Base metal casting alloys Flashcards
What do base metal casting alloys NOT contain?
no Au, Ag, Pt or Pd
In dentistry what do base metal casting alloys comprise of?
- Co/Cr
- Ni/Cr
What is Co/Cr mainly used for?
mainly partial denture frameworks and complete denture bases
What does Ni/Cr have applications in?
crown and bridge
What are the constituents of Co/Cr?
- cobalt
- chromium (no less than 25%)
- molybdenum (no less than 4%)
- cobalt + nickel + chromium (no less than 85%)
- plus traces of elements Be, Si and C
What are the actions of the cobalt and nickel constituents of Co/Cr?
confer hardness and strength
What are the actions of the chromium constituent of Co/Cr?
hardness and corrosion resistance (passivating effect)
What does the passivating effect of chromium mean?
interaction of chromium and oxygen —> chromium oxide which is fairly resistant to corrosion
What are the actions of the trace inclusion constituents of Co/Cr?
improve casting and handling
What are the constituents of Ni/Cr?
- nickel (main constituent)
- chromium (no less than 20 %)
- molybdenum (no less than 4 %)
- beryllium (no less than 2 %)
- nickel + Cobalt + Chromium (no less than 85 %)
What constituent is there concern about being contained in dental alloys?
beryllium - carcinogen
What is the fusion temperature for base casting alloys Co/Cr and Ni/Cr?
1200-1500 degrees C
- higher than gold alloys
How are fusion temperatures for Co/Cr and Ni/Cr achieved?
either electrical induction or any acetylene torch
(1200-1500 degrees C)
What is the density of Co/Cr and Ni/Cr compared to gold alloys?
lower density than gold alloys
Why does the casting machine for Co/Cr and Ni/Cr need to be different to that for gold alloys?
Co/Cr and Ni/Cr lower density
- need to avoid miscasting by having :
- additional casting thrust
- porous investment material
How are Co/Cr and Ni/Cr finished?
time consuming to finish
- sandblasting possible
- for Co/Cr alone electrolytic polishing
- wary of finishing heat gain if fused to porcelain
What are steel and stainless steel most widely used as?
wrought alloys
- instruments, burs and wires, and sometimes denture bases
What can be done to steel and stainless steel shape it?
hammered, drawn or bent into shape at temperatures well below the metal recrystallisation temperature
- may lead to work hardening (advantage/disadvantage)
What are the pros and cons of work hardening of steel and stainless steel?
pro: toughens up the alloy
con: if over work hardened more predisposed to fracture
What is steel?
an alloy of iron and less than 2% carbon
What happens if the carbon content of steel is increased beyond 2%?
- brittle
- unsuitable for cold working
In solid state, a variety of strcutures are adopted by steel, depending on what?
carbon content and temperature
When does steel have an austenite structure?
above 723 degrees C
What is an austenite structure of steel?
an interstitial solid solution of carbon in a FCC iron matrix
At what temperature do 2 phases of steel exist?
below 723 degrees C
What 2 phases of steel exist below 723 degrees C?
Ferrite
- dilute solid solution of C (up to 0.02%) in Fe
Cementite
- specific compound Fe3C
What is a mixture of ferrite and cementite termed?
pearlite
What does 0.8% carbon in steel give?
eutectoid alloy
What is the critical temperature of steel?
Tc = 723 degrees C
How is the hardness of steel customised?
heat treatments
In a phase diagram where is the hypo eutectoid alloy?
to the left of the eutectic alloy line
In a phase diagram where is the hyper eutectoid alloy?
to the right of the eutectic alloy line
What does being a hypo or hyper eutectoid alloy depend on?
percentage of carbon present
What is an example of a hypereutectoid alloy?
cementite - hard, brittle
- cutting instruments e.g. burs
What is an example of a hypoeutectoid alloy?
ferrite - softer and more ductile
- non cutting instruments such as forceps
What is stainless steel?
steel with the addition of chromium
In stainless steel, what does chromium improve?
improves corrosion resistance by passivating effect
What may be added to stainless steel?
may also add Ni to increase corrosion resistance and strength
What effect does the addition of both chromium and Ni to steel have?
reduces the Tc
What is 18/8 stainless steel?
18% chromium and 8% Ni
Is 18/8 stainless steel amenable to heat treatment? Why?
no - Tc is below where atomic movements are possible
- therefore rely upon cold working to form wrought structure e.g. non-cutting instruments, wires, denture bases
What can 18/8 be soldered and welded?
yes
What grades is 18/8 stainless steel available in?
3 grades: soft, half hard, hard
depends upon carbon content, these grades exist where the carbon content is less than 0.15%
How may 18/8 stainless steel be joined?
- solder - with Ag solder
- welding - pressed together and heated
In welding, what is the temperature change proportional to?
I^2Rt
where I = current, R= electrical resistance and t= time current passes
What is martensitic stainless steel?
- no Ni, 12% chromium
- heat treatments possible
- probes and cutting instruments
What is springback ability of wire?
ability to undergo large deflections without permanent deformation
- springback potential = yield stress/modulus of elasticity
thickness affects stiffness
