stainless steel and wrought alloys Flashcards
name two uses for stainless steel in dentistry
in orthodontic appliances
as a denture base material
define a ‘wrought alloy’
an alloy which can be manipulated/shaped by cold working
what are the components of steel?
98% iron
2% carbon
0.5-1% chromium
also contains manganese, molybdenum, silicon, nickel, cobalt
what would steel have to be classed as if it contained any more than 2% carbon?
cast iron or PIG iron
what is the role of chromium in steel?
improves tarnish resistance
what is the role of manganese in steel?
sulphur scavenger
name a use for steel in dentistry?
instruments e.g. forceps or cutting instruments
explain how iron is allotropic
in a solid state, it can exist in two crystalline forms
<900C/>1400C= body centred cubic crystalline structure
>900C/<1400C= forms a face centred cubic lattice structure
what causes the step change in the Fe-C phase diagram?
a drop in the lattice volume as it reconfigures to an FCC form from a BCC form
what happens to the iron lattice during the step change in the Fe-C phase diagram?
it expands
describe the structure of austenite
an interstitial solid solution
lattice has iron in rows and columns with some C atoms interspersed
at what temperature does austenite exist?
> 720C
what is pearlite a mixture of?
an eutectoid mixture of ferrite and cementite
name 4 types of solid solution
- random substitutional solid solution
- ordered solid solution
- grains of solid solution
- interstitial solid solution
describe a random substitutional solid solution
both types of atoms un the lattice structure are arranged in random fashion
describe a grains of solid solution
lattices/grains made up of two types of atom
the grains are random substitutional solid solutions
describe an interstitial solid solution
the two atoms are markedly different in size
e.g. iron occupies all the lattice sites and carbon fits in the spaces, in random fashion
when cooling steel rapidly, which grain structure is locked in?
austenite
according to the Fe-C phase diagram, quenching steel should give us austenite but this is not what happens in practice- what is produced?
martensite
why does martensite have a distorted lattice structure?
carbon is unable to diffuse normally within the array of iron atoms in each grain - forms a hard and brittle material
describe the difference in mechanical properties of ferrite and cementite?
F- soft and ductile
C- hard and brittle
what are the 4 components of stainless steel?
iron
carbon
chromium
nickel
what are the roles of chromium in stainless steel?
- lowers the austenite to martensite temperature and the conversion rate
- decreases the percentage of carbon at which eutectoid is formed
- corrosion resistance
what are the roles of nickel in stainless steel?
- lowers austenite to martensite transition temperature
- improves fracture strength
- improves corrosion resistance
what are the two forms of stainless steel and their uses?
autenitic- ortho wire
martensitic- dental instruments
what are the advantages of using stainless steel as a denture base?
thin
light
fracture resistant
corrosion resistant
high polish obtainable- conducts heat readily
high impact strength
high abrasion resistance
what are the disadvantages of using stainless steel as a denture base?
possible dimensional inaccuracy
elastic recovery of steel - can lead to 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
what is the purpose of stainless steel wire stress relief?
to ensure the configuration of the metal atoms in each of the alloy grains settle into an equilibrium
what may happen if stainless steel exceeds 450C during stress relief?
grain structure may be changed or carbides may form at the grain boundaries
what temperature does weld decay occur at?
500-900C
what happens during weld decay?
chromium carbides precipitate at grain boundaries- alloy becomes brittle
less chromium in central region of solid solution- more susceptible to corrosion
what happens if cobalt chromium forms at the grain boundaries of stainless steel?
becomes brittle
more likely to corrode
how are the risks of weld decay minimised?
low carbon content steels
stabilised s/steel with small amount of titanium/niobium forming carbides preferentially- limits prospects of cobalt chromium forming at grain boundaries
what are the components of cobalt chromium wire?
40% cobalt
20% chromium
15% nickel
16% iron
what are the components of gold wire?
60% gold
15% silver
15% copper
what are the components of nickel-titanium wire?
55% nickel
45% titanium
also some cobalt
define ‘spring back ability’
the ability of a material to undergo large deflections without permanent deformation
how is springiness of a material calculated?
EL/YM
what characteristics should the chosen wire (e.g. for ortho) have?
appropriate rigidity
high level of ductility so it can be manipulated
good strength and corrosion resistance
at what ratio of chromium and nickel is the transition of austenite to martensite suppressed in austenitic s/steel ?
18:8 or 12:12%
what % of martensitic stainless steel is chromium?
12%
describe some of the features of 18:8 stainless steel
does not heat harden
soft and malleable when cast
work hardens quickly
slip can sometimes occur during cold working. what does this mean?
defects/dislocations in a grain being moved along the grain boundaries
grains cant jump to another grain meaning alloy has fewer defects in the lattice structure making up each grain
alloy is now stronger and harder
