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BDS3 CLINICAL DMS > Stainless Steel & Wrought Alloys > Flashcards

Stainless Steel & Wrought Alloys Flashcards

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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
What is an interstitial solid solution
§ The two atoms are markedly different in size - smaller atom occupies the spaces in between the large ones
26
What does quenching give us
• Quenching should give us a supersaturated austenite solution according to the phase diagram but in practice we get martensite which behaves differently
27
Describe the structure of martensite
• 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
28
What does slow cooling of austenite result in
pearlite
29
What is tempering
• Tempering is when the martensite is heated to 450 degrees followed by quenching
30
What does tempering result in
• The temperature and duration affect conversion to: ○ Ferrite ○ Cementite
31
What are the mechanical properties of ferrite
(soft, ductile)
32
What are the mechanical properties of cementite
(hard, brittle)
33
What is the composition of stainless steel
Fe C Cr Ni
34
How much chromium is required to classify steel as stainless
>12%
35
What is the function of chromium in stainless steel
§ It lowers the austenite to martensite temperature § It lowers austenite to martensite conversion rate § Decreases % carbon at which eutectoid formed
36
What is the function of nickel in stainless steel
§ Lowers austinite to martensite transition temperature § It improves fracture strength § It improves corrosion resistance
37
How is stainless steel corrosion resistant
• It is corrosion resistant due to the formation of a chromium oxide layer but it can be attacked by chlorides
38
What are the two types of stainless steel
martensitic | austenitic
39
What is martensitic stainless steel
§ 12-13% chromium + a little carbon § Heat hardenable (tempering process) Can be used to make dental instrumen
40
How is austenitic stainless steel formed
§ 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
What are the uses of austenitic stainless steel
dental equipment and instruments that need to be sterilized wires sheet forms for denture bases
42
How are stainless steel wires made for orthodontic
eadily cold worked (so can be bent into shape)
43
How is stainless steel made into denture bases
Swaged (adapted to a die)
44
What is the composition of 18/8 stainless steel
○ 18% chromium ○ 8% nickel ○ 0.1% carbon ○ 74% iron
45
Does stainless steel heat harden
• It does NOT heat harden
46
Is stainless steel hard when cast
• It is soft (malleable) when cast
47
How quickly does stainless steel work harden
• Work hardens rapidly so can't be repeatedly work hardened
48
What is cold working
• Work done on metal/alloy at LOW temperature - below the recrystallisation temperature e.g bending, rolling, swaging
49
What does cold working cause
* 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
What is cold working aka
• It is also known as work or strain hardening
51
What are wrought alloys
* It is manipulated/shaped by cold working e.g drawn into wire * It's use is orthodontic wires and partial denture clasps
52
What are the grades of stainless steel wires
``` • Grades depend on degree of bending required ○ Soft ○ Half hard ○ Hard ○ Spring temper ```
53
What are the different types of alloys used for wires
* Stainless steel * Gold (similar to type 4) * Nickel-titanium * Cobalt chromium * Titanium (variations of this may be used)
54
What is composition of authentic stainless steel used for wires
18% Cr 8% Ni 0.1% C 74% Fe
55
What is the composition of cobalt chromium (not partial denture)
40% Co 20% Cr 15% Ni 16% Fe
56
What is the composition of gold
Au 60% Ag 15% Cu 15% Pt/Pd 10%
57
What is the composition of Nickel titanium
Ni 55% Ti 45% Cobalt
58
What is the composition of beta-titanium
titanium and some molybdenum
59
What is springiness
• Ability of material to undergo large deflections (to form arc) without permanent deformation (i.e it returns to its original shape)
60
How is springiness calculated
EL/YM
61
What are the requirements of a wire
``` high springiness stiffness high ductility easily joined corrosion resistance ```
62
Why should a wire have high springiness
○ Undergo large deflections without permanent deformation
63
Why should a wire be stiff
○ Depends on required force for tooth movement
64
Why should a wire be highly ductile
so it can bend without fracture
65
What alloys are least and most stiff
most - stainless steel and cocr | least - nickel titanium
66
Which alloy is most and least springy
most - NiTi | least - gold and cocr
67
Which alloy is most and least ductile
most - CoCr | least - niti
68
What alloy is most easily joined and least easily joined
most - gold | least - niti/cocr
69
How can stainless steel wires be soldered
• 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
How do we avoid recrystlalisation in soldering
• Quenching rapidly can be done to avoid this and also to maintain the ultimate tensile strength
71
What is weld decay
* 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
How can weld decay be minimized
○ 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
What is stress relief annealing
• Stress relief annealing process is needed for stainless steel wires to ensure configuration of metal atoms
74
How is stress relief annealing done
• Possible (need care) | ○ 450 degrees Celsius, 1-2 minutes
75
What temp is the grain structure affected in stress relief annealing in stainless steel wires
650
76
What temp does precipitation of carbides happen in stress relief annealing in stainless steel wires
500
77
What is swaging
done for denture
78
What re advantages of stainless steel for denture
``` ○ Thin 0.1mm - acrylic 1.52mm ○ Light ○ Fracture resistant ○ Corrosion resistant ○ High polish obtainable ○ High thermal conductivity ○ High impact strength ○ High abrasion resistance ```
79
What are disadvantages of stainless steel for a denture
○ 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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