Ss in Orthodon

BASIC PROPERTIES OF METALS

& APPLICATION OF STAINLESS STEEL IN ORTHODONTICS

History

Orthodontists need a variety of devices, made from a large array of materials, which, insofar as possible, must be harmless. This obvious demand also is probably the most difficult to meet.

HistoryMaterial Scarcity, Abundance of Ideas (1750-1930)The scarcity of adequate dental materials at the end of 19 th century launched E.H. Angle on his quest for new sources.Before Angel began his search for new materials, orthodontist made attachments from noble metals and their alloys.

Historynoble metalsGold, platinum, iridium and silver alloysgood corrosion resistanceacceptable estheticslacked the flexibility and tensile strength Wood , rubber, vulcanite, piano wire and silk thread . No restrictions.

History

In 1887 angle tried to replace noble metals with German Silver{ a type of brass}

This was opposed by J.N.Farrar and others as it discolored in mouth.In 1888 Angle discovered Neusileber brass {German silver ,Cu 65%,Ni 21%,Zn} by varying composition of CU,Ni,Zn, as well as by applying cold working operation at various digress of plastic deformation

HistoryStainless steel (entered dentistry -1920)By 1920 Dr. F Hauptmeyer,a dentist used it to fabricate prosthesis. SS replaced gold Opposition Emil Herbstgold wire was stronger than stainless steel. (1934)Steel as ligature wire

HistoryAbundance of materials, Refinement of Procedures (1930 1975)Improvement in metallurgy and organic chemistry mass production(1960)Farrars dream(1878)Cobalt chrome (1950s)-Elgin watch coRocky Mountain Orthodontics- ElgiloyNitinol (1970s)- Buehler, into orthodontics- Andreasen. Unitek

HistoryThe beginning of Selectivity (1975 to the present)Orthodontic manufacturersManual and analog machines have been replaced by digitally controlled models, increasing both production and quality{ CAD/CAM}Composites and Ceramics Beta titanium (1980)The rise of large number of products on the market has led to iatrogenic damage. release of Nickel from attachment etc New products- control of govt agencies, private organizations.

Basic Properties of MaterialsElements all particles identical Atoms-smallest Atoms interact via electronsElectrons orbits around nucleusFloating in shells of diff energy levels Electrons form the basis of bonds

In metals, the energy levels are very closely spaced and the electrons tend to belong to the entire assembly rather than a single atom. So metals are seen as a network of +ions in a sea of electrons.

Basic Properties of MaterialsElectrons free to move throughout metal

As a result of these free electrons metals are good conductors of heat & electricity

The can be readily deformed

Basic Properties of MaterialsMolecules 2 or more atoms

CRYSTALIn solids atoms gather together in different ways,

Amorphous materials similar properties in all directions isotropy Glass

Crystals on other hand have specific geometry created by particular arrangement of their atoms in to a lattice.anisotropy

Basic Properties of Materials CRYSTALS the difference in the mechanical properties of solid results from the directional arrangement of the atoms.

Perfect crystals: the atoms occupies well-defined positions and their movement is hindered ~ cation-anion-cation-anionextremely strongIf like ions are forced together, breakage results. Unlike metals, crystals cannot deform

Basic Properties of MaterialsIn common metals the crystals penetrate one another such a way that the shape of the crystals can hardly be detected, microscopically these can be seen as grains~ ranging from microns to centimeters

The areas where crystals meet ,known as grain boundary

Grain boundary are the areas where atoms are irregularly arranged leading to weaker non crystalline (amorphous) structure.

boundaries are responsible not only for decreasing in mechanical strength but also for increasing in corrosion.

Basic Properties of MaterialsStages in the formation of metallic grains during the solidification of a molten metal

Polycrystalline- each crystal - grain

Basic Properties of MaterialsIn most cases crystal imperfections such as vacancies,interstitials,and dislocations contribute to the weakness of metals .

Vacancies These are empty atom sites

Interstitials Smaller atoms that penetrate the lattice Eg Carbon, Hydrogen, Oxygen, Boron. Often distort the metal structure

Basic Properties of MaterialsSubstitutial Element another metal atom can substitute one of the same or similar size. E.g. - Nickel or Chromium substituting iron in stainless steel.

Imperfections- although they lower the cleavage strength of the metal , increase its resistance to deformation

LATTICEThe three dimensional arrangement of lines that can be visualized as connecting the atoms in undisrupted crystals, is called a lattice.In its common representation a lattice is made up of spherical atoms distributed in a Unit cellCrystal combination of unit cells, in which each cell shares faces, edges or corners with the neighboring cells14 crystal lattices The most common lattice in orthodontic material is cubic lattice which includes, FCC&BCC,monoclenic&close-packed hexagonal lattice.

Basic Properties of Materials

Basic Properties of MaterialsThe atoms, which are represented as points, are not static. Instead, they oscillate about that point and are in dynamic equilibrium.

Lattice deformations:Because metals with simple bcc or fcc cells are densely packed ,they show a large number of slip planes that makes possible the plastic deformation, yet maintain the integrity of the crystal. slip planes-along which dislocation occurs

Basic Properties of Materialsshear stress atoms of the crystals can glide along these planes

more the slip planes easier is it to deform

Slip planes intercepted at grain boundaries-increases the resistance to further deformation

Basic Properties of MaterialsIf the shearing force is:-Small - atoms slip, and return back to their original position (elastic deformation)

Beyond the elastic limit - crystal suffers a slight deformationpermanent (plastic deformation)Greater stress - fracture

Basic Properties of MaterialsDuring deformation - atomic bonds within the crystal get stressed resistance to more deformation

Number of atoms that get stressed also increases resistance to more deformation

Work hardening

Forced interlocking of grains and atoms of metal.Locked in and under pressure/tensionCarried at room temperature. Strain or work hardening or cold work

Hard and strong, tensile strengthBrittle.

Basic Properties of Materials Annealing The effects associated with cold working, such as strain hardening ,low ductility ,and distorted grains, can be reversed by simply heating the metal bellow its melting point, this process is is called annealing

More the cold work, more rapid the annealingHigher melting point higher annealing temp. the melting temperature (oK)

Before Annealing

Recovery Relief of stresses

Recrystallization New grains from severely cold worked areas-original soft and ductile condition

Grain Growth large crystal eat up small ones-ultimate coarse grain structure is produced

Basic Properties of MaterialsTwinningIn certain metals with Closed packed hexagonal type of crystals deformation occurs by twinning, a movement that divides the lattice in to two symmetrical parts Fixed angle NiTi - multipleSubjected to a higher temperature, de - twinning occurs (shape memory)

Basic Properties of MaterialsMetals made of large grains are weak ,the smaller the grains , the more the intergranular boundaries that oppose the slip planes Various methods of obtaining smaller grain size Enhancing crystal nucleation by adding fine particles with a higher melting point, around which the atoms gather. Preventing enlargement of existing grains. Abrupt cooling (quenching) of the metal.Dissolve specific elements at elevated temperatures. Metal is cooledSolute element precipitates barriers to the slip planes.

Basic Properties of MaterialsPolymorphismMetals and alloys exist as more than one type of structureTransition from one to the otherAllotropy - reversibleAt higher temperature, iron FCC structure (austenite) lower temperatures, BCC structure (ferrite)

Transition of IronIron FCC stable (austenite), 912*c-1394*cLattice spaces greater, Carbon atom can easily be incorporated into the unit cell

Transition of IronOn Cooling

Transition of Ironcooled (quenched)Carbon Rapidly cannot escapeHighly strained, distorted body centered tetragonal lattice called martensiteBecause it occurs without diffusion or chemical change this transition is the result of a specific crystallographic relationship between the parent phase and the new phase ,rearrangement of the atoms in the unit cells that has been named the Bain distortion


Interstitials are intentionally incorporated into the alloy to make it hard when it is quenched.Unlike boiling and melting temperate martensitic transformation do not occur in precise temperature but rather within a range known as temperature transition range (TTR)Ms-martensite start Ms- martensite finishAs-Austenite start Af- austenite finish

Elastic PropertiesThe elastic behavior of any material is defined in terms of its stress-strain response to an external load .Stress is the internal distribution of the load ,defined as force per unit area

Strain- internal distortion produced by load defined as deflection per unit length.

Elastic PropertiesTypes of stress/strainTensile induced force that resist deformation caused by a load that tends to stretch or elongate a body Compressive internal force that resists a loade which compress/towards each otherShear a force that tends to resist a twisting motion or sliding of one portion of a body over another .Complex force systems

Elastic PropertiesStrainStressElastic limitThe greatest stress to which a material can be subjected such that it returns to it original dimensions when forces are released

Elastic Properties proportional limit: It is defined as the greatest stress which may be produced in a material such that the stress is directly proportional to strain { Hookes law}Yield strength: the point at which a deformation of 0.1% is measured UTS.:the maximum load the wire can sustain is reached after some permanent deformation it is greater than the yield strength .

Elastic PropertiesStrainStressElastic LimitProportional LimitYield strength0.1%

Elastic PropertiesStrainStressUltimate TensileStrengthFracture Point

Elastic Propertiesultimate tensile strength is higher than the yield strength

important clinically maximum force that the wire can deliver

Ultimate tensile strength higher than the stress at the point of fracture

The slope of the stress- strain curve is the modulus of elasticity ,E. stiffness and springiness are proportional to E

Elastic PropertiesStrainStressSlope StiffnessStiffness 1 .Springiness

Elastic PropertiesStrainStressRangeSpringbackPoint of arbitrary clinical loadingYield point

Elastic PropertiesRange: is defined as the distance that the wire will bend elastically before permanent deformation occurs.Clinically, ortho wires are deformed beyond their elastic limit.Spring back property is between the elastic limit and UTS Strength = Stiffness x Range

Elastic PropertiesResiliency it is defined as the amount of energy absorbed by a structure when it is stretched no to exceed its proportional limit .the energy is released back when the stress is released .But a part of the energy is disputed in the form of heat .Strength + springiness

Elastic PropertiesStrainStressResilienceFormabilityProportional limitYield strength

Elastic Properties

Formability - amount of permanent deformation that the wire can withstand without breakingIndication of the ability of the wire to take the shape Also an indication of the amount of cold work that they can withstand

Elastic PropertiesFlexibility large deformation (or large strain) with minimal force, within its elastic limit.

Maximal flexibility is the strain that occurs when a wire is stressed to its elastic limit.Max. flexibility = Proportional limit Modulus of elasticity.

Elastic PropertiesToughness required to fracture a material. Total force area under the stress strain graph.Brittleness opposite of toughness. A brittle material, is elastic, but cannot undergo plastic deformation. eg: GlassFatigue Repeated cyclic stress of a magnitude below the fracture point of a wire can result in fracture. This is called fatigue.

Corrosion

A chemical or electrochemical process through which a metal is attacked by natural agents such as air & ,water resulting in partial or complete dissolution , deterioration, or weakening of any solid substance.

Steel resist corrosion primarily because of passivating effect of the chromium.For passivation to occur , a thin ,transparent, but tough and impervious oxide layer of CrO3 forms on the surface of the alloy when it is subjected to an oxidizing atmosphere .If this layer ruptured bay means of mechanical or chemical means a temporary loss of protection against corrosion will occur.

CorrosionUniform attack the entire wire reacts with the environment,Chlorides can penetrate and destroy the passivity that is responsible for corrosive resistance of stainless steel Aggressive chlorides are part of our diet and also found in saliva over 500mg/Lof Cl ions.Pitting Corrosion manufacturing defectssites of easy attack

CorrosionStainless SteelNiTi Pitting corrosion

Corrosion3.Crevice corrosion or gasket corrosion -.the common type of attack on stainless steel takes the form of crevices ,which develop at the interface between metal appliances and food debris or non adherent polymers

Electrometric ligatures ,acrylic plates .

After pit has formed the access of oxygen is limited and the protective layer of chromium oxide cannot be regenerated.

Corrosion4. Galvanic /Electrochemical CorrosionIn the presence of an electrolyte ,the joining of an anodic metal to another, less noble metal leads to the formation of an electric cell .the less noble metal oxidizes by releasing electrons and becomes anode ,the noble metal becomes cathode

even the same metal after different type of treatment (soldering etc) difference in the reactivity Galvanic cell. Less Reactive More Reactive (Cathodic) (Anodic) less noble metal

CorrosionAnodicLooses ElectronsSoluble ionsLeach outCathodic (nobel)Accepts electronsEven less reactiveS.Steel- active and passive areas : depletion & regeneration of passivating film

Corrosion5. Intergranular corrosion Sensitization (400c-900c ) ppt of CrC

6. Fretting corrosion Wire and brackets contact Friction surface destructionPressure rupture of the oxide layerDebris get deposited at grain boundaries, grain structure is disturbed.

Corrosion7. Microbiologically influenced corrosion Microorganisms such as the sulfate-reducing bacteroides corrodens and the acid producing streptococcus are know to attack dental alloys in mouthThe bacteria attack .AdhesiveCraters at the base of bracketsOr wires directly bonded on to teeth shown by Matasa.Certain bacteria dissolve metals directly form the wires. Others affect surface structure.

Micro-0rganisms on various dental materials

Corrosion8. Stress corrosion Similar to galvanic corrosionBending of wires different degress of tension and compression. Alter the electrochemical behavior anode cathode

Stainless Steel1919 Germany used to make prostheses. Extremely chemically stable High resistance to corrosion. Chromium content. The chromium gets oxidized, Impermeable, corrosion resistant layer.

Stainless SteelVariety of stainless steels Varying the degree of cold work and annealing during manufactureFully annealed stainless steel extremely soft, and highly formableLigature wireDead soft

Stainless SteelStructure and compositionChromium (11-26%)improves the corrosion resistance Stabilizes BCC phaseNickel(0-22%) austenitic stabilizercopper, manganese and nitrogen - similaramount of nickel added to the alloy adversely affect the corrosion resistance.

Stainless SteelCarbon (0.08-1.2%) provides strength

Reduces the corrosion resistance Sensitization. During soldering or welding, 425-815ocChromium diffuses towards the carbon rich areas (usually the grain boundaries)Chromium carbides Chromium carbide is soluble, intergranular corrosion.

Stainless SteelStabilization Element which precipitates carbide more easily than Chromium. Titanium is often used for stabilizationTi 6x> CarbonNo sensitization during soldering. Most steels used in orthodontics are not stabilized.

Stainless SteelSilicon (low concentrations) improves the resistance to oxidation and carburization at high temperatures.Sulfur (0.015%) increases ease of machiningPhosphorous allows sintering at lower temperatures.But both sulfur and phosphorous reduce the corrosion resistance.

Ingot formationPorosities due to dissolved gases (produced / trapped)Vacuum voids due to shrinking of late cooling interior.Important to control microstructure at this stage basis of its phy properties and mechanical performance

Making of ss ortodonticwiresStepsMeltingIngot formationRollingDrawing

Melting: various metals and alloys are melted the composition of metals influence the final mechanical properties .

Ingot formation :

The molten alloys poured in to a mould to produce a ingotAn in got is a uniform chunk of metal with a various degree of porosity and inclusions of slang in different parts .In got made up of granular structure that controls ultimate mechanical properties .

it is very impartment to control the microstructure of the in got as it is basis for physical properties and mechanical performance. Rolling: it is the first mechanical step ingot is rolled in a long bar by series of rollers gradually reducing the ingot diameter. each pass through the roller increases work hardening , then metal is annealed to releave the stresses.Drawing final size of the wire is obtained by drawing the wire is pulled through a small hole in a die .The whole is slightly smaller in diameter than the stating diameter of the wire for squeezing the wire uniformly. annelid several times along to relieve stresses .

Stainless SteelClassificationAmerican Iron and Steel Institute (AISI)Unified Number System (UNS)German Standards (DIN).

Stainless SteelThe AISI numbers used for stainless steel range from 300 to 502Numbers beginning with 3 are all austeniticHigher the number More the iron content More expensive the alloyNumbers having a letter L signify a low carbon content

Stainless SteelAustenitic steels (the 300 series)Better corrosion resistance -attachmentsFCC structure non ferromagneticNot stable at room temperature, Austenite stabilizers Ni, Mn and N 302& 304 is Known as the 18-8 stainless steels, these are most comely used ss in orthodonticsType 316L is ordinarily employed for implants.

AISI 316L is currently used for bracket manufacturing

this alloy contains 16-18%-Cr 10-14% Ni 2-3% Mo and maximum of 0.03% C

Stainless SteelMartensitic steel FCC BCC BCC structure is highly stressed. More grain boundaries, StrongerLess corrosion resistantMaking instrument edges which need to be sharp and wear resistant. Super ferritics contain 19% to 30 % chromium and are used in several nickel free brackets. These are highly resistant to chlorides and alloys contain small amounts of aluminium and molybdenum and very little carbon.

Stainless SteelFerritic steels (the 400 series)Good corrosion resistance Low strength. Not harden able by heat treatment. Not readily cold worked.

Stainless SteelAustenitic steels more preferable :-Greater ductility and ability to undergo more cold work without breaking. Substantial strengthening during cold work. (Cannot be strengthened by heat treatment). Strengthening effect is due partial conversion to martensite.Easy to weldEasily overcome sensitizationEase in forming.

Stainless SteelDuplex steels It consists of an assembly of both austenite and ferrite grains. They also contain molybdenum and chromium and lower nickel content. Their yield strength is more than twice that of similar austenitic stainless steels. These steels have been used for the manufacture of one-piece brackets (Eg: Bioline low nickel brackets).

Manufacturing low nickel attachments Increased toughness and ductility than Ferritic steels

Stainless steelPrecipitation hardened steels Certain elements added to them precipitate and increase the hardness on heat treatment. The strength is very high Resistance to corrosion is low. Used to make mini-brackets.

cobalt containing alloys they are used in orthodontic to make wires and bracketsElgiloy and flexiloy nickel free alloys are used primarily to make attachments such as prestige(pyramid ortodontics),Nu-EdgLN(TPorthodontics)

General properties of Stainless SteelRelatively stiff materialYield strength and stiffness can be variedAltering the carbon content and Cold working and AnnealingHigh forces - dissipate over a very short amount of deactivation (high load deflection rate).

Stainless SteelClinical terms:-Loop - activated to a very small extent so as to achieve optimal forceDeactivated by only a small amount (0.1 mm)Force level will drop tremendouslyNot physiologicMore activations

Stainless SteelForce required to engage a steel wire into a severely mal-aligned tooth. Either cause the bracket to pop out, Or the patient to experience pain. Overcome by using thinner wires, which have a lower stiffness High stiffness Maintain the positions of teethHold the corrections achieve

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High Tensile Australian Wires HistoryA.j.wilcock of Australia originally developed this special orthodontic wire at the request of Dr.P.R.Begg .Available in a verity of diameter sizes ,grades of resiliency, coiled or un coiled or st.lengths.each grade is colure coded .Regular grade white :used for auxiliaries Regular plus-green: relatively easy to form Special grade-black: highly resilient , used as starting arch wire Special plus-orange: for supporting anchorage

High Tensile Australian Wires

Newer grades were introduced after the 70s.Premium, premium +, supreme Premium grade-high tensile strength Brittle. Softening , loss of high tensile properties

High Tensile Australian Wires Bauschinger effect.Described by Dr. Bauschinger in 1886.Material strained beyond its yield point in one direction, then strained in the reverse direction, its yield strength in the reverse direction is reduced.

solders for stainless steel For a gold soider a fineness of approximately 250 (10 karat solder) is necessary. Silver solder are essentially alloys of silver, copper and zinc to which elements such as tin and indium my be added to lower fusion temperature and improves solderability.The soldering temperature for orthodontic silver solders are in the range of 620C-665CFlux contains a fluoride to dissolve the passivating film suplied by the chromium . Potassium fluoride is one of the most active chemical in this respect .

Welding of Steel3 useful properties Comparatively low melting point, High electrical resistance and Low conductivity of heat.

Welding of SteelImportant to minimize the time of passing the currentminimize the area of heatingSensitization - between 425 and 815oC Chromium carbides need time for their formation.

Lattice- arrangements of points in a regular periodic pattern2D or 3D mannerMonoclinic and closed hexagonal latticeSecondary electron images of as-received wires. Excessively porous surfaces with a high susceptibility to pitting corrosion attributed to manufacturing defects.

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Ss in Orthodon