Stainless Steel and Wrought Alloys Flashcards
wrought alloys
manipulated/ shaped by cold working e.g. drawn into wire
wrought alloys uses
- wires (orthodontic)
- partial denture clasps
composition of steel
Iron >98%
Carbon <2%
- Above 2% carbon–> cast iron, pig iron
Other constituents - Chromium (0.5 - 1%) improve tarnish resistance - Manganese sulphur scavenger - Molybdenum, Silicon, Nickel, Cobalt etc
chromium role in SSteel
improve tarnish resistance
maganese role in SSteel
sulphur scavenger
uses of SSteel
Cutting Instruments (>0.8% C) (medical instruments) Forceps etc. (<0.8% C)
iron
key component in steel
iron is
Allotropic - undergoes TWO solid state phase changes with temperature.
- In a solid state is can exist as 2 crystalline forms/phases depending on temperature
- Temp. > 1400C
BCC lattice structure; low Carbon solubility (0.05%) - 900 < Temp. < 1400C
FCC lattice; higher Carbon solubility (2%) - Temp < 900C:
BCC lattice structure; low Carbon solubility (0.05%)
drop in the lattice volume between 900 and 140C – as the lattice re-configures to an FCC form – from a BCC form.
- Note that between these two temperatures the IRON lattice will expand, which you’d expect.
austenite
interstitial solid solution, FCC;
lattice has iron inrows and columns
exists at high temp (ie >720 C)
ferrite
very dilute solid solution; e
exists at low temp
cemenite
Fe3C ;
exists at low temp
pearlite
Eutectoid mixture of Ferrite and Cementite
alloy is
TWO metals that form a COMMON LATTICE structure
- are SOLUBLE in one another
form a SOLID SOLUTION
substitutional solid solution types (2)
RANDOM
- where both types of atoms in the lattice structure – are arranged in random fashion
ORDERED
- Here we can predict the type of atom based on its location.
interstitial solid solution
two atoms are markedly different in size (requirement )
Here IRON occupies all the lattice sites – and the CARBON fits in the spaces, in random fashion
cooling of Fe-C rapidly
grain structure that is locked in is that of AUSTENITE.
So QUENCHING should give us Austenite – according to the phase diagram.
BUT in practice we get MARTENSITE, which behaves quite differently.
quenching of austentite ->
MARTENSITE
NOT supersaturated austenite solution
martensite
- No time for diffusion of Carbon
- Distorted Lattice
- Hard, Brittle
Undesired for dentistry
tempering of steel
heating (450 C) followed by quenching
temperature and duration affect conversion to:
- ferrite (soft, ductile)
- & cementite (hard, brittle)
control over mechanical properties through heat treatment
VERSATILE ALLOY
martensite uses
very useful – in non-dental applications.
- produce materials that are soft or hard – or somewhere in between.
achieved by TEMPERING the material.
- Altering its temperature, and the duration you maintain it at a specific temperature and then quenching it
- will determine the proportion of ferrite and cementite produced.
4 constituents of StSteel
Fe
C
Cr
Ni
chromium in StSteel
STAINLESS if > 12% Cr
- lowers Austenite to Martensite temperature
- lowers Austenite to Martensite rate
- decreases % carbon at which Eutectoid formed
Corrosion resistance
- very relevant in dentistry.
risk of any metal in the oral environment experiencing corrosion – the presence of saliva, liquids with acidic pH levels – all ingredients designed to provoke an electrochemical reaction.
CRUCIALLY S/Steel forms a chromium oxide layer on its surface, which protects it from corrosion. It’s vital BUT can be attacked by chlorides
key role of chromium in StSteel dental appliances
Corrosion resistance
- very relevant in dentistry.
risk of any metal in the oral environment experiencing corrosion – the presence of saliva, liquids with acidic pH levels – all ingredients designed to provoke an electrochemical reaction.
CRUCIALLY S/Steel forms a chromium oxide layer on its surface, which protects it from corrosion. It’s vital BUT can be attacked by chlorides
nickel in StSteel
lowers Austenite to Martensite transition temperature
improves fracture strength
improves corrosion resistance
martensitic StSteel
12 - 13% chromium + little carbon
heat hardenable (tempering process)
dental instruments (not relevant to this lecture)
austenitic StSteel
Earlier in the IRON-CARBON phase diagram, on quenching the alloy, contrary to expectations MARTENSITE not AUSTENITE is produced, away that transition can be suppressed.
- the right proportions of Cr and Ni - specifically, either 18:8 or 12:12 ratio.
Uses
Dental equipment and - instruments -to be sterilised (NOT cutting edge)
- corrosion resistance more important than strength and hardness
- withstands autoclaving
wires e.g. orthodontics
- readily cold worked and corrosion resistant
sheet forms for denture bases
- swaged (adapted to a die)
3 dental uses of austenitic
Dental equipment and - instruments -to be sterilised (NOT cutting edge)
- corrosion resistance more important than strength and hardness
- withstands autoclaving
wires e.g. orthodontics
- readily cold worked and corrosion resistant
sheet forms for denture bases
- swaged (adapted to a die)
stainless steel in wires
18-8 Stainless Steel
18% Chromium
8% Nickel
0.1% Carbon
74% Iron
18:8 stainless steel in wires properties
does NOT heat harden unlike martensitic version
- soft (malleable) when cast
BUT work hardens rapidly
Cold Working
work done on metal /alloy at LOW TEMPERATURE
- below recrystallisation temperature:
e.g. bending, rolling, swaging
- causes SLIP - dislocations collect at grain boundaries
- hence stronger, harder material
aka WORK or STRAIN HARDENING
cold working
work done on metal /alloy at LOW TEMPERATURE
- below recrystallisation temperature:
e.g. bending, rolling, swaging
- causes SLIP - dislocations collect at grain boundaries
- hence stronger, harder material
aka WORK or STRAIN HARDENING
wrought alloys
manipulated/ shaped by cold working e.g. drawn into wire
Use :
- wires (orthodontic)
- partial denture clasps
18:8 stainless steel wires uses
orthodontic appliances - springs & clasps
partial dentures - clasp arms, wrought rests
18:8 stainless steel wires grades
depends on degree of bending required (manipulation etc)
- Soft Half Hard
- Hard Spring Temper
5 alloys - wires
Stainless Steel
Cobalt-Chromium
Gold
Nickel-Titanium (similar to Type IV)
beta-Titanium
stainless steel (austenitic) components
Cr 18%
Ni 8%
C 0.1%
Fe 74%
cobalt chromium (not as partial denture) components
Co 40%
Cr 20%
Ni 15%
Fe 16%
gold (similar to Type IV) components
Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%
Ni-Ti components
Ni 55%
Ti 45%
+ some Cobalt
beta- titanium components
Ti,
some molybdenum
springiness (EL/YM)
Ability of a material to undergo large deflections (to form arc) without permanent deformation (i.e. it returns to its original shape)
Diagram (a) shows we have a straight wire, an alloy.
Diagram (b) shows force applied at each end of the wire to make it form an arc shape – the sort of shape that fits a patient’s dentition
c) And in c) note that when the force at each end is released – the wire rebounds, it springs back to being straight – there’s no deformation
SPRINGINESS is calculated as the ratio ( EL / YM).
We’ll see shortly what the springiness of various alloys are
wires - requirements (5)
high springiness ( EL / YM) - i.e. 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 resistant
alloy properties
stainless steel
stiffness - high spring back ability - good ductility - ok ease of joining - reasonable
alloy properties
gold
stiffness - medium spring back ability - ok ductility - ok ease of joining - easy, solder
alloy properties
Ni Ti
stiffness - low spring back ability - excellent ductility - good ease of joining - difficult
alloy properties
Beta - Ti
stiffness - medium spring back ability - good ductility - ok ease of joining - weld
alloy properties
CoCr
stiffness - high (heat treated) spring back ability - OK ductility - GOOD ease of joining - difficult
comparisons of alloys
S/Steel is satisfactory across the board; CoCr is pretty similar except for the ease of joining several wire components.
Gold is satisfactory too, though its rigidity is less – this makes it suited to scenarios where a more restrained rate of movement of the dentition is needed.
NiTi is excellent for its springiness, and is ideal for moving teeth slowly. The main challenge here is in joining NiTi wires together.
S/Steel soldering
By - Gold solder - Silver solder (Melting point < 700 0C) avoid recrystallisation – adversely affect mechanical properties quench rapidly to maintain UTS
Care has to be taken though as the temperature rise created is close to the melting point of s/steel.
weld decay
Occurs between 500 - 900 C
Chromium carbides precipitate at grain boundaries
- alloy becomes brittle - limits the amount of manipulation of the wire to match the desired configuration
- more susceptible to corrosion
less chromium in central region of solid solution
minimise weld decay by (2)
- Low carbon content steels - expensive
- Stabilised stainless steel
- contain small quantities of TITANIUM or NIOBIUM
- forms carbides preferentially
- not at grain boundaries
stress relieved anneal - stainless steel wires
Possible (need care)
- 450 C, 1 - 2 min – does not exceed – change in grain structure or metal carbides at grain boundaries
Grain structure affected above 650 C
Precipitation of carbides above 500 C
Hence different grades
Care needed
swaging of S/STeel denture base
S/steel sheet positioned between a die and counter-die.
When these are pressed together the sheet of alloy is SWAGED – taking on board the shape of the denture base.
advantages of swaged SSteel denture base (8)
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 Swages S/Steel (6)
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
