Seminar by

DHANYA MENON M

STAINLESS STEEL

CONTENTS

BRIEF HISTORY

GLOSSARY OF TERMS

ALLOYS

CARBON STEELS

STAINLESS STEELS

SOLDERING

WELDING

HISTORY

It was discovered accidentally during

the early part of the first world war in

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.

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.

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 the 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.

Stress and Strain : When an external force acts

upon a solid body, a reaction force results within

the body that is equal in magnitude but opposite in

direction to the external force (Load).This internal

force divided by the area over which it acts within

the body is the

resultant stress.(Psi, Mpa)

 

And the deformation is

called strain.(in/in,cm/cm)

Strain may be either plastic or elastic

or a combination of the two. Elastic strain is

reversible and disappears when the force is

removed. Whereas plastic strain is

irreversible and represents permanent

deformation of the material that never

recovers when force is removed.

Types of stresses and strains: Stress can be defined according to its direction and magnitude.

By means of their directions stresses are :

(i)Tensile stress: It is carried by a load that tends to stretch/elongate a body. It is always accompanied by tensile strain.

(ii)Compressive stress : If a body is placed under a load that tends to compress or shorten it, internal resistance to such a load is called compressive stress. It is always accompanied by compressive strain.

In both the above stresses, forces are applied at right angle to the area over which they act.

(iii)Shear stress : Stress that tends to result from a twisting motion or sliding of one portion of a body over another.

Shear stresses result from force that tend to act parallel to surface of objects.

Elastic limit (Psi, Mpa)

 

If small tensile stresses are induced in

a wire,the wire will return to its original

length when the load 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 wire has been stressed beyond

elastic limit.

Proportional limit (PL) (Psi, Mpa)May be defined as greatest stress

that may be produced in a material such that the stress is directly proportional to strain. 

To determine the yield strength of a

material at a particular offset of .1% a line

drawn parallel to the straight line region

starting at a value of .001 or 0.1% of the

plastic strain along the straight axis and is

extended until it intersects the stress strain

curve. The stress corresponding to this point

is the yield strength. 

Yield strength (YS) (Psi, Mpa)

Is the stress required to produce the particular offset chosen or a point at which a deformation of .1% is measured. 

Modulus of elasticity, Young’s modulus or elastic modulus (E) (Psi, Mpa)  This describes the relative stiffness or

rigidity of a material which is measured by the slope of

theelastic region of the stress strain diagram. 

It is the ratio of stress to the strain.

It is a slope of stress/ strain curve.

If the slope is more horizontal- it is springier.

If slope is more vertical- it is stiffer.

Wire with high MOE is difficult to bend.

Less the strain for a given stress, greater will be the MOE.

Flexibility

It is the property of elastic deformation under loading. Maximum flexibility may be defined as the strain that occurs when the material is stressed to its proportional limit.

Resiliency

Can be defined as amount of energy absorbed by a structure when it is stressed not to exceed its proportional limit.(or)It can also be defined as maximum amount of energy a material can absorb without undergoing permanent deformation.

-     It is the property of the material itself and is not related to the size or form of the wire.

-     It is the area under the stress-strain curve at the

given maximum stress required to fracture a

structure.

-     It is the greatest force that can be sustained.

Strength, ultimate tensile strength, shear strength, compressive strength or flexural strength 

Is the maximum stress required to fracture a structure (can be tensile, compressive or shear, depending upon the predominant type of stress present).  

Toughness

It is defined as the amount of elastic and plastic deformation energy required to fracture a material and it is a measure of the resistance to fracture.

Can be defined as energy required to fracture a material.

Is related to the total area within the elastic and plastic regions.

Brittleness

Opposite to toughness, brittle material is apt to fracture at or near to proportional limit.

Can be defined as the relative inability to of a material to sustain plastic deformation before fracture of a material takes place.

Stiffness /Load Deflection Rate

-      Is the measure of resistance to deformation.

-      It is measure of force required to bend or otherwise deform the material over a definite distance.

-      Other things being equal, a stiffer wire can store proportionately more force.

Working Range

Is the measure of how far a wire or material can be deformed without exceeding the elastic limit.

This helps in knowing how far a tooth can be moved with single adjustment.

 

  Malleability  Is the ability of a material to withstand permanent deformation without rupture under compression as in hammering or rolling.

It is not as dependent upon strength as is ductility.

Ductility

Is the ability of a material to withstand permanent deformation under tensile load without rupture.

In general, ductility decreases with increase in temp, whereas malleability increases with increase temp.

It is dependent on

tensile strength.

Poisson’s ratio 

During axial loading in tension or

compression there is simultaneous axial and

lateral strain. Under tensile loading, there is

a reduction in cross section. Within the

elastic range, the ratio of lateral to axial

strain is called poisson’s ratio. In tensile

loading the poisson’s ratio indicates that

reduction in cross section is proportional to

the elongation during the elastic

deformation. The reduction in cross section

continues until the material is fractured.

HardnessIt is defined as resistance to

indentation.

Factors influencing the hardness of a material are its :

Proportional limitDuctilityMalleabilityResistance to abrasion

The different types of hardness tests are : MACRO HARDNESS TESTS

Brinell test wherein a hardened steel ball is pressed under a specified load into thepolished surface of the material.The load is divided by the area of the projected surface of the indentation and the quotient is referred to as brinell hardness number (BHN). Thus for a given load the smaller the indentation the larger is the number and harder is the material.

In Rockwell hardness test (RHN), it is similar to Brinell except for the use of a steel ball , a conical diamond point is used.

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).

Vickers hardness test is done using a diamond in the shape of a square based pyramid. 

Phase 

A homogeneous portion of a material

system that has uniform physical and

chemical characteristics.

Phase Transformation

 

A change in the number and or

character of phases that constitute the

microstructure of an alloy by a change in

crystalline structure. 

Pseudoelasticity 

Is the mechanical analogue of

thermoelasticity in which at constant

temperature, the austenitic to martensitic

phase transformation occurs with increasing

applied force.

As the force is subsequently removed,

the reverse phase transformation occurs.

 

 

Thermoelasticity

 

The thermal analogue of pseudo

elasticity in which the martensitic phase

transformation occurs from austenitic as the

temperature is decreased.

This can be reversed by increasing the

temperature to its original value.

 

Transition temperature Range

 

That temperature range over which the

alloy structure changes from the martensitic

to the austenitic phase is known as the

transition temperature range.

 

Spring BackAlso referred to as maximum elastic

deflection, maximum flexibility, range of activation, range of deflection (or) working range.

Higher springback values provide the ability to apply large activation with a resultant increase in working time of the appliance.

This is turn implies that fewer arch wires have to be changed or adjustments will be required.

Springback is also a measure of how far a wire can be deflected without causing permanent deformation exceeding the limits of the material.

Formability  

High formability provides the ability to bend a wire into desired configuration such as loops, coils ,stops etc without fracturing the wire.

The property relates to the area under the graph between the yield point and the failure point.

Biocompatibility And Environmental Stability 

Biocompatibility includes resistance to corrosion and tissue tolerance to elements in the wire.

Environmental stability ensures the maintenance of desired properties of the wire for extended period of time after manufacture.

Both in turn, ensure a predictable behaviour of the wire when in use.

Joinability

The ability to attach auxiliaries to orthodontic wires by welding or soldering to provide additional advantage when incorporating modifications to the appliance. 

Friction 

Space closure and canine retraction in continuous arch wire technique involves a relative motion of wire over bracket.

Excessive amount of bracket/wire friction may result in loss of anchorage or binding accompanied by little or no tooth movement.

The preferred wire material for moving a tooth relative to the wire would be one which produces the least amount of friction at the bracket/wire interface.

Space Lattice 

Can be defined as any arrangement of atoms in space such that every atom is situated similarly to every other atom. They may be result of primary or secondary bonds.

There are 14 possible lattice types and

forms but many of the metals used in

dentistry belong to cubic system ie the

atoms may crystalize in cubic arrangements.

Metals are made up of thousands of

tiny crystals and each crystal is known as

grain.

Strain hardening/Work hardening/Cold working 

Deformation of space lattice of metals by mechanical manipulation at room temperature is called cold working and hardening of metal by cold working is called strain hardening or work hardening.

Surface hardness, strength, proportional limit of metals are increased with strain hardening.

Whereas ductility and resistance to corrosion are decreased but elastic modulus is not changed appreciably.

Tensile strength increases but ductility decreases during cold working/work hardening/strain hardening.

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

 

A) Annealing Effects associated with cold working ( eg

strain hardening, lowered ductility and distorted

grains ) can be reversed by simple heating the metal. This

process is called annealing. The more severe the cold

working, more rapidly the effects can be reversed by

annealing.

Annealing in general comprises of three stages :

1) Recovery

2) Recrystallization

3) Grain growth

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. 

RECRYSTALLIZATION

It occurs after recovery stage. A radical change occurs microstructurally old grains disappear completely and are replaced by a new set of strain free grains. These grains nucleate in the most severely cold worked regions in the metal usually grain boundaries or where lattice is most deformed. On completion the metal essentially attains its original soft and ductile condition.

GRAIN GROWTH

The recrystallized structure has a certain average grain size depending on the number of nuclei. The more severe the cold working the greater the number of such nuclei. Thus the grain size for the completely recrystallized material can range from fine to fairly coarse. If fine grain structure is further annealed the grain begins to grow an in effect large grains consume the smaller grains. The process continues till a course grain structure is produced.

Stress relief annealing

Is the heat treatment to relieve stress without affecting the physical properties. The wire can be given new shape and can resist deformation to a greater degree.

 Procedure

Heat upto 260ºC for 20 min or 399ºC for 10 min

3-4V of Electric current is passed between two terminal blocks (Usually 2.5V is required but for springs or coils having greater length and increase in voltage greater than 2.5V is needed)

Colour change to a medium straw

Switch off current

If heat exceeds 500°C weld decay

takes place which can be avoided by adding

columbium. If complete ductility of wire is

required it should be heated more than

950°C. this causes recrystallization thus

giving it a equivaxed structure but at the

same time destroying the fibrous structure

of the wire on which the springiness

depends.

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.

 

Polymorphism 

A few metals and many compounds crystallize into more than one structure. If the changed structure is reversible as in iron, it is called allotropy. At higher temp iron unit cells belong to F C C system (austenite) whereas at lower ones it has BC C ( ferrite) 

ALLOYS

An alloy is defined as a combination of two or more metals, which are (generally) soluble in molten condition. 

Can also be defined for dental purposes as a metal containing two or more elements, at least one of which is a metal and all of which are mutually soluble in the molten state.

 

Various properties of alloys  Not different from those of pure metals

Most alloys solidify over at a temperature range rather than a single temperature with in this range a two phase solid and liquid system exists.

The presence of more than one metal can bring about certain reactions in the solid state that cannot occur in presence of a single metal.

Classification

1)According to the use (such as metal inlays, crowns and bridges, metal ceramic restorations, RPD, implants)

2)Major element (Au, Ag, Pd, Ti, Ni)

3)Nobility (high noble, noble and predominantly base metal)

4)Principle three elements (Au-Pd-Ag, Fe-Ni-Cr, Pd-Ag-Sn)

5) The dominant phase ( isomorphous single phase), eg. Eutectic, peritectic alloys, intermetallic compounds and combinations

6) According to number of elements like binary, tertiary quaternary etc

Eutectic alloy

This is an alloy having a fusion temperature, which is lower than that of its components. When solidifying, the components of alloy separate out, even though they were soluble in molten state. Generally these alloys are brittle and have a very low resistance to tarnish and corrosion. Are mainly used in solders Hypo Eutectic alloys

This is a eutectic alloy having a composition of less than eutectic.  

Hyper eutectic alloys

This is a eutectic alloy having a composition of more than eutectic.

Peritectic alloys

Is an alloy which solidifies while an atomic diffusion occurs, on slow cooling changes into beta phase.

Solid solution alloys

Is an alloy in which atoms of the solute were randomly distributed in space lattice on the solvent.

Dental alloys are normally of this type. 

PROPERTIES OF AN IDEAL ORTHODONTIC ALLOY Formability

Large elastic deflection for more constant force for tooth movement

A high yield strength since it is directly proportional to maximum elastic deflection.

Weld ability and soldering

Corrosion resistant

Stable in oral environment and biocompatible

Cost effective

Easily available

CARBON STEEL 

Steels are iron based alloys that contain less than 1.2% carbon ( More than 2% carbon containing alloys are called PIG IRON). The different classes of steels evolve from 3 possible lattice arrangements of iron.

Pure iron at room temperature has body centred cubic (BCC) structure and is referred to as FERRITE.

This phase is stable upto 912c.The spaces between atoms in BCC structure are small and oblate hence carbon has very low solubility in ferrite (.02wt % max.)

At temperature between 912c and 1394c, the stable form of iron is face centred cubic (FCC) structure called AUSTENITE.

The interstices in FCC lattice are larger than those of BCC structure. However the size of carbon atom is such that the resulting lattice strain still limits the maximum carbon solubility to 2.11wt%.

Austenitic form of iron is the stable form of

iron and is called GAMMA IRON or AUSTENITE

after the well known metallurgist, ROBERT

AUSTEN.

Transformation of Austenite to BCT

structure called MARTENSITE is a highly distorted

and strained, resulting in an extremely hard, strong

and brittle alloy.

Formation of martensite is an important

strengthening mechanism for carbon steels.

The process of decomposition of

martensite to form ferrite and carbide

[cementite and pearlite] can be accelerated

by appropriate heat treatment to reduce the

hardness but it is counter-balanced by an

increase in toughness. Such a heat

treatment process is called TEMPERING. 

STAINLESS STEEL

 

When the chromium (generally 12 to

30% )is added to steel, alloy is commonly

defined as stainless steel.

Elements other than iron, carbon and

chromium may also be present, resulting in

a wide variation of composition and

properties of stainless steel. 

MODIFYING ELEMENTS AND THEIR

FUNCTION

 

Chromium is added to increase tarnish and

corrosion resistance. It also increases

hardness, tensile strength and proportional

limit.

Nickel strengthens the alloy and helps in

increasing the tarnish and corrosion

resistance.  

Cobalt decreases the hardness

Manganese acts as scavenger and increases the hardness during quenching.

Silicon acts as a deoxidizer and also as scavenger

Titanium inhibits the precipitation of chromium carbide.

So these elements are added to stainless steel to modify the physical properties and also to make the unstable phase stable at room temperature.

STANDARDIZATION  

All standard stainless steels are classified and numbered for identification according to standardized system set up for steels by American Iron and steel Institute (AISI).

This system uses numbers from 300-502 for stainless steel number depends on composition and physical properties

 

TYPE AISI NO. ferritic 400

austenitic 302,304,316L (300 series)

martensitic 400

TYPES OF STAINLESS STEEL 

Basically the steels used in dentistry are divided into 3 types (based on lattice structure)

TYPE (space

lattice)

chromium

nickel carbon

Ferritic (BCC) 11.5-27 0

0.20max

Austenitic (FCC) 16.0-26

7-22 0.25max

Martensitic (BCT) 11.5-17

0-2.5 0.15-1.20

AISI UNS EXAMPLE

Cr Ni Mn

Mo C P Si S

303 S-30300 OrmcoDiamond

17-19

8-10 2 0.6 .15 0.2 1.00

.15

304L S-30403 Advanced orthod

18-20

8-12 2 - .03 0.04

1.00

.03

316L S-31603 “A” CoStand twins

16-18

10-14

2 2.5 .03 0.04

1.00

.03

types Approx. yield strength(kg/cm3)

Approx. tensileStrength(kg/cm3)

% elongation

Martensitic - annealed - heat treated

46009000-14000

700011000-21000

3012

Austenitic -annealed -cold worked

28004000-10000

67007000-12000

7050

FERRITIC STAINLESS STEELS (400series) Microstructure of these steels is similar to iron at

room temperature (BCC). The difference being that

in ferritic steel chromium is substituted for some

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

(eg: Carbon, Nitrogen, Nickel)

Modern super ferrites contain 19-30% chromium

and are used in several nickel free brackets.

Highly resistant to chlorides these alloys contain small amounts of aluminium, molybdenum and very little carbon.

Ferritic alloys provide good corrosion resistance atlower cost, provided high strength is not required.

Since temperature change induces no phase change inthe solid state, the alloy is not hardenable by heat treatment.

Ferritic stainless steels are not readily work hardenable.

This series of alloys find little application in dentistry.

MARTENSITIC STAINLESS STEELS

(400series)They can be heat treated in the same manner as

plain carbon steels with similar results.Because of their high strength and hardness

martensitic stainless steels are used for surgical

&

cutting instruments.Corrosion resistance of martensitic stainless

steel is

less than that of other types and is reduced

following

hardening heat treatment.As usual, when the strength and hardness

increase

ductility decreases. It may go as low as 2%

elongation for a high carbon martensitic

stainless

steel.

AUSTENITIC STAINLESS STEELS (300series)

These are most corrosion resistant of

stainless steels.

AISI 302 is the basic type containing

18% Cr, 8% Ni and .15% carbon.

Type 304 has similar composition, chief

difference

being that the carbon content is limited

to .08%.

Both 302 and 304 may be designated as 18/8

stainless

steel and are most commonly used in orthodontics

in

form of bands and wires.

Type 316 L (.03% max. carbon) is the type

ordinarily

employed for implants

The 316 & 316 L types have been recently

introduced

and 316 differs in that it contains 2% more Nickel

in

addition to about 2% Mb, thus improving its

corrosion

resistance.

Generally, austenitic stainless steel is preferable to the ferritic alloy because of :

Greater ductility & ability to undergo more could work without breakage.

Substantial strengthening during cold working.

Greater ease of welding.

Ability to fairly readily overcome senstization.

Less critical grain growth.

Comparative ease in formation.

DUPLEX STEELS

Consists of an assembly of both austenite and ferrite grains.

Along with Fe these steels have Mo and Cr and low amounts of Ni.

As opposed to austenitic ones these steels are attracted to magnets.

Duplex structure (γ+α') results in improvement in ductility and toughness compared to ferritic ones, while the yield strength is more than twice that of similar austenitic steels.

High corrosion resistant

When improperly heat treated, these steels have a tendency to form a brittle phase that diminishes their corrosion resistance.

Combining a lower Ni content with superior mechanical properties it is used for manufacture of one piece brackets (eg. Bioline “low nickel” by CEOSA, madrid)

PRECIPITATION-HARDENABLE (PH) STEELS

PH steels can be hardened by heat treatment the process being an aging treatment which promotes the precipitation of some elements which are added.

High tensile strength PH stainless steel is widely used for “mini” brackets

Ormco uses PH to make its edgelock brackets

The added metals lower the corossion resistance

COBALT CONTAINING ALLOYS

Commonly used in orthodontics e.g. Elgiloy and Flexiloy

Some contain large amounts of Ni others however are Ni free

Ni free steels are used to make arch wires

Generally corrosion resistant

To manufacture attachments such as Prestige (pyramid orthodontics), NU Edge LN ( TP orthodontics) and Elite –opti- mim (ortho organisers).

MANGANESE CONTAINING STEELS

Known as austenizing element

Manganese acts by interstitially solubilizing the really “austenitizing” element, nitrogen thus replacing Ni.

Unfortunately high proportions of Mn increases the alloys susceptibility to corrosion.

ACCORDING TO

E.C. COOMBE C.P. ADAMS

1 Soft

Very soft/ fully annealed

2 ½ hard Hard

3 Hard High tensile/super hard/hard spring

SENSITIZATION, WELD DECAY & STABILIZATION

At temperature in excess of 500c (exact

temperature depends upon its carbon content)

[ Range 400c -900C according to skinner’s]

chromium and carbon react to form chromium

carbide (Cr3 c), which precipitate at the grain

boundaries causing brittle behaviour. Also the

corrosion resistance decreases due to depletion of

the central regions of the crystals off chromium,

which has migrated to the boundaries to form the

carbides. This process in known as sensitization or

weld decay.

SOFT STAINLESS STEEL WIRE

Thoroughly annealed to release any work hardening 0.009”, 0.010”, 0.011”, 0.012”, 0.014”Enables ligature to be tied around the arch wireMaintains groups of teeth togetherBecause of its low yield strength there may be some extension in the length during its use and the wire should be stretched as it is placed to reduce the amount of lengthening.

Should be used to ligate NiTi wire to teeth that are displaced from the line of the arch, for rotation ties and to secure ties when full expression of torquing arch wire is required

Replaced by elastic modules

Criteria for selection of a proper wire/alloy Load deflection rate required in the appliance.

Magnitude of force and movements required.

Stiffness of the alloy its relative formability.

AUSTRALIAN ARCHWIRES

In 1952, Dr. Begg in collaboration with an Australian metallurgist Mr. A.J Wilcock, developed a high tensile stainless steel wire that is heat treated and cold drawn to yield its now familiar and excellent clinical properties the A.J.

It was made thin enough to distribute force at an optimal level for tooth movement over a considerable period of time, over long distance and with minimal loss of force intensity while doing so.

The diameter of the wire initially produced was progressively decreased from .018” to .014”.

There are 6 types of Australian arch wires :

Regular grade (white label)lowest gradeeasiest to bendused for practice bending and forming

auxillaries.

Regular plus (green label)Relatively easy to form, yet more resilient than

regular grade.Used for auxillaries and arch wires when more

pressure and resistance to deformation are

desired.

Special grade (black label)highly resilient yet can be formed into shape with

little danger of breakage

Special plus grade (orange label)Hardness and resiliency of .016” wire is excellent

for

supporting anchorage and reducing deep

overbites.Must be bent with care.Routinely used by experienced operators.

.014” Premium plus

In addition to the routine use of .014” wire as

up righting and torquing springs, these wires

have

been suggested for use in high angle cases as a

means to prevent molar extrusion.

.016” Premium plus

Is claimed to undergo less distortion in the mouth compared to older grades.

Is actually suitable for mesiofacial types.

.018” Premium plusThis wire has two good uses:

Where the VTO indicates that the upper incisors need intrusion form the outset. This is infact the most common requirement when the case requires reduction of a severe overbite before cessation of growth.

It is usually dictated by lip line considerations since the lower lip grows up on the average 4mm relative to the symphysis during the pubertal growth spurt. Otherwise the case will suffer a gummy smile after treatment.

2 nd use is for lower bypass arch wires.

.020” Premium

Claimed to be superior to .022” special plus for maintains the arch form.

Is also very effective in correcting arch form.

Compared to .022” special plus, this is equally stiff and yet more formable.

Ultra high tensile wires

Ultra high tensile wires of supreme grade (.008”, .009”, .010”, .011”, .022”) were used initially for aligning in lingual orthodontics.

It has been shown that supreme grade wires have similar flexibility values to -titanium and are approximately three times more resilient..

Only Nitinol has a superior property in terms of flexibility in comparison to the supreme grade wire but at the expense of good formability and at a purchase price many time that of stainless steel.

.008” / .009” Supreme (A.J. Wilcock Jr. 1984)

Is an ultra high tensile S.S. fine round wire, the

supreme grade.

Initially introduced in .010”, was further reduced

to

.009” diameter.

May be used to form a boxed reciprocal torquing

mechanism to lip one tooth against the adjacent

tooth

torque wise.

.010” Supreme

May also be used to form reciprocal torquing

springs.

Wire is best indicated for incisally activated

mouse

traps. These result in a shortened duration of

stage III

treatment.

Gentle force developed with .010” mousetraps

are not

associated with root restoration as happens with

heavier wires despite the rapid movement.

These wires are excellent for making minisprings. Coil size may be decreased with these wires. The advantage of minisprings is that they can be inserted behind the stage III brass pins in the slot, rear to stage III Archwire giving greater control and ease of placement. They are available as preformed.

Also they produce such light forces that they truly realise Begg’s concept of light differential forces.

The disadvantage is that they tend to get distorted easily.

.011” / .012” supreme

Mollenhauer states that this wire appears to be

strong

yet flexible for anterior teeth. He claims the wire

is

excellent for aligning second molars towards the

end

of stage III. This helps to establish good molar

contact at the end of treatment.

In the anterior section the .011” wire is tied

Piggyback

gingival to the main arch wire between central

incisors.

Some use these wires for complete aligning

arches.

MULTI STRANDED OR BRAIDED WIRES

 Initial orthodontic leveling arch wires

require great working range to accommodate the usualmalalignment of bracket slots in the untreated malocclusion.

Low stiffness is advantageous so that the force can be kept as gentle as possible.

High strength is desirable so that normal masticatory forces will not render the wire uselessthrough plastic deformation of fracture.

Braided or twisted wires are able to sustain large elastic deflection in bendings.

Because of their low apparent modulus bending these wires apply low forces for a given deflection when compared with solid stainless steel.

CO-AX WIRES

One of most efficient wires available for

edgewise or light wire technique to align

crowded or rotated

anterior teeth.

Has a central core wire for stability with

five outer

wires wrapped around for resilience and

flexibility.

Co-ax wires provide light continuous forces over a longer period and can be deflected to great degree (without taking a set).

The tightly wound, smooth, bright finish allows brackets to slide freely.

Mechanical properties of stainless steel

Modulus of elasticity – 26 x 106 psiYield strength- 229 x 103 psiUltimate tensile strength – 307 x 103 psiNo. of 90 cold bends without fracture for .017 x .025 wire = 5

DEFINITION

Is a group of processes that join metals by heating them to a suitable temperature below the solidus of the substrate metals and applying a filler metal having a liquidus not exceeding 450°C that melts and flows by capillary attraction between the parts without appreciably affecting the dimensions of the joined structure. (metals handbook desk edition 1992)

Soldering is the process of joining metals by use of filler material with a fusion temperature of less than 450°C.

Solidus

In a phase diagram, is the temperatures at which metals of an alloy system become completely solidified on cooling or start to melt on heating. (metals handbook desk edition 1992)

Liquidus

In a equilibrium phase diagram, it is the temperatures at which metals of an alloy system begin to freeze on cooling or at which the metals completely molten on heating.(metals handbook desk edition 1992)

Brazing

The process of joining closely approximated solid metal parts by heating them to a suitable temperature below the solidus temperature`of the parts and allowing a filler metal having a liquidus temperature above 450°C to melt and flow by capillary attraction between the parts without appreciably affecting the dimensions of the joined structure. (metals handbook desk edition 1992)

It is the process of joining metals by using a filler metal with a fusion temperature of more than 450 °C

The soldering process involves the:

substrate or parent metal to be joined

soldering filler metal (solder)

a flux

a heat source

Substrate metalKnown as the basis metal, it is the original pure metal or alloy that is prepared for joining to another substrate metal or alloy

Its composition determines its melting range

Also determines the oxide layer formed during the procedure and the flux that must be used to reduce the oxide, inhibit further oxidation or facilitate its removal.

Its composition also determines the wettability of the substrate by the molten solder alloy. The solder chosen must wet the metal at as low a contact angle as possible to ensure wetting at the joint area.

SolderDental solders are alloys that are used as intermediary or a filler metal to join two or more metallic parts.Fusion temperature should be lower than that of the parts to be joined. Rule of thumb is that the flow temperature of the filler metal should be 56°C (100°F) lower than the solidus temperature of the substrate metal.Free flowing and should adequately wet the metal parts it unites so that good adhesion is achieved. Strength of the solder should be similar to that of metals being joined. Should exhibit excellent tarnish and corrosion resistance in oral environment.

COMPOSITION

Usually fineness of a solder is less than that of the alloy being used. Previously, solders were commonly referred to by carat number, the number did not describe the actual carat of solder but rather the carat of the gold alloy on which the solder was to be used. In recent years, the degree of fineness has been used to describe the various solders. General rule is that the fineness or actual carat of the solder should be slightly less than the actual carat/fineness of the parts being joined since the solder of reduced fineness has a lower melting range and improved fluidity.

Gold solders

Contains gold (45-80%), silver (8-12%), copper (7-

12%) with tin (2-3%) and zinc (2-4%)Zinc and tin reduce the fusion temp of the solder

below the casting alloys. Also increase fluidity of

solder in molten state and improve the

mechanical

properties.Copper is added to improve strength and lower

the

fusion temp and to make it amenable to age

hardening.

Silver in large proportion than copper

improves

wetting of gold solders. Also decreases the

fusion

temperature.

Nickel may be added instead of copper if a

white

alloy is desired.

Silver solders

Are essentially alloys of silver (46-60%),

copper (15

-30%) and zinc (15-20%) to which

elements such as

tin and indium may be added to lower

fusion

temperature and improve solder ability.

  soft

Types of solders 

hard

Soft soldersInclude lead – in having low m.p. Known as plumber’s solder.Has low fusion range of about 260 c or less which permits them to be applied by simple means such as by a hot soldering iron.Soft solders lack corrosion resistance hence impracticable for dental application.

Hard solders

Have much higher melting temperature

Also possess greater hardness and strength properties e.g dental gold and silver solders which also possess good and corrosion resistance.

Flux

flux means flow

purpose of flux is to remove any oxide coating on the parent metal surface when the filler metal is fluid and ready to flow into place.

The most commonly used flux in

dentistry has the following

composition borax glass-55%, boric

acid- 35%, silica-10%

Anti flux

It prevents flow of solder and is used to confine the

solder to the work area. Graphite from a lead pencil is convenient antiflux

however it is removed by oxidation at higher

temperature.An effective antiflux for prolonged heating or higher temps can be made from a suspension of rouge (ferric oxide) or chalk (calcium carbonate) in alcohol.

TECHNIQUE

Involves several critical steps

A) Cleaning and preparing the surfaces to be joined.B) Assembling the parts to be joined.C) Preparing and fluxing the gap surfaces between the

parts.D) Maintaining the proper position of the parts during

the procedure.E) Controlling the proper temperature.F) Controlling the time to ensure adequate flow of

solders and complete filling the solder joint.

Based on technique used, soldering can be :

1.Investment soldering

2.Free hand soldering

3.Infra red soldering

Investment soldering

Recommended for precise arrangement of parts for

bridge work or partial denture with wrought wire clasp

arm. Used when area of contact between the metallic

parts being joined is large and whenever precision is needed in joining the metals.

Procedure involves embedding of the metallic parts

in an investment leaving a gap of about 0.13mm between the metals.

Free hand soldering

Most commonly used in orthodontics.

Is done without use of an investment.

Solder is generally melted onto one of the parts,

then

they are held together and joint is heated.

Named so because torches can be placed on

bench so

that both hands are free to hold the parts in

position.

Infra red soldering

Instead of using a torch to provide heat, an infra

red

heating unit is available specifically for dental

soldering.

Unit uses the light from 1000 walt tungsten

filament

quartz iodine bulb, which is mounted at the

primary

focal point of a gold plated elliptical reflector.

Material to be soldered is placed at the reflectors secondary focal point, at which the reflected infra red energy of the tungsten source is focused.

The main problem in the use of this unit is locating the focal centre of the light on the spot to be soldered. Failure to focus at the right spot can result in cold joints that are porous.

Joint design

Whenever possible, wires should be joined by turning one wire around the other and soldering the joint.

The excess bulk formed can used to advantage as in the stop lock for intermaxillary and extraoral traction.

When soldering wires, the joint should be encased completely in solder.

Soldered joint should not be polished as polishing removes the outer layer of solder and exposes the wire thereby breaking the continuity of solder which generally leads to failure of the joint.

Flux should be removed from soldered joints when the joint has barely cooled, by picking it away with a probe.

The solder will be found to have a bright smooth surface which is perfectly clean and hygienic.

Heat control

Most convenient method of melting solder for stainless steel is by means of miniature butane blow lamp.

Jet of blow lamp should be small enough to produce a fine, needle flame (1cm long).

A soft, quiet, blue flame melts the solder adequately as well as gives the operator time to observe the flow of solder and manipulate the wires.

Even slight overheating of the joint produces burning of the wire and solder resulting in weak joint and rough pitted surface of the solder.

If possible soldering procedure should be done in one heating. Remelting a joint to add more solder and make adjustments increase the rise of burning the solder and wire.

Localization of heat to site of soldering is important to avoid annealing of a large section of wire.

Availability of solders

Dental solders are supplied in variety of shapes and forms such as strips, rods, wires or cubes.

Choice of solder depends largely on the operation to be performed and each form is available in range of fineness.

Thin strips represent the conventional form for general applications.

Fine wire forms are most desirable for orthodontic applications.

Small cubes, approximately 1mm square are convenient for soldering a contact area on an inlay or for other small soldering procedures.

Rod forms are often notched along two sides, which permit the rod to hold flux better than smooth polished rod or strip and notched rods do not roll back into a ball when melted.

General considerations

Gap

Should be neither too great nor too small.

If the gap is too great, joint strength will be strength of the filler metal.

If it is too narrow, strength will probably be limited by flux inclusions, porosity caused by incomplete flow of the filler metal or both.

Flame Should be neutral or slightly reducing portion of the flame.

Flame application to the joint should be continuous and not to be removed until the brazing is complete.

Flame gives protection from oxidation especially at the brazing temperature.

TemperatureShould be the minimum required to complete the brazing operations.

Time

Longer time increases the possibility of diffusion

between parent metal and filler metal.

Shorter time increases possibility of incomplete filling

of joint and possibility of flux inclusion in the joint.

Both conditions result in weaker joints.

MICROSTRUCTURE

EXCESSIVE HEATING TIME AND TEMPERATURE

RECRYSTALLIZATION TO VARYING DEGREES

REDUCTION OF MECHANICAL PROPERTIES

IF EXCESSIVE DRAMATIC LOSS OF MECHANICAL PROPERTIES

TENDENCY TO BECOME BRITTLE AT AREAS IN WHICH RECRYSTALLIZATION HAS TAKEN PLACE

Therefore to prevent changes in microstructure the heating operation should be kept at minimum to achieve a successful operation.

Defective soldering

Overheating of wires during soldering can lead to diffusion between solder and wire, recrystallization, surface pitting, internal porosity and microstructure changes.

Failure to flow is generally due to one or more of following :

Parts were too cool when solder was applied.

Flux insufficient to cover the joint.

Contamination owing to improper cleaning or sulfur released from overheated investment.

Oxidation from improperly adjusted torch or oxidation caused by removing the reduced portion of the flame from the joint before the solder flows.

Soldering application in orthodontics:

Quad helix with spring

Hooks for arch wires

Labial bow soldered to adam’s clasp

Lingual arches

Retention appliances

DefinitionIt is joining two pieces of metal without the

use of an intermediatory alloy.

Methods of welding

There are 3 methods of welding used in dentistry. Each of them achieve metal to metal contact differently. Pressure weldingLaser weldingSpot welding

Pressure welding

If two metals are placed together and a sufficiently large pressure is applied rectangular to the surface, pressure welding occurs. Pure gold has no surface oxides but adsorbed gases prevent metal to metal contact.

If the force is applied rapidly so that the exposed surfaces can be compressed together before surface gases adsorb and if the applied force has a sufficiently large component parallel to the surface to produce permanent distortions that expose film – free metal, pressure welding results.

In pressure welding the problems of surface roughness are overcome by large compressive forces. eg gold foil (foil, mat or powdered pure gold) restorations are pressure welded by hand or mechanical condenses.

Laser welding

A laser generates a coherent, high intensity pulse of light that can be focused.

By selecting the duration and intensity of the pulse, metals can be melted in a small region without extensive microstructural damage to surrounding areas.

In laser welding of metals the beam is focused at the joint to melt the opposing surfaces.

Owing to the expansion form the locally high temperature, and change of state, two liquid surfaces contact and form a weld on solidification.

Lasers can be directed at small regions and can apply high energy to these regions in a very short amount of time. This means there is very little heating of the total appliance, except at the point of application.

This procedure can be performed on the master cast.

Spot welding

Is a convenient method for uniting pieces

of metal of the same kind.

Method is clean and quick and produces

joints which are strong and reliable.

Most metals may be spot welded.

Process consists of varying the temperature of pieces of metal to be joined until the metal becomes plastic but not molten at the site of joint and immediately applying pressure so that the metal parts are squeezed together in their plastic state and become one.

In spot welding, the pieces of metal to be united are held together in the required position and placed between two copper alloy electrodes which press the parts together.

In small bench machines. Spring pressure is

usually employed, when current is passed from one

electrode to the other through the metal, heat is

generated in and between the metal parts which is

sufficient to make them plastic. The pressure of the

electrodes then, forces the metal parts together, so

creating the weld.

Since the current is constant, so more heat will

be generated at the contact areas than in the

interior parts, therefore, the metals will become

plastic first at the contact point.

Small welds are generally considered

better since bonding is achieved with a

minimum of change in the original grain

structure.

Orthodontic welder design

Pioneer work on the design of a welder for orthodontic purpose was done by Friel (1993) and Mckeag (1939).

Welding machines from orthodontic purposes are designed to deliver heavy currents for accurately predetermined, very short time.

Spot welding is carried out without the aid of flux or any other protecting material.

Heat required for spot welding is generated at the interface of the workpieces.

Longer the time allowed for a weld, the greater the opportunity for heat developed at the interface to spread into the surrounding metal and greater the possibility for the full work piece thickness rising to a temperature at which loss of temper and softening can occur.

Feature of an orthodontic welder include turret electrodes which make various shapes of electrodes quickly available, a feature which holds the electrodes together so that both hands are free.

Timing switch may be automatic or controlled electronically or by capacitor discharge.

Electrode design is important feature in orthodontic welders, variety of electrodes tips are required for welding of wires, latches and attachments in fixed appliance construction which are usually provided on rotating turrets so that any pair of u & l may be selected.

When welding a light part to a heavy part the bulk of the heavy part is not raised to welding temp, only a skin at the surface making contact with the light part becomes plastic.

Light tapes can be welded to heavy wires using flat electrodes.

When welding light wires at heavy wires precautions should be taken against overheating the finer wire this can be overcome by using a grooved electrodes to weld fine wire.

Another method to overcome the problem is to make an attachment with a strap or loop of tape, so avoiding actually welding the fine wire. This tape can be welded to the arch wire making a strong joint.

A properly welded joint does not need any reinforcement.

It is impossible to solder the welded joint properly as small extrusions of metal which is tarnished prevent the flow of solder into the interstices of the joint.

In general welds are more susceptible to corrosion than are the metals surrounding them and spot welding in dentistry has been confined to temporary appliances, where the results have been satisfactory.