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BDS3 CLINICAL DMS > Stainless Steel & Wrought Alloys > Flashcards

Stainless Steel & Wrought Alloys Flashcards

1
Q

What is the definition of a wrought alloy

A

that it can be manipulated/shaped by cold working e.g drawn into wire

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2
Q

What are uses of wrought alloys

A

Its uses include orthodontic wires or partial denture clasps

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3
Q

What is the composition of steel

A

○ >98% iron

<2% carbon

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4
Q

If the steel contains more than 2% carbon what is it regarded as

A

means it is not steel but cast iron

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5
Q

What are the other constituents of steel

A 
○ Chromium (0.5-1%) 
		○ Manganese 
		○ Molybdenum
		○ Silicon
		○ Nickel 
		○ Cobalt
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6
Q

What is the function of chromium in steel

A

to improve its tarnish resistance

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7
Q

What is the function of manganese in steel

A

to act as a sulphur scavenger

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8
Q

What are uses of steel in medicine

A

as a cutting instrument (>0.8% C) and forceps (<0.8% C)

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9
Q

What does iron being allotropic mean

A

this means it undergoes two solid state phases (two crystalline forms) depending on its temperature

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10
Q

What is the crystalline form of iron below 900 degrees

A

body centred cubic crystalline structure

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11
Q

What is the crystalline form of iron above 1400 degrees

A

body centred cubic crystalline structure

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12
Q

What is the crystalline form of iron between 900 and 1400 degrees

A

has a face centred cubic lattice structure

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13
Q

What are the several phases seen in an iron/carbon phase diagram

A

austenite
ferrite
cementite
pearlite

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14
Q

What is austenite

A

§ Interstitial solid solution (iron is in rows and columns with some carbon atoms interspersed)
§ FCC
§ Exists at a high temperature (>720 degrees Celsius)

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15
Q

What is ferrite

A

§ Very dilute solid solution

§ Exists at low temperatures

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16
Q

What is cementite

A

§ Fe3C

§ Exists at low temperatures

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17
Q

What is pearlite

A

§ Eutectoid mixture of ferrite and cementite

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18
Q

From the phase diagram, what does cooling rapidly, quenching and reaching solidus, what grain structure should there be

A

austenite

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19
Q

When do ferrite and cementite come into play according to the phase diagram

A

• Ferrite and cementite are what come into play if you cool a molten alloy extremely slowly but we do not want this as it generates large grains with poor mechanical properties

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20
Q

What is an alloy

A

• Two metals that form a common lattice structure and are soluble in one another form a SOLID solution

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21
Q

What are the different types of solid solution

A

substitutional solid solution

interstitial solid solution

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22
Q

What are the different types of substituional solid solutions

A

random solid solution

ordered solid solution

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23
Q

What is a random solid solution

A

both types of atoms in the lattice structure are arranged in a random fashion

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24
Q

What is an ordered solid solution

A

can predict the type of atom based on its location

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25
Q

What is an interstitial solid solution

A

§ The two atoms are markedly different in size - smaller atom occupies the spaces in between the large ones

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26
Q

What does quenching give us

A

• Quenching should give us a supersaturated austenite solution according to the phase diagram but in practice we get martensite which behaves differently

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27
Q

Describe the structure of martensite

A

• Martensite has a distorted lattice structure as a result of carbon being unable to diffuse normally within the array of iron atoms in each grain resulting in a hard and brittle material

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28
Q

What does slow cooling of austenite result in

A

pearlite

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29
Q

What is tempering

A

• Tempering is when the martensite is heated to 450 degrees followed by quenching

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30
Q

What does tempering result in

A

• The temperature and duration affect conversion to:
○ Ferrite
○ Cementite

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31
Q

What are the mechanical properties of ferrite

A

(soft, ductile)

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32
Q

What are the mechanical properties of cementite

A

(hard, brittle)

33
Q

What is the composition of stainless steel

A

Fe
C
Cr
Ni

34
Q

How much chromium is required to classify steel as stainless

A

> 12%

35
Q

What is the function of chromium in stainless steel

A

§ It lowers the austenite to martensite temperature
§ It lowers austenite to martensite conversion rate
§ Decreases % carbon at which eutectoid formed

36
Q

What is the function of nickel in stainless steel

A

§ Lowers austinite to martensite transition temperature
§ It improves fracture strength
§ It improves corrosion resistance

37
Q

How is stainless steel corrosion resistant

A

• It is corrosion resistant due to the formation of a chromium oxide layer but it can be attacked by chlorides

38
Q

What are the two types of stainless steel

A

martensitic

austenitic

39
Q

What is martensitic stainless steel

A

§ 12-13% chromium + a little carbon
§ Heat hardenable (tempering process)
Can be used to make dental instrumen

40
Q

How is austenitic stainless steel formed

A

§ Earlier we saw in the phase diagram that quenching the alloy, contrary to expectation, martensite was produced instead of austenite
§ This transition can be suppressed by having sufficient chromium and nickel to suppress austenite to martensite transition
§ This is done by having the right proportions (either 18:8 ratio or 12:12)

41
Q

What are the uses of austenitic stainless steel

A

dental equipment and instruments that need to be sterilized
wires
sheet forms for denture bases

42
Q

How are stainless steel wires made for orthodontic

A

eadily cold worked (so can be bent into shape)

43
Q

How is stainless steel made into denture bases

A

Swaged (adapted to a die)

44
Q

What is the composition of 18/8 stainless steel

A

○ 18% chromium
○ 8% nickel
○ 0.1% carbon
○ 74% iron

45
Q

Does stainless steel heat harden

A

• It does NOT heat harden

46
Q

Is stainless steel hard when cast

A

• It is soft (malleable) when cast

47
Q

How quickly does stainless steel work harden

A

• Work hardens rapidly so can’t be repeatedly work hardened

48
Q

What is cold working

A

• Work done on metal/alloy at LOW temperature - below the recrystallisation temperature e.g bending, rolling, swaging

49
Q

What does cold working cause

A
  • It causes slip - dislocations collect at grain boundaries and so can’t jump to another grain
    • As a consequence of this, the alloy has fewer defects in the lattice structure that makes up a grain and this makes the alloy a stronger and harder material
50
Q

What is cold working aka

A

• It is also known as work or strain hardening

51
Q

What are wrought alloys

A
  • It is manipulated/shaped by cold working e.g drawn into wire
    • It’s use is orthodontic wires and partial denture clasps
52
Q

What are the grades of stainless steel wires

A 
• Grades depend on degree of bending required
		○ Soft
		○ Half hard
		○ Hard
		○ Spring temper
53
Q

What are the different types of alloys used for wires

A
  • Stainless steel
    • Gold (similar to type 4)
    • Nickel-titanium
    • Cobalt chromium
    • Titanium (variations of this may be used)
54
Q

What is composition of authentic stainless steel used for wires

A

18% Cr
8% Ni
0.1% C
74% Fe

55
Q

What is the composition of cobalt chromium (not partial denture)

A

40% Co
20% Cr
15% Ni
16% Fe

56
Q

What is the composition of gold

A

Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%

57
Q

What is the composition of Nickel titanium

A

Ni 55%
Ti 45%
Cobalt

58
Q

What is the composition of beta-titanium

A

titanium and some molybdenum

59
Q

What is springiness

A

• Ability of material to undergo large deflections (to form arc) without permanent deformation (i.e it returns to its original shape)

60
Q

How is springiness calculated

A

EL/YM

61
Q

What are the requirements of a wire

A 
high springiness 
stiffness 
high ductility 
easily joined 
corrosion resistance
62
Q

Why should a wire have high springiness

A

○ Undergo large deflections without permanent deformation

63
Q

Why should a wire be stiff

A

○ Depends on required force for tooth movement

64
Q

Why should a wire be highly ductile

A

so it can bend without fracture

65
Q

What alloys are least and most stiff

A

most - stainless steel and cocr

least - nickel titanium

66
Q

Which alloy is most and least springy

A

most - NiTi

least - gold and cocr

67
Q

Which alloy is most and least ductile

A

most - CoCr

least - niti

68
Q

What alloy is most easily joined and least easily joined

A

most - gold

least - niti/cocr

69
Q

How can stainless steel wires be soldered

A

• Can be soldered using gold or silver but care must be taken as the temperature rise created is close to the melting point of stainless steel and there is a risk that the stainless steel grains will recrystallise which can adversely effect mechanical properties

70
Q

How do we avoid recrystlalisation in soldering

A

• Quenching rapidly can be done to avoid this and also to maintain the ultimate tensile strength

71
Q

What is weld decay

A
  • When the stainless steel temperature is raised to 500-900, it can push chromium and carbon atoms to grain boundaries allowing chromium carbide to precipitate there
    • It causes the alloy to become brittle so is only able to undergo a small amount of deformation before fracture
    • There is less chromium in the central region of the solid solution
    • Makes it more susceptible to corrosion
72
Q

How can weld decay be minimized

A

○ Low carbon content steels can be used but they are expensive
○ Stabilised stainless steel can be used, it contains small amounts of
§ Titanium or niobium
§ Forms carbides preferentially
§ Not at grain boundaries

73
Q

What is stress relief annealing

A

• Stress relief annealing process is needed for stainless steel wires to ensure configuration of metal atoms

74
Q

How is stress relief annealing done

A

• Possible (need care)

○ 450 degrees Celsius, 1-2 minutes

75
Q

What temp is the grain structure affected in stress relief annealing in stainless steel wires

A

650

76
Q

What temp does precipitation of carbides happen in stress relief annealing in stainless steel wires

A

500

77
Q

What is swaging

A

done for denture

78
Q

What re advantages of stainless steel for denture

A 
○ Thin 0.1mm - acrylic 1.52mm 
		○ Light
		○ Fracture resistant 
		○ Corrosion resistant 
		○ High polish obtainable
		○ High thermal conductivity
		○ High impact strength
		○ High abrasion resistance
79
Q

What are disadvantages of stainless steel for a denture

A

○ Possible dimensional inaccuracy (contraction of die not matched by model expansion)
○ Elastic recovery of steel - inaccuracy
○ Damage of die under hydraulic pressure
○ Loss of fine detail during the many stages
○ Difficult to ensure uniform thickness
○ Uneven pressure on die and counter die resulting in wrinkling of steel

BDS3 CLINICAL DMS (9 decks)

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