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BDS3 DMS > Stainless Steel & Wrought Alloys > Flashcards

Stainless Steel & Wrought Alloys Flashcards

1
Q

What is the definition of a ‘wrought’ alloy?

A
  • It is an alloy which can be manipulated/shaped by cold working
  • So it can be drawn into a wire
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2
Q

What are 2 uses of wrought alloys?

A
  • Wires (orthodontic)

- Partial Denture Clasps

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3
Q

What elements is a steel alloy composed of? (2)

A
  • Iron

- Carbon

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4
Q

What % of iron is in a steel alloy?

A

> 98%

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5
Q

What % of carbon is in a steel alloy?

A

<2%

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6
Q

IF there is more than 2% carbon this would mean the material is no longer classed as steel. What would it be classed as?

A
  • Cast iron or pig iron
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7
Q

What are the other minor constituents of steel? (6)

A
  • Chromium (0.5-1%)
  • Manganese
  • Molybdenum, Silicon, Nickel, Cobalt
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8
Q

What is the function of Chromium in Steel?

A
  • Improves tarnish resistance
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9
Q

What is the function of Manganese in Steel?

A
  • Sulphur Scavenger
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10
Q

What are 2 uses of steel?

A
  • Cutting instruments

- Forceps

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11
Q

Iron is ‘Allotropic’. What does this mean?

A
  • IT undergoes TWO solid state phase changes with temperature
  • In other words, in a solid state, it can exist in TWO crystalline forms - two phases - depending on its temperature
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12
Q

What crystalline structure is Iron in at a temperature of >1400 or <900 degrees Celsius?

A
  • Body centred cubic (BCC) crystalline structure

- Low carbon solubility (0.05%)

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13
Q

What crystalline structure is Iron in at a temperature of >900 and <1400 degrees Celsius?

A
  • Face centred cubic (FCC) crystalline structure

- Carbon solubility higher (2%)

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14
Q

What is austenite in the Fe-C Phase diagram?

A
  • Interstitial solid solution (lattice has iron in rows and columns with come C atoms interspersed) , FCC
  • Exists at a high temp (i.e. >720 degrees celsius)
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15
Q

What is ferrite in the Fe-C Phase diagram?

A
  • Very dilute solid solution

- Exists at low temps

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16
Q

What is cementite in the Fe-C Phase diagram?

A
  • Fe3C

- Exists at low temps

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17
Q

What is pearlite in the Fe-C Phase diagram?

A
  • Eutectoid mixture of Ferrite and Cementite
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18
Q

What forms a solid solution?

A
  • TWO metals that form a common lattice structure and are soluble in one another
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19
Q

What is a random substitutional solid solution?

A
  • Where both types of atoms in the lattice structure are arranged in a random fashion
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20
Q

What is an ordered substitutional solid solution?

A
  • We can predict the type of atom based on its location in the lattice structure
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21
Q

What is an interstitial solid solution?

A
  • The 2 different atoms are markedly different in size - that’s a prerequisite
  • The larger atom will occupy all the lattice sites and the smaller atom fits in the spaces, in random fashion
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22
Q

From the Fe-C phase diagram it shows us that quenching should give us austenite however what does it actually give us?

A
  • Actually gives us Martensite, which behaves quite differently
  • (martensite is NOT a supersaturated austenite solution)
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23
Q

What are the features of Martensite? (3)

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
  • This means it forms a hard and brittle material - we don’t want this
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24
Q

What does fast cooling of austenite produce?

A
  • Martensite which can be turned into Pearlite by tempering
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25
Q

What does slow cooling of austenite produce?

A

Pearlite

  • Ferrite
  • Cementite
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26
Q

Martensite is very useful in non-dental applications. It can be used to produce materials that are soft or hard, or somewhere in between. How is this achieved?

A
  • Achieved by tempering
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27
Q

What is tempering?

A
  • Altering its temperature, and the duration you maintain it at a specific temperature and then quenching it
  • Heating to 450 degrees Celsius followed by quenching
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28
Q

When tempering martensite, the temperature and duration affect the conversion. What can it convert to? (2)

A
  • Ferrite (soft, ductile)
  • Cementite (hard, brittle)
  • Control over mechanical propertied through heat treatment
  • Versatile alloy
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29
Q

What are the 4 main components of stainless steel?

A
  • Iron (Fe)
  • Carbon (C)
  • Chromium (Cr)
  • Nickel (Ni)
30
Q

Stainless steel is only considered stainless steel if it has at least what % of Chromium?

A
  • At least 12%
31
Q

What are the properties of Chromium in stainless steel? (3)

A
  • Lowers austenite to martensite temperature
  • Lowers austenite to martensite rate
  • Decreases % carbon at which eutectoid formed
32
Q

What is one of stainless steels key attributes?

A
  • Corrosion resistance due to chromium oxide layer
  • There’s a risk of any metal in the oral environment experiencing corrosion – the presence of saliva, liquids with acidic pH levels – all ingredients designed to provoke an electrochemical reaction.
  • CRUCIALLY S/Steel forms a chromium oxide layer on its surface, which protects it from corrosion. It’s vital.
33
Q

Stainless steel is resistant to corrosion but what can it be attacked by?

A
  • Can be attacked by chlorides
34
Q

What are the properties of Nickel in stainless steel? (3)

A
  • Lowers austenite to martensite transition temperature
  • Improves fracture strength
  • Improved corrosion resistance
35
Q

What are the 2 different types of stainless steel?

A
  • Martensitic

- Austenitic

36
Q

What is the martensitic composition of stainless steel?

A
  • 12-13% Chromium + little carbon
37
Q

What is the property of martensitic stainless steel?

A
  • Heat hardenable (tempering process)
38
Q

What is martensitic stainless steel used to make?

A
  • Dental instruments (but this is not the type of stainless steel that interests us)
39
Q

How can we suppress the transition of austenite to martensite?

A
  • There is a way that transition – austenite to martensite - can be suppressed.
  • By having the right proportions of Cr and Ni - specifically, either 18:8 or 12:12 ratio.
40
Q

What are the 3 uses of austenitic stainless steel?

A
  • Dental equipment and instruments
  • Wires (ortho)
  • Sheet forms for denture bases
41
Q

Why is austenitic stainless steel good for making dental equipment and instruments?

A
  • Its corrosion resistance ensures it withstands the autoclave process
  • Corrosion resistance more important than strength and hardness
  • (NOT cutting edge instruments)
42
Q

Why is austenitic stainless steel used to produce orthodontic wires? (2)

A
  • IT is readily cold worked so can bend into shape

- It is corrosion resistant

43
Q

Stainless steel can be used to from denture bases. What needs to be done to it for this to happen?

A
  • This requires it to be swaged (adapted to a die)
44
Q

What is swaging?

A
  • Swaging is a forging process in which the dimensions of an item are altered using dies into which the item is forced. Swaging is usually a cold working process, but also may be hot worked.
45
Q

What are the components of 18-8 Stainless steel? (4)

A

18% Chromium
8% Nickel
0.1% Carbon
74% Iron

46
Q

Does 18-8 stainless steel heat harden?

A
  • NO
47
Q

When cast, what is 18-8 stainless steel like?

A
  • IT is soft (malleable) when cast

BUT
- IT work hardens rapidly - so it can’t be repeatedly manipulated to form the desired shape

48
Q

What is another name for cold working?

A
  • Work or strain hardening
49
Q

What is work hardening?

A
  • It’s work done on metal/alloy at LOW TEMPERATURE - below recrystallisation temperature. Work done could be:- bending, rolling, swaging.
50
Q

What does work hardening cause?

A
  • These processes cause SLIP – that is defects/dislocations in a grain being moved along to the grain boundaries; where they stay, they can’t jump to another grain
  • As a consequence of this, the alloy has fewer defects within the lattice structure making up each grain – this makes the alloy a stronger, harder material.
51
Q

What are wrought alloys?

A
  • Ones that can be manipulated/shaped by cold working

- The goal is to draw them into the form of a wire. The diameter of which depends on how this is done

52
Q

What are the uses of wrought alloys? (2)

A
  • Wires (orthodontic)

- Partial denture clasps

53
Q

What are the uses of 18-8 stainless steel wires? (2)

A
  • Orthodontic appliances - springs & clasps

- Partial dentures - Clasp arms, wrought rests

54
Q

What are the different grades of 18-8 stainless steel wires? (4)

A
  • Soft
  • Half hard
  • Hard
  • Spring temper
  • The grade you select depends on how much bending, shape manipulation you require for your dental appliance
55
Q

As well as stainless steel wires, what other materials can be used? (4)

A
  • Gold
  • Cobalt Chromium
  • Nickel-titanium
  • beta-titanium
56
Q

What is the composition of cobalt chromium used for wires?

A

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

57
Q

What is the composition of Gold used for wires?

A

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

58
Q

What is the composition of NiTi used for wires?

A

Ni 55%
Ti 45%
+ some cobalt

59
Q

What is the composition of Beta-Ti used for wires?

A

Ti

Some molybdenum

60
Q

What is Springiness (EL/YM)?

A
  • Ability of a material to undergo large deflections (to form arc) without permanent deformation (i.e. it returns to its original shape)
61
Q

What are the properties required for a wire? (5)

A
  • High Springiness (i.e. undergo large deflections without permanent deformation)
  • Stiffness (YM) - depends on required force of tooth movement
  • High ductility - bending without fracture
  • Easily joined without impairing properties - soldered, welded
  • Corrosion resistant
62
Q

Look at the properties table in the lecture!

A

:)

63
Q

What can stainless steel be soldered using? (2)

A
  • Gold solder

- Silver solder (melting point <700 degrees celsius)

64
Q

Why must care be taken when stainless steel is soldered?

A
  • Care has to be taken as the temperature rise created is close to the melting point of s/steel.
  • So there is a risks the s/steel grains may recrystallize – which adversely affects its mechanical properties.
  • Quenching the alloy would avoid this and maintain UTS.
65
Q

Between what temperatures does weld decay occur and what can this cause?

A
  • Occurs between 500-900 degrees celsius
  • This can push the Cr and C atoms to grain boundaries - allowing CrC to ‘precipitate’ there
  • If CrC forms at the grain boundaries it causes the S/steel to become brittle – so its able to undergo just a small amount of deformation before fracturing.
  • This limits the amount of manipulation of the wire to match the desired configuration. That’s a severe restriction.
  • It also makes s/steel more likely to corrode.
66
Q

How can we minimise the disadvantages of weld decay? (2)

A
  1. Low carbon content steels - EXPENSIVE
  2. Stabilised stainless steel - contain small quantities of TITANIUM or NIOBIM
    - Forms carbides preferentially
    - Not at grain boundaries
67
Q

Why do stainless steel wires need stress relief annealing?

A
  • As with other alloys, which have undergone various processes – such as cold working – a stress relief annealing process is needed for s/steel wires.
  • This is to ensure the configuration of the metal atoms - Ni, Cr and Fe - in each of the alloy grains settle into an equilibrium
  • This process requires the temperature of s/steel needs to be held at around 450C for a minute or two. It’s crucial that the temperature of the s/steel DOES NOT exceed this value – otherwise it may be adversely affected :- either through a change in the grain structure; OR due to metal carbides forming at the grain boundaries. SO care is required at each stage.
  • Grain structure affected above 650 degrees
  • Precipitation of carbides above 500 degrees
68
Q

Look at swaging diagram slide

A

:)

69
Q

What are the advantages of using stainless steel as a denture base? (8)

A
  • Thin 0.11mm - acrylic is 1.52mm
  • Light
  • Fracture resistant
  • Corrosion resistant
  • High polish obtainable
  • Hight thermal conductivity
  • High impact strength
  • High abrasion resistance
70
Q

What are the drawbacks of using stainless steel as a denture base? (6)

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 -> wrinkling of steel

BDS3 DMS (5 decks)

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