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BDS3 DMS > Stainless steel and wrought alloys > Flashcards

Stainless steel and wrought alloys Flashcards

1
Q

What is the definition of Wrought alloy?

A
  • An alloy which can be manipulated/shaped by cold working
  • So it can be drawn into wire
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2
Q

What are some uses of wrought alloys?

A
  • Wires for orthodontics
  • Partial denture clasps
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3
Q

What is steel composed of?

A
  • Iron >98%
  • Carbon <2%
    *Above 2% carbon is regarded as cast iron or pig iron
  • Chromium 0.5-1% to improve tarnish resistance
  • Manganese
  • Nickel, cobalt
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4
Q

What are some uses of steel?

A
  • Cutting instruments (<0.8% C)
  • Forceps (<0.8% C)
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5
Q

What does allotropic mean?

A
  • Undergoes two solid state phases changes with temperature
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6
Q

What temperature does Iron have a Body centred cubic crystalline structure?

A
  • Temp >1400C (low carbon solubility 0.05%)
  • Temp <900C
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7
Q

What temperature does Iron have Face centred cubic lattice structure?

A
  • Temp in between 900 and 1400C
  • Has higher carbon solubility 2%
  • During this temp the Iron lattice expands which is what you’d expect
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8
Q

What is Austenite?

A
  • Interstitial solid solution of Iron
  • Face centred cubic lattice
  • Lattice has iron in rows and columns and some C atoms interspersed
  • Exists at high temp >720C
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9
Q

What is Ferrite?

A
  • Very dilute solid solution of Iron
  • Exists at low temp
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10
Q

What is Cementite?

A
  • Fe3C
  • Exists at low temp
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11
Q

What is Pearlite?

A
  • Eutectoid mixture of Ferrite and Cementite
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12
Q

What is a solid solution?

A
  • Two metals that form a common latice structure
  • Are soluble in one another
  • Two types of solid solution
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13
Q

What are the two types of Substitional solid solution?

A

-Random ( both types of atoms in lattice structure arranged in random fashion)

  • Ordered ( Can predict type of atom based on its location)
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14
Q

What are grains of solid solution?

A
  • Several lattices together
  • Can be random or ordered
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15
Q

What is an Interstitial solid solution?

A
  • Atoms of different sizes (prerequisite)
  • Solute atoms occupy the interstitial (holes between atoms) in crystal lattice of the solute
  • E.g. Fe and C
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16
Q

Findings of a Fe-C phases diagram?

A
  • When Iron cools rapidly the grain structure is Austenite
  • Ferrite and Cementite exist if you slow the molten alloy down extremely slowly (not interested)
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17
Q

What is quenching?

A
  • Rapid cooling of metal to adjust the mechanical properties of its original state
18
Q

What does quenching of steel give us?

A
  • Should give us Austenite according to phase diagram
  • But in practice gives us Martensite
19
Q

What is Martensite?

A
  • Has distorted lattice structure due to carbon being unable to diffuse normally within array of iron atoms in each grain
  • This is hard and brittle
  • Don’t want this
20
Q

What is the composition of stainless steel?

A
  • Iron 72%
  • Carbon <2%
  • Chromium 17-19%
  • Nickel 7-9%
21
Q

When is steel regarded as stainless steel?

A
  • If there is at least 12% chromium
22
Q

What is the role of Chromium in stainless steel?

A
  • Corrosion resistance due to chromium oxide layer it forms on surface
  • Vital as oral environment high risk for corrosion due to saliva, liquids with acidic pH levels
  • But it can be attacked by chlorides
  • Also lowers Austenite to Martensite temp and rate
  • Decreases % C at which Eutectoid formed
23
Q

What is the role of Nickel in stainless steel?

A
  • Lowers Austenite to Martensite transition temp
  • Improves fracture strength
  • Improves corrosion resistance
24
Q

What two types of stainless steel are there?

A
  • Martensitic
  • Austenitic
25
Q

What is Martensitic stainless steel?

A
  • 12-13% Chromium and little carbon
  • Can be hardened by heat during tempering process
  • Useful for dental instruments
26
Q

How can Austenitic stainless steel be made during the process of quenching instead of Martensite?

A
  • Need to have the right proportions of Chromium and Nickel to suppress Austenite to Martensite transition
  • Either 18% Cr and 8% Ni
  • Or 12% Cr and 12%Ni
27
Q

What are some Austenitic stainless steel uses?

A
  • Dental equipment and instruments to be sterilised
  • No useful for cutting edge
  • Corrosion resistance is more imp than strength and hardness
  • Wires for orthodontics (readily cold worked and corrosion resistant)
  • Sheet forms for denture bases (requires it to be swaged)
28
Q

What does swaged mean?

A
  • Stainless steel sheet positioned between die and counter die
  • When these are pressed together a sheet of alloy is swaged
29
Q

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

A
  • Iron 74%
  • Chromium 18%
  • Nickel 8%
  • Carbon 0.1%
30
Q

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

A
  • Does not heat harden
  • Soft (malleable) when cast
  • But work hardens rapidly
31
Q

What is cold working?

A
  • Process of hardening and strengthening metals through plastic deformation at temps below their recrystallisation temp
  • Metal subjected to mechanical stress that causes dislocations in crystal structure
  • The dislocations hinder the motion of atoms and create barriers to slip
  • Makes the metal harder and stronger
  • Excessive cold working can make metal brittle and prone to fracture
32
Q

What are some uses of 18-8 stainless steel wires?

A
  • Orthodontic appliances like springs and clasps
  • Partial dentures like clasp arms and wrought rests
33
Q

What are the grades of stainless steel wire?

A

Soft - Half hard
Hard - Spring temper
- Grade you select depends on how much bending, shape and manipulation required for dental appliance

34
Q

What is springiness/ Springback ability?

A
  • Ability of material to undergo large deflections (to form an arc) without permanent deformation (returns to original shape)
  • Calculated as ratio EL/YM
35
Q

How can stainless steel wires be soldered?

A
  • Use Gold or silver solder
  • Careful as temp rise created is close to melting point of steel so risk of recrystalisation which adversely affects mechanical properties
  • Quench rapidly to avoid this and maintain UTS (ultimate tensile strength)
36
Q

What is Weld decay?

A
  • Occurs between 500-900C
  • Chromium carbides precipitate at grain boundaries particularly where soldering occurred
  • Alloy becomes brittle and more prone to fracture
37
Q

How is weld decay minimised?

A
  1. Low carbon content sheets but this is expensive
  2. Stabilised stainless steel
    - Contain small quantities of Titanium or Niobium
    - Forms carbides preferentially not at grain boundaries
38
Q

Why do stainless steel wires need a Stress relief annealing process?

A
  • Ensures configuration of metal atoms in each of the alloy grains settle into equilibrium
  • Requires temp of stainless steel to be held around 450C for 1-2mins
  • Crucial temp does not exceed this as
  • Grain structure affected if above 650C
  • Precipitation of carbides above 500C
  • Which produce different grades
39
Q

What are the properties of stainless steel denture base?

A
  • Thin 0.11mm
  • Light
  • Fracture resistant
  • Corrosion resistant
  • High polish obtainable
  • High thermal conductivity
  • High impact strength
  • High abrasion resistance
40
Q

What are some disadvantages of stainless steel denture base?

A
  • Poss dimensional inaccuracy as contraction of die not matched by model expansion
  • Elastic recovery of steel leads to inaccuracy
  • Difficult to ensure uniform thickness
  • Uneven pressure on die and counter die leads to wrinkling of steel

BDS3 DMS (10 decks)

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