Stainless Steel Flashcards
what category does stainless steel come under
it is a wrought alloy
what is a wrought alloy
can be manipulated / shaped by cold working i.e. it can be drawn into a wire
uses of stainless steel
orthodontic wires and partial denture clasps
components of steel
> 98% iron
<2% carbon
0.5-1% chromium (to increase tarnish resistance)
what is cast / pig iron
when the carbon content is above 2%
uses of stainless steel
cutting instruments >0.8% C
medical instruments e.g. forceps <0.8% C
iron is allotropic, what does this mean
in a solid state it can exist in 2 crystalline forms (2 phases) depending on the temperature
examples of iron acting as an allotrope
900 < temp < 1400C = face centred cubic lattice (FCC) with a higher carbon solubility (2%)
temp <900 or >1400C = body centred cubic lattice (BCC) with a low carbon solubility (0.05%)
what 4 things are found in a Fe-C diagram
- austenite
- ferrite
- cementite
- pearlite
austenite
interstitial solution, FCC; exists as high temp i.e. >720 C
ferrite
very dilute solid solution, exists at low temp
cementite
Fe3C exists at low temp
pearlite
eutectoid mixture of ferrite & cementite
what is an alloy
2 metals that form a common lattice structure, are soluble in one another and form a solid solution
what are 2 types of solid solution
substitutional & interstitial
2 types of substitutional solid solution
a) random - where both types of atoms in the lattice structure are arranged in a random fashion
b) ordered - can predict the type of atom based on its location
interstitial solid solution
2 atoms are markedly different in size e.g. iron occupies all the lattice sites and the carbon fits in the spaces in a random fashion
rapidly cooling FeC gives
grain structure locked in to make austenite
slowly cooling molten alloy FeC gives
ferrite & cementite but also large grains with poor mechanical properties which is not ideal
what happens when you quench FeC
should give austenite but in reality gives martensite
what is martensite
has a distorted lattice structure as C is unable to diffuse normally within the array of Fe atoms in each grain meaning it forms a hard & brittle material - don’t want
what is tempering
heating followed by quenching which allows martensite to become a versatile alloy but not for dental application
slow cooling austenite gives
pearlite (ferrite & cementite)
fast cooling austenite gives
martensite (distorted lattice)
tempering martensite gives
pearlite
what components make up stainless steel
Fe
C
Cr (>12%)
Ni
Cr function in stainless steel
only stainless if >12% Cr
lowers austenite to martensite temp and conversion
decreases % C at which eutectoid forms
Ni function in stainless steel
to improve fracture strength UTS and corrosion resistance
lowers austenite to martensite temperatures
what 2 forms does stainless steel come in
- martensitic SS
- austenitic SS
martensitic SS
- 12-13% Cr & little C
- used to make dental instruments
- tempered to produce very hard materials
austenitic SS
contains sufficient Cr & Ni to suppress austenite to martensite transition e.g. 18% Cr + 8% Ni
uses of austenitic SS
- dental equipment & instruments - to be sterilised (not cutting edge) corrosion resistance important
- wires e.g. orthodontics, readily cold worked, corrosion resistant
- sheet forms e.g. for denture bases
characteristics of 18-8 SS
doesn’t heat harden unlike martensitic version, it is soft & malleable when cast but work hardens rapidly so it cannot be repeatedly manipulated to form desired shape
what does cold working only apply to
alloys
what is cold working
work done on alloy at low temperature i.e. below recrystallisation temp. work done inc bending, rolling, swaging which causes slip (dislocations in grain being moved to grain boundaries where they stay). alloy then has fewer defects within lattice making up each grain so it is harder and stronger
how are wrought alloys shaped
by cold working and drawn into wires; diameter of which depends on how this is done
what is spring back ability
(EM/YM) ability of material to undergo large deflections (to form an arc) without permanent deformation i.e. returns to its original shape
requirements for wires
- high springiness
- stiffness (for force for tooth movement)
- high ductility
- easily joined
- corrosion resistance
properties of SS
high stiffness
good springback ability
okay ductility
reasonable ease of joining
properties of gold alloy
medium stiffness
okay springback ability
okay ductility
ease of joining good, solder
properties of CoCr
high stiffness (heat treated)
okay springback ability
good ductility
ease of joining difficult
properties of NiTi
low stiffness
great springback ability
poor ductility
ease of joining is difficult
properties of beta Ti
medium stiffness
good springback ability
okay ductility
ease of joining - weld
what is a risk when welding SS
weld decay
what is weld decay
occurs between 500 - 900C. chromium carbides precipitate at grain boundaries forming CrC thats causes SS to become brittle so it is able to undergo only a small amount of deformation before fracturing thus limiting manipulation of the wire & makes it more likely to corrode
how to minimise weld decay
- low carbon content steels but this is expensive
- stabilised SS which contains small amounts of titanium/niobium to form carbides preferentially so limiting prospects of CrC forming at grain boundaries
why stress relief anneal SS
to ensure configuration of metal atoms (Ni, Cr, Fe) in each of the alloy grains to settle into an equilibrium
process of stress relief annealing of SS
requires temp of SS to be held at 450C for 1-2mins. crucial that it doesn’t exceed this value as it will adversely affect properties either through a change in grain structure or due to metal carbides forming at grain boundary
what is swaging
a metal forming technique where the metal of one part is deformed to fit around another part by hammering or forcing the material through a die. SS denture base requires swaging
advantages of SS denture base
thin compared to acrylic
light
fracture resistant
corrosion resistant
high polish attainable
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 detail during many stages
- difficult to ensure uniform thickness
- uneven pressure on die & counter die - wrinkling of steel
