Dental materials: Base metal casting alloys Flashcards
What metals make up base metal alloys?
Nickel, Chromium, Cobalt
What are base metal alloys used for in dentistry?
Ni/Cr- crown + bridge frameworks
Co/Cr- partial denture components
What is the action of Cobalt, Nickel and Chromium?
Co + Ni= hardness and strength
Cr= hardness + corrosion resistance through passivating effect
Why are trace metals included in these alloys?
Improves casting and handling
What is the fusion temp. for both alloys?
1200-1500
How is fusion of both alloys achieved?
electrical induction
oxyacetylene torch
Which has a lower density, gold or base metal alloys?
base metal
What is required to avoid miscasting?
additional casting thrust requires
porous investment required
What is true for both Co/Cr and Ni/Cr?
time consuming to finish
-can use sandblasting or electrolytic polishing (Co/Cr only)
-be careful when using heat for finishing if fused to porcelain
What is steel made up of ?
Iron + 2% Carbon
What happens to steel in the carbon content is above 2%?
brittle
unsuitable for cold working
What can steel be used for?
burs, instruments, wires and some denture bases
how is steel manipulated?
hammered, drawn, bent into shape at temps lower than the metals recrystallisation temp.
What does steel become at a temp above 723?
AUSTENITIC
-carbon is an interstitial solid solution in an FCC iron matrix
What are the 2 phases of steel under 723?
FERRITE-> dilute solid solution of C in Fe
CEMENTITE-> specific compound of Fe3C
FERRITE+ CEMENTITE= PEARLITE
What are hypoeutectoids and what are they used in?
e.g Ferrite- softer, more ductile
used in non cutting instruments like forceps
What are hypereutectoids and what are they used in?
e.g cementite- more hard and brittle
used in cutting instruments like burs
What is a eutectic alloy?
an alloy in which its mixtures are completely soluble in the LIQUID state but have limited solubility in the solid state
what is the percentage of carbon in a eutectoid alloy?
0.8%
What is the critical temperature for carbon?
723
How do you get stainless steel?
steel + chromium
can also add Ni- increase corrosion resistance + strength
what is the benefit of adding chromium to steel?
improves corrosion by the passivating effect
Does adding Ni + Cr increase of decrease the critical temperature?
reduce
What is 18/8 stainless steel?
18% Cr
8%Ni
Why can you not heat treat 18/8 stainless steel?
critical temp is below where atomic movements are possible- therefore can only use cold working
What is the percentages of Ni + Cr in martensitic stainless steel?
ni- 0%
cr- 12%
how can 18/8 stainless steel be wrought together?
soldering
welding- pressed together and heated
temp change is proportional to
I2 R t» Where I = Current, R =Electrical resistance and t = time currentpasses
