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BDS3 DMS > 2 - Steel alloys > Flashcards

2 - Steel alloys Flashcards

1
Q

Define a wrought alloy.

A

Manipulated by cold working (ie drawn into a wire)

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2
Q

Define steel.

A
  • iron >98%
  • carbon <2%
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3
Q

What are some common materials added to steel?

A
  • chromium (improve tarnish resistance)
  • manganese (sulphur scavenger)
  • molybdenum
  • silicon
  • nickel
  • cobalt
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4
Q

Define allotropic.

A

Undergoes 2 solid states, phase changes with temperature.

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5
Q

Describe iron’s structure at > 1400 degrees.

A
  • body centric cubic lattice
  • low carbon solubility
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6
Q

Describe iron’s structure at < 1400 degrees.

A
  • face centred cubic lattice
  • high carbon solubility
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7
Q

Define austenite.

A
  • interstitial solid solution (FCC)
  • exists at high temperature
  • iron in lattice, carbon interspersed
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8
Q

Define ferrite.

A
  • very dilute solid solution
  • exists at low temperature
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9
Q

Define cementite.

A
  • Fe3C
  • exists at low temperature
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10
Q

Define pearlite.

A

Eutectoid mixture of ferrite and cementite

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11
Q

Define eutectoid.

A

The minimum temperature at which a material exists as a single solid solution phase

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12
Q

Define martensite.

A
  • when austenite is quenched
  • C atoms aren’t able to diffuse into lattice
  • distorted lattice
  • hard and brittle
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13
Q

How does the cooling process affect austenite?

A

Fast cooling = martensite
Slow cooling = pearlite

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14
Q

How does martensite turn into pearlite?

A
  • tempering
  • heated to 450 degrees then quenched
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15
Q

What is the composition of stainless steel?

A
  • Fe
  • C
  • Cr
  • Ni
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16
Q

Describe chromium’s role in stainless steel.

A

> 12% Cr
- lowers austenite to martensite temperature
- lowers austenite to martensite rate
- decreases the % of C at which the eutectoid is formed
- chromium oxide prevents corrosion

17
Q

Describe nickel’s role in stainless steel.

A
  • lowers austenite to martensite transition temperature
  • improves fracture strength
  • improves corrosion resistance
18
Q

Define martensitic stainless steel.

A
  • 12-13% chromium, little carbon
  • heat hardenable
  • used in dental instruments
19
Q

Define austenitic stainless steel.

A
  • contains enough chromium and nickel to suppress austenite to martensite transition
  • 18% Cr, 8% Ni
  • 12% Cr, 12% Ni
20
Q

What are the uses of austenitic stainless steel?

A
  • dental equipment and instruments (not cutting)
  • orthodontic wires
  • denture bases (swaged into a sheet)
21
Q

What is the composition of 18-8 stainless steel?

A

18% chromium
8% nickel
0.1% carbon
74% iron

22
Q

What are the properties of 18-8 stainless steel?

A
  • does not heat harden
  • malleable when cast but work hardens rapidly, cannot be repeated manipulated to get desired shape
23
Q

What are the uses of 18-8 stainless steel?

A
  • orthodontic springs and clasps
  • RPD clasp arms and rests
24
Q

What are the grades of 18-8 stainless steel?

A

Chosen based on degree of bending required
- soft
- hard
- half hard
- spring temper

25
Q

What are examples of alloys used in wires?

A
  • stainless steel
  • cobalt chromium
  • gold
  • nickel titanium
  • beta-titanium
26
Q

What is the composition of cobalt chromium (wire)?

A

40% cobalt
20% chromium
15% nickel
16% iron

27
Q

What is the composition of gold alloy (wire)?

A

60% gold
15% silver
15% copper
10% palladium

28
Q

What is the composition of nickel titanium?

A

55% nickel
45% titanium
+ some cobalt

29
Q

What is the composition of beta-titanium?

A
  • titanium
  • molybdenum
30
Q

Define springiness.

A

Ability of materials to undergo large deflections (form an arc) without permanent deformation

31
Q

What is the best properties for a wire?

A
  • high springiness
  • high ductility
  • easily joined
  • corrosion resistant
  • stiffness depends on function
32
Q

What is used to solder stainless steel wires?

A
  • gold solder
  • silver solder
33
Q

What is weld decay?

A
  • occurs between 500 - 900 degrees
  • chromium carbines precipitate at grain boundaries
  • alloy becomes brittle
  • more susceptible to corrosion
34
Q

How can weld decay be avoided?

A
  • low carbon content steel (expensive)
  • stabilised stainless steel (by titanium or niobium)
35
Q

Define swaging.

A

Thin sheet of metal is shaped by a die and counter-die

36
Q

What are the advantages of stainless steel as a denture base?

A
  • thin
  • light
  • fracture resistant
  • corrosion resistant
  • high polish obtainable
  • high thermal conductivity
  • high impact strength
  • high abrasion resistance

BDS3 DMS (9 decks)

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