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Engineering Materials > Microstructure and Metallography > Flashcards

Microstructure and Metallography Flashcards

1
Q

What is a phase?

A

A specific arrangement of atoms in a regular repeating array, commonly know as crystalline structure.

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2
Q

What does a phase diagram show?

A

A visual representation of phases with varying temperature, composition or pressure

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3
Q

How can the iron-carbon phase diagram be described and what does it allow us to obtain?

A

various phases which allows us to attain great variability in the microstructures we form in steels which allows us to attain strengths anywhere from 200MPa to 2GPa

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4
Q

Why do we study the Fe-C phase diagram?

A

Understanding Fe-C phase diagram allow us to
process alloys by control temperature and
concentration of Fe and C
Only constructed for C contents of less than 6.67wt.% (25
at.%)
– composition of Fe3C (Cementite), an extremely hard and brittle intermetallic phase
– Alloys with C contents greater than 6.67 are of little
commercial interest, and therefore this region of the phase diagram is rarely presented

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5
Q

At what temperature does solidification of pure iron occur at?

A

1534 ºC

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6
Q

What occurs at solidification?

A

Pure iron form as bcc phase δ.

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7
Q

What two polymorphic changes does pure iron undergo and at what temperatures?

A

First at ~1400 ºC to the fcc austenite γ phase

Then at 910 ºC to bcc ferrite α phase.

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8
Q

What is the maximum solubility of C in

(a) δ
(b) γ
(c) α

A

(a) 0.1
(b) 2.0
(c) 0.02 wt.%

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9
Q

What can steels be decided as?

A

Alloys with a C content composition less than 2.06 wt.%

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10
Q

What can cast irons be described as?

A

Alloys greater than 2.06 wt.% C

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11
Q

What do the majority of commercial steels contain?

A

A carbon content less than 1 wt.%.

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12
Q

What are the phases that appear on the Iron-Carbon equilibrium phase diagram?

A 
Ferrite (F)
Austenite (A)
Cementite (Fe3C)
Pearlite (P) =F+Fe3C C 0.77%
Martensite*
Ledeburite (Le) =A+Fe3C C 4.3%
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13
Q

What is Ferrite (α iron) known as and what kind of solid solution is it?

A

α (alpha) solid solution.
It is an interstitial solid solution of a small amount of carbon dissolved in α (BCC) iron.
It is the softest structure that appears on the diagram
It is the softest structure that appears on the
diagram

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14
Q

When is Ferrite a stable form of iron?

A

below 912 deg.C

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15
Q

What is the maximum solubility of Ferrite and at what temperature does this occur?

A

0.03 % C at 723C

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16
Q

What is the solubility of Ferrite at room temperature?

A

only 0.008 % C

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17
Q

What is (a) Tensile strength, (b) Elongation and (c) Hardness of Ferrite?

A

Tensile strength = 2.75x10^7 Pa;
Elongation = 40 % in 2 in;
Hardness > Rockwell C 0 or > Rockwell B 90

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18
Q

What type of mixture is Pearlite and how is it formed?

A

eutectoid mixture containing 0.80 % C and is formed at 723°C on very slow cooling

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19
Q

What is Pearlite a mixture of?

A

It is a very fine platelike or lamellar mixture of ferrite and cementite.
The white ferritic background or matrix contains thin plates of cementite (dark).

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20
Q

What is (a) Tensile strength, (b) Elongation and (c) Hardness of Pearlite?

A

(a) Tensile strength = 8.27 x10^8 Pa;
(b) Elongation = 20 % in 2 in.;
(c) Hardness = Rockwell C 20, Rock­well B 95-100, or BHN 250-300.

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21
Q

What type of solid solution is Austenite (γ iron) and what does it contain?

A

Austenite is an interstitial solid solution of Carbon dissolved in (F.C.C.) iron

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22
Q

What is the maximum solubility of Austenite and at what temperature does this occur?

A

2.0 % C at 1130°C

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23
Q

How could the formability of Austenite be described?

A

High formability, most of heat treatments begin with this single phase

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24
Q

How does Austenite perform at room temperature?

A

It is normally not stable at room temperature. But, under certain conditions it is possible to obtain austenite at room temperature

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25
Q

What is (a) Tensile strength, (b) Elongation, (c) Hardness and (d) Toughness of Austenite?

A

Tensile strength = 1.03x10^9 Pa;
Elongation = 10 percent in 2 in.;
Hardness = Rockwell C 40, approx; and
Toughness = high

26
Q

What is Cementite? (mechanical properties, type of compound, etc)

A

Cementite or iron carbide, is very hard, brittle intermetallic compound of iron & carbon, as Fe3C, contains 6.67 % C.

27
Q

What is the melting point of Cementite?

A

It is the hardest structure that appears on the diagram, exact melting point unknown.

28
Q

What type of structure is Cementite?

A

Its crystal structure is orthorhombic

29
Q

What is the tensile and compressive strength of Cementite?

A

It is has low tensile strength (approx. 3.44x10^7 Pa) but a high compressive strength

30
Q

What kind of mixture is Ledeburite?

A

The eutectic mixture of austenite and cementite

31
Q

What percentage of C does Ledeburite have and at what temperature?

A

Contains 4.3 percent C and is formed at 1130°C

32
Q

What is the crystal structure and characteristics of Ferrite?

A

BBC

Soft, ductile, magnetic

33
Q

What is the crystal structure and characteristics of Austenite?

A

FCC

Soft, moderate strength, non-magnetic

34
Q

What is the crystal structure and characteristics of Cementite?

A

Compound of Iron & Carbon Fe3C

Hard & brittle

35
Q

What are some structure sensitive properties?

A

yield strength and fracture toughness

The word structure in this context (usually) implies microstructure.

36
Q

What are some structure insensitive properties?

A

density and Young’s modulus

The word structure in this context (usually) implies microstructure.

37
Q

What is metallography?

A

The process by which a sample is prepared so as to reveal the microstructure under a optical microscope

38
Q

What are 3 metallographic specimen preparations?

A

Sampling - selecting (say by cutting) representative regions of sample
Polishing - uniform material removal to make the sample surface flat
Etching - selective electro-chemical removal of the material to create a non-uniform surface

39
Q

Why do some regions look bright and some dark in an optical micrograph?

A

In an optical micrograph from a metallic specimen the main mechanism of contrast is reflection contrast.
Regions which reflect into the optical axis (i.e. in a bright field image) look bright and which reflect away from the optical axis look dark.
A rough surface reflects in a diffuse manner while a smooth surface gives rise to ‘specular’ reflection (like a mirror).
A polished surface is flat, while an etched surface is not uniform and has a ‘topography’.

40
Q

What does the ‘electro-chemical’ action (leading to a ‘etched’ surface) depends on?

A

the crystallographic orientation of the surface plane

41
Q

How do you achieve equilibrium conditions in reality?

A

Placing the steel into a furnace, heating the steel into the austenite phase field and then turning the furnace off and letting the steel cool down in the furnace.
If we change the speed of cooling, the microstructures of steel will be totally different. The product will have different mechanical properties from equilibrium cooling of steels.

42
Q

What is heat treatment?

A

A method used to alter the physical, and sometimes chemical properties of a material.
The most common application is metallurgical

It involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material.

It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally.

43
Q

What is the most common application of heat treatment and what does it invlove?

A

Metallurgica - involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material.
It applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally

44
Q

What is Annealing?

A

primary purpose is to soften the steel and prepare it for additional processing such as cold forming or machining
If already cold worked - allows recrystallisation

45
Q

What does Annealing do?

A 
Reduce hardness
Remove residual stress (stress relief)
Improve toughness
Restore ductility
Refine grain size
46
Q

What is process annealing.

A

Low carbon steels may harden through cold working. They can be heated to around 100 degrees below lower critical temp., soaked and allowed to cool in air

47
Q

What is Spheroidising?

A

High carbon steels may be annealed just below the lower critical temp. to improve machinability

48
Q

What is Normalising?

A

Internal stresses caused by rolling and rolling or forging are removed. Steels are heated above upper critical temp., soaked and cooled in air. The cooling rate is faster than annealing giving a smaller grain structure

49
Q

What is stress relieving?

A

The component is reheated and held at temperature for a period of time and cooled slowly

50
Q

What is quenching?

A

hardening

51
Q

What happens to medium and high carbon steels when quenched?

A

Medium and High carbon steels (0.4 – 1.2%) can be heated until red hot and then quenched in water producing a very hard and brittle metal.

52
Q

What happens to BCC ferrite at 723 degrees and what does quenching the metal prevent?

A

At 723 degrees, the BCC ferrite changes into Austenite with a FCC structure. Quenching the metal quickly in water prevents the structure from changing back into BCC.

53
Q

What happens FCC ferrite once it has been quenched ?

A

A different structure, Body Centre Tectragonal (BCT) is formed.
It is called Martensite and is extremely hard and brittle with a needle-like microstructure.

54
Q

What is Martensite and how is it formed?

A

A super-saturated solid solution of carbon in ferrite.
It is formed when steel is cooled so rapidly that the change from austenite to pearlite is suppressed.
The interstitial carbon atoms distort the BCC ferrite into a BC-tetragonal structure (BCT).; responsible for the hardness of quenched steel

55
Q

What is almost always done following quenching

A

Tempering

56
Q

How does heat treatment impact the mechanical properties of metals?

A

Changes the yield point of a material

57
Q

What is tempering and why is it carried out?

A

Because of lack of adequate toughness and ductility after
quenching, high carbon martensite is not a useful material despite
its great strength (too brittle).

Tempering imparts a desired amount of toughness and ductility (at the expense of strength)

The metal is heated to the range of 220-300 degrees and cooled.

Tempering colours are an indicator of temperature on polished
metals. Colours range from yellow to brown to violet and blue.

58
Q

What are the properties and applications of carbon steels with a carbon content 0.01-0.1 wt%?

A

P- Soft, ductile, not useful hardening by heat treatment except by normalising, but can be work-hardened. Weldable.
A- Pressings where high formability is required

59
Q

What are the properties and applications of carbon steels with a carbon content 0.1-0.25 wt%?

A

P - (same as 0.01-0.1) Ductile-brittle transition temperature just below room temperature.
A- General engineering uses for mild steel

60
Q

What are the properties and applications of carbon steels with a carbon content 0.25-0.6 wt%?

A

P- Very strong, heat treatable to produce a wide range of properties in quenched and tempered conditions. Difficult to weld. Can become brittle below room temperature
A- Bars and forgings for a wide range of engineering components, connecting rods, springs, hammers, axle shafts requiring strength and toughness

61
Q

What are the properties and applications of carbon steels with a carbon content 0.6-0.9 wt%?

A

P- Strong, whether heat treated or not. Ductility lower when less carbon is present
A- Use where maximum strength rather than toughness is important. Tools, wear resisting components (piano wire and silver steels are in this group)

62
Q

What are the properties and applications of carbon steels with a carbon content 0.9-2.0 wt%?

A

A- Wear resistant and can be made very hard at expense of toughness and ductility. Cannot be welded. Tend to be brittle if the structure is not carefully controlled.
A- Cutting tools like wood chisels, files and saw blades

Engineering Materials (20 decks)

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