Stainless steel, soldering & welding. corrosion

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Dr.Abhideep Singhai
PG Ist year
Dept. of Orthodontics & Dentofacial
Orthopaedics

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CONTENT
INTRODUCTION
BRIEF HISTORY
MECHANICAL PROPERTY CONCEPT FOR ORTHODONTIC MATERIALS
ALLOYS
CARBON STEELS
STAINLESS STEELS
SOLDERING
WELDING
CORROSION

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INTRODUCTION
• Several different metals are used in the orthodontic appliances .
• Stainless steel is one of the most widely used materials in current
orthodontics.
• Archwires, brackets, bands, ligatures, tubes, among other
appliances, are manufactured using different types of this alloy.
• The first evidence of the use of this alloy in the orthodontic field
dates back to the mid-1920s, when it was introduced as a material
to manufacture wires.
• The alloy has ever since gained popularity among orthodontists and
its further development has led to its widespread use in today’s
different orthodontic techniques.

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• Steel is an alloy of Iron and Carbon. Carbon content should not
exceed 2.1% max.
• When it contains 12 to 13% chromium it is called
• Steel exists in three forms -Ferritic, Austenitic and Martensitic .

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HISTORY -
• It was discovered accidentally during the early part of W.W.I, in the
U.K. by the Sheffield Metallurgist Harry Brearly, of the Brown Firth
Research Lab , who noticed that a discarded steel sample was not
rusting – Result was a chrome alloy steel. (Dated-4th June, 1912).
• Two months later stainless steel was cast for first time in August 20,
1912.

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• Stainless steel entered dentistry in 1919, introduced at Krupp’s
Dental Polyclinic in Germany by F. Hauptmeyer, who first used it
to make a prosthesis and called it Wipla (Wie Platin; in German,
like Platinum).
• Becket of U.S., Strauss and Edward Maurer of Germany also
shared the development of the material between 1903-1921.

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• Application of stainless steel to the fabrication of appliances was
credited to a Belgian Lucien de Coster.
• Research study related to metallurgy with particular references to
orthodontic applications was done by Metallurgist R.M.Williams.
• Angle used it in his last year (1930) as ligature wires.
• By 1937, the value of stainless steel as an orthodontic material had
been confirmed.

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• Stainless steel today is used to make arch wires , ligature wires,
band material, brackets and buccal tubes.
• A variety of stainless steels have been developed and at least 10
are or were used to manufacture orthodontic instruments and
attachments.
• Stainless steel brackets are most widely used because of their
durability says Dr. Robert Waxler (American Association of
orthodontics).

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Mechanical properties of importance to dentistry include
STRESS & STRAINS
• TENSILE STRESS
• COMPRESSIVE
STRESS
• SHEAR STRESS
• FLEXURAL (BENDING)
STRESS
ELASTIC PROPERTIES
• ELASTIC MODULUS/YOUNG’S
MODULUS
• DYNAMIC YOUNG’S MODULUS
• FLEXIBILITY
• RESILIENCE
• POISSON’S RATIO

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STRENGTH PROPERTIES
• PROPROTIONAL LIMIT
• ELASTIC LIMIT
• YIELD STRENGTH (PROFF STRESS)
• BAUSCHINGER EFFECT
• ULTIMATE TENSILE STRENGTH
• PERMANENT ( PLASTIC) DEFORMATION
• COLD WORKING ( STRAIN HARDENING OR WORK HARDENING)
• FLEXURAL STRENGTH

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OTHER IMPORTANT PROPERTIES
• TOUGHNESS
• FRACTURE TOUGHNESS
• BRITTLENESS
• DUCTILITY AND MALEABILITY
• HARDNESS

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General properties of stainless steel.
• SENSITIZATION.
• STABILIZATION.
• DUCTILITY AND MALLEABILITY.
• SOLDERING AND WELDING.
• STRAIN HARDENING.
• HEAT TREATMENT.
• ANNEALING.
• HARDENING HEAT TREATMENT.

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MECHANICAL PROPERTIES
• All mechanical properties are measures of the resistance of a
material to deformation, crack growth, or fracture under an
applied force or pressure and the induced stress.
• An important factor in the design of a dental prosthesis is
STRENGTH, a mechanical property of a material, which ensures
that the prosthesis serves its intended functions effectively and
safely over extended periods of time.

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WHAT ARE MECHANICAL PROPERTIES?
• Mechanical properties are defined by the laws of mechanics—that
is, the physical science dealing with forces that act on bodies and
the resultant motion, deformation, or stresses that those bodies
experience.
• Mechanical properties are the measured responses, both
ELASTIC (reversible upon force reduction) and PLASTIC
(irreversible or nonelastic), of materials under an applied force,
distribution of forces, or pressure.
• Mechanical properties are expressed most often in units of STRESS
and/or STRAIN.

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• The STRESSING RATE is also of importance since the
“strength of brittle materials increase with an increase in
the rate at which stress is induced within their structures”.
• They represent measures of :-
1. Elastic or reversible deformation (e.g., proportional limit,
resilience, and modulus of elasticity);
2. Plastic or irreversible deformation (e.g., percent elongation
and hardness); or
3. A combination of elastic and plastic-deformation (e.g.,
toughness and yield strength).

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STRESSES AND STRAINS
• STRESS—Force per unit area within a structure subjected to a
force or pressure
• PRESSURE—Force per unit area acting on the surface of a
material
• STRAIN—Change in dimension per unit initial dimension. For
tensile and compressive strain, a change in length is measured
relative to the initial reference length.
• Based on Newton’s third law of motion (i.e., for every action
there is an equal and opposite reaction), when an external force
acts on a solid, a reaction occurs to oppose this force which is equal
in magnitude but opposite in direction to the external force.

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• The stress produced within the solid material is equal to the applied
force divided by the area over which it acts.
• The SI unit of stress or pressure is the pascal, which has the symbol
Pa, that is equal to 1 N/m2.
• A tensile force → tensile stress,
• A compressive force → compressive stress, and
• A shear force → shear stress.
• A bending force can produce all three types of stresses, but in most
cases fracture occurs because of the tensile stress component.

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• When stress is induced by an external force or
pressure, deformation or strain occurs.
• STRAIN - or the change in length per unit
length, is the relative deformation of an object
subjected to a stress.
• Strain may be either
1. ELASTIC,
2. PLASTIC,
3. ELASTIC AND PLASTIC,
4. VISCOELASTIC.
• Strain is a dimensionless quantity, is measured
in inch per inch, foot per foot, and so forth.

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• Elastic strain is reversible. The object fully recovers its original
shape when the force is removed.
• Plastic strain represents a permanent deformation of the
material; it does not decrease when the force is removed.
• When an adjustment is made by bending an orthodontic wire, the
plastic strain is permanent but the wire springs back a certain
amount as elastic strain recovery occurs.
• Viscoelastic materials deform by exhibiting both viscous and
elastic characteristics. These materials exhibit both properties and a
time-dependent strain behavior.

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• Elastic strain (deformation) typically results from stretching but not
rupturing of atomic or molecular bonds in an ordered solid,
• whereas the viscous component of viscoelastic strain results
from the rearrangement of atoms or molecules within
amorphous materials.

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TENSILE STRESS COMPRESSIVE STRESS SHEAR STRESS
• A stress caused by a
load that tends to
stretch or elongate a
body.
• A tensile stress is
always accompanied
by tensile strain
• When a body is placed
under a load that tends to
compress or shorten it,
the internal resistance to
such a load is called a
compressive stress.
• A compressive stress is
associated with a
compressive strain.
• To calculate compressive
stress, the applied force
is divided by the cross-
sectional area
perpendicular to the
axis of the applied
force.
• This type of stress tends to resist
the sliding or twisting of one
portion of a body over another.
• Shear stress can also be
produced by a twisting or
torsional action on a material.
• For example, if a force is applied
along the surface of tooth enamel
by a sharp-edged instrument
parallel to the interface between
the enamel and an orthodontic
bracket, the bracket may debond
by shear stress failure of the
resin luting agent.
• Shear stress is calculated by
dividing the force by the area
parallel to the force direction.

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FLEXURAL (BENDING) STRESS
• These stresses are produced by bending forces in dental appliances in one
of two ways:
1. By subjecting a structure such as an FDP to three-point loading, whereby
the endpoints are fixed and a force is applied between these endpoints, as
in figure 4-1, A; and
2. By subjecting a cantilevered structure that is supported at only one end to
a load along any part of the unsupported section, as in figure 4-1, B.

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ELASTIC PROPERTIES
ELASTIC MODULUS (YOUNG’S MODULUS OR MODULUS OF
ELASTICITY)
• Elastic modulus describes the relative
stiffness or rigidity of a material,
• Measured by the slope of the elastic region of
the stress-strain graph.
• If the tensile stress below the proportional limit
in Figure 4-3 or the compressive stress (below
the proportional limit) in Figure 4-5 is divided
by its corresponding strain value, that is,
tensile stress/tensile strain or compressive
stress/compressive strain,

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• A constant of proportionality will be obtained
that is known as the ELASTIC MODULUS,
MODULUS OF ELASTICITY, OR YOUNG’S
MODULUS.
• These terms are designated by the letter E.
• The units of E are usually expressed as Mpa
for highly flexible materials
• Or Gpa for most stiffer restorative materials.

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• Because the elastic modulus of a material is a constant, it is
unaffected by the amount of elastic or plastic stress induced in the
material.
• It is independent of the ductility of a material, since it is
measured in the linear region of the stress-strain plot. Thus,
elastic modulus is not a measure of its plasticity or strength.
• Because the elastic modulus represents the ratio of the elastic
stress to the elastic strain, it follows that the lower the strain for a
given stress, the greater the value of the modulus.

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• The elastic modulus (E) of a tensile test specimen can be calculated
as follows:

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• For example, if one wire is much more difficult to bend than another
of the same shape and size, considerably higher stress must be
induced before a desired strain or deformation can be produced in
the stiffer wire. Such a material would possess a comparatively high
modulus of elasticity.

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DYNAMIC YOUNG’S MODULUS
• Elastic modulus can be measured by –
1. A dynamic method as well as the
2. Static techniques.
• Since the velocity at which sound travels through a solid can be
readily measured by ultrasonic longitudinal and transverse wave
transducers and appropriate receivers, the velocity of the sound
wave and the density of the material can be used to calculate the
elastic modulus and poisson’s ratio.
• Method of determining dynamic elastic moduli is less complicated
than conventional tests of tensile or compressive strength

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FLEXIBILTY
• For materials used to fabricate dental appliances and restorations, a
high value for the elastic limit (the stress above which a material
will not recover to its original state when the force is released) is a
necessary requirement because the structure is expected to return
to its original shape after it has been stressed and the force is
removed (elastic recovery).
• For example, in an orthodontic appliance, a spring is often bent a
considerable distance under the influence of a small stress. In such
a case, the structure is said to be flexible and to possess the
property of flexibility.

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RESILIENCE
• As interatomic spacing increases, internal
energy increases. As long as the stress is not
greater than the proportional limit, this energy
is known as RESILIENCE.
• It means precisely the amount of energy
absorbed within a unit volume of a
structure when it is stressed to its
proportional limit.
• The area bounded by the elastic region is a
measure of resilience and the total area under
the stress-strain curve is a measure of
TOUGHNESS.

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POISSON’S RATIO
• Because of the conservation of mass, an
object, such as a cylinder, becomes
longer and thinner when a tensile force is
applied to it.
• Conversely, a compressive force acts to
make such an object shorter and thicker.

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• An axial tensile stress, σz, along the
z (long axis) of a mutually
perpendicular xyz coordinate system
produces an ELASTIC TENSILE
STRAIN and an accompanying
ELASTIC CONTRACTION in the x
and y directions (εx and εy,
respectively).
• The ratio of εx / εz or εy / εz is an
engineering property of the material
called the Poisson’s ratio (ν).

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STRENGTH PROPERTIES
• STRENGTH - is equal to the degree of stress necessary to cause
either fracture (ULTIMATE STRENGTH) or a specified amount of
plastic deformation (YIELD STRENGTH).
• The strength of a material can be described by one or more of the
following properties:
1. PROPORTIONAL LIMIT- the stress above which stress is no
longer proportional to strain;
2. ELASTIC LIMIT- the maximum stress a material can withstand
before it becomes plastically deformed;

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3. YIELD STRENGTH OR PROOF STRESS- the stress required to
produce a given amount of plastic strain; and
4. ULTIMATE TENSILE STRENGTH, SHEAR STRENGTH,
COMPRESSIVE STRENGTH, AND FLEXURAL STRENGTH-
each of which is a measure of stress required to fracture a
material.

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PROPORTIONAL LIMIT
• PROPORTIONAL LIMIT- (stress
corresponding to point P) is the greatest
elastic stress possible in accordance with
Hooke’s law, it represents the maximum
stress above which stress is no longer
proportional to strain.
• When a straight edge is laid along the
straight-line portion of the curve from O to P,
the stress value at P, the point above which
the curve deviates from a straight line, is
known as the proportional limit.

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• When a certain stress value corresponding
to point P is exceeded, the line becomes
nonlinear and stress is no longer
proportional to strain.
• The initial region of the stress-strain plot
must be a straight line. Because direct
proportionality between two quantities is
graphically represented by a straight line,
the linear portion of the graph in Figures 4-
3 satisfies this law.

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ELASTIC LIMIT
• The elastic limit of a material is defined as the greatest stress to
which the material can be subjected such that it returns to its
original dimensions when the force is released.
• When a small tensile stress is induced in a wire, the wire will return
to its original length when the load/FORCE is removed. If the load is
increased progressively in small increments and then released, after
each increase in stress, a stress value will be reached at which the
wire does not return to its original length after it is unloaded.
• At this point the wire has been stressed beyond its elastic limit.

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YIELD STRENGTH (PROOF STRESS)
• Yield strength often is a property that
represents the stress value at which a
small amount (0.1% or 0.2%) of PLASTIC
STRAIN has occurred.
• A value of either 0.1% or 0.2% of the plastic
strain is often selected and is referred to as
the percent offset.
• The yield strength is the stress required to
produce the particular offset strain (0.1% or
0.2%) that has been chosen.

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BAUSCHINGER EFFECT
• This denotes the phenomenon when the material is strained beyond
its yield point in one direction, and then strained in the reverse
direction, its yield strength in the reverse direction is reduced.

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ULTIMATE TENSILE STRENGTH
• If a material continues to have more and more weight applied to it, it
will eventually break.
• If the material is being stretched , the stress at breakage is called
the ultimate tensile strength.
• This is the entire area under the stress – strain curve is a measure
of the energy required to fracture the material.
• It is a measure of the energy required to propagate critical flaws in
the structure

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SHEAR STRENGTH
• SHEAR STRENGTH—Shear stress at the point of fracture.
• SHEAR STRESS—Ratio of shear force to the original cross-
sectional area parallel to the direction of the applied force.
• TENSILE STRESS—Ratio of tensile force to the original cross-
sectional area perpendicular to the direction of applied force.
• COMPRESSIVE STRENGTH—Compressive stress at fracture.

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• Toughness can be measured as the total
area under the stress strain curve.
Toughness depends on strength and
ductility.
• The greater the strength and higher the
ductility, greater the toughness.

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PERMANENT (PLASTIC) DEFORMATION
• The stress-strain graph is no longer a straight line above the
proportional limit (PL); instead, it curves until the structure
fractures.
• If the material is deformed by stress at a point above the proportional
limit before fracture:-
1. Removal of the applied force will reduce the stress to zero,
2. But the plastic strain (deformation) remains.
• Thus the object does not return to its original dimension when the force
is removed. It remains bent, stretched, compressed, or otherwise
plastically deformed.

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COLD WORKING (STRAIN HARDENING OR WORK HARDENING)
• The process of plastically deforming metal at room temperature.
• When most metal alloys have been stressed beyond their proportional
limits :-
• Their HARDNESS AND STRENGTH  at the area of deformation,
but their DUCTILITY .
• As dislocations move and pile up along grain boundaries, further
plastic deformation in these areas becomes more difficult.

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• As a result, repeated plastic deformation of the metal, as occurs
during the bending of orthodontic wire or adjustment of a clasp arm
on a removable dental prosthesis, can lead to embrittlement of the
deformed area of the wire, and it may fracture when further
permanent adjustment is attempted.
• The key to minimizing the risk of reduced plasticity (embrittlement) is
to deform the metal in small increments so as not to plastically
deform the metal excessively.

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FLEXURAL STRENGTH
• Flexural strength, which is also called
TRANSVERSE STRENGTH and
MODULUS OF RUPTURE, is essentially a
strength test of a bar supported at each
end or a thin disk supported along a lower
support circle under a static load.
• For a bar with a rectangular cross section
subjected to THREE-POINT FLEXURE
(upper central loading in Figure 4-8), the
following equation may be used to
calculate the flexural strength (maximum
flexural stress at the lower midpoint
surface):

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• This test is, A COLLECTIVE MEASUREMENT OF TENSILE,
COMPRESSIVE, AND SHEAR STRESSES SIMULTANEOUSLY.

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OTHER IMPORTANT PROPERTIES
TOUGHNESS
• TOUGHNESS is defined as the amount of
elastic and plastic deformation energy
required to fracture a material.
• FRACTURE TOUGHNESS is a measure of
the energy required to propagate critical
flaws in the structure.
• Toughness is measured as the total area
under the stress-strain graph (such as
shown in Figure 4-6) from zero stress to the
fracture stress.

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• In comparison, the MODULUS OF RESILIENCE is the energy
required to stress a structure to its proportional limit.
• Fracture toughness, or the critical stress intensity, is a mechanical
property that describes the resistance of brittle materials to the
catastrophic propagation of flaws under an applied stress.

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BRITTLENESS
• BRITTLENESS is the relative inability of a material to sustain
plastic deformation before fracture of a material occurs.
• In other words, a brittle material fractures at or near its proportional
limit.

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DUCTILITY AND MALLEABILITY
• DUCTILITY- The ability of a solid to be
elongated or thinned plastically without
fracturing.
• Represents the ability of a material to
sustain a large permanent deformation
under a tensile load up to the point of
fracture.
• For example, a metal that can be drawn
readily into a long thin wire is considered to
be ductile.

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• MALLEABILITY- is the ability of a material to sustain
considerable permanent deformation without rupture under
compression, as in hammering or rolling into a sheet.
• Three common methods are used to determine ductility:
1. The percent elongation after fracture,
2. The reduction in area of tensile test specimens, and
3. The maximum number of bends performed in a cold bend test.

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HARDNESS
• It is defined as resistance to indentation.
• Factors influencing the hardness of a material
are its :
Compressive strength
Proportional limit
Ductility
The different types of hardness tests are :

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MACRO HARDNESS TESTS
• BRINELL HARDNESS TEST - In this test, a
hardened steel ball is pressed under a specified
load into the polished surface of a material, as
diagrammed in Figure 4-14.
• The load is divided by the area of the projected
surface of the indentation, and the quotient is
referred to as the Brinell hardness number, usually
abbreviated as HB or BHN, or more recently, HBW.
• Thus, for a given load, the smaller the indentation,
the larger is the number and the harder the material.

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• ROCKWELL HARDNESS TEST (RHN)
• It is similar to Brinell except for the use of steel ball or a conical
diamond point is used.

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MICRO HARDNESS TESTS
• KNOOP HARDNESS TEST - Employs a
Diamond indenting tool that is cut in the
geometric configuration.
• The impression is rhombic in outline and the
length of the largest diagonal is measured.
• The projected area is divided into the load to
give knoop hardness number (KHN).

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• VICKERS HARDNESS TEST -
• The Vickers hardness test employs the
same principle of hardness testing that
is used in the Brinell test. However,
instead of a steel ball, a square-based
pyramid is used.
• The method for calculating the Vickers
hardness number (usually abbreviated
as HV or VHN) is the same as that for
the BHN in that the load is divided by
the projected area of indentation.

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ALLOYS
• ALLOY is a metallic intimately mixed solid mixture of two or
more different elements one of which is at least essentially a
metal.
• Different methods used to prepare alloys are
1. Fusion method
2. Electro deposition
3. Reduction method
4. Powder metallurgy or compression method

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The General Requirements of a Dental Alloy
1. The alloys chemical nature should not produce harmful toxologic
or allergic effects in the patient or the operator.
2. The chemical properties of the appliances should provide
resistance to corrosion and physical changes when in the oral
fluids.
3. The physical and mechanical properties such as strength,
modulus of elasticity, coefficient of thermal expansion
conductivity should all be satisfactory.
4. The technical expertise needed for fabrication and use should be
feasible for the average dentist and skilled technician.
5. The metals, alloys and companion materials for fabrication should
be plentiful, relatively inexpensive and readily available even in
periods of emergency.

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CARBON STEELS
• Stainless steels are the major alloy system used in
orthodontics. However the metallurgy and
terminology of these alloys are intimately
connected to the binary iron carbon alloy
system and to carbon steel alloys.
• STEELS are iron based alloys that usually
contain less than 2.1% carbon by weight. The
different classes of steels are based on three
possible lattice arrangements of iron.
• Pure iron at room temp. has a BODY CENTRED
CUBIC STRUCTURE and is referred to as
FERRITE. This phase is stable up to 912oc.

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• The solubility of carbon in BCC is very low and
reaches a maximum of 0.02% at 723 °C.
• At temp. between 912oc – 1394oc the stable
form of iron is FACE CENTERED CUBIC
STRUCTURE [FCC] called AUSTENITE.
• Named after the British. Metallurgist – Robert
Austen.
• The maximal solubility of carbon in FCC
matrix is 2.1%.
• When a plain carbon steel containing 0.8%
carbon is cooled slowly in the austenitic
phase to 723 °C, it undergoes a solid-state
eutectoid transformation to yield a
microstructural constituent called PEARLITE,

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• which consists of alternating fine-scale lamellae of ferrite and iron
carbide (Fe3C), referred to as CEMENTITE, or simply, CARBIDE.
• The Fe3C phase is much harder and more rigid than austenite or
ferrite.
• When the carbon content is less than 0.8%, the microstructure
consists of FERRITE AND PEARLITE,
• whereas carbon steels containing more than 0.8% carbon will
yield much harder alloys with microstructures consisting of
CARBIDE AND PEARLITE.

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• If the AUSTENITE is cooled very rapidly
(quenched), it will undergo a spontaneous
transformation to a BODY CENTRED
TETRAGONAL [BCT] structure called
MARTENSITE.
• The lattice is highly distorted and strained
resulting in an extremely hard, strong and brittle
alloy named after the German metallurgist Adolf
Marten.
• Martensite is a metastable phase that
transforms to ferrite and carbide when it is
heated to elevated temperatures. This process is
called TEMPERING; it reduces the hardness of the
alloy but increases its toughness.

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STAINLESS STEEL
• When approx. 12% - 30% chromium by weight is added to carbon
steel, the alloy is commonly called
• A typical composition of the alloy consists of :-
18% chromium,
8% Nickel,
71% iron,
0.2% carbon
and other metals are also there like Titanium, Manganese (2%)
Silicon (1%) Sulphur (0.15%) molybdenum, niobium and tantalum
are present.

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• The CORROSION RESISTANCE of stainless steel is largely due to
the presence of CHROMIUM.
• About 11% of chromium is needed to produce corrosion resistance
in pure iron.
• Chromium resists corrosion well because of the formation of a
strongly adherent coating of Cr2O3 on the surface through a
process called PASSIVATION.
• Which provides a barrier to diffusion of oxygen and other
corrosive species and it prevents further corrosion of the
underlying alloy.
• Cr favours the stability of the [BCC] unit cells.

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• NICKEL stabilizes at low temperature into a homogenous mass and
FAVOURS corrosion resistant austenitic phase. Ni , Mn and N
favour the FCC structure.
• The alloys resistance to pitting corrosion is based on the content
of chromium, Mo, and Ni.
• CARBON provides STRENGTH AND HARDNESS and it increases
corrosion.
• SILICON IMPROVES RESISTANCE TO OXIDATION AND
CARBURIZATION at higher temperature and to corrosion.

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• SULPHUR allows easy machining of the alloy parts.
• PHOSPHOROUS allows for a lower temperature for Sintering, a
process in powder metallurgy in which the particles are heated to
coalesce just under their melting point.
• MANGANESE stabilizes the austenitic phase, but it decreases
the corrosion resistance.

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CLASSIFICATION OF STAINLESS STEEL
• Classified according to the American Iron and Steel Institute
(AISI) system. This classification parallels the unified number
system (UNS) and the German standards.
• (a) Austenitic stainless steels (300 series)
• (b) Martensitic steels (400 series)
• (c) Ferritic stainless steel (400 series)
• (d) Duplex steels [SAF 2205]
• (e) Types [500 series]
• (f) Precipitation Hardenable steels [PH steels] [600 series]
[630/17-4] [631/17-7]

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(A) AUSTENITIC STAINLESS STEELS (300 SERIES)
• The addition of nickel to the iron-chromium-carbon composition
stabilizes the austenite phase on cooling.
• Type 18-8 stainless steel, which contains 18% chromium and 8%
nickel by weight, is the most commonly used alloy for orthodontic
stainless-steel wires and bands.
• Austenitic stainless steel is preferable to ferritic stainless steel
for dental applications because it has the following properties:
1. GREATER DUCTILITY and ability to undergo MORE COLD
WORK without fracturing

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2. SUBSTANTIAL STRENGTHENING during cold working (some
transformation to martensite)
3. GREATER EASE OF WELDING
4. ABILITY TO OVERCOME SENSITIZATION
5. LESS CRITICAL GRAIN GROWTH
6. COMPARATIVE EASE OF FORMING

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Properties of Austenitic, 18–8 Stainless Steels
BIOCOMPATIBILITY
• High corrosion resistance below 400°C
• Chemically stable in oral or implant environments
• Superior mechanical properties
• Yield strength (YS) 1,100–1,750 MPa
• Ultimate tensile strength up to 2200 MPa
• Modulus of elasticity 170,000–200,000 MPa
• Surface hardness about 250–400 KHN
• Density 8.5 gm/cc
• Percentage elongation up to 35%
• Undergoes work hardening by large amount. That is why thinner wires
have higher mechanical properties.
• Fairly high formability factor, spring back qualities = YS/Q

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THERMAL PROPERTIES
• Melting temperature ranges = 1240–1260°C.
• Responds to heat treatments
• Can be welded and soldered
• When heated above 400°C undergoes sensitization, which can be
remedied to certain extent.
• Easily available in various forms, wires, sheets, bands, etc. and not
expensive.
• Most of these properties are suitable for selection for reactive
(passive) orthodontic appliances.

75
• These alloys are the MOST CORROSION RESISTANT OF THE
STAINLESS STEELS.
• AISI 302 is the basic type containing 18% chromium, 8% Nickel and
0.15% carbon.
• Type 304 has similar composition, but carbon content is (0.08%).
• Both 302 and 304 may be designated as 18-8 STAINLESS
STEEL.
• Type 316 L (0.03%) is used for implants 'L' signifies low carbon
content.

76
• Austenitic FCC structure is unstable at lower temperature where it
tends to turn into BCC (ferrite).
• If austenitizing elements (Ni, Mn and N) are added the highly
corrosion resistant solid solution phase can be preserved even at
room temperature.
• If these elements are absent these steels even with high
chromium content are ferritic at room temperature.

77
• GRAIN—A single crystal in the
microstructure of a metal.
• GRAIN BOUNDARY—The interface
between adjacent grains in a
polycrystalline metal. Dental alloys are
polycrystalline solids consisting of many
individual grains (crystals) separated by
grain boundaries.

78
Corrosion Resistance of Austenitic Stainless Steels
PASSIVATION
• It is the formation of an impervious, Cr2O3 layer on the surface,
when the base metal alloys containing Cr (also Al, Ti) are exposed
to atmosphere.
• This takes place immediately and even if it is scratched, new layer
forms instantly.

79
SENSITIZATION
It is the loss of corrosion resistance
When austenitic stainless steel is heated to
between approximately 400 °C and 900 °C,
iron-chromium carbide precipitate along
the grain boundaries and chromium is
depleted near the grain boundaries below
concentrations necessary for protection.
Thus, the stainless steel becomes
susceptible to INTERGRANULAR
CORROSION, and partial disintegration of
the weakened alloy may result.
• This phenomenon is called
SENSITIZATION.

80
• These temperatures are within the range of soldering and
welding temperatures.
• Corrosion taking place at the soldered joints and weld-nuggets, is
due to the loss of passivation and localized stress in the welded,
or soldered interfaces. These lead to failures known as weld-decay.
• In addition, filler materials that constitute brazed or soldered joints in
orthodontic appliances can also form galvanic couples in vivo.

81
Two methods can be used to minimize sensitization.
1. REDUCE THE CARBON CONTENT of the steel to such an extent
that carbide precipitation cannot occur.
• If the stainless steel is severely cold-worked and heated within the
sensitization temperature range, the iron-chromium carbides
precipitate instead at dislocations, which are located on slip
planes within the bulk grains.
• As a result, the carbide formation leads to a uniform distribution
throughout the alloy rather than by preferentially depleting
chromium near the grain boundary precipitates.

82
2. STABILIZATION
• Addition of small amounts (about 6 times carbon) of niobium or
titanium with tantalum stabilizes stainless steel.
• Elements such as Titanium And Tantalum, preferentially form
carbides, can be added to the stainless steel to preserve the level of
chromium when the metal is exposed to elevated temperatures. This
process is called STABILIZATION.
• During work hardening, i.e. drawing into wires, the carbon atoms
are shifted to dislocations. This causes uniform distribution of
carbon atoms (rather than concentrated at grain boundaries) and
reduces chance of corrosion.

83
MECHANICAL PROPERTIES OF AUSTENITIC STAINLESS STEEL
• Approximate values of the mechanical properties of a stainless-steel
orthodontic wire are listed in Table 17-3.

84
• STRESS RELIEF—The reduction of residual stress by heat
treatment.
• COLD WORKING—The process of plastically deforming metal at
room temperature.

85
RECOVERY HEAT TREATMENT
• An increase in the elastic properties of a stainless-steel wire can
be obtained by heating it to temperatures between 400 °C and 500
°C for 5 to 120 seconds after it has been cold-worked.
• This stress-relief heat treatment promotes the recovery annealing
stage, which removes residual stresses introduced during
manipulation of the wire.
• Thus, It Stabilizes The Shape Of The Appliance.
• This is important clinically, since such residual stresses can promote
fracture when the appliance is being adjusted by the clinician.

86
HEAT TREATMENT-
• Process of subjecting a metal to a given controlled heat followed by
controlled sudden or gradual cooling to develop desired qualities of
metal.
• 2 types of heat treatment
SOFTENING HEAT TREATMENT → ANNEALING
HARDENING HEAT TREATMENT → TEMPERING

87
A) ANNEALING
• ANNEALING—The process of controlled heating and cooling that is
designed to produce desired properties in a metal.
• Typically, The Annealing Process Is Intended To Soften Metals, To
Increase Their Ductility, Stabilize Shape, And Increase
Machinability.
• Effects associated with cold working ( e.g. strain hardening, lowered
ductility and distorted grains ) can be reversed by simple heating the
metal to an appropriate elevated temperature without melting it.This
process is called ANNEALING.

88
• The YIELD STRENGTH and the RANGE OF ELASTIC
DEFORMATION (working range) of the wire, necessary for a
satisfactory orthodontic appliance, are greatly reduced after such
annealing, which is a decided disadvantage clinically.
• However, it can be minimized by using LOW-FUSING SOLDERS
and MINIMIZING THE SOLDERING AND WELDING TIMES.
• Any softening that occurs under such conditions can be remedied
considerably by contouring and polishing of soldered areas.
• The more severe the degree of cold working, the more rapidly the
effects can be reversed by annealing.
• Also the higher the melting point of the metal, the higher the
temperature needed for annealing.

89
• Annealing takes place in three successive stages:
1) RECOVERY
2) RECRYSTALLIZATION
3) GRAIN GROWTH

90
1.RECOVERY-
• It is considered the stage at which the cold work properties begin
to disappear. There is slight decrease in tensile strength and no
change in ductility.

91
2.RECRYSTALLIZATION-
• When cold-worked metal specimens are annealed, recrystallization
occurs after the recovery stage.
• The atoms at this stage are rearranged into a lower energy
configuration
• A radical change occurs microstructurally the old, deformed grains
disappear completely and a new structure of strain-free grains with
a lower density of grain boundary per volume emerges.
• After completion of recrystallization, the metal retains microstructures
resembling that before cold-work and essentially attains its original
soft, ductile condition.

92
3.GRAIN GROWTH-
• When the cold-worked metal is annealed at an elevated
temperature, the grain size increases (see Figure 17-7, row 4 to
row 6).
• This increase is called GRAIN GROWTH, which is a process by
which the grain boundary area is minimized; large grains grow at
the expense of small grains.
• Grain growth does not proceed indefinitely to yield a single
crystal; rather, it ceases after a relatively coarse grain structure has
been produced.

93
• A larger grain size is not necessarily
beneficial, a large grain may have an
orientation where the resolved shear
stress results in a low proportional limit
and a substantial amount of local
permanent deformation.

94
B) TEMPERING
• Stainless steel cannot be hardened like carbon steel by quenching
or by any other heat treatment because of stability of austenitic
steel.
• Can be hardened only by cold working.

95
(b) Martensitic steels (400 series)
• This is the hardest variety.
• It has Cr = 11.5– 17%,
Ni = 0–2.5%,
C = 0.15–1.2%,
Fe = balance.
• When austenitic stainless steel is suddenly cooled , it has distorted
BCT structure formed by diffusion less transformation.
• Hence, it has very high slip resistance or strength.
• Has the lowest corrosion resistance out of the three varieties.

96
• Such steels could only be used for short contacts with the oral
environment.
• Because of their high yield strength and hardness, martensitic
stainless steels are used for surgical and cutting instruments.
• The cutting edges or the blades of hand cutting instruments or
dental burs, etc. are heat treated to form suitable amount of
martensite. The blades of knives, saw-teeth, sickles, etc. have
been martensiticed.
• The corrosion resistance is less than that of the other types and
is reduced further following a hardening heat treatment.

97
• It can be heat hardened by quenching and tempered by slow
cooling.
• It has very high mechanical properties, depending on carbon
content,
• Yield strengths 500–1900 MPa and
• Hardness 250–1100 KHN.
• It also has low ductility or percentage elongation <2%.

98
(c) Ferritic stainless steel (400 series)
• These alloys provide good corrosion resistance at a low cost
when high strength is not required.
• Compositions,
Cr = 11.5–27%,
C = 0–0.2%,
Mn = 2%,
Iron = balance.
• This is a low cost variety having fairly good corrosion resistance, but
comparatively poor mechanical properties.
• They cannot be hardened by heat treatment or readily work-
hardened. Consequently they have little application in dentistry.

99
• The NAME derives from the fact that the microstructure of these
steels is the same as that of iron at room temperature.
• The difference is that chromium is substituted for some of the
iron atoms in the unit cells.
• The degree of substitution can go as high as 30% in the presence of
small amounts of other elements like C, N, Ni.

100
• The modern SUPER FERRITES which belong to this category
contain 19-30% chromium and are used in several nickel free
brackets.
• Highly resistant to chlorides these alloys contain small amounts of
aluminium and molybdenum and very little carbon.

101
PERCENTAGE COMPOSITION OF THREE BASIC TYPES OF
STAINLESS STEEL

102
(d) Duplex steels [SAF 2205]
• Duplex steels consist of an assembly of both AUSTENITIC and
FERRITIC grains.
• Besides iron these steels contain molybdenum and chromium and
they have lower nickel content.
• Their duplex structures results in improved TOUGHNESS and
DUCTILITY compared to ferritic steels.
1. Cr-22%,
2. Ni-5.5%,
3. Mn – 2%,
4. Mo-3%,
5. C-0.03%,
6. P-0.03%,
7. Si-1%,
8. S-0.02% and others

103
• Their YIELD STRENGTH is more than twice that of austenitic
stainless steel.
• They are HIGHLY CORROSION RESISTANT.
• Combining a lower nickel content with superior mechanical
properties duplex steel has been used for the manufacture of one-
piece brackets. [eg: Bioline low nickel by CEOSA]

104
(e) Types [500 series]
• 501 and 502 are low chromium [4-6%] steel not used for orthodontic
appliances.

105
(f) Precipitation Hardenable steels [PH steels] [600 series]
[630/17-4] [631/17-7]
• Unlike most stainless steels PH STEELS can be hardened by heat
treatment.
• The process actually is an aging treatment. Which promotes the
precipitation of some elements purposely added.
• Because of its high tensile strength 17 - 4 PH stainless steel is
widely used for “MINI BRACKETS”.
• The manufacturer ORMCO has used another steel from the same
class PH 17 - 7 to make its “EDGE LOCK BRACKETS”. The added
metals lower their corrosion resistance.

106

107
STAINLESS STEEL IN ORTHODONTICS
• Stainless steel has many uses in orthodontics for the fabrication of
brackets, archwires, bands, ligatures, among other appliances

108
SOME ORTHODONTIC APPLICATIONS
SUPER STAINLESS STEELS
• Despite the fact that austenitic stainless steels are the most widely
used alloys for orthodontic applications, there are concerns among
orthodontists about allergic reactions caused by nickel.
• In addition, the need for alloys with higher corrosion resistance,
higher strength, and improved formability is increasing among
professionals.

109
• Therefore, a super austenitic stainless steel, known as SR-50A,
has been reported as having localized corrosion resistance like
that of titanium alloys because the passive film is enhanced by
the synergistic effect of high concentrations of nitrogen
(0.331%) and molybdenum (6.77%).
• This alloy has been used experimentally for manufacturing
orthodontic brackets and wires with very promising results

110
ORTHODONTIC BRACKETS
• Most manufacturers of orthodontic products use different stainless-
steel alloys for the fabrication of the numerous brackets they offer.
• Austenitic stainless steel, such as AISI 304L and 316l, remains
as the first choice for the manufacturing of brackets. However,
orthodontic brackets are also manufactured using alternative
stainless-steel alloys, such as 17-4 PH stainless steel and 2205
alloy.

111
ORTHODONTIC MINI IMPLANTS
• Although most mini implants or screws used as anchorage devices
in the orthodontic field are made of titanium alloys due to this
metal’s outstanding characteristics, stainless steel is still used by
some manufacturers.
• They claim that the mini implant made of surgical grade stainless
steel can be easily removed once that action has been performed
since this material will not induce osseointegration, which is
advantageous because a second surgical procedure will not be
necessary.

112
ORTHODONTIC WIRES
• Archwires are the base wires, which are
engaged in brackets of the various appliance
systems.
• These are used to provide a proper arch form
and / or provide a stable base to which the
auxiliaries can be attached to generate the tooth
moving forces.

113
CLASSIFICATION OF ARCHWIRE MATERIALS
Classification of archwire materials
based on material constituent
A. Gold
B. Stainless steel
C. Chrome-cobalt
D. Nickel-titanium
 Martensitic, and austenitic
 Super elastic, and thermodynamic /
temperature transforming.
E. Beta titanium
F. Alpha titanium
G. Titanium niobium alloy
H. Multi-stranded archwires
I. Composite/coated wires
J. Optiflex archwires

114
Classification of archwires according to CROSS-SECTION:
a. Round
b. Square
c. Rectangular
d. Miscellaneous

115
Classification of archwires based on the DIAMETER OF THE
ARCHWIRE
A. ROUND • 0.08” • 0.10” • 0.12” • 0.14” ETC.
B. SQUARE • 0.16” × 0.16” • 0.17” × 0.17” ETC.
C. RECTANGULAR • 0.17” × 0.25” • 0.17” × 0.28” ETC.
Classification of archwires according to the MICROSTRUCTURAL
ARRANGEMENT
A. SIMPLE CUBIC
B. FACE CENTERED CUBIC
C. BODY CENTERED CUBIC.

116
Stainless steel wires
• Introduced in 1929 by Wilkinson.
• Used for making clasps, springs, bows, arch wires etc.
• It is dispensed in various thickness or gauges.
• Mainly the austenitic form is made use of in orthodontics.
• Both, round and rectangular wires are made from stainless steel.
• Their use is dependent on the technique practiced, the stage of
treatment and the stiffness required (the purpose for which it is
being used- retraction/ aligning/ finishing etc.).

117
ADVANTAGEOUS PROPERTIES
1. High stiffness
2. High yield strength- 1400 MPa approx.
3. High resilience
4. Good formability
5. Good environmental stability
6. Good joinability
7. Adequate springback
8. Biocompatible
9. Corrosion resistant, except at weld sites
10.Economical.

118
DISADVANTAGEOUS PROPERTIES
1. Soldering is demanding
2. Lower springback than Nickel-titanium alloys.
3. High modulus of elasticity.
4. More frequent activations are required to maintain the same force
levels.
5. Heating to temperatures of 400-900 degrees causes the release of
nickel and chromium, thereby decreasing the corrosion resistance
of the alloy.

119
ADA. SPECIFICATION NO.32
ALLOY
MODULUS OF
ELASTICITY
YIELD
STRENGTH
(Mpa)
ULTIMATE
TENSILE
STRENGTH
SS 179 GPA 1759 MPA 2117

120
VARIATION OF PROPERTIES OF STAINLESS-STEEL WIRES
• VARIATION IN DIAMETER
• The force that can be developed in a given length of wire increases
16 times per unit of deflection when diameter is doubled.
• If the diameter of the given length of wire is doubled total load
will increase by 8 times.
• Range decreases as the diameter is doubled.

121
• VARIATION IN LENGTH
• The force that can be developed decreases to 1/8th when the
length of the wire is doubled
• Increase in length will proportionately decrease the maximum
load on a one for one ratio.
• If the amount of length of wire is doubled the amount of
deflection increases 4 times.

122
MULTI STRANDED OR BRAIDED WIRES
• To increase the strength and to decrease the stiffness wire is
braided or twisted together by the manufacturer which increases
flexibility and can sustain large elastic deflection in bending.
• CO-AXIAL WIRE •
• Has got a central core wire with 5 outer wires wrapped around.
• It increases resiliency & flexibility.
• It applies light continuous force.

123
WHEN TO USE MULTISTRANDED STAINLESS STEEL ARCHWIRES
• By using multi-stranded stainless steel arch wires one can employ
stainless steel arch wires in the initial stage of tooth alignment and
leveling without the need for loops.
• The elastic recovery of multi-stranded wires is 25% higher than
that of a conventional stainless steel wire of identical diameter.
• The rigidity of interbracket segments is much lower than that of
conventional stainless steel wires of identical diameter.
• Although less formable than conventional steel wires multistranded
wires are responsive to contours and bends, such as omega loops
for posterior tying, thus preventing tooth projection.
Orthodontic wires: knowledge ensures clinical Optimization - Dental Press J. Orthod 2009

124
AUSTRALIAN ORTHODONTIC ARCH WIRES
• Claude Arthur J Wilcock developed an orthodontic arch wire for use in the
Begg technique
• Unique characteristics different from usual orthodontic arch wires.
• They are ultra high tensile austenitic stainless steel arch wires.
• The wires are highly resilient.
• When arch wire bends are incorporated and pinned to the teeth the
stress generated within the wire which generate a light force which
is continuous in nature.

125
Unique Characteristic Of A J Wilcock Wire Different From
Usual Stainless Steel Wire
• Ultra high tensile austenitic stainless-steel arch wire
• The wire is resilient – certain bends when incorporated into the arch
form and pin to the teeth become activated by which stress are
produced within the wire which generates the force.
• The stress relaxation of Wilcock wire are significantly lesser
than Elgiloy wires.
• The Magnitude and continuous application of force are vital for
efficient function of appliances

126
AUSTRALIAN ORTHODONTIC ARCH WIRES
Types:
• Regular
• Regular plus
• Special
• Special plus
• Extra special plus
• Supreme
• premium
• Premium plus

127
Regular Grade – white Label
• Lowest grade and easiest to bend
• Used for practice bending and forming axillaries
Regular Plus Grade - Green Label
• Easy to form and more resilient than regular grad
• Used for axillaries and arch wires when more pressure and
resistance to deformation is required
Special Grade – Black Label
• Highly resilient, yet can be formed into intricate shapes with little
danger of breakage

128
Special Plus Grade – Orange Label
• Hardness and resiliency of the wire are excellent for supporting
anchorage and reducing deep overbite
Extra Special Plus Grade – ESP Blue Label
• Highly resilient and hard
• Difficult to bend and subject to fracture
Supreme Grade – Blue Label
• Further develop by Mr. A. J Wilcock Jr. In 1982 on request of Dr.
Mollenhauer of Australia.
• Is ultra light tensile fine round stainless steel wire.

129
• Was initially introduced in the .010” diameter and was further
reduced to .009” diameter.
• Is primarily used in early treatment for rotation , alignment and
leveling.
• Although the supreme wire exceeds the yield strength of the ESP it
is intended to use in either short section or full arches where sharp
bends are not required

130
NEWER WILCOCK ARCH WIRES
• Recently, A.J. Wilcock scientific and engineering company, the
manufactures of this wire, have introduced a new series of wire
grade and sizes with superior properties by use of new
manufacturing process called pulse straightening.
• The new grades available now are :
• Premium .020”
• Premium plus .010”, .011”, .012”, .014”, .016” .018”
• Supreme .008”, .009”, .010”, .011”

131
Care to be taken when handling A J Wilcock wire
• The wire should be held 12mm away from the tip of the beak and
wire
• Subsequently, the wire should be bent around the flat beak of
Mollenhauer plier.
• Coils are made by bending the wire towards the flat end of the beak
for the first 800 and completing the coil with round end of the beak

132
SOLDERING
• A process of joining two or more metal components by heating
them to a temperature below their solidus temperature and filling the
gap between them using a molten metal with a liquidus temperature
below 450 °C.
• Bonding of the molten solder to the metal parts results from flow by
capillary action between the parts without appreciably affecting the
dimensions of the joined structure.
• If the process is conducted above 450 °C, it is called BRAZING.
• In dentistry, many metals are joined by brazing, although the term
soldering is commonly used.

133
Types of solders
Soft Solders
• These are alloys of lead, bismuth and tin, having low fusion
temperatures, 200–260°C.
• It can be easily melted with narrow gas flame, or electric soldering
irons, it is used for soldering parts of electronic gadgets
(equipment's).
• It is also known as lead solder, or tin makers’ solder. It is not used in
dentistry due to Very low tarnish and corrosion resistance Poor
inadequate mechanical properties.

134
Hard Solders (Brazing fillers)
• These are alloys of gold, silver, copper, zinc, etc.
• Tarnish and corrosion resistance
• Mechanical properties, tensile strength, shear strength and abrasive
resistances
Liquidus temperature >450°C
Fusion temperature, ranges around :
• 700–870°C for gold solders
• 620–700°C for silver solders

135
• Ideally silver solders are used- alloy of silver, copper, zinc to
which tin and indium are added to lower the fusion temperature and
improve solderability

136
FLUX:
• Compound applied to metal surfaces that dissolves or prevents the
formation of oxides and other undesirable substances that may
reduce the quality or strength of a soldered or brazed area.
• The soldering flux should be fluoride fluxes, to reduce Cr2O3 film for
better flow of solder to improve wetting and bonding.
FUNCTIONS OF FLUX:
• Aids in removing the oxide coating so as to increase the flow.
• Dissolves any surface impurities.
• Reduces the melting point of the solder

137
Three types of fluxes can be distinguished -
• Type I–protective flux: When applied on the surface and heated,
the BORAX present melts forming a thin protective low fusing glass
layer, covering the surface, which protects it from oxidation.
• Type II–reducing flux: SODIUM TETRA BORATE or its dehydrated
form borax reduces the oxides, like CuO in metal alloy surfaces.
• Type III–dissolving (solvent) flux: FLUORIDES (like KF) can
dissolve the oxides of Cr, Co, Ni, etc. formed on the surface of base
metal alloys.

138
Composition of fluoride, fluxes for base metal alloys
• Potassium (or sodium) fluoride = 50–60%
• Boric acid = 25–35%
• Borax = 6–8%
• Potassium or sodium carbonates = 8–10%
• This is reducing and dissolving flux.
• The fluxes are applied as thin coating on the surface. If the coating
is too thin it will get burnt out. If it is too thick, it will be trapped as
bubbles in between, and bonding is decreased.

139
ANTIFLUX:
• A substance such as graphite that prevents flow of molten solder on
areas coated by the substance.
• Before soldering procedure the flux is heated, melted and flowed to
wet the surface area of soldering. This fluxed area helps molten
brazing filler to flow, to this entire area.
• To prevent the molten brazing alloys flowing to unwanted regions,
antifluxes are used.

140
• Demarking lines drawn by lead pencil, or application of suspension
of rouge (Fe2O3) or whiting (CaCO3), to the areas to be protected,
prevents the molten brazing filler, crossing into this region. Hence,
before brazing, first apply the flux to the surface and then antiflux
coating to the unwanted areas

141
Heat Sources for Melting Brazing Fillers
• Various types of mixtures of air-gas flames, electrical heaters,
infrared sources, etc. are used.
• Gas brazing: Special types of gas torches with adjustments for gas-
air (oxygen) mixing and reducing narrow pencil-like flame are
available. The gases should have high flame temperature as well as
high heat contents or calorific values. Eg: butane gas ,
Oxyacetylene gas , hydrogen gas.

142
Methods of soldering
Free Hand Soldering
• Usually employed for quick soldering of orthodontic wires, brackets,
and sometimes repairing the perforated castings.
• One part, say the thicker wire is held in a vice or a clamp support.
Flux and antifluxes are applied. A drop of molten solder is placed at
the required place on the thicker part. The thinner wire to be
soldered, is applied with the flux and is held in hand in contact with
the solder.
• The two wire parts are heated by narrow reducing gas flame until
the solder melts and flows into the adjusted gap. The assembly is
cooled and then quenched

143
Investment Soldering
• Suitable investment material quartz-gypsum bonded or phosphate
bonded investment material is mixed with water and poured on a
glass plate or a tile. The assembly is carefully transferred to this
soldering investment mix, which is allowed to set.
• Apply the antiflux and fluxes properly and place the solder in the
gap. The parts are heated by reducing gas flame until the solder
liquid flows into the gap. It is then cooled, devested, polished and
delivered

144
WELDING
• Process of fusing two or more metal parts through the application of
heat, pressure, or both, with or without a filler metal, to produce a
localized union across an interface between the workpieces
• Spot welding is used to join various components in orthodontics. A
heavy current is allowed to pass through a limited area on the
overlapping metals to be welded
• The resistance of the material to the flow of current produces
intense localized heating and fusion of metals.

145
• ELECTRIC-RESISTANCE SPOT WELDING– is used in dentistry,
for stainless steel, orthodontic bands, etc.
• The welded area becomes susceptible to corrosion due Chromium
carbide precipitation and loss of passivation
• The grain structure is not affected
• Increased weld area increases the strength

146
Electric resistance or spot welding
• The instrument consists of a step down transformer, to supply large
current (i Amps) through two thick copper electrodes C1 and C2
• The article to be welded (say matrix band), is held under pressure
between the two electrodes.
• Large AC or DC of about 750 amperes is passed for a short time
1/50 sec, through C1 and C2 by just pressing and releasing key K.
• As all other metals like stainless steel have higher resistance ‘R’
compared to thick copper rods C1 and C2, large heat produced
raises the temperature and melts the parts momentarily at the
contact point, forming a nugget.

147
CORROSION OF STAINLESS STEEL
• Defined as destruction or deterioration of material by a chemical or
electrochemical reaction.
• CAUSES: If surface in-homogenesity is present it allows corrosion
cells to form in presence of saliva.
RUST-
• It’s the formation of iron oxide when iron & steel alloy corrodes.
• It may be brown, black or reddish in color.
• Can take form of pits and blisters .

148
• House et al, in their review article, summarize corrosion types as follows:
1. UNIFORM ATTACK- It is the most common form, affecting all metals.
The metal undergoes a redox reaction with the surrounding
environment.
2. PITTING AND CREVICE CORROSION- It is formed on the surface
since wires and brackets are not perfectly smooth. Pits and
crevices may harbor plaque-forming microorganisms. Crevice corrosion
may also occur in removable appliances when wires or components of
expansion screws enter the acrylic.
3. GALVANIC CORROSION- This type occurs when two metals are
placed together in an electrolyte, such as wires and brackets made of
different alloys in the oral cavity.

149

150
4. INTERGRANULAR CORROSION- As discussed below, stainless
steel is particularly susceptible to this form of corrosion during
brazing and welding (see sensitization).
5. FRETTING CORROSION- It takes place in areas of metal contact
that are subject to load, as the archwire/bracket-slot interface.
6. CORROSION FATIGUE- This type of corrosion occurs when
metals are subject to cyclic stresses. The phenomenon is
accelerated if the alloy is in a corrosive medium, for instance,
when archwires are left in the oral cavity for long periods under
load.

151
7. MICROBIOLOGICALLY-INFLUENCED CORROSION- It may
occur due to the fact that microorganisms and their by-products
can affect metals in two ways: FIRST, some species absorb and
metabolize metal from alloys, which leads to corrosion. SECOND,
they can alter environmental conditions (e.g. by increasing local
acidity levels), making them more favorable to cause corrosion of
metals
8. CATASTROPHIC CORROSION- When stainless steel is
sensitized as in brazing or welding & then exposed to chemical
agents then Cr depleted boundaries are readily attacked by
oxygen. This phenomenon is called as catastrophic corrosion.

152
CONCLUSION
• Stainless steel is still one of the widest used materials in the
orthodontic field. Different types of this alloy are available in the
market to achieve diverse purposes. Austenitic stainless steel is the
preferred alloy for the manufacturing of wires, brackets, bands, and
mini-implants due to its good corrosion resistance and notable
mechanical properties. However, this material is not perfect and some
drawbacks can be found, such as allergies, sensitization, and cold
welding. Nonetheless, this alloy has been part of the orthodontic
armamentarium for many decades and is an active part of today’s
orthodontic practice. Therefore, a better knowledge of this material is
of paramount importance for the orthodontic practitioner.

153

Stainless steel, soldering & welding. corrosion

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