Stainless Steel & Wrought Alloys Flashcards
(70 cards)
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
2
Q
What are 2 uses of wrought alloys?
A
- Wires (orthodontic)
- Partial Denture Clasps
3
Q
What elements is a steel alloy composed of? (2)
A
- Iron
- Carbon
4
Q
What % of iron is in a steel alloy?
A
> 98%
5
Q
What % of carbon is in a steel alloy?
A
<2%
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
7
Q
What are the other minor constituents of steel? (6)
A
- Chromium (0.5-1%)
- Manganese
- Molybdenum, Silicon, Nickel, Cobalt
8
Q
What is the function of Chromium in Steel?
A
- Improves tarnish resistance
9
Q
What is the function of Manganese in Steel?
A
- Sulphur Scavenger
10
Q
What are 2 uses of steel?
A
- Cutting instruments
- Forceps
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
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%)
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%)
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)
15
Q
What is ferrite in the Fe-C Phase diagram?
A
- Very dilute solid solution
- Exists at low temps
16
Q
What is cementite in the Fe-C Phase diagram?
A
- Fe3C
- Exists at low temps
17
Q
What is pearlite in the Fe-C Phase diagram?
A
- Eutectoid mixture of Ferrite and Cementite
18
Q
What forms a solid solution?
A
- TWO metals that form a common lattice structure and are soluble in one another
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
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
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
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)
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
24
Q
What does fast cooling of austenite produce?
A
- Martensite which can be turned into Pearlite by tempering
25
What does slow cooling of austenite produce?
Pearlite
- Ferrite
- Cementite
26
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?
- Achieved by tempering
27
What is tempering?
- 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
28
When tempering martensite, the temperature and duration affect the conversion. What can it convert to? (2)
- Ferrite (soft, ductile)
- Cementite (hard, brittle)
- Control over mechanical propertied through heat treatment
- Versatile alloy
29
What are the 4 main components of stainless steel?
- Iron (Fe)
- Carbon (C)
- Chromium (Cr)
- Nickel (Ni)
30
Stainless steel is only considered stainless steel if it has at least what % of Chromium?
- At least 12%
31
What are the properties of Chromium in stainless steel? (3)
- Lowers austenite to martensite temperature
- Lowers austenite to martensite rate
- Decreases % carbon at which eutectoid formed
32
What is one of stainless steels key attributes?
- 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
Stainless steel is resistant to corrosion but what can it be attacked by?
- Can be attacked by chlorides
34
What are the properties of Nickel in stainless steel? (3)
- Lowers austenite to martensite transition temperature
- Improves fracture strength
- Improved corrosion resistance
35
What are the 2 different types of stainless steel?
- Martensitic
| - Austenitic
36
What is the martensitic composition of stainless steel?
- 12-13% Chromium + little carbon
37
What is the property of martensitic stainless steel?
- Heat hardenable (tempering process)
38
What is martensitic stainless steel used to make?
- Dental instruments (but this is not the type of stainless steel that interests us)
39
How can we suppress the transition of austenite to martensite?
- 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
What are the 3 uses of austenitic stainless steel?
- Dental equipment and instruments
- Wires (ortho)
- Sheet forms for denture bases
41
Why is austenitic stainless steel good for making dental equipment and instruments?
- Its corrosion resistance ensures it withstands the autoclave process
- Corrosion resistance more important than strength and hardness
- (NOT cutting edge instruments)
42
Why is austenitic stainless steel used to produce orthodontic wires? (2)
- IT is readily cold worked so can bend into shape
| - It is corrosion resistant
43
Stainless steel can be used to from denture bases. What needs to be done to it for this to happen?
- This requires it to be swaged (adapted to a die)
44
What is swaging?
- 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
What are the components of 18-8 Stainless steel? (4)
18% Chromium
8% Nickel
0.1% Carbon
74% Iron
46
Does 18-8 stainless steel heat harden?
- NO
47
When cast, what is 18-8 stainless steel like?
- IT is soft (malleable) when cast
BUT
- IT work hardens rapidly - so it can't be repeatedly manipulated to form the desired shape
48
What is another name for cold working?
- Work or strain hardening
49
What is work hardening?
- It’s work done on metal/alloy at LOW TEMPERATURE - below recrystallisation temperature. Work done could be:- bending, rolling, swaging.
50
What does work hardening cause?
- 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
What are wrought alloys?
- 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
What are the uses of wrought alloys? (2)
- Wires (orthodontic)
| - Partial denture clasps
53
What are the uses of 18-8 stainless steel wires? (2)
- Orthodontic appliances - springs & clasps
| - Partial dentures - Clasp arms, wrought rests
54
What are the different grades of 18-8 stainless steel wires? (4)
- Soft
- Half hard
- Hard
- Spring temper
- The grade you select depends on how much bending, shape manipulation you require for your dental appliance
55
As well as stainless steel wires, what other materials can be used? (4)
- Gold
- Cobalt Chromium
- Nickel-titanium
- beta-titanium
56
What is the composition of cobalt chromium used for wires?
Co 40%
Cr 20%
Ni 15%
Fe 16%
57
What is the composition of Gold used for wires?
Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%
58
What is the composition of NiTi used for wires?
Ni 55%
Ti 45%
+ some cobalt
59
What is the composition of Beta-Ti used for wires?
Ti
| Some molybdenum
60
What is Springiness (EL/YM)?
- Ability of a material to undergo large deflections (to form arc) without permanent deformation (i.e. it returns to its original shape)
61
What are the properties required for a wire? (5)
- 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
Look at the properties table in the lecture!
:)
63
What can stainless steel be soldered using? (2)
- Gold solder
| - Silver solder (melting point <700 degrees celsius)
64
Why must care be taken when stainless steel is soldered?
- 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
Between what temperatures does weld decay occur and what can this cause?
- 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
How can we minimise the disadvantages of weld decay? (2)
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
Why do stainless steel wires need stress relief annealing?
- 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
Look at swaging diagram slide
:)
69
What are the advantages of using stainless steel as a denture base? (8)
- 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
What are the drawbacks of using stainless steel as a denture base? (6)
- 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
