Stainless Steel and Wrought Alloys Flashcards
how are wrought alloys shaped
by cold working
what are uses of wrought alloys
wires (orthodontic) and partial denture clasps
what is the composition of steel
iron - 98%
carbon - 2%
chromium
manganese
what is the function of chromium in steel
improves tarnish resistance
how many solid state changes does iron go through with temperature
2
this is known as allotropic
what happens to iron at temperatures over 1400 degrees
BCC lattice structure, low carbon solubility
what happens to iron at temperatures between 900 and 1400
FCC lattice - higher carbon solubility
what happens to iron at temperatures under 900 degrees
BCC lattice structure - low carbon solubility
what is austenite and when does it exist
it is the initial solid solution which exists at high temperature
what is ferrite and when does it exist
it’s a very dilute solid solution
exists at a low temperature
when does cemenite exist
low temperatures
what is pearlite
eutectoid mixture of ferrite and cemenite
what is the definition of an alloy
2 metals that form a common lattice structure, are soluble in one another and form a solid solution
why is marsenite not a desired product
it is hard and brittle
what does fast cooling of austenite result in
pearlite
what does fast cooling of austenite result in
martensite
what does tempering of martensite, a distorted lattice thats hard and brittle, give
pearlite
how is martensite tempered
heated to 450 then quenched
what are the four constituents of stainless steel
iron
carbon
chromium
nickel
what level of chromium is required to regard steel as stainless
at least 12%
what causes stainless steel to be corrosion resistant
chromium oxide layer
what can stainless steel be attacked by that is very important to remember
chlorides
what are the benefits of nickel
improves fracture strength
improves corrosion resistance
lowers austenite to martensite transition temperature
what two forms does stainless steel come in
autentic
martensitic
what is the martensitic stainless steel used for
dental instruments
how does autentic stainless steel prevent from converting to martensite
contains sufficient chromium and nickel
what is autentic stainless steel used for
dental equipment that isn’t cutting edge
orthodontic wires
sheet forms for denture bases
does stainless steel heat harden
no
how is the presentation of stainless steel when cast
soft/ malleable
why can stainless steel NOT be manipulated repeatedly
due to rapid work hardening
what is cold working
work done on metal/ alloy at a low temperature below the recrystallisation temperature and causes slip
what is slip
dislocations collect at grain boundaries which results in a stronger/ harder material
what are wrought alloys used for
wires and RPD clasps
what are 18 chromium -8 nickel stainless steel wires used for
ortho appliances (springs and clasps)
partial dentures (clasp arms)
what are the grades of 18-8 stainless steel
soft
half hard
hard
spring temper
when does weld decay occur
between 500 and 900 degrees celsius
what causes weld decay
chromium carbides precipitate at grain boundaries and the alloy becomes brittle which limits manipulation of the wire - less chromium in the centre of the solid solution makes it more susceptible to corrosion
how do you minimise weld decay
low carbon content steel
stabilised stainless steel
what does stabilised stainless steel consist of
contain small amounts of titanium or niobium
forms carbides preferentially
not at grain boundaries
advantages of using stainless steel as denture base
fracture resistant
corrosion resistant
high polish attainable
high thermal conductivity
high impact strength
what are disadvantages of stainless steel as a denture base
possible dimensional inaccuracy
loss of fine detail during many stages
difficulty to ensure uniform thickness
