Stainless Steel Properties and Orthodontic Applications

Stainless Steel and it’s
Application in Orthodontics.
www.indiandentalacademy.com

 Introduction
 History of stainless steel
 Metallurgy
 Composition and functions of each ingredient.
 Types and grade of stainless steel.
Synopsis

 General properties of stainless steel.
 Sensitization.
 Stabilization.
 Ductility and malleability.
 Soldering and welding.
 Strain hardening.
 Heat treatment.

Annealing.

Hardening heat treatment
Synopsis

 Strength Properties
 Stiffness
 Strength
 Stress, Strain, Proportional Limit
 Elastic Limit
 Yield Point and Yield strength
 Plastic deformation
 Tensile strength
 Fatigue Strength
 Impact Strength
 Ultimate Tensile Strength
Synopsis

 Mechanical Properties based on clinical significance
 1. Elastic Modulus
 2. Resilience
 3. Flexibility
 4. Poisson’s Ratio
 5. Spring back
 6. Load deflection Rate
 7. Stress Relaxation
 8. Working Range
 9. Friction
Synopsis

 Other Properties
 Toughness
 Modulus of resilience
 Brittleness
 Bio-host ability
 Stainless Steel wires
 Ideal requirement of orthodontic wires
 Properties of stainless steel orthodontic wires
 Variation of properties
Synopsis

 Australian orthodontic arch wire.
 Unique characteristics.
 Manufacture, grading and color
coding.
 Nomograms
 Other Applications
 Conclusion.
Synopsis

Introduction
 Steel is an alloy of Iron and Carbon. Carbon
content should not exceed 0.2% max.
 When it contains 12 to 13% chromium it is
called stainless steel.
 Steel exists in three Ferritic, austenitic and
martensitic forms.

History
 First developed accidently by Harry Brearley in
Sheffield, England.
 Stainless steel entered dentistry in 1919, introduced
at Krupp’s dental poly clinic in Germany by F. Haupt
Meyer.
 In 1930 Angle used it to make ligature wires.
 By 1937 the value of stainless steel as an orthodontic
wire had been confirmed
 Stainless steel today is used to make arch
wires,ligature wires, band material, brackets and
buccal tubes www.indiandentalacademy.com

Nature of metallic bonding
Structure of solidification and grain structure.
Crystal lattice Types in Stainless Steel
Crystal imperfections.
Metallurgy

Composition
TYPES CHROMIUM NICKEL CARBON
FERRITIC 11.5-27% 0 0.2% MAX
AUSTENITIC 16-26% 7-22% 0.25%
MARTENSITIC 11.5-27% 0-2.5% 0.15-1.2%
Minor quantities of Silicon, phosphorous, sulphur, Manganese,
Tantalum.
In addition to Iron

 Chromium:
 Increases tarnish and corrosion resistance. A thin
transparent, tough, impervious oxide layer of Chromium
oxide forms on the surface of the alloy when subjected to
room air - “Passivating film effect”
 Increases hardness, tensile strength and proportional limit
 Nickel:
 Increases strength
 Increases tarnish and corrosion resistance
Functions

Functions
 Cobalt:
 Decreases hardness
 Manganese:
 Scavenger for Sulphur
 Increases hardness during quenching
 Silicon:
 Deoxidizer and scavenger.
 Titanium:
 Inhibits the precipitation of Chromium carbide.

Grades of Stainless
Steel
 SOFT
 HALF HARD OR SPRING HARD
 HARD

Austenitic Stainless Steel
 Most corrosion resistant of all types of stainless steel
 Formed between 912 – 13940
C
 AISI 302,304 – Chromium18%, Nickel 8% and Carbon
0.15%(302) 0r 0.08%(304) – 18-8 stainless steel
 Austenite is preferred to Ferritic because of greater ductility,
ability to undergo more cold work without fracture. Increased
strength during cold working, ease of welding, readily
overcomes sensitisation, less critical grain growth and ease
of forming
 When austenite is allowed to cool slowly to room
temperature it forms Fe3C and ferrite. The iron carbide
compound is called cementite and the solid solution of
ferrite along with cementite is called pearlite
FACE CENTERED
CUBIC LATTICE

Ferritic Stainless Steel
 Stable between room temperature and 912 C.
 Carbon has low solubility in this structure.
 Interstices in BCC are very small.
 AISI 400
 Good corrosion resistance at low cost provided increased
strength is not required.
 Temperature change does not induce phase change in solid
state
 The alloy is not hardenable by heat treatment.
 Not readily work hardenable.
 Little application in Dentistry.
BODY
CENTERED
CUBIC LATTICE

Martensitic Stainless Steel – Body
centered tetragonal
 If austenite is cooled rapidly (Quenched) it will undergo
spontaneous diffusion less transformation to a Body Centered
Tetragonal
 The lattice is highly distorted, strained resulting in a hard strong
brittle alloy
 Martensite decomposes into ferrite and carbide
 Decomposition is accelerated by appropriate heat treatment to
reduce hardness but this is counter balanced by increased
toughness – “Tempering”
 AISI 400

Properties of Martensite
 Increased strength and hardness – used for surgical and
cutting instruments
 Yield strength of 492 MPa (annealed). Hardened – 1898
Mpa
 Brinell’s hardness range- 230 – 600
 Elongation – less than 2%
 Reduced ductility
 Corrosion resistance is the least. Reduced further with
Hardening heat treatment.

 SENSITISATION:
 When heated between 400 and 900 C 18-8 stainless
steel loses it’s resistance to tarnish and corrosion.
 Carbon atoms migrate to grain boundaries and
combine with chromium to form chromium carbide
where the energy is the highest
 If the stainless steel is severely cold worked the
carbide precipitate along slip planes, as a result the
areas deficient in chromium are less localized and
carbides are more uniformly distributed
General Properties

General Properties
 Stabilization:
 Introduction of any element which precipitates
as carbide instead of chromium
 Titanium approximately six times the carbon
content prevents the accumulation of chromium
carbide at the grain boundaries

 A group of process of fusing two similar or dissimilar
metals by heating them to a suitable temperature
below the solidous of the substrate metals and
applying filler metals having a liquidous not exceeding
4500
C that melts and flows by capillary attraction
between the parts without appreciably affecting the
dimension of joined structure
 Soldering temperature – 620 – 6650
C
 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
Soldering

Soldering
 Flux:
 Material used to prevent formation of, or to
dissolve and facilitate removal of oxides,
impurities that may reduce the quality or strength
of the solder metals.
 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

 Composition:

Borax glass – 55%

Boric acid – 35%

Silica – 10%

Potassium flouride is added to dissolve the
passivating effect of Chromium.

Potassium fluoride and Boric acid should be in
1:1 concentration
Flux

Welding
 Joining of two or more similar metal pieces by applying heat,
pressure without introduction of an intermediary or a filler
material to produce localized union across the interface thro’
fusion or diffusion
 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
 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

Heat treatment
 As a result of cold working the stainless steel
is strain hardened. The method of treatment
to remove the unwanted strain hardening is
heat treatment. The effect of such treatment
depend entirely on temperature
 Hardening heat treatment
 Softening heat treatment - Annealing

Annealing
 The effect associated with cold working such as strain
hardening, low ductility and distorted grains can be
reversed by simply heating the metal
 The greater the amount of cold working the more rapidly
the effects can be reserved by annealing
 Temperature: 399 0
C for 11 minutes. Metal should have
a straw colored appearance on optimum heat treatment.
- Funk
 Stages of annealing:

Recovery

Recrystallisation

Grain growth

Annealing
 Recovery:
 Cold work properties begin to disappear.
 Slight decrease in tensile strength and no change in ductility.
 All the residual stress is relaxed
 Recrystallisation:
 Old grains disappear totally and are replaced with strain free grains.
 Occurs mostly in regions where defects have accumulated.
 It attains it’s soft and ductile condition at the end of this stage.
 Grain Growth
 The Grain size and number of the recrystallised structure depends on the
amount of prior cold working.
 On repeated annealing larger grains consume smaller grains. At the end of
annealing the number of grains decrease and size increases.

Hardening heat treatment
 There is no hardening heat
treatment for austenitic steel due
to it’s stability
 It can only be hardened by cold
working.

General
Properties
 Ductility:
 Ability of a material to be drawn into wires.
 Ability of a material to withstand permanent
deformation under tensile load without
fracture
 Malleability:
 Ability of metal to be made in sheets
 Ability of a metal to withstand permanent
deformation under compressive forces
without fracturing

Mechanical
Properties

Mechanical properties of Stainless Steel

Strength Properties
 Stress:
 Internal distribution of the load
 Force per unit area.
 Tensile, compressive or shear stress.
 Strain:
 Internal distortion produced by the load
 Deflection per unit length
 Proportion of change in dimension to the applied stress.
 Elastic strain: Original shape is regained.
 Plastic strain: Original shape is not regained.
 Elasticity:
 Ability of the stressed material to return to it’s original form
 Elastic limit:
 The greatest stress to which a material can be subjected so that it will return to it’s
original dimension when the forces are released.
 Hooke’s law:
 Stress is proportional to strain within the proportional limit.
 Proportional limit:
 Greatest possible stress that can be induced in a material such that stress is directly
proportional to strain.

Strength Properties
 Modulus of Elasticity: It is the measure of relative stiffness or rigidity of
the material. The mechanical property that determines the load deflection rate
is the modulus of Elasticity 179 GPa
 Strength: Capacity of a material to resist a deforming load without exceeding
the limits of plastic deformation. Strength is proportional to the resiliency of the
material
 Yield strength: The stress at which increase in strain is disproportionate to
stress. 1579 MPa 0.2% plastic deformation.
Ultimate strength: The strength at which the material fractures. 2117 MPa
Tensile strength – 200 MPa
Resilience: Total energy storage capacity. The amount of energy absorbed
by a structure when it is stressed within it’s proportional limit.
Knoop hardness: 600
 Stiffness: Force/ distance. It is the measure of resistance to deformationwww.indiandentalacademy.com

Property and Uses: British standard
3507:1962
DIAMETE
R
TENSILE
STRENGTH
(tons/in)
APPLICATION
0.9 TO
1.5mm
100-120 BOWS AND ARCHES
0.5mm to
0.8mm
120-130 CLASPS, FINGER SPRINGS
AND SELF SUPPORTING
SPRINGS
0.3 to
0.4mm
130-140 SPRINGS SUPPORTED ON
HEAVY ARCHES
0.15 to
0.25mm
140-150 COIL SPRING
0.4 to
0.55mm
160 or more. ARCHES FOR MULTIBAND
APPLIANCE

Characteristics of Clinical
relevance
 Spring back (maximum elastic deflection):
 The extent to which the range recovers upon
deactivation of an activated arch wire.
 A measure of how far a wire can be deformed
without causing permanent deformation or
exceeding the limits of the material.
 Higher the spring back, grater the working range
and lesser are the requirements of frequent
activations.
 Stainless steel has a spring back lesser than
Nickel-titanium or beta titanium

relevance
 Resilience:
 The capacity of a material to absorb
energy when the material is elastically
deformed
 It is measured by the area under the
stress strain curve

relevance
 Load deflection rate:
 For a given load the deflection observed
within the elastic limit
 The force magnitude delivered by an
appliance and is proportional to the modulus
of elasticity
 Low load deflection rate provides ability to
apply low forces, a more constant force over
time while deactivation, greater ease and
accuracy in applying a given force

Working range and Flexibility
 The distance a wire will bend elastically
before permanent deformation occurs
 Measured in millimeter or other length units
 Flexibility is the measure of the amount at
which the wire can be strained without
undergoing plastic deformation
 D x PL3
/ T4

Formability
The ability to bend wires into
desired configurations as loops, coils
and stops without fracturing the wire

Stress relaxation
When a wire has been deformed
and held in a fixed position the stress may
diminish with time even though the total
strain may remain constant.

 Toughness: The amount of elastic and plastic
deformation energy required to fracture. It is the
measure of resistance to fracture
 Modulus of resilience : Energy required to stress
a structure to stress to its proportional limit
 Brittleness : It is a relative inability of the material
to sustain plastic deformation before fracture of
material occurs. A stainless steel wire can undergo
five 900
cold working bends before fracture.
 Biocompatability: It is biocompatible. But Park
and Shearer have demonstrated the release of
Nickel and Chromium from stainless steel
appliances.
Other Properties

Ideal requirements of
Orthodontic arch wires
 Esthetic
 Good range
 Tough
 Poor biohost
 Good springback
 Low friction
 Weldable
 Springy
 Formable
 Biocompatible
 Resilient
 Strong

Properties of Stainless steel
arch wires:
 High stiffness.
 Low resiliency.
 Moderate spring back.
 Moderate range of action.
 Low friction.
 Good formability.
 Biocompatible.
 Good joinability.
 Less springy.

 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.
 Variation in Length

The force that can be developed decreases 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.
 Modification in arch wire – Multistranded arch wires:

Low load deflection rate.
 Increased flexibility and range.

Low force level.
Variation of properties of
Stainless Steel wires

Variation of properties of
Stainless Steel wires
 Cold working:
 Increased hardness.
 Reduced ductility.
 Increased yield strength.
 Increased modulus of elasticity.
 Annealing.

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.

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

Australian Orthodontic arch
wires
 Types:
 Regular
 Regular plus
 Special
 Special plus
 Extra special plus
 Supreme
 Premium plus

 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

 Special Plus Grade – Orange Label
 Hardness and resiliency of the wire are excellent
for supporting anchorage and reducing deep
overbite
 Extra Special Grade – ESP Blue Label
 Highly resilient and hard
 Difficult to bend and subject to fracture
 Supreme Grade – Blue Label
 Used for 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 requiredwww.indiandentalacademy.com

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
andcompleting the coil with round
end of the beak

 NOMOGRAM
 Nomograms are fixed charts which display the
mathematical functions, provided each scales is
adjusted in space appropriately with normal range
from one
 when constructed properly the relationship
between the parameters will be given in a straight
line
 In other words the extended the line between the
two will yield the third
 Strength = stiffness x range

Conclusion
Stainless steel is generally used
orthodontic wire because of its greater ease
of forming, greater ductility and malleability,
cold workable, ease of joining can be heat
treated and readily overcome sensitization.

Stainless Steel Properties and Orthodontic Applications

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