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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?

A

tempering

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
Q

What percentage chromium is required for steel to be considered stainless?

A

> 12%

26
Q

What is the function of chromium in stainless steel?

A
  • lowers the transition temperature and conversion rate of austenite to martensite
  • decreases % carbon at which eutectoid forms
27
Q

Why is stainless steel relevant to dentistry?

A
  • corrosion resistant
  • can withstand saliva and an acidic pH which usually provoke electrochemical reactions
28
Q

How does chromium make stainless steel corrosion resistant?

A

it creates a chromium oxide layer

  • however it can be attacked by chlorides
29
Q

What is the function of nickel in stainless steel?

A
  • lowers the austenite to martensite transition temperature
  • improves fracture strength
  • improves corrosion resistance
30
Q

Describe martensitic stainless steel and its role in dentistry

A
  • 12-13% chromium
  • small quantity of carbon
  • heat hardenable through tempering
  • used for dental instruments
31
Q

Describe austenitic stainless steel

A
  • contains sufficient chromium and nickel to suppress austenite to martensite transition
32
Q

What ratios of chromium to nickel can be used for austenitic stainless steel?

A
  • 18:8
  • 12:12
33
Q

What are the uses of austenitic stainless steel in dentistry?

A

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
Q

What are the components of 18-8 stainless steel wire?
What are its properties?

A

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

  • high stiffness
  • good spring back ability
  • ok ductility
  • reasonably easy joining
35
Q

What are the properties of 18-8 stainless steel wire?

A
  • does not heat harden
  • malleable when cast
  • work hardens rapidly (cannot be repeatedly manipulated)
36
Q

What is cold working?

A

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
Q

Describe the use of 18-8 stainless steel wires

A

orthodontic appliances
- springs
- clasps

partial dentures
- clasp arm
- wrought rests

38
Q

What is the grade of stainless steel wire? Give examples

A

the grade depends on how much bending and manipulation is required

  • soft
  • hard
  • half hard
  • spring temper
39
Q

What alloys can be used for wires?

A
  • stainless steel
  • cobalt-chromium
  • gold
  • nickel titanium
  • beta titanium
40
Q

What is the composition of cobalt chromium wire?
What are its properties?

A

Cobalt - 40%
Chromium - 20%
Nickel - 15%
Iron - 16%

  • high stiffness
  • ok spring back ability
  • good ductility
  • difficult to join
41
Q

What is the composition of gold wire?
What are its properties?

A

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
Q

What is the composition of nickel titanium wire?
What are its properties?

A

Nickel - 55%
Titanium - 45%
+ some cobalt

  • low stiffness
  • excellent spring back ability
  • poor ductility
  • difficult to join
  • used for slow movements
43
Q

What is the composition of beta titanium wire?
What are its properties?

A

Mostly titanium with some molybdenum

  • medium stiffness
  • good spring back ability
  • ok ductility
  • required welding
44
Q

What is springiness?

A

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
Q

What are the desired properties of a wire?

A
  • 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
Q

Describe the process of soldering and its risks?

A
  • 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
Q

What is weld decay?

A

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
Q

What is weld decay?

A

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
Q

How can weld decay be minimised?

A
  • low carbon content steels
    • very expensive
  • stabilised stainless steel
    • contain small quantities titanium or niobium
    • preferential carbide formation
    • reduced formation at grain boundaries
50
Q

Describe the process of stress relief annealing for stainless steel wires

A
  • 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
Q

How are stainless steel denture bases formed?

A
  • 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
Q

What are the advantages of a stainless steel denture base?

A
  • considerably thinner than acrylic
  • light
  • fracture resistant
  • corrosion resistant
  • high polish obtainable (very smooth)
  • high thermal conductivity
  • high impact strength
  • high abrasion resistance
53
Q

What are the disadvantages of a stainless steel denture base?

A
  • 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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