Wrought CS and SS Flashcards
describe cold working
a way to shape metal;
a metal is bent or hammered into a new shape below it’s crystallization temp –> usually RT –> wrought
usually for ortho apps, and wires, endo instruments (files, reamers) prefabricated crowns, hand instruments
wrought alloys are relatively ductile
ye
how does plastic deformation change a metal’s properties?
- elongates grains
- - increases in deislocation density and reduction in porosity–>INCREASES hardness and strength
what is strain hardening?
a process of making a metal harder and stronger through plastic deformation
must happen at a temp LOW enoguh so the atoms can’t rearrange themselves
why is hot working not useful in strengthening?
atoms can rearrange themselves at the high temp so little strengthening is achieved
what is softening annealing?
the process of losing the strength gained from strength hardening
stages:
recovery–elevated temps = increase n atomic diffusion; relieves some intenral strain energy–>recover normal lattice position; at this point there is no appreciable loss of strength and hardness, and corrosion resistance improves
recrystallization - at even higher temps, new, strain-free graisn nucleate and grow at the grain boundaries or inside the old grains; replace the deformed grains; now the mechanical properties return to their original state; recrystallization depends on temp and time
grain growth–grain begins to grow in size if specimen is left at a high temp; larger grains reduce the strength and toughness of a material
dependent of temp and time
what properites make a good spring?
high yield strength (stress at which a mateiral begins to deform)
low Elastic limit (stiffness)
what matierals are used as ortho wires?
- stainless teel
- cobalt-chromium
- beta-titanium
- nickel-titanium
Ti clasps are more flexible than CoCr –> Ti clapss can also engage deeper undercuts and can be used where shorter clasps arems are needed (e.g. premolars)
ye
define steel
an alloy of iron and carbon where C content is less than 2%
C acts as the hardening component, prevents iron atoms from dislocating
what does C amount in steel control?
hardness, ductility, elasticity, and tensile strength
the more carbon = harder, but more brittle
steel vs cast iron?
cast iron has MORE CARBON
CAST IRON is WEAK and BRITTLE and unsuitable for cold working
ye
what is hardest alloy known?
tungsten carbide
the GREATER the carbon content, the MORE BRITTLE, and thus the material cannot be COLD WORKED
ye
why is steel one of the most widely used materials?
- wide range of properties
- ease of controlling its carbon content
- annealing (hardening by heat tx)
- low cost
what are the major crystal types of carbon steel phases?
- austenite
- ferrite
- cementite iron carbide
describe the austenite or gamma phase of carbon steel
- a face centred cubic structure
- more ductile, weak and soft
- carbon cannot fit in matrix–>distorts iron
describe the ferrite phase of carbon steel
- -can hold very little carbon (0.02%)
- has BCC structure;
stronger, harder, and less ductile than austenite
used for non-cutting instruments like forceps
describe the cementite iron carbide phase of carbon steel
much harder, stronger, and is brittle compared to the other phases
-used for cutting instruments like burs
what is pearlite?
a mix of ferrite and cementite (softer)
how to prevent solid state transformation from austenite to ferrite?
add nickel
describe how martensite is made
alloy containing 0.02%-2% carbon is heated above critical temp but below solidus temp and quenched–severe distortion of lattice–body centred tetragon
very hard, strong, but brittle
ghow to convert marteniste to ferrite and cementite (pearlite?)
low temp heat
clinical significance of heat and transformation of materials?
if you sterilize metal materials, you can significantly change their properties
what is the purpose of Cr in steel?
creates stainless steel
- overcomes corrosion with passivating effect
with stainless you can either have austenitic and martenistic used in dent–can determine which phase will be present at room temp by adding both Cr and Ni
ye
what are the good properties of 18-8 Cr-Ni?
- excellent corrosion resistance
- superior ductility
- can be soldered and welded
what is austenitic SS used for? give examples
- where moderately high strength is needed
- ortho wires, instrument handles, impression trays, SS crowns, root canal (reamers and files)
- denture base
what is martinsitic SS used for?
when you need retention of a cutting edge and strength
- e.g. scalers, curettes, scalpals, chisels, bone rongeurs, scissors, retroctors, extraction forceps, root elevators, etc
how much Cr is needed for form the Cr2O3 passivation layer?
at leat 12%
chloride can destroy the passivation layer
ye`
what is sensitization
the loss of SS corrosion resistance by overheating
-occurs at grain boundaries; grain boundaries can be depleted of free Cr and corrosion occurs
in order to prevent, a strong carbide former is added (e.g. Ti)
