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DMS :( > stainless steel > Flashcards

stainless steel Flashcards

1
Q

uses

A

ortho wires

partial denture clasps

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2
Q

wrought alloy

A

manipulated/shaped by cold working e.g. drawn into wire

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3
Q

steel uses

A 
cutting instruments (>0.8% C)
forceps (<0.8% C)
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4
Q

steel composition

A 
iron >98%
carbon <2% (above - cast/pig iron)
chromium 0.5-1% - improve tarnish resistance
manganese - sulphur scavenger
molybdenum, silicon, nickel, cobalt
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5
Q

iron is allotropic

A

undergoes 2 solid state phase changes with temp
- can change from one solid state to another
undergoes a change in its crystal structure depending on temp

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6
Q

iron at >1400 degrees

A

BCC lattice

low carbon solubility (0.05%)

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7
Q

iron at 900-1400 degrees

A

FCC lattice

higher carbon solubility (2%)

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8
Q

iron at <900 degrees

A

BCC lattice

low carbon solubility (0.05%)

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9
Q

Fe-C phase diagram

A

austenite
ferrite
cementite
pearlite

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10
Q

austenite

A

interstitial solid solution, FCC

exists high temp >720 degrees

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11
Q

ferrite

A

v dilute solid solution

exists low temp

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12
Q

cementite

A

Fe3C ppt when solubility exceeded

exists low temp

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13
Q

pearlite

A

eutectoid mixture of ferrite and cementite

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14
Q

alloys

A

2 metals that form a common lattice structure

  • are soluble in one another
  • form a solid solution
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15
Q

types of solid solution

A

substitutional - random/ordered

interstitial (Fe + C)

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16
Q

liquidus

A

temp at which they begin to crystallise

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17
Q

solidus

A

crystallisation ends, now a solid material

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18
Q

quenching of austenite

A 
martensite
NOT supersaturated austenite solution
no time for diffusion of carbon and rearrangement of atoms so can't get ferrite and cementite
distorted lattice
hard, brittle
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19
Q

austenite slow cooling (not usually done)

A

pearlite

  • ferrite
  • cementite
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20
Q

martensite to pearlite

A

tempering

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21
Q

martensite tempering

A

reheating (450 degrees) then quenching
temp and duration affect conversion to pearlite
- ferrite (soft, ductile)
- cementite (hard, brittle)
control over mechanical properties through heat tx
versatile alloy

22
Q

main aspect of SS

A

resistant to corrosion

23
Q

components of SS

A

Fe
C
Cr
Ni

24
Q

types of SS

A

austenitic

martensitic

25
Q

chromium in SS

A

stainless if >13%
reduces austenite to martensite temp
reduces austenite to martensite rate
reduces % carbon at which eutectoid formed
corrosion resistance - passivation chromium oxide layer
BUT can be attacked by chlorides

26
Q

nickel in SS

A

lowers austenite to martensite transition temp
improves UTS
improves corrosion resistance

27
Q

martensitic SS

A 
12-13% chromium and little carbon
heat hardenable (tempering)
dental instruments
cutting instruments
hard
maintains sharp edge
28
Q

austenitic SS

A

sufficient Cr and Ni to suppress austenite to martensite transition
e.g. 18% Cr 8% Ni 12% Cr 12% Ni
dental equipment and instruments - to be sterilised (not cutting edge)
- corrosion resistance more important than strength and hardness
wires e.g. ortho, readily cold worked, corrosion resistant
sheet forms for denture bases - swaged - adapted to a die

29
Q

SS wires 18-8 SS composition

A 
18% Cr
8% Ni
0.1% C
74% Fe
austenitic
30
Q

18-8 SS properties

A 
doesn't heat harden 
can't stress relief anneal
malleable when cast
but work hardens (cold work) rapidly
corrosion resistance
31
Q

18-8 SS wires uses

A

ortho appliances - springs and clasps

partial dentures - clasp arms, wrought rests

32
Q

18-8 SS grades

A

depends on degree of bending required

  • soft
  • hard
  • half hard
  • spring temper
33
Q

cold work

A 
work done on metal at low temp
 - below recrystallisation temp
e.g. bending, rolling, swaging
causes slip - dislocations correct at grain boundaries
stronger, harder material
work/strain hardening
34
Q

alloys - wires CoCr (not RPD) composition

A

Co 40%
Cr 20%
Ni 15%
Fe 16%

35
Q

alloys - wires

A 
SS austenitic
CoCr
NiTi
B-Ti
gold (approx T4)
36
Q

alloys - wires NiTi composition

A

Ni 55%
Ti 45%
and some Co

37
Q

alloys - wires gold composition

A

Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%

38
Q

alloys - wires B-Ti composition

A

Ti

some molybdenum

39
Q

requirements of wires

A

high springiness (EL÷YM) - undergo large deflections without permanent deformation
stiffness (YM) - depends on required force for tooth movement
high ductility - bending without fracture
easily joined without impairing properties - soldered, welded
corrosion resistance

40
Q

springiness

A

EL÷YM
ability of a material to undergo large deflections (to form arc) without permanent deformation i.e. returns to original shape

41
Q

SS wires soldering

A

use: gold solder, silver solder (mp <700 degrees)
- avoid recrystallisation
- quench rapidly to maintain UTS

since melting point of solder is near SS mp - may use NiCr (20% 80%) alloy “Nichrome” -£££

42
Q

SS properties

A

high stiffness
springback ability good
ductility ok
reasonable ease of joining

43
Q

gold properties

A

med stiffness
springback ability ok
ok ductility
easy to join - solder

44
Q

CoCr properties

A

high stiffness (heat txed)
springback ability ok
good ductility
ease of joining - hard

45
Q

NiTi properties

A

low stiffness
excellent springback ability
poor ductility
ease of joining - hard

46
Q

B-Ti properties

A

med stiffness
good springback ability
ok ductility
ease of joining - weld

47
Q

weld decay

A

occurs between 500-900 degrees
chromium carbides ppt at grain boundaries
alloy becomes brittle
less chromium in central region of solid solution
more susceptible to corrosion

48
Q

minimising weld decay

A 
low carbon content steels - £££
stabilised SS
 - contain small quantities of titanium or niobium 
 - forms carbides preferentially
 - not at grain boundaries
49
Q

SS wires - stress relief anneal

A

possible (need care) - 450 degrees, 1-2min
grain structure affected >650 degrees
ppt of carbides >500 degrees
therefore different grades

50
Q

SS denture base - swaging

A

pressure applied to a die
SS sheet
counter die

51
Q

advantages of SS denture base

A 
thin 0.11mm (acrylic 1.52mm)
light
fracture resistant
corrosion resistant
high polish obtainable
high thermal conductivity
high impact strength
high abrasion resistance
52
Q

disadvantages of SS denture base

A

possible dimensional inaccuracy (contraction of die not matched by model expansion)
elastic recovery of steel - 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

DMS :( (18 decks)

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