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dental material science > stainless steel and wrought alloys > Flashcards

stainless steel and wrought alloys Flashcards

1
Q

name two uses for stainless steel in dentistry

A

in orthodontic appliances
as a denture base material

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2
Q

define a ‘wrought alloy’

A

an alloy which can be manipulated/shaped by cold working

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3
Q

what are the components of steel?

A

98% iron
2% carbon
0.5-1% chromium
also contains manganese, molybdenum, silicon, nickel, cobalt

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4
Q

what would steel have to be classed as if it contained any more than 2% carbon?

A

cast iron or PIG iron

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5
Q

what is the role of chromium in steel?

A

improves tarnish resistance

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6
Q

what is the role of manganese in steel?

A

sulphur scavenger

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7
Q

name a use for steel in dentistry?

A

instruments e.g. forceps or cutting instruments

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8
Q

explain how iron is allotropic

A

in a solid state, it can exist in two crystalline forms
<900C/>1400C= body centred cubic crystalline structure
>900C/<1400C= forms a face centred cubic lattice structure

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9
Q

what causes the step change in the Fe-C phase diagram?

A

a drop in the lattice volume as it reconfigures to an FCC form from a BCC form

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10
Q

what happens to the iron lattice during the step change in the Fe-C phase diagram?

A

it expands

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11
Q

describe the structure of austenite

A

an interstitial solid solution
lattice has iron in rows and columns with some C atoms interspersed

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12
Q

at what temperature does austenite exist?

A

> 720C

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13
Q

what is pearlite a mixture of?

A

an eutectoid mixture of ferrite and cementite

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14
Q

name 4 types of solid solution

A
  1. random substitutional solid solution
  2. ordered solid solution
  3. grains of solid solution
  4. interstitial solid solution
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15
Q

describe a random substitutional solid solution

A

both types of atoms un the lattice structure are arranged in random fashion

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16
Q

describe a grains of solid solution

A

lattices/grains made up of two types of atom
the grains are random substitutional solid solutions

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17
Q

describe an interstitial solid solution

A

the two atoms are markedly different in size
e.g. iron occupies all the lattice sites and carbon fits in the spaces, in random fashion

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18
Q

when cooling steel rapidly, which grain structure is locked in?

A

austenite

19
Q

according to the Fe-C phase diagram, quenching steel should give us austenite but this is not what happens in practice- what is produced?

A

martensite

20
Q

why does martensite have a distorted lattice structure?

A

carbon is unable to diffuse normally within the array of iron atoms in each grain - forms a hard and brittle material

21
Q

describe the difference in mechanical properties of ferrite and cementite?

A

F- soft and ductile
C- hard and brittle

22
Q

what are the 4 components of stainless steel?

A

iron
carbon
chromium
nickel

23
Q

what are the roles of chromium in stainless steel?

A
  • lowers the austenite to martensite temperature and the conversion rate
  • decreases the percentage of carbon at which eutectoid is formed
  • corrosion resistance
24
Q

what are the roles of nickel in stainless steel?

A
  • lowers austenite to martensite transition temperature
  • improves fracture strength
  • improves corrosion resistance
25
Q

what are the two forms of stainless steel and their uses?

A

autenitic- ortho wire
martensitic- dental instruments

26
Q

what are the advantages of using stainless steel as a denture base?

A

thin
light
fracture resistant
corrosion resistant
high polish obtainable- conducts heat readily
high impact strength
high abrasion resistance

27
Q

what are the disadvantages of using stainless steel as a denture base?

A

possible dimensional inaccuracy
elastic recovery of steel - can lead to 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

28
Q

what is the purpose of stainless steel wire stress relief?

A

to ensure the configuration of the metal atoms in each of the alloy grains settle into an equilibrium

29
Q

what may happen if stainless steel exceeds 450C during stress relief?

A

grain structure may be changed or carbides may form at the grain boundaries

30
Q

what temperature does weld decay occur at?

A

500-900C

31
Q

what happens during weld decay?

A

chromium carbides precipitate at grain boundaries- alloy becomes brittle
less chromium in central region of solid solution- more susceptible to corrosion

32
Q

what happens if cobalt chromium forms at the grain boundaries of stainless steel?

A

becomes brittle
more likely to corrode

33
Q

how are the risks of weld decay minimised?

A

low carbon content steels
stabilised s/steel with small amount of titanium/niobium forming carbides preferentially- limits prospects of cobalt chromium forming at grain boundaries

34
Q

what are the components of cobalt chromium wire?

A

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

35
Q

what are the components of gold wire?

A

60% gold
15% silver
15% copper

36
Q

what are the components of nickel-titanium wire?

A

55% nickel
45% titanium
also some cobalt

37
Q

define ‘spring back ability’

A

the ability of a material to undergo large deflections without permanent deformation

38
Q

how is springiness of a material calculated?

A

EL/YM

39
Q

what characteristics should the chosen wire (e.g. for ortho) have?

A

appropriate rigidity
high level of ductility so it can be manipulated
good strength and corrosion resistance

40
Q

at what ratio of chromium and nickel is the transition of austenite to martensite suppressed in austenitic s/steel ?

A

18:8 or 12:12%

41
Q

what % of martensitic stainless steel is chromium?

A

12%

42
Q

describe some of the features of 18:8 stainless steel

A

does not heat harden
soft and malleable when cast
work hardens quickly

43
Q

slip can sometimes occur during cold working. what does this mean?

A

defects/dislocations in a grain being moved along the grain boundaries
grains cant jump to another grain meaning alloy has fewer defects in the lattice structure making up each grain
alloy is now stronger and harder

dental material science (2 decks)

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