stainless steel Flashcards
uses
ortho wires
partial denture clasps
wrought alloy
manipulated/shaped by cold working e.g. drawn into wire
steel uses
cutting instruments (>0.8% C) forceps (<0.8% C)
steel composition
iron >98% carbon <2% (above - cast/pig iron) chromium 0.5-1% - improve tarnish resistance manganese - sulphur scavenger molybdenum, silicon, nickel, cobalt
iron is allotropic
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
iron at >1400 degrees
BCC lattice
low carbon solubility (0.05%)
iron at 900-1400 degrees
FCC lattice
higher carbon solubility (2%)
iron at <900 degrees
BCC lattice
low carbon solubility (0.05%)
Fe-C phase diagram
austenite
ferrite
cementite
pearlite
austenite
interstitial solid solution, FCC
exists high temp >720 degrees
ferrite
v dilute solid solution
exists low temp
cementite
Fe3C ppt when solubility exceeded
exists low temp
pearlite
eutectoid mixture of ferrite and cementite
alloys
2 metals that form a common lattice structure
- are soluble in one another
- form a solid solution
types of solid solution
substitutional - random/ordered
interstitial (Fe + C)
liquidus
temp at which they begin to crystallise
solidus
crystallisation ends, now a solid material
quenching of austenite
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
austenite slow cooling (not usually done)
pearlite
- ferrite
- cementite
martensite to pearlite
tempering
martensite tempering
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
main aspect of SS
resistant to corrosion
components of SS
Fe
C
Cr
Ni
types of SS
austenitic
martensitic
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
nickel in SS
lowers austenite to martensite transition temp
improves UTS
improves corrosion resistance
martensitic SS
12-13% chromium and little carbon heat hardenable (tempering) dental instruments cutting instruments hard maintains sharp edge
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
SS wires 18-8 SS composition
18% Cr 8% Ni 0.1% C 74% Fe austenitic
18-8 SS properties
doesn't heat harden can't stress relief anneal malleable when cast but work hardens (cold work) rapidly corrosion resistance
18-8 SS wires uses
ortho appliances - springs and clasps
partial dentures - clasp arms, wrought rests
18-8 SS grades
depends on degree of bending required
- soft
- hard
- half hard
- spring temper
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
alloys - wires CoCr (not RPD) composition
Co 40%
Cr 20%
Ni 15%
Fe 16%
alloys - wires
SS austenitic CoCr NiTi B-Ti gold (approx T4)
alloys - wires NiTi composition
Ni 55%
Ti 45%
and some Co
alloys - wires gold composition
Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%
alloys - wires B-Ti composition
Ti
some molybdenum
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
springiness
EL÷YM
ability of a material to undergo large deflections (to form arc) without permanent deformation i.e. returns to original shape
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” -£££
SS properties
high stiffness
springback ability good
ductility ok
reasonable ease of joining
gold properties
med stiffness
springback ability ok
ok ductility
easy to join - solder
CoCr properties
high stiffness (heat txed)
springback ability ok
good ductility
ease of joining - hard
NiTi properties
low stiffness
excellent springback ability
poor ductility
ease of joining - hard
B-Ti properties
med stiffness
good springback ability
ok ductility
ease of joining - weld
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
minimising weld decay
low carbon content steels - £££ stabilised SS - contain small quantities of titanium or niobium - forms carbides preferentially - not at grain boundaries
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
SS denture base - swaging
pressure applied to a die
SS sheet
counter die
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
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
