Wrought CS and SS

Question 1

describe cold working

Answer 1

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

Question 2

wrought alloys are relatively ductile

Answer 2

ye

Question 3

how does plastic deformation change a metal’s properties?

Answer 3

• elongates grains

- - increases in deislocation density and reduction in porosity–>INCREASES hardness and strength

Question 4

what is strain hardening?

Answer 4

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

Question 5

why is hot working not useful in strengthening?

Answer 5

atoms can rearrange themselves at the high temp so little strengthening is achieved

Question 6

what is softening annealing?

Answer 6

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

Question 7

what properites make a good spring?

Answer 7

high yield strength (stress at which a mateiral begins to deform)

low Elastic limit (stiffness)

Question 8

what matierals are used as ortho wires?

Answer 8

• stainless teel
• cobalt-chromium
• beta-titanium
• nickel-titanium

Question 9

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)

Answer 9

ye

Question 10

define steel

Answer 10

an alloy of iron and carbon where C content is less than 2%

C acts as the hardening component, prevents iron atoms from dislocating

Question 11

what does C amount in steel control?

Answer 11

hardness, ductility, elasticity, and tensile strength

the more carbon = harder, but more brittle

Question 12

steel vs cast iron?

Answer 12

cast iron has MORE CARBON

Question 13

CAST IRON is WEAK and BRITTLE and unsuitable for cold working

Answer 13

ye

Question 14

what is hardest alloy known?

Answer 14

tungsten carbide

Question 15

the GREATER the carbon content, the MORE BRITTLE, and thus the material cannot be COLD WORKED

Answer 15

ye

Question 16

why is steel one of the most widely used materials?

Answer 16

• wide range of properties
• ease of controlling its carbon content
• annealing (hardening by heat tx)
• low cost

Question 17

what are the major crystal types of carbon steel phases?

Answer 17

• austenite
• ferrite
• cementite iron carbide

Question 18

describe the austenite or gamma phase of carbon steel

Answer 18

• a face centred cubic structure
• more ductile, weak and soft
• carbon cannot fit in matrix–>distorts iron

Question 19

describe the ferrite phase of carbon steel

Answer 19

• -can hold very little carbon (0.02%)
• has BCC structure;

stronger, harder, and less ductile than austenite

used for non-cutting instruments like forceps

Question 20

describe the cementite iron carbide phase of carbon steel

Answer 20

much harder, stronger, and is brittle compared to the other phases

-used for cutting instruments like burs

Question 21

what is pearlite?

Answer 21

a mix of ferrite and cementite (softer)

Question 22

how to prevent solid state transformation from austenite to ferrite?

Answer 22

add nickel

Question 23

describe how martensite is made

Answer 23

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

Question 24

ghow to convert marteniste to ferrite and cementite (pearlite?)

Answer 24

low temp heat

Question 25

clinical significance of heat and transformation of materials?

Answer 25

if you sterilize metal materials, you can significantly change their properties

Question 26

what is the purpose of Cr in steel?

Answer 26

creates stainless steel

• overcomes corrosion with passivating effect

Question 27

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

Answer 27

ye

Question 28

what are the good properties of 18-8 Cr-Ni?

Answer 28

• excellent corrosion resistance
• superior ductility
• can be soldered and welded

Question 29

what is austenitic SS used for? give examples

Answer 29

• where moderately high strength is needed
• ortho wires, instrument handles, impression trays, SS crowns, root canal (reamers and files)
• denture base

Question 30

what is martinsitic SS used for?

Answer 30

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

Question 31

how much Cr is needed for form the Cr2O3 passivation layer?

Answer 31

at leat 12%

Question 32

chloride can destroy the passivation layer

Answer 32

ye`

Question 33

what is sensitization

Answer 33

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)