Base Metal Casting Alloys Stainless Steel Flashcards
What is 18/8 stainless steel? How is it manipulated and what is it used for?
- 18% Cr, 8% Ni.
- Cannot be heat treated hence rely on COLD WORKING
- non-cutting instruments, wires, denture bases.
What does steel consist of?
- Alloy of IRON and less than 2% carbon.
- If more than 2% would make it BRITTLE and unsuitable for cold working.
What can be done to avoid miscasting in CoCr and NiCr alloys?
- Additional casting thrust.
- Porous investment material.
What determines whether there will be a hypo or hyper eutectoid alloy?
The % of carbon.
(more carbon = hyper)
What are the 5 constituents of Nickel Chrome?
- Nickel (main constituent).
- Chromium (NO LESS THAN 20%).
- Molybdenum (NO LESS THAN 4%).
- Beryllium (NO LESS THAN 2% concern as CARCINOGEN).
- Cobalt + Nickel + Chromium (NO LESS THAN 85%).
What happens if steel is above Tc?
AUSTENITE - Interstitial solid solution of carbon in a face centered cubic iron matrix.
What is the passivating effect?
Chromium oxide PROTECTS AGAINST EROSION.
Give an example of a hyper eutectoid alloy and what it is used for in dentistry.
Cementite - hard and brittle.
Cutting instruments like burs.
What happens if steel is below Tc?
2 forms:
- Ferrite (dilute solid solution of carbon 0.02% in Fe).
- Cementite (specific compound with satisfied valence requirements Fe3C).
What 2 methods can be used to join 18/8 stainless steel?
- SOLDERING (Ag solder)
- WELDING
What is the function of Chromium in CoCr?
HARDNESS AND CORROSION RESISTANCE via PASSIVATING EFFECT
What is the temperature change of welding proportional to?
I^2Rt
I = current
R = electrical resistance
t = time current passes
What is the function of trace elements in CoCr?
IMPROVE CASTING AND HANDLING.
What are two examples of wrought alloys? What are these used for in dentistry?
Steel and Stainless Steel.
Used for instruments, burs, wires and denture bases.
Give an example of a hypo eutectoid alloy and what it is used for in dentistry.
Ferrite - soft and ductile.
- used for NON CUTTING INSTRUMENTS, ex. forceps.
What is martensitic stainless steel composed of? How is it manipulated and what is it used for?
No Nickel, 12% Chromium
- Heat treatment possible, used for probes and cutting instruments.
How are Co/Cr or Ni/Cr fused?
By ELECTRICAL INDUCTION or OXYACETYLENE TORCH.
What is the function of each of the components of stainless steel?
- Chromium: CORROSION RESISTANCE by PASSIVATING EFFECT
- Nickel: CORROSION RESISTANCE + STRENGTH
What affects wire stiffness?
Thickness.
What is springback ability?
Ability to undergo large deflections without permanent deformations.
What are Steel and Stainless Steels?
WROUGHT ALLOYS
What can be used to speed up NiCr finishing?
Sandblasting.
How are wrought alloys manipulated? What may occur due to this manipulaiton?
Hammered, bent, drawn into shape at temperatures way below their RECRYSTALLIZATION TEMPERATURE.
Work Hardening.
What are 3 common traits of CoCr and NiCr
- Fusion temperature 1200-1500.
- Difficult to finish.
- Density lower than gold alloys.
What is the function of Nickel in CoCr?
Adds hardness and strength.
What is a concern with beryllium?
CARCINOGEN.
What is the effect of the addition of chromium and nickel to stainless steel?
Reduce the critical temperature.
What are the 3 grades of 18/8 Stainless steel?
Where less than 0.15% C:
- 3 Grades: Soft, Half Hard, Hard.
When does the steel eutectoid alloy form?
At critical temperature when carbon concentration is 0.8%
What is the mixture of cementite and ferrite called?
Pearlite.
What is stainless steel composed of?
Steel and Chromium
May also add nickel.
What is the formula for springback potential?
Springback potential = yield stress/ modulus of elasticity.
What can be used to speed up CoCr finishing?
Sandplasting and electrolytic polishing.
What are the 5 constituents of Cobalt Chrome?
- Cobalt.
- Chrome (no less than 25%)
- Molybdenum (no less than 4%).
- Cobalt + Nickel + Chromium (no less than 85%).
- All plus TRACE ELEMENTS Silicone, Carbon, Beryllium.
What is the function of Cobalt in CoCr?
Adds hardness and strength.
What elements do base metal casting alloys not contain?
Ag, Au, Pt, Pd.
What is the critical temperature of steel?
723 celsius
What is the carbon content of ferrite?
up to 0.02%
Chemical formula of cementite
specific compound with satisfied valence requirements Fe3C
