ss wire properties / orthodontic courses by indian dental academy

MECHANICAL PROPERTIES & STRUCTURE OF STAINLESS

STEEL WIRE

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INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

ContentsIntroductionHistorical BackgroundKey termsStructural propertiesPhysical propertiesStainless Steel• Classification of stainless steel• Applications of Stainless Steel

Conclusionwww.indiandentalacademy.com

Introduction

•Recent advances in orthodontics wire alloys have resulted in a varied array of wires that exhibit a wide spectrum of properties. Up until the 1930’s the only orthodontic wire available were made of gold.

•Austenitic stainless steel,was introduced as an orthodontic wire in 1929 and shortly afterwards gained popularity over gold.


Historical Background of Stainless steel :

•1903 to 1921 was the year that stainless steel was

developed and perfected by Brearley of Sheffield and

Becket of the U.S.

•Stainless steel entered dentistry in 1919, being introduced

at Krupp’s Dental Polyclinic in Germany by the

Company’s dentists, F. Hauptmeyer. He first used it to

make a prosthesis and called it Wipla ( Wie platin; in German, like platinum),


•Angle used it in his last year (1930) as ligature wire. By 1937 the value of Stainless steel as an orthodontic material had been confirmed.


KEY TERMSLATTICES

•The three dimensional network of lines that can be visualized to connect the atoms in undisturbed crystal is called a lattice.

•In the 19th century BRAVAIS showed that only a limited number of crystal lattices exist.


MECHANISM OF CRYSTALLIZATION

SOLID STATE OF DIFFUSION

ATOMIC DIFFUSION

GRAIN SIZE

NUMBER OF NUCLEI

RATE OF CRYSTALLIZATION

MOULD USED


LATTICE IMPERFECTIONS

POINT DEFECT OR LINE DEFECT

DISLOCATIONS


STRAIN HARDENING

INCREASE IN SURFACE HARDENESS STRENGTH PROPORTIONAL LIMIT

DECREASE IN DUCTILITY RESISTANCE TO CORROSION

RESIDUAL STRESS BUILTUP


ANNEALING

RECOVERY482ºC 3Min

RECRYSTALLIZATION

GRAIN GROWTH


Carbon steels:• Steels are iron-based alloys that usually contain less than 1.2%

carbon.

• The major classes of carbon steel are based on three possible

crystal structures that can occur for the iron carbon alloys:-

• Pure iron at room temperature has a body-centered cubic (BCC)

structure and is referred to as ferrite. This phase is stable in

temperatures as high as 912° C.


•The spaces between atoms in the BCC structure (interstices)

are small and oblate; hence, carbon has a very low solubility in

ferrite (maximum of 0.02 wt%).


• At temperatures between 912° C and 1394° C, the stable

form of iron is a face centered cubic (FCC) structure called

austenite. The interstices in the FCC lattice are larger than

those in the BCC structure. However, the size of the carbon

atom limits the maximum carbon solubility to 2.1 wt%.


• When austenite is cooled slowly from high temperatures, the

excess carbon that is not soluble in ferrite forms iron carbide

(Fe3C) to yield a microstructural constituent referred as

pearlite which consists of alternating fine scale lamellae of

ferrite and iron carbide (Fe3C), called cementite / simply

carbide. The Fe3C phase has an orthorhombic crystal

structure and is much harder and more rigid than austenite /

ferrite.


• If the austenite is cooled rapidly (quenched), it

undergoes spontaneous, diffusionless

transformation of a body-centered tetragonial (bct)

structure called martensite.

• The arrangement of the iron atoms in martensite is

highly distorted by the carbon atoms, resulting in a

very hard, strong, brittle alloy.


•The formation of martensite is an important

strengthening mechanism for carbon steels. The

cutting edge of carbon steel instruments are

ordinarily martensitic because the high hardness of

this structure allows the grinding of a sharp edge

that is retained in use.


IRON (bcc)

Heat 9120 C to 13940 C

AUSTENITE (fcc)

Gradually cooled

Ferrite + cementite

Rapidly cooled

Martensite


HARDNESS OF STEEL

carbon content

percent of austenite which transforms to martensite during cooling

CRITICAL COOLING RATE


Classification of Stainless Steel (depending on the lattice)

• Ferrtic

• Martensitic

• Austenitic

Other

• Duplex steels

• PH steel

• Cobalt containing steels

• Manganese containing steels


STAINLESS STEEL:

When approximately 12% to 30% chromium is added to iron, the alloy is commonly called stainless steel. Elements other than iron, carbon and chromium may be present, resulting in a wide variation in composition and properties of the stainless steel.


Composition ( %age ) of three types of stainless steel

Type of stainless steel (crystal structure formed by iron atoms)

Chromium

Nickel Carbon

Ferritic (bcc) 11.5 - 27.0

0 0.20 max

Austenitic (fcc)

16.0 - 26.0

7.0 - 22.0 0.25 max

Martensitic (bct)

11.5 - 17.0

0 to 2.5 0.15 - 1.20

Note : Silicon phosphorous, sulfur mangnese, tantalum may also be present in small amounts. Balance is iron.


Function of each component

Carbon : Provides strength.

Chromium : Passivating property.

Nickel : Stabilizes at lower temperature-depresses the

martensitic formation temperature and slows atomic

diffusion.


Silicon : Improves resistance to oxidation and carbonization.

Sulfur : Allows easy machining of wrought parts.

Phosphorous : Allows use of lower temperature for sintering.

Manganese : Is used as replacement for nickel to stablize the

austenite.


FERRITIC STAINLESS STEEL:

• These alloys are designated as American Iron and Steel

Institute (AISI) series 400 stainless steel. This series number is

shared with the martensitic stainless steel.

• These stainless steel provide good corrosion resistance at low

cost.

• These stainless steels are not readily work-hardenable, these

stainless steels have numerous industrial uses; they have little

application in dentistry.


MARTENSITIC STAINLESS STEEL:

• Because of their high strength and hardness, martensitic

stainless steels are used for surgical and cutting

instruments.

• Corrosion resistance is low-Dec with heat treatment

• Low ductility-2% elongation


AUSTENITIC STAINLESS STEELS:

The austenitic stainless steels are the most corrosion resistant

alloys of the three major types and are the stainless steels used

for:-

• Orthodontic wires

• Endodontic instruments

• Crowns in pediatric dentistry

The austenitic stainless steel is achieved by the addition of

Nickel to the iron-chromium - carbon composition.


Austenitic type

Chromium Nickel Carbon Molybdenum

Type 302 17% to 19% 8% to 10% 0.15% (max) -

Type 304 18% to 20% 8% to 12% 0.08% (max) -

Type 316L( low carbon)

16% to 18% 10% to 14% 0.03% (max) 2% to 3%

AUSTENITIC TYPES OF STAINLESS STEEL


Austenitic stainless steel is preferable to ferritic stainless steel

• Greater ductility and ability to undergo more cold work without fracturing.

• Substantial strengthening during cold working ( some transform into a martensite phase )

• Greater ease of welding

• Ability to overcome sensitization

• Less critical grain growth

• Comparative ease in forming


CORROSION RESISTANCE AND PROPERTIES OF AUSTENITIC STAINLESS STEEL:

• The resistance of stainless steel to tarnish and corrosion is

associated with the passivating effect of chromium.

• A very thin, transparent, adherent layer of Cr2O3 forms on

the surface of stainless steel when it is exposed to oxidizing

atmosphere such as room air.

• This protective layer provides a barrier to oxygen diffusion

and other corrosion environments and prevents further

corrosion of the underlying alloy.


SENSITIZATION

•Austenitic stainless steel may lose its resistance to corrosion if

it is heated between approximately 4000C and 9000C, the exact

temperature depending on its carbon content. Such temperature

are within the range used by the orthodontist for soldering and

welding.

•The decrease in corrosion resistance is caused by the

precipitation of chromium-carbide at the grain boundaries at

these high temperatures.


•The small carbon atoms rapidly diffuse to the grain boundary

region to combine with the chromium and iron atoms to form

(CrFe)4C, resulting in loss of the corrosion resistance provided

by chromium. This is called as sensitization.

•Corrosion resistance is reduced in regions adjacent to the

grain boundaries in which the chromium level is depleted

below that necessary for protection (approximately 12%). The

stainless steel becomes susceptible to intergranular corrosion,

and partial disintegration of the weakened alloy may result.


STABILIZATION

• Reduce the carbon content of the steel to an extent that

such carbide precipitation cannot occur, but this remedy is

not economically feasible.

• By stabilization if the stainless steel in which some

element is introduced that precipitates as a carbide in

preference to chromium. Titanium is commonly used. If

titanium is introduced six times the carbon content, the

precipitation of chromium carbide be inhibited at soldering

temperatures. These are called stabilized stainless steel.


•If the stainless steel is severely cold worked, the carbides

precipitate along the slip planes. As a result, the areas deficient

in chromium are less localized and the carbides are more

uniformly distributed so that the resistance to corrosion is

greater than when they precipitate only along the grain

boundaries.

•Such a method is presumably used in the processing of

orthodontic stainless steel wires.


•Any surface inhomogeneity is a potential source of tarnish or

corrosion- CORROSION CELL

•A common cause of the corrosion of stainless steel is the

incorporation of bits of carbon steel or similar metal in its surface.

•Such a situation results in an electric couple that may cause

considerable corrosion

•Chlorine-containing cleansers should not be used to clean

removable appliances fabricated from stainless steel.


MECHANICAL PROPERTIES OF FOUR TYPES OF ORTHODONTIC WIRES

Alloy Modulus of elasticity (Gpa)

Yield strength(0.2% offset)Gpa

Ultimate tensile strength (Gpa)

Number of 900 cold bends without fracture

Stainless steel

179 1.6 2.1 5

Cobalt-chromium-Nickel

184 1.4 1.7 8

Nickel-titanium

41 0.43 1.5 2

Bita-titanium

72 0.93 1.3 4


STRESSStress = Force/Area

STRAIN Strain = Deformation/Original length

MODULUS OF ELASTICITY:MOE = Stress/Strain

ELASTIC LIMIT







Stainless steel

179 1.6 2.1 5


184 1.4 1.7 8

Nickel-titanium

41 0.43 1.5 2

Bita-titanium

72 0.93 1.3 4


YIELD STRENGTHDeformation of 0.1% is measured







Stainless steel

179 1.6 2.1 5


184 1.4 1.7 8

Nickel-titanium

41 0.43 1.5 2

Bita-titanium

72 0.93 1.3 4


ULITIMATE TENSILE STRENGTH







Stainless steel

179 1.6 2.1 5


184 1.4 1.7 8

Nickel-titanium

41 0.43 1.5 2

Bita-titanium

72 0.93 1.3 4


FORMABILITY FLEXIBILITY & RESILIENCE strength and springiness


STIFFNESS OF THE WIRE

A knowledge of the stiffness of wire, tells us how far the tooth

will be moved by a specific initial force and how much force will

be applied at a certain deflection.

SPRINGINESSKapila, Sachdeva (1989), Drake et a1., (1992) referred to spring back as the maximum elastic deflection.

Springback = ys / E

Where, ys = Yield strength E = Modulus of elasticity


STRENGTH

KUSY (1997) defines it as the force required to activate an arch wire to a specific distance. Profit - Defines strength as the product of stiffness and range

Strength = Stiffness x range. The size and shape of the cross section of a wire have profound effects on the stiffness, strength and working range of a wire.


Comparison of some desirable clinical characteristics of orthodontic wires


•In orthodontic wires strength and hardness may increase with a

decrease in the diameter because of the amount of cold working

induced in forming the wire.

•The property of being readily strain hardened is a

characteristic of austenitic stainless steel.

•Phase change from a face-centered to a body-centered lattice.


•It is unfortunate that after strain hardening, a stainless steel wire

can become fully annealed in a few seconds at a temperature of

7000 C t6 8000 C. After such an annealing procedure, it has lost

much of the range of elasticity or working range that is necessary

to produce a satisfactory orthodontic appliance.

•Low-fusing solders and by confining the time for soldering and

welding procedures


DUPLEX STEELS

• Duplex steels consists of an assembly of both austenite and

ferrite grains Besides iron these steels contain molybdenum,

chromium, and they have lower nickel content. As a result of the

ferrite content, these steels ( as opposed to austenitic ones ) are

attracted by magnets.

• Their duplex structure results in improved toughness and

ductility compared to ferritic steels. While their yield strength is

more than twice that of similar austenitic stainless steels.


•They also are highly stress-corrosion resistant.

•Combining a lower nickel content with superior mechanical

properties, duplex steel has been used for the manufacture of

one-piece brackets.


PRECIPITATION - HARDENABLE (PH) STEEL

Unlike most stainless steel, the 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, PH stainless steel is

widely used for “mini” brackets.


COBALT CONTAINING ALLOYS

• Several cobalt containing alloys are commonly used in orthodontics, both for wires and brackets.

• Some, such as Eligiloy and Flexiloy, still contain a large proportion a nickel.

• Others, however are almost nickel free, having been developed to replace their allergenic counterparts.

• Although both types are used to make archwires, nickel free steels are used primarily to manufacture attachments.

• These alloys generally are corrosion resistant.


MANGANESE - CONTAINING STEELS

Known as an “austenitizing” element, manganese acts by

interstitially solubilizing the really” austenitizing” element

nitrogen, thus replacing nickel. Unfortunately, when used in a high

proportion, manganese increases the alloys susceptibility to

corrosion.


Wires used in orthodontics may be classified in three ways.

BY DESIGN/CROSS SECTION

• Round• Rectangular• Square • Double cross sectioned ( wonder arches )

BY DIAMETER

•0.016”•0.018”•0.020” etc

BY ALLOY

•Stainless steel •NiTi•Elgiloy•TMA•B – Ti•Polymeric wires etc


MANUFACTURING OF ORTHODONTIC WIRES

THUROW has described the various steps in manufacturing

process as follows:

a.Melting

b.Ingot Formation

c.Rolling

d.Drawing

BRANTLEY – Thermomechanical processing.


MELTING

The selection and melting of the components of alloys influence

the physical properties of metals.

INGOT FORMATION

An ingot is produced by the powering of mother alloy into a mold.

It differs from any other casting, by being a non uniform

chunk of metal. Different parts of the ingot possess varying

degrees of porosities.


ROLLING

• It is the first mechanical step in the manufacturer of a wire from the

ingot.

• The ingot is rolled into a long bar by a series of rollers that gradually

reduce it to a relatively small diameter.

• Orthodontic wires with rectangular/ square cross-sectional are

fabricated by rolling round wires, using a TURK’S head apparatus

that consist of pair of rollers positioned at right angles.


•The squeezing and massaging action of rolling the ingot, alters

the shape and arrangement of the crystals. This action causes

them to elongate into finger like shapes, closely meshed with

each other.

•Each pass through the rollers increases the work hardening until

finally, the structure becomes so locked - up that it can no longer

adjust enough to adapt to the squeezing of the rollers.


DRAWING

•It is a more precise process by which the ingot is reduced to its

final size.

•The wire is pulled through a small hole in a dye. The size of the

hole is slightly smaller than the starting diameter of the wire, in

order to facilitate uniform squeezing of the wire from all sides by

the walls of the die as it passes through, reducing the wire to the

diameter of the division.


•Drawing is a more precise process them rolling, as it subjects

the entire surface of the wire to the same pressure instead of

squeezing it from only two opposite sides as in rolling.

•Before the wire is reduced to its orthodontic size, it is drawn

through a series of dies and annealed several times along the

way to retrieve work hardening.


STAINLESS STEEL WIRES

• In the 1940’s Austenitic stainless steel began to displace gold as

the primary alloy for orthodontic wires.

• The most commonly used types are 302 and 304 stainless steel,

which contain approximately 18% chromium, 8% Nickel and

carbon of 0.08%. to 0.15 (max).

• 17-7 precipitation hardenable stainless steel alloy-high strength

& resiliency.


•These alloys derive most of their strength from cold working

and carbon interstitial hardening.

•The microstructure demonstrates the typical ‘fibrous’

appearance associated with extensively strains.

•A two-phase structure was found for some SS wires. where

the austenitic phase was accompanied by a body-centered

cubic (bcc) martensitic phase.


Ormco's H.T. Gold is a heat treated stainless steel wire which

provides higher force level and greater springback than traditional

stainless steel. It should be considered in applications where

resistance to deformation is a primary factor. The higher force

level and rigid nature of this wire makes it an excellent choice for

transverse arch form control.


MULTISTRANDED STAINLESS STEEL WIRE

• Flexibility of stainless steel wire can be increased by building

up a strand of stainless steel wire around a core of 0.0065” wire

along with 0.0055” wire used as wrap wires. This produces an

overall diameter of approximately 0.0165”.

• The strand of stainless steel wire is more flexible due to the

contract slip between adjacent wrap wires and the core wire of

the strand.


Twisted

Braided


• When the strand if deflected the wires which are both under

tension and torsion will slip with respect to the core wire and

each other. If there is no plastic deformation wire returns to

its normal position giving the elasticity to the strand of the

wire.

• Multistranded wires are available in round, rectangular,

square cross sections.

• Multistranded wire can be used as a substitute to the newer

alloy wire considering the cost of nickel-titanium wire.


•Kusy and Dilley noted that the stiffness of a triple stranded

0.0175" (3 X 008") stainless steel arch wire was similar to that

of 0:010" single stranded stainless steel arch wire.

•The multistranded archwire was also 25% stronger than

the .010" stainless steel wire.

•Then .0175" triple stranded wire and .016" Nitinol

demonstrated a similar stiffness.However nitinol tolerated 50%

greater activation that the multistranded wire.

•The triple stranded wire was also half as stiff as .016" B-

titanium. www.indiandentalacademy.com

Some of the multistranded wire available are:

• Dentaflex- Dentaurum is available in triple strand, co-

auxial, six strand, and braided eight strand.

• Twist flex-unitex

• Force-9-Ormco

• D-rect Ormco

• Respond- Ormco


D-rect is an 8 stranded, interwoven braided rectangular

wire. Its high flexibility, together with 3-dimensional control and

slot filling capabilities, makes it ideally suitable for multiple

applications like:

1.Initial torque control

2.Picking up second molars later in treatment

3. A finishing arch wire, where torque control is desired yet

resilient to permit interarch occlusal settling.

4.Torque control with vertical or anterior Box elastics.


FORCE 9 is a 9-strand, interwoven, braided rectangular wire. It

delivers 50% more force than the 8-stranded D-rect. Its selection

can be based upon similar applications where slightly more force

seems to be indicated.

RESPOND is a 6-strand, spiral wrap with a central core wire.

Respond can deliver light, initial forces while filling the arch

wire slot for greater control. Its resistance to permanent

deformation makes respond an excellent choice as an initial

archwire in more severe dental malalignments.


Currently a stainless steel combination wire is also available. It

consists of an anterior rectangular wire and a posterior round

wire. The anterior rectangular wire gives better torque control

and acts as brakes to burn out the anchorage. These wires are

also known as dual flex wires or wonder wires.


CONCLUSION

• In the last few decades, a variety of new wire alloys has been

introduced into orthodontics.

• These wires demonstrate a wide spectrum of mechanical

properties and have added to the verstaility of orthodontic

treatment.


•Appropriate use of all the available wire types may enhance patient

comfort and reduce chairside time and duration of treatment.

•The future lies in finding newer materials which gives more

physiologic tooth moving forces. This is just an milestone, But the

quest for newer orthodontic wires continues.



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