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BDS3 DMS > Stainless Steel & Wrought Alloys > Flashcards

Stainless Steel & Wrought Alloys Flashcards

1
Q

What are wrought alloys?

A

alloys that are manipulated and shaped by cold working , often drawn into wire

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2
Q

What are wrought alloys used for?

A
  • orthodontic wires
  • partical denture clasps
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3
Q

What is the composition of steel?

A

Iron - >98%
Carbon - <2%
Chromium - 0.5-1%

Additionally
- manganese
- molybdenum
- silicon
- nickel
- cobalt

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4
Q

What is iron with more than 2% carbon called?

A

cast iron or pig iron

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5
Q

What is the function of chromium in alloys?

A

improved tarnish resistance

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6
Q

What is the function of manganese in alloys?

A

sulphur scavenging

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7
Q

What are the dental functions of steel?

A
  • cutting instruments (>0.8% carbon)
  • medical instruments e.g. forceps (<0.8% carbon)
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8
Q

Describe the allotropic nature of iron

A

Iron undergoes two solid state phase changes with temperature

1 - below 900 degrees
- body centred cubic (BCC) lattice structure
- low carbon solubility (0.05%)

2 - between 900 and 1400 degrees
- face centred cubic (FCC) lattice
- higher carbon solubility (2%)

3 - above 1400 degrees
- same as , below 900 degrees
- body centred cubic (BCC) lattice structure
- low carbon solubility (0.05%)

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9
Q

What happens to the lattice volume of iron between 900-1400 degrees?

A
  • lattice volume decreases
  • exists as a face centred cubic (FCC) lattice
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10
Q

What are the four phases of Fe-C

A
  1. austenite
  2. ferrite
  3. cementite
  4. pearlite
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11
Q

Describe austenite

A
  • interstitial solid solution (FCC)
  • lattice contains iron in rows and columns with carbon interspersed
  • exists at high temperatures (>720 degrees)
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12
Q

Describe ferrite

A
  • very dilute solid solution
  • exists at low temperatures
  • soft and ductile
  • large grains with poor mechanical properties
  • not relevant to dentistry
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13
Q

Describe cementite

A
  • Fe3C
  • exists at low temperatures
  • hard and brittle
  • large grains with poor mechanical properties
  • not relevant to dentistry
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14
Q

Describe pearlite

A
  • eutectoid mixture of ferrite and cementite
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15
Q

What is a solid solution?

A

two metals that are soluble in one another forming a common lattice structure

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16
Q

Describe a random substitutional solid solution

A

the atoms of two metals are arranged in a lattice structure with a random order

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17
Q

Describe an ordered substitutional solid solution

A

the atoms of two metals are arranged in a lattice structure and the type of atom can be predicted based on its location

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18
Q

Describe an interstitial solid solution

A

formed by two different types of atoms which are markedly different in size, the smaller atoms occupy the space between the larger atoms in a lattice in a random manner

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19
Q

What does quenching steel create?

A

martensite

  • not austenite as the Fe-C phase diagram would suggest
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20
Q

Why is martensite created by quenching steel?

A
  • there is no time for carbon to diffuse normally through the iron atoms
  • distorted lattice created
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21
Q

What are the properties of martensite?

A
  • hard
  • brittle
  • not desirable in dentistry
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22
Q

By what process can martensite be made into other forms?

23
Q

Describe the process of tempering

A
  • martensite is heated to 450 degrees then quenched
  • depending on temperature, time heated and quenching, either ferrite or cementite can be produced.
  • mechanical properties can be controlled through heat treatment
24
Q

What is the composition of stainless steel?

A

Iron - 74%
Carbon - 0.1%
Chromium - 12-18%
Nickel - 8-12%

25
What percentage chromium is required for steel to be considered stainless?
>12%
26
What is the function of chromium in stainless steel?
- lowers the transition temperature and conversion rate of austenite to martensite - decreases % carbon at which eutectoid forms
27
Why is stainless steel relevant to dentistry?
- corrosion resistant - can withstand saliva and an acidic pH which usually provoke electrochemical reactions
28
How does chromium make stainless steel corrosion resistant?
it creates a chromium oxide layer - however it can be attacked by chlorides
29
What is the function of nickel in stainless steel?
- lowers the austenite to martensite transition temperature - improves fracture strength - improves corrosion resistance
30
Describe martensitic stainless steel and its role in dentistry
- 12-13% chromium - small quantity of carbon - heat hardenable through tempering - used for dental instruments
31
Describe austenitic stainless steel
- contains sufficient chromium and nickel to suppress austenite to martensite transition
32
What ratios of chromium to nickel can be used for austenitic stainless steel?
- 18:8 - 12:12
33
What are the uses of austenitic stainless steel in dentistry?
Dental equipment and instruments - requiring sterilisation - not used for cutting edges - corrosion resistant rather than strong and hard Wires - orthodontics - readily cold worked (can be bent) - corrosion resistant Denture Bases - sheet form - swaged (adapted to a die)
34
What are the components of 18-8 stainless steel wire? What are its properties?
Chromium - 18% Nickel - 8% Carbon - 0.1% Iron - 74% - high stiffness - good spring back ability - ok ductility - reasonably easy joining
35
What are the properties of 18-8 stainless steel wire?
- does not heat harden - malleable when cast - work hardens rapidly (cannot be repeatedly manipulated)
36
What is cold working?
Work done on a metal/alloy at a low temperature below the recrystallisation temperature - can involve bending, rolling, swaging - cause dislocations to collect at grain boundaries (slip) - dislocations cannot transfer to other grains - stronger, harder material produced
37
Describe the use of 18-8 stainless steel wires
orthodontic appliances - springs - clasps partial dentures - clasp arm - wrought rests
38
What is the grade of stainless steel wire? Give examples
the grade depends on how much bending and manipulation is required - soft - hard - half hard - spring temper
39
What alloys can be used for wires?
- stainless steel - cobalt-chromium - gold - nickel titanium - beta titanium
40
What is the composition of cobalt chromium wire? What are its properties?
Cobalt - 40% Chromium - 20% Nickel - 15% Iron - 16% - high stiffness - ok spring back ability - good ductility - difficult to join
41
What is the composition of gold wire? What are its properties?
Gold - 60% Silver - 15% Copper - 15% Pt/Pd - 10% - medium stiffness - ok springback ability - ok ductility - easy joining, can be soldered - used for slow, restrained tooth movements
42
What is the composition of nickel titanium wire? What are its properties?
Nickel - 55% Titanium - 45% + some cobalt - low stiffness - excellent spring back ability - poor ductility - difficult to join - used for slow movements
43
What is the composition of beta titanium wire? What are its properties?
Mostly titanium with some molybdenum - medium stiffness - good spring back ability - ok ductility - required welding
44
What is springiness?
the ability of a material to undergo large deflections (to form an arc) without permanent deformation - term specific to wires - calculated as elastic limit/Young's modulus
45
What are the desired properties of a wire?
- high springiness - undergo large deflections - no permanent deformation - stiffness - depends on the required force for tooth movement - high ductility - ability to bend without fracture - allows for manipulation to desired configuration - easily joined - joining by soldering or welding - characteristics should not be compromised - corrosion resistant - must not rust in the oral cavity
46
Describe the process of soldering and its risks?
- gold solder or silver solder available - must be careful with silver solder as it must be heated to 700 degrees which is close to the melting point of stainless steel - increased risk of recrystallisation which must be avoided - quench rapidly to maintain ultimate tensile strength
47
What is weld decay?
precipitation of chromium carbides at grain boundaries when heated to between 600-900 degrees. - results in brittle alloy - less chromium located in central region of solid solution - more susceptible to corrosion - limits manipulation possible before fracture
48
What is weld decay?
precipitation of chromium carbides at grain boundaries when heated to between 600-900 degrees. - results in brittle alloy - less chromium located in central region of solid solution - more susceptible to corrosion - limits manipulation possible before fracture
49
How can weld decay be minimised?
- low carbon content steels - very expensive - stabilised stainless steel - contain small quantities titanium or niobium - preferential carbide formation - reduced formation at grain boundaries
50
Describe the process of stress relief annealing for stainless steel wires
- wire heated to 450 degrees for 1-2 minutes - atoms in each grain settle into equilibrium - allows different grade of wire to be created - temperature must not exceed this - precipitation of carbides occurs above 500 degrees - grain structure is affected above 650 degrees
51
How are stainless steel denture bases formed?
- sheet of stainless steel placed between die and counter-die - considerable force applied - sheet of alloy swaged - alloy takes on shape of denture base
52
What are the advantages of a stainless steel denture base?
- considerably thinner than acrylic - light - fracture resistant - corrosion resistant - high polish obtainable (very smooth) - high thermal conductivity - high impact strength - high abrasion resistance
53
What are the disadvantages of a stainless steel denture base?
- potential for dimensional inaccuracy - elastic recovery of steel resulting in inaccuracy - damage caused to die under hydraulic press - loss of fine detail - difficult to ensure uniform thickness - wrinkling of steel due to uneven pressure on die and counter die

BDS3 DMS (10 decks)

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