boundaries.

27
Q

What does manganese do in plain carbon steel?

A

Manganese is often added to reduce the sulphur content and to increase the presence of manganese sulphide in preference to ferrous sulphide.

28
Q

What are the three critical lines of the heat treatment of plain carbon steels?

A

Three critical lines
– the upper‐critical‐temperature lines, A3
and Acm
– the lower‐critical temperature line A1.

29
Q

What is the process of heat treatment (4)?

A 
Heat treatment processes:
– Annealing 
• Full
• Process 
– Normalising
– Hardening 
– Tempering
30
Q

Why is annealing carried out? (3)

A

– Relieve stresses
– Increase softness, ductility, and toughness
– Produce a specific microstructure

31
Q

What are the three stages of annealing?

A

– Heating to the desired temperature
– Holding or soaking at the temperature
– Cooling, to room temperature

32
Q

What can annealing to in hypoeutectoid steel?

A

Annealing is given to produce a ductile condition in hypo eutectoid steel.

33
Q

How does full annealing occur? (3)

A

It involves heating the steel to a temperature within the range which is 30‐50°C above the A3 line
and allowing the steel to cool slowly within the furnace.

• This process is often necessary after severe cold working, deep drawing, rolling or heavy machining.

The heating reverts the steel to a more stable condition, all the previous distorted structures are removed and the microstructure produced by full annealing is shown in the above figure.

34
Q

Why must heating be slow in full annealing?

A

The rate of heating should be fairly slow with the time for soaking at temperature being dependent on the thickness and volume of the component or batch of components. A guide is to allow one hour per 25 mm of ruling section.
• Slower heating conditions should be applied to highly stressed items to avoid distortion.

35
Q

How does annealing occur in steel?

A
  • This is a stress relieving heat treatment process for mild steel, and is particularly relevant to sheet metal products that have been cold worked.
  • It is carried by heating the steel to a temperature below the A1 line (about 650°C) and allowing the slow cooling as previously described.
36
Q

What is the purpose of normalising?

A

The purpose of normalising is to produce harder and stronger steel than that provided by full annealing, or in other words to bring it back to “normal” after previous working operations.

37
Q

How does normalising occur?

A

Normalising consists of heating the steel 25‐30°C above the A3 line temperature for hypo eutectoid steels and 50°C above Acm line for hypereutectoid steels. So the process is similar to full annealing, differing in the cooling that is done in still (normal) air.

38
Q

What happens when you increase the rate of cooling in normalising?

A

With the increasing rate of cooling the pearlite structure is finer compared to that obtained with annealing.

39
Q

What happens if a piece of steel with sufficient carbon is cooled rapidly?

A

If a piece of steel containing sufficient carbon is cooled rapidly from a temperature above the A3 line it becomes considerably harder.

40
Q

How is the degree of hardness produced?

A

The degree of hardness produced is largely dependent on the percentage of carbon present, but this can vary depending upon such factors as the initial quenching temperature, the size of the workpiece, the type of quenching medium, its temperature and the amount of agitation.

41
Q

What is the martensite phase?

A

Basically, the rate of quenching does not provide sufficient time for the carbon to diffuse out of the austenitic structure (F.C.C.) to form the ferritic structure (B.C.C.). The carbon is trapped in this solid solution, the lattice structure distorted. This phase is known as martensite.

42
Q

Why is martensite phase the hardest produced in steels?

A

Due to the extreme distortion, dislocation movement is virtually prevented with the result that the phase martensite is one of the hardest produced in steels. However, it is extremely brittle.

43
Q

What range of tempering to allow ductility to be restored?

A

Tempering in the range of 200‐400°C allows ductility to be restored but decreases the hardness. In the range of 400‐600°C provides steel with maximum toughness.

44
Q

What does tempering to 200C do?

A

Tempering to 200°C reduces some of the brittleness but retains the high hardness.

45
Q

What happens when you heat treat martensite to form tempered martensite?

A

Heat treat martensite to form tempered martensite

  • tempered martensite less brittle than martensite
  • tempering reduces internal stresses caused by quenching
46
Q

What are 5 iron-carbon schematic structures resulting from various heat treatments?

A 
– Gf flake graphite
– Gr, graphite rosettes 
– Gn, graphite nodules 
– P, pearlite
–  ferrite
47
Q

How is pearlite formed from austenite?

A

by slow cooling

( α+ Fe3C layers + a proeutectoid phase)

48
Q

How is bainite formed from austenite?

A 
moderate cool
(α + elong. Fe3C particles)
49
Q

How is Martensite and tempered martensite formed from austenite?

A

Martensite is formed by rapid quenching (BCT phase diffusionless transformation)

reheating martensite forms tempered martensite (α + very fine Fe3C particles)

50
Q 
Rank from strongest to weakest
Martensite 
T Martensite
bainite
fine pearlite 
coarse pearlite
spheroidite
A 
Martensite 
T Martensite
Bainite
Fine pearlite 
Coarse pearlite
Spheroidite
51
Q 
Rank the ductility of these metals 
T Martensite
bainite
fine pearlite 
coarse pearlite
spheroidite
A 
spheroidite
coarse pearlite
fine pearlite 
bainite
T Martensite 
Martensite
52
Q

Whats the difference between ferrite and martensite?

A

ferrite has a BCC crystal structure and is soft.

Martensite has a Body cantered tetragonal (BCT) structure and is hard.

53
Q

What is the primary strengthening mechanism in the high strength “aircraft quality” aluminium alloys?

A

primary strengthening method is precipitation hardening (age hardening) produced by heat treatment

54
Q

Briefly explain why there may be significant scatter in the fracture strength for some given ceramic material, and why fracture strength increases with decreasing specimen size.

A

Because the mechanical properties of ceramics are dependent on the presence and size of internal and surface flaws and cracks. The lower the volume of the ceramic, ie. the smaller the piece, the lower the probability that the piece contains a critically sized flaw and therefore less chance of a reduction in strength – so an increase in strength is observed. Hence the introduction of a probability factor requires the use of a statistical analysis and therefore the observed scatter in data on fracture strength.

55
Q

In a fibre-reinforced composite, what are the primary roles of;

i. the matrix?
ii. the fibres?

A

The Matrix supports and transmits loads to he fibres and provides ductility and toughness
The fibres provide strength by carrying most of the load

Materials module 2 (10 decks)

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