stainless steel

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Stainless Steel and it’s Application in Orthodontics.

By

Post graduate studentDepartment of Orthodontics

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Introduction History of stainless steel Metallurgy Composition and functions of each

ingredient. Types and grade of stainless steel.

Synopsis

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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

Synopsis

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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

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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

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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

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Australian orthodontic arch wire. Unique characteristics. Manufacture, grading and color

coding. Nomograms Other Applications Conclusion.

Synopsis

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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.

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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

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Nature of metallic bondingStructure of solidification and grain structure.Crystal lattice Types in Stainless SteelCrystal imperfections.

Metallurgy

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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

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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

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Functions Cobalt:

Decreases hardness Manganese:

Scavenger for Sulphur Increases hardness during quenching

Silicon: Deoxidizer and scavenger.

Titanium: Inhibits the precipitation of Chromium

carbide.

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Grades of Stainless Steel

SOFT

HALF HARD OR SPRING HARD

HARD

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Austenitic Stainless Steel

Most corrosion resistant of all types of stainless steel

Formed between 912 – 13940C

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

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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

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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

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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.

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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

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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

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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 4500C that melts and flows by capillary attraction between the parts without appreciably affecting the dimension of joined structure

Soldering temperature – 620 – 6650C

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

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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

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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

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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

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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

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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

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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.

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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.

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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

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Mechanical Properties

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Mechanical properties of Stainless Steel

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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.

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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 deformation

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Property and Uses: British standard 3507:1962

DIAMETER

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

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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

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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

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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

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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

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Formability

The ability to bend wires into desired

configurations as loops, coils and stops

without fracturing the wire

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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.

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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

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Ideal requirements of Orthodontic arch wires

Esthetic Good range Tough Poor biohost Good springback Low friction Weldable Springy Formable Biocompatible Resilient Strong

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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.

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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

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Variation of properties of Stainless Steel wires

Cold working: Increased hardness. Reduced ductility. Increased yield strength. Increased modulus of elasticity.

Annealing.

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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.

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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

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Australian Orthodontic arch wires

Types: Regular Regular plus Special Special plus Extra special plus Supreme Premium plus

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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

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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 required

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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

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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

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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.