Download Lecture #6 / Dr.Nadia (Cast and wrought base

Cast Co/Cr alloys

RPD framework
 Porcelain-metal restorations
Crowns
 Bridges
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
Cast Ni/Cr alloys
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RPD framework
 Cr and bridges
 Porcelain metal restorations
Endodontic instruments
 Orthodontic wires and
brackets
 Preformed crowns
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
Cast Ti and Ti alloys

Crowns
 Bridges
 RPD framework
 Implants
Wrought Ti and Ti alloys

Implants
Wrought Co/Cr/Ni alloys
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Wrought S/S alloys
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Ortho wires and endo files
Wrought Ni/Ti alloys
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
Ortho wires and endo files
Wrought beta Ti alloys(
Ti/Mo)
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
Ortho wires
- Non toxic non allergic.
- Resistant to corrosion.
- Satisfactory physical and mechanical properties.
- Easy to process and handle.
- Readily available, relatively inexpensive
constituents
- Almost all RPD frameworks are constructed in
Co/Cr. Small percentage of frameworks are
constructed in Ni/Cr.
- Chromium should not be less than 20% and Co,
Cr and Ni altogether should not be less than
85% of the alloy by weight.
- Physical properties are controlled by the
presence of minor alloying elements such as:
carbon, molybdenum, beryllium, tungsten, and
aluminum.
- Cr is responsible for tarnish and corrosion
resistance, but if content is higher than 30% the
alloy is difficult to cast and it becomes brittle. So
the maximum allowed Cr content should be 28
or 29%.
- Co and Ni are interchangeable. Co increases
elastic modulus (rigidity), strength and hardness
more than Ni.
- C is essential in the alloy, if quantity is >0.2%,
the alloy becomes too hard and brittle for dental
use. If quantity < 0.2%, the alloy’s yield and
ultimate tensile strengths become very low
- When the alloy is Ni containing alloy, its carbon
content is usually made significantly lower if it
was to used for porcelain metal restorations.
- Mo in 3-6% contributes to the strength of the alloy.
- Al in Ni containing alloys forms a compound of Ni and
Al (Ni3Al) which increases the ultimate tensile and
yield strengths of the alloy considerably.
- As little as 1% to 2% beryllium to Ni based alloys lowers
the fusion range by about100 C, but this will affect the
alloys ductility and corrosion resistance
- Si and Manganese increase fluidity and castability of
these alloys.
- Ni content is difficult to control unless castings are made
in vacuum or under argon. Ni more than 0.1% will
reduce the ductility of the casting.

Elevating temperature 100 degrees above melting
temperature will result in a casting with poor surface
due to increased reaction with the investment.
- Generally: heating cobalt-based alloys reduce
yield strength and elongation. However,
annealing Co/Cr alloys (by heating the alloy to
1225° C) will lead to increased yield strength
and ductility.
- When performing soldering and welding
procedures, the lowest possible temperature
should be used with the shortest possible time of
heating.
- Ranges from 1400° C -1500 ° C.
- Addition of 1%-2% beryllium lowers the melting
temperature by 100 ° C.
- Because of their toughness, partial denture clasps cast of
alloys with a high elongation and tensile strength do
not fracture in service as often as do those with low
elongation.
- The higher the elastic modulus, the more rigid a
structure
- Elastic modulus of base-metal alloys is
approximately double the modulus of Type IV
cast dental gold alloys
- Hardness is an indication of the ease of finishing
the structure and its resistance to scratching in
service.
- The higher hardness of the cast base-metal
alloys as compared with gold alloys requires the
use of special polishing equipment, which may
be considered a disadvantage, but the finishing
operation can be completed without difficulty
by experienced operators.
- It is a common practice to use electrolytic polishing for a
portion of the finishing process, which reduces the time
and effort necessary for mechanical finishing
operations.
- Clasp arms of RPDs are most prone to fracture.
- Comparisons among cobalt-chromium, titanium,
and gold alloys shows that cobalt-chromium
alloys possess superior fatigue resistance
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
Precautions should be taken to avoid exposure to
metallic vapor, dust, or grindings containing beryllium
and nickel. The safety standard for beryllium dust is 2
ug/m3 of air for a timeweighted, 8-hour day. A higher
limit of 25 ug/m3 is allowed for a minimum exposure
time of less than 30 minutes.
Physiological responses may range from contact
dermatitis to severe chemical pneumonitis.

A cobalt-chromium alloy without nickel or other
non-nickel containing alloy should be used on
patients with a medical history indicating an
allergic response to nickel.
-An alloy that is wrought is one that is worked by being
forged or hammered.
A cast alloy is when the molten alloy is poured into a
mold to give it its shape.
A wrought alloy is stronger as it has been cold-worked,
pounded into shape. It may have been heated and then
cooled slowly to anneal it to make it stronger. Between
working it and annealing it the molecules are brought
closer together giving it more strength.
Wrought Ti and Ti alloys
 Implants
 Crowns
 Bridges
 Wrought S/S alloys
 Endodontic instruments
 Orthodontic wires and
brackets
 Preformed crowns
 Wrought Co/Cr/Ni alloys
 Ortho wires and endo

files
 Wrought Ni/Ti alloys
 Ortho wires and endo
files
 Wrought beta Ti alloys(
Ti/Mo)
 Ortho wires
- Steel is an iron-carbon alloy.
- Stainless steel are alloys of iron and carbon that contain
chromium, nickel, manganese, and other metals to
improve properties and give the stainless quality to the
steel.
- Usually, stainless steel alloys are not cast, but instead are
used in the wrought form.
-Applications include: Orthodontic appliances and
fabrication of endodontic instruments, such as files and
reamers, temporary space maintainers, prefabricated
crowns, and the various clinical and laboratory
instruments.
- Ferritic alloys: chromium steel alloys (15-25%), C, Si,
Mo. Used in the construction of instruments and
equipment.
- Martensitic alloys: Cr steel alloys with lower content of
Cr (12-18%). Can be hardened with heat treatment,
used in the construction of instruments and sometimes
orthodontic appliances.
- Austenitic alloys: chromium 18% and nickel 8%, carbon
(0.08% and 0.20%), titanium, manganese, silicon,
molybdenum, niobium, and tantalum (in minor
amounts) to give important modifications to the
properties. The balance (= 72%) is, of course, iron.
These have the highest corrosion resistance.
- Chromium gives corrosion resistance.
Approximately 13% chromium is needed to
produce corrosion resistance in pure iron, and
the necessary proportion is increased with the
addition of carbon to form steel.
- Chromium resists corrosion well because of the
formation of a strongly adherent coating of
chromium oxide on the surface, which prevents
further reaction with the metal below the
surface.
-The formation of such an oxide layer is called
passivation. The surface coating is not visible,
even at high magnification, but the film adds to
the metallic luster of the metal.
- Increasing chromium content more than 28%
will lead to the formation of a carbide layer at
the boundaries of the grains, instead of the
oxide, lowering the corrosion resistance and
producing a brittle alloy. This process is called
sensitization.
- The degree of passivity is influenced by a number
of factors, such as alloy composition, heat
treatment, surface condition, stress in the
appliance, and the environment in which the
appliance is placed.
- In dental applications, the stainless
characteristics of the alloys can therefore be
altered or lost by excessive heating during
assembly or adaptation, using abrasives or
reactive cleaning agents, which can alter the
surface conditions of the appliance; and even by
poor oral hygiene practices over prolonged
periods.
- Molybdenum increases resistance to pitting
corrosion.
- The elements present in small quantities
(stabilizing elements) such as titanium, niobium
and tantalum tend to prevent the formation of
iron or chromium carbides.
- Chemical action is reduced if the surface is
smooth and polished.
- Soldering with gold or silver produce
electrogalvanic action which reduce the stainless
properties.
- Heat treatment above 650° C (annealing)
produces recrystallization of the micro structure
of the alloy and the formation of chromium
carbides with lower corrosion resistance.
- Heat treatment between 400-500 °C for 5 -120
seconds removes the internal residual stresses
produced by cold working of the alloy.
- Cold worked using pliers.
- Once prepared the wires should be heat treated
to 450 °C for one minute.
- Soldering is difficult and requires skill. Borax
fluxes are not satisfactory and the use of
fluoride containing fluxes is required.
- Gold or silver solders can be performed more
easily.
- Silver has lower melting point reducing the
chance for annealing of steel.
- Numerous endodontic instruments are classified
for hand use or as motor-driven instruments.
- The most common instruments are the K type of
root canal files and reamers. These are
manufactured by machining a stainless steel
wire into a pyramidal blank, either square or
triangular in cross section, and then twisting the
blank to form a spiral cutting edge.
- A file with a rhombohedral cross section has also
been introduced.
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Mechanical properties are dependant on file
geometry, direction of loading and composition.
E.g Rhomohedral are less stiff than K-files. Both
are weaker in counterclockwise, so twist <¼ turn.
- Contain about 56% Ni and 44% Ti by weight, which
calculate to be 50% of each by atoms. In some
instances, <2% of cobalt may be substituted for nickel
- These alloys can change their structure from austenitic
(body-centered cubic) to martensitic (close-packed
hexagonal) as a function of stress during root canal
preparation.
- The modulus of Ni-Ti austenite is 120 GPa, and that of
martensite is 50 GPa. This effect results in what is
termed super-elasticity, and when stress is removed the
alloy returns to original shape without permanent
deformation and the alloy becomes austenite again.
The super-elasticity of Ni-Ti permits
deformations of 8% strain in endodontic files
with complete recovery (only less than 1% is
allowed in SS alloys).
- Has higher strength, lower elastic modulus and
comparable corrosion resistance to SS.
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-
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SS crowns are recommended to restore primary
teeth.
Have reasonable strength and hardness.
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40% Co, 20% Cr, 15% Ni, and some other
elements.
Used for Orthodontic wires
Formed and shaped then heat-treated for 7 min
at 482 C.
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Nitinol orhtodontic wire was introduced in 1972.
Has high resiliency, limited formability, and thermal
memory.
55% Ni, 45 Ti.
Possesses a temperature transition range (TTR). At
temperatures below the TTR, the alloy can be
deformed plastically. When the alloy is then heated
from below to above the TTR, a temperature-induced
crystallographic transformation from a martensitic to
an austenitic microstructure occurs and the alloy will
return to its original shape. Hence, nickel-titanium is
called a shape-memory alloy.
Has comparable properties to SS, but has maximum
elastic deflection.
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-
-
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A titanium-molybdenum alloy known as betatitanium was introduced in 1979 as a wrought
orthodontic wire.
Beta Ti: Body centered cubic crystal lattice.
composition: 78% titanium, 11.5% molybdenum, 6%
zirconium, and 4.5% tin and is supplied as wrought
wire.
Has lower strength and elastic modulus, max.
deflection, lower yield strength, and good ductility,
weldability, and corrosion resistance.
Has large working range