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stainless steel Flashcards

(52 cards)

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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

Study These Flashcards
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

Study These Flashcards
A

resistant to corrosion

23
Q

components of SS

24
Q

types of SS

Study These Flashcards
A

austenitic

martensitic

25
chromium in SS
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
nickel in SS
lowers austenite to martensite transition temp improves UTS improves corrosion resistance
27
martensitic SS
``` 12-13% chromium and little carbon heat hardenable (tempering) dental instruments cutting instruments hard maintains sharp edge ```
28
austenitic SS
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
SS wires 18-8 SS composition
``` 18% Cr 8% Ni 0.1% C 74% Fe austenitic ```
30
18-8 SS properties
``` doesn't heat harden can't stress relief anneal malleable when cast but work hardens (cold work) rapidly corrosion resistance ```
31
18-8 SS wires uses
ortho appliances - springs and clasps | partial dentures - clasp arms, wrought rests
32
18-8 SS grades
depends on degree of bending required - soft - hard - half hard - spring temper
33
cold work
``` 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
alloys - wires CoCr (not RPD) composition
Co 40% Cr 20% Ni 15% Fe 16%
35
alloys - wires
``` SS austenitic CoCr NiTi B-Ti gold (approx T4) ```
36
alloys - wires NiTi composition
Ni 55% Ti 45% and some Co
37
alloys - wires gold composition
Au 60% Ag 15% Cu 15% Pt/Pd 10%
38
alloys - wires B-Ti composition
Ti | some molybdenum
39
requirements of wires
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
springiness
EL÷YM ability of a material to undergo large deflections (to form arc) without permanent deformation i.e. returns to original shape
41
SS wires soldering
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
SS properties
high stiffness springback ability good ductility ok reasonable ease of joining
43
gold properties
med stiffness springback ability ok ok ductility easy to join - solder
44
CoCr properties
high stiffness (heat txed) springback ability ok good ductility ease of joining - hard
45
NiTi properties
low stiffness excellent springback ability poor ductility ease of joining - hard
46
B-Ti properties
med stiffness good springback ability ok ductility ease of joining - weld
47
weld decay
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
minimising weld decay
``` low carbon content steels - £££ stabilised SS - contain small quantities of titanium or niobium - forms carbides preferentially - not at grain boundaries ```
49
SS wires - stress relief anneal
possible (need care) - 450 degrees, 1-2min grain structure affected >650 degrees ppt of carbides >500 degrees therefore different grades
50
SS denture base - swaging
pressure applied to a die SS sheet counter die
51
advantages of SS denture base
``` thin 0.11mm (acrylic 1.52mm) light fracture resistant corrosion resistant high polish obtainable high thermal conductivity high impact strength high abrasion resistance ```
52
disadvantages of SS denture base
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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