Base metal casting alloys

Question 1

What do base metal casting alloys NOT contain?

Answer 1

no Au, Ag, Pt or Pd

Question 2

In dentistry what do base metal casting alloys comprise of?

Answer 2

1. Co/Cr
2. Ni/Cr

Question 3

What is Co/Cr mainly used for?

Answer 3

mainly partial denture frameworks and complete denture bases

Question 4

What does Ni/Cr have applications in?

Answer 4

crown and bridge

Question 5

What are the constituents of Co/Cr?

Answer 5

• 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

Question 6

What are the actions of the cobalt and nickel constituents of Co/Cr?

Answer 6

confer hardness and strength

Question 7

What are the actions of the chromium constituent of Co/Cr?

Answer 7

hardness and corrosion resistance (passivating effect)

Question 8

What does the passivating effect of chromium mean?

Answer 8

interaction of chromium and oxygen —> chromium oxide which is fairly resistant to corrosion

Question 9

What are the actions of the trace inclusion constituents of Co/Cr?

Answer 9

improve casting and handling

Question 10

What are the constituents of Ni/Cr?

Answer 10

• 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 %)

Question 11

What constituent is there concern about being contained in dental alloys?

Answer 11

beryllium - carcinogen

Question 12

What is the fusion temperature for base casting alloys Co/Cr and Ni/Cr?

Answer 12

1200-1500 degrees C
- higher than gold alloys

Question 13

How are fusion temperatures for Co/Cr and Ni/Cr achieved?

Answer 13

either electrical induction or any acetylene torch

(1200-1500 degrees C)

Question 14

What is the density of Co/Cr and Ni/Cr compared to gold alloys?

Answer 14

lower density than gold alloys

Question 15

Why does the casting machine for Co/Cr and Ni/Cr need to be different to that for gold alloys?

Answer 15

Co/Cr and Ni/Cr lower density
- need to avoid miscasting by having :
- additional casting thrust
- porous investment material

Question 16

How are Co/Cr and Ni/Cr finished?

Answer 16

time consuming to finish
- sandblasting possible
- for Co/Cr alone electrolytic polishing

• wary of finishing heat gain if fused to porcelain

Question 17

What are steel and stainless steel most widely used as?

Answer 17

wrought alloys
- instruments, burs and wires, and sometimes denture bases

Question 18

What can be done to steel and stainless steel shape it?

Answer 18

hammered, drawn or bent into shape at temperatures well below the metal recrystallisation temperature
- may lead to work hardening (advantage/disadvantage)

Question 19

What are the pros and cons of work hardening of steel and stainless steel?

Answer 19

pro: toughens up the alloy
con: if over work hardened more predisposed to fracture

Question 20

What is steel?

Answer 20

an alloy of iron and less than 2% carbon

Question 21

What happens if the carbon content of steel is increased beyond 2%?

Answer 21

• brittle
• unsuitable for cold working

Question 22

In solid state, a variety of strcutures are adopted by steel, depending on what?

Answer 22

carbon content and temperature

Question 23

When does steel have an austenite structure?

Answer 23

above 723 degrees C

Question 24

What is an austenite structure of steel?

Answer 24

an interstitial solid solution of carbon in a FCC iron matrix

Question 25

At what temperature do 2 phases of steel exist?

Answer 25

below 723 degrees C

Question 26

What 2 phases of steel exist below 723 degrees C?

Answer 26

Ferrite - dilute solid solution of C (up to 0.02%) in Fe Cementite - specific compound Fe3C

Question 27

What is a mixture of ferrite and cementite termed?

Answer 27

pearlite

Question 28

What does 0.8% carbon in steel give?

Answer 28

eutectoid alloy

Question 29

What is the critical temperature of steel?

Answer 29

Tc = 723 degrees C

Question 30

How is the hardness of steel customised?

Answer 30

heat treatments

Question 31

In a phase diagram where is the hypo eutectoid alloy?

Answer 31

to the left of the eutectic alloy line

Question 32

In a phase diagram where is the hyper eutectoid alloy?

Answer 32

to the right of the eutectic alloy line

Question 33

What does being a hypo or hyper eutectoid alloy depend on?

Answer 33

percentage of carbon present

Question 34

What is an example of a hypereutectoid alloy?

Answer 34

cementite - hard, brittle - cutting instruments e.g. burs

Question 35

What is an example of a hypoeutectoid alloy?

Answer 35

ferrite - softer and more ductile - non cutting instruments such as forceps

Question 36

What is stainless steel?

Answer 36

steel with the addition of chromium

Question 37

In stainless steel, what does chromium improve?

Answer 37

improves corrosion resistance by passivating effect

Question 38

What may be added to stainless steel?

Answer 38

may also add Ni to increase corrosion resistance and strength

Question 39

What effect does the addition of both chromium and Ni to steel have?

Answer 39

reduces the Tc

Question 40

What is 18/8 stainless steel?

Answer 40

18% chromium and 8% Ni

Question 41

Is 18/8 stainless steel amenable to heat treatment? Why?

Answer 41

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

Question 42

What can 18/8 be soldered and welded?

Answer 42

yes

Question 43

What grades is 18/8 stainless steel available in?

Answer 43

3 grades: soft, half hard, hard depends upon carbon content, these grades exist where the carbon content is less than 0.15%

Question 44

How may 18/8 stainless steel be joined?

Answer 44

- solder - with Ag solder - welding - pressed together and heated

Question 45

In welding, what is the temperature change proportional to?

Answer 45

I^2Rt where I = current, R= electrical resistance and t= time current passes

Question 46

What is martensitic stainless steel?

Answer 46

- no Ni, 12% chromium - heat treatments possible - probes and cutting instruments

Question 47

What is springback ability of wire?

Answer 47

ability to undergo large deflections without permanent deformation - springback potential = yield stress/modulus of elasticity thickness affects stiffness