STAINLESS STEEL Seminar by DHANYA MENON M CONTENTS BRIEF HISTORY GLOSSARY OF TERMS ALLOYS CARBON STEELS STAINLESS STEELS SOLDERING WELDING Two months later stainless steel was cast for first time in August 20. of the Brown Firth Research Lab .K. by the Sheffield Metallurgist Harry Brearly. 1912). 1912.HISTORY It was discovered accidentally during the early part of the first world war in U. . (Dated-4th June. who noticed that a discarded steel sample was not rusting – Result was a chrome alloy steel. who first used it to make a prosthesis and called it Wipla (Wie Platin.S. like Platinum). Strauss and Edward Maurer of Germany also shared the development of the material between 1903-1921. Hauptmeyer. introduced at Krupp’s Dental Polyclinic in Germany by F.Stainless steel entered dentistry in 1919. in German.. . Becket of U. By 1937.M. Research study related to metallurgy with particular references to orthodontic applications was done by the metallurgist R.Williams. Angle used it in his last year (1930) as ligature wires.Application of stainless steel to the fabrication of appliances was credited to a Belgian Lucien de Coster. the value of stainless steel as an orthodontic material . Terminology . cm/cm) .(in/in.Stress and Strain : When an external force acts upon a solid body.(Psi.This internal force divided by the area over which it acts within the body is the resultant stress. a reaction force results within the body that is equal in magnitude but opposite in direction to the external force (Load). Mpa) And the deformation is called strain. 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.Strain may be either plastic or elastic or a combination of the two. . .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. It is always accompanied by compressive strain. forces are applied at right angle to the area over which they act. internal resistance to such a load is called compressive stress. In both the above stresses. . Shear stresses result from force that tend to act parallel to surface of objects. .(iii)Shear stress : Stress that tends to result from a twisting motion or sliding of one portion of a body over another. At this point wire has been stressed beyond elastic limit. If the load is increased progressively in small increments and then released after each increase in stress . . Mpa) If small tensile stresses are induced in a wire.the wire will return to its original length when the load is removed. a stress value will be reached at which the wire does not return to its original length after it is unloaded.Elastic limit (Psi. Mpa) May be defined as greatest stress that may be produced in a material such that the stress is directly proportional to strain. .Proportional limit (PL) (Psi. 1% of the plastic strain along the straight axis and is extended until it intersects the stress strain curve. .To determine the yield strength of a material at a particular offset of .001 or 0.1% a line drawn parallel to the straight line region starting at a value of . The stress corresponding to this point is the yield strength. Yield strength (YS) (Psi. .1% is measured. Mpa) Is the stress required to produce the particular offset chosen or a point at which a deformation of . 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 the elastic region of the stress strain diagram. . Wire with high MOE is difficult to bend. If the slope is more horizontal. It is the ratio of stress to the strain. .it is stiffer. It is a slope of stress/ strain curve. Less the strain for a given stress. If slope is more vertical.it is springier. greater will be the MOE. . Maximum flexibility may be defined as the strain that occurs when the material is stressed to its proportional limit.Flexibility It is the property of elastic deformation under loading. .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 greatest force that can be . - It is the area under the stress-strain curve at the given maximum stress required to fracture a structure.- It is the property of the material itself and is not related to the size or form of the wire. . compressive or shear. compressive strength or flexural strength Is the maximum stress required to fracture a structure (can be tensile. ultimate tensile strength.Strength. shear strength. depending upon the predominant type of stress present). Is related to the total area within the elastic and plastic regions. Can be defined as energy required to fracture a material. . 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. . brittle material is apt to fracture at or near to proportional limit.Brittleness Opposite to toughness. Can be defined as the relative inability to of a material to sustain plastic deformation before fracture of a material takes place. a stiffer wire can store proportionately more force. - Other things being equal. - It is measure of force required to bend or otherwise deform the material over a definite distance. .Stiffness /Load Deflection Rate - Is the measure of resistance to deformation. This helps in knowing how far a tooth can be moved with single adjustment. .Working Range Is the measure of how far a wire or material can be deformed without exceeding the elastic limit. 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 decreases with increase in temp. It is dependent on tensile strength. . whereas malleability increases with increase temp. In general.Ductility Is the ability of a material to withstand permanent deformation under tensile load without rupture. there is a reduction in cross section. Within the elastic range. 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 . Under tensile loading. the ratio of lateral to axial strain is called poisson’s ratio.Poisson’s ratio During axial loading in tension or compression there is simultaneous axial and lateral strain. Factors influencing material are its : the hardness Proportional limit Ductility Malleability Resistance to abrasion of a .Hardness It is defined as resistance to indentation. The different types of hardness tests are : MACRO HARDNESS TESTS Brinell test wherein a hardened steel ball is pressed under a specified load into the polished 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. . a conical diamond point is used.In Rockwell hardness test (RHN). it is similar to Brinell except for the use of a steel ball . The impression is rhombic in outline and the length of the largest diagonal is measured.MICRO HARDNESS TESTS Knoop hardness test employs a Diamond indenting tool that is cut in the geometric configuration. 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 Transformation A change in the number and or character of phases that constitute the microstructure of an alloy by a change in crystalline structure.Phase A homogeneous portion of a material system that has uniform physical and chemical characteristics. . the austenitic to martensitic phase transformation occurs with increasing applied force. .Pseudoelasticity Is the mechanical analogue of thermoelasticity in which at constant temperature. 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. . . maximum flexibility. range of deflection (or) working range.Spring Back Also referred to as maximum elastic deflection. range of activation. 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 . coils .stops etc without fracturing the wire.Formability High formability provides the ability to bend a wire into desired configuration such as loops. The property relates to the area under the graph between the yield point and the failure point. . Both in turn.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. . 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. . Excessive amount of bracket/wire friction may result in loss of anchorage or binding accompanied by little or no tooth movement.Friction Space closure and canine retraction in continuous arch wire technique involves a relative motion of wire over bracket. . 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. Metals are made up of thousands of tiny crystals and each crystal is known as grain. .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. proportional limit of metals are increased with strain hardening.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. strength. . Surface hardness. 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 . lowered ductility and distorted grains ) can be reversed by simple heating the metal. This process is called annealing.A) Annealing Effects associated with cold working ( eg strain hardening. The more severe the cold working. 1) Recovery 2) Recrystallization 3) Grain growth . more rapidly the comprises effects can be reversed Annealing in general of three stages : by annealing. 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. . These grains nucleate in the most severely cold worked regions in the metal usually grain boundaries or where lattice is most deformed. A radical change occurs microstructurally old grains disappear completely and are replaced by a new set of strain free grains. On completion the metal essentially attains its original soft and ductile condition. Thus the grain size for the completely recrystallized material can range from fine to fairly coarse.GRAIN GROWTH The recrystallized structure has a certain average grain size depending on the number of nuclei. . If fine grain structure is further annealed the grain begins to grow an in effect large grains consume the smaller grains. The more severe the cold working the greater the number of such nuclei. 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. 5V is needed) Colour change to a medium straw . 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. 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. . Can be hardened only by cold working.B) Tempering Stainless steel cannot be hardened like carbon steel by quenching or by any other heat treatment because of stability of austenitic steel. 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) .Polymorphism A few metals and many compounds crystallize into more than one structure. .ALLOYS An alloy is defined as a combination of two or more metals. at least one of which is a metal and all of which are mutually soluble in the molten state. which are (generally) soluble in molten condition. Can also be defined for dental purposes as a metal containing two or more elements. . 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.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. Classification 1)According to the use (such as metal inlays. crowns and bridges. metal ceramic restoration RPD. Ni) 3)Nobility (high noble. Fe-Ni-Cr. implants) 2)Major element (Au. Pd. Ti. Pd-Ag-Sn) . Ag. noble and predominantly metal) 4)Principle three elements (Au-Pd-Ag. Eutectic. intermetallic compo and combinations 6) According to number of elements like binary. eg. terti quaternary etc . peritectic alloys.5) The dominant phase ( isomorphous single phase). Generally these alloys are brittle and have a very low resistance to tarnish and corrosion. which is lower than that of its components.Eutectic alloy This is an alloy having a fusion temperature. Are mainly used in solders Hypo Eutectic alloys This is a eutectic alloy having a composition of less than eutectic. even though they were soluble in molten state. When solidifying. the components of alloy separate out. . on slow cooling changes into beta phase. Peritectic alloys Is an alloy which solidifies while an atomic diffusion occurs. .Hyper eutectic alloys This is a eutectic alloy having a composition of more than eutectic. 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 Pure iron at room temperature has body centred cubic (BCC) structure and is referred to as FERRITE. This phase is stable upto 912c.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.CARBON STEEL Steels are iron based alloys that contain less than 1.The spaces between atoms in BCC structure are . However the size of carbon atom is such that the resulting lattice strain still limits the maximum carbon solubility to 2. . The interstices in FCC lattice are larger than those of BCC structure.At temperature between 912c and 1394c. the stable form of iron is face centred cubic (FCC) structure called AUSTENITE.11wt%. strong and brittle alloy. ROBERT AUSTEN. Formation of martensite is an important strengthening mechanism for carbon steels.Austenitic form of iron is the stable form of iron and is called GAMMA IRON or AUSTENITE after the well known metallurgist. . Transformation of Austenite to BCT structure called MARTENSITE is a highly distorted and strained. resulting in an extremely hard. .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. alloy is commonly defined as stainless steel. Elements other than iron. resulting in a wide variation of composition and properties of stainless steel.STAINLESS STEEL When the chromium (generally 12 to 30% )is added to steel. carbon and chromium may also be present. . 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. 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 .Cobalt decreases the hardness Manganese acts as scavenger and increases the hardness during quenching. This system uses numbers from 300-502 for stainless steel number depends on composition and physical properties TYPE ferritic austenitic martensitic AISI NO.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). 400 302.316L (300 series) 400 .304. 20max 0.0-26 11.5 carbon 0.5-17 nickel 0 7-22 0-2.25max 0.5-27 16.20 .15-1.TYPES OF STAINLESS STEEL Basically the steels used in dentistry are divided into 3 types (based on lattice structure) TYPE (space lattice) Ferritic (BCC) Austenitic (FCC) Martensitic (BCT) chromiu m 11. AISI UNS EXAMPL E Cr Ni M n Mo C 303 S-30300 Ormco Diamond 1719 8-10 2 0.0 .2 1.6 .0 .03 0.0 .0 1.15 0.03 0.0 1.03 4 0 1618 P Si S .5 .03 4 0 .15 0 304L S-30403 Advanced 1820 orthod 8-12 2 - 316L S-31603 “A” Co Stand twins 1014 2 2. types Approx. tensile Strength (kg/cm3) % elongation Martensitic . yield strength (kg/cm3) Approx.annealed .heat treated 4600 9000-14000 7000 11000-21000 30 12 Austenitic -annealed -cold worked 2800 4000-10000 6700 7000-12000 70 50 . 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 at lower cost, provided high strength is not required. Since temperature change induces no phase change in the solid state, the alloy is not hardenable by heat treatment. Ferritic stainless hardenable. steels are not readily work 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% 8% Ni and . Type 304 has similar composition.15% carbon.AUSTENITIC STAINLESS STEELS (300series) These are most corrosion resistant of stainless steels. AISI 302 is the basic type containing 18% Cr. chief difference being that the carbon content is limited to . Type 316 L (.03% max.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. 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 . Greater ease of welding. Comparative ease in formation. austenitic stainless steel is preferable to the ferritic alloy because of : Greater ductility & ability to undergo more could work without breakage. . Ability to fairly readily overcome senstization. Less critical grain growth.Generally. Substantial strengthening during cold working. 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 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. Unfortunately high proportions of Mn increases the alloys susceptibility to corrosion. nitrogen thus replacing Ni. ADAMS Very soft/ fully annealed Hard High tensile/super hard/hard spring .C. COOMBE 1 Soft 2 ½ hard 3 Hard C.P.ACCORDING TO E. which has migrated to the boundaries to form the carbides. Also the corrosion resistance decreases due to depletion of the central regions of the crystals off chromium. This process in known as sensitization or weld decay. which precipitate at the grain boundaries causing brittle behaviour. 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). .SENSITIZATION. 010”.014” Enables ligature to be tied around the arch wire Maintains groups of teeth together Because 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.012”.009”. . 0. 0.011”.SOFT STAINLESS STEEL WIRE Thoroughly annealed to release any work hardening 0. 0. 0. for rotation ties and to secure ties when full expression of torquing arch wire is required Replaced by elastic modules .Should be used to ligate NiTi wire to teeth that are displaced from the line of the arch. .Criteria for selection of a wire/alloy Load deflection rate required appliance. Magnitude required. of force and proper in the movements Stiffness of the alloy its relative formability. AUSTRALIAN ARCHWIRES In 1952. over long distance and with minimal loss of force intensity while doing so. Begg in collaboration with an Australian metallurgist Mr.J. It was made thin enough to distribute force at an optimal level for tooth movement over a considerable period of time.018” to . . The diameter of the wire initially produced was progressively decreased from . Dr.J Wilcock. A. 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.014”. There are 6 types of Australian arch wires : Regular grade (white label) lowest grade easiest to bend used for practice bending and forming auxillaries. Regular plus (green label) Relatively easy to form. . Used for auxillaries and arch wires when more pressure and resistance to deformation are desired. yet more resilient than regular grade. Routinely used by experienced operators.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. these wires have been suggested for use in high angle cases as a means to prevent molar extrusion.014” wire as up righting and torquing springs. Is actually suitable for mesiofacial types.. . .016” Premium plus Is claimed to undergo less distortion in the mouth compared to older grades.014” Premium plus In addition to the routine use of . 018” Premium plus This wire has two good uses: Where the VTO indicates that the upper incisors need intrusion form the outset.. 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. This is infact the most common requirement when the case requires reduction of a severe overbite before cessation of growth. . Compared to ..022” special plus for maintains the arch form.020” Premium Claimed to be superior to . . this is equally stiff and yet more formable. Is also very effective in correcting arch form.022” special plus. 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”.Ultra high tensile wires Ultra high tensile wires of supreme grade (.010”. .009”. Wilcock Jr.008” / . 1984) Is an ultra high tensile S. was further reduced to .009” Supreme (A.010”. fine round wire. May be used to form a boxed reciprocal torquing mechanism to lip one tooth against the adjacent tooth torque wise. the supreme grade.S. Initially introduced in .J.009” diameter.. . 010” Supreme May also be used to form reciprocal torquing springs. Gentle force developed with . Wire is best indicated for incisally activated mouse traps. These result in a shortened duration of stage III treatment..010” mousetraps are not associated with root restoration as happens with . rear to stage III Archwire giving greater control and ease of placement. The advantage of minisprings is that they can be inserted behind the stage III brass pins in the slot.These wires are excellent for making minisprings. Also they produce such light forces that they truly realise Begg’s concept of light differential forces. They are available as preformed. . Coil size may be decreased with these wires. He claims the wire is excellent for aligning second molars towards the end of stage III.012” supreme Mollenhauer states that this wire appears to be strong yet flexible for anterior teeth. .011” wire is tied Piggyback gingival to the main arch wire between central incisors. This helps to establish good molar contact at the end of treatment.011” / . In the anterior section the .. 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. . High strength is desirable so that normal masticatory forces will not render the wire uselessthrough plastic deformation of fracture. Low stiffness is advantageous so that the force can be kept as gentle as possible. . Because of their low apparent modulus bending these wires apply low forces for a given deflection when compared with solid stainless steel.Braided or twisted wires are able to sustain large elastic deflection in bendings. Has a central core wire for stability with five outer wires wrapped around for resilience and flexibility. .CO-AX WIRES One of most efficient wires available for edgewise or light wire technique to align crowded or rotated anterior teeth. Co-ax wires provide light continuous forces over a longer period and can be deflected to great degree (without taking a set). . smooth. bright finish allows brackets to slide freely. The tightly wound. 229 x 103 psi Ultimate tensile strength – 307 x 103 p No. of 90 cold bends without fracture for .017 x .Mechanical properties of stainless stee Modulus of elasticity – 26 x 106 psi Yield strength.025 wire = 5 . Soldering . (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.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. . is the temperatures at which metals of an alloy system become completely solidified on cooling or start to melt on heating.Solidus In a phase diagram.(metals handbook desk edition 1992) . 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) Liquidus In a equilibrium phase diagram. (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 .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. The soldering process involves the: substrate or parent metal to be joined soldering filler metal (solder) a flux a heat source . inhibit further oxidation or facilitate its removal. Its composition also determines the wettability of the substrate by the molten solder alloy. 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. The solder chosen must wet the metal at as low a contact .Substrate metal Known as the basis metal. 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. Fusion temperature should be lower than that of the parts to be joined.Solder Dental solders are alloys that are used as intermediary or a filler metal to join two or more metallic parts. Should exhibit excellent tarnish and . 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. COMPOSITION Usually fineness of a solder is less than that of the alloy being used. the degree of fineness has been used to describe the various solders. 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. Previously. solders were commonly referred to by carat number. 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 . In recent years. Copper is added to improve strength and lower the fusion temp and to make it amenable to age hardening. silver (8-12%). .Gold solders Contains gold (45-80%). Also increase fluidity of solder in molten state and improve the mechanical properties. copper (712%) with tin (2-3%) and zinc (2-4%) Zinc and tin reduce the fusion temp of the solder below the casting alloys. Also decreases the fusion temperature. Nickel may be added instead of copper if a white alloy is desired. .Silver in large proportion than copper improves wetting of gold solders. 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.Silver solders Are essentially alloys of silver (46-60%). . 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. Known as plumber’s solder.soft Types of solders hard Soft solders Include lead – in having low m.p. 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. 35%. silica-10% . boric acid.The most commonly used flux in dentistry has the following composition borax glass-55%. 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. Graphite from a lead pencil is convenient antiflux however it is removed by oxidation at higher temperature.Anti flux It prevents flow of solder and is used to confine the solder to the work area. . .TECHNIQUE Involves several critical steps A) Cleaning and preparing the surfaces to be joined. F) Controlling the time to ensure adequate flow of solders and complete filling the solder joint. B) Assembling the parts to be joined. E) Controlling the proper temperature. D) Maintaining the proper position of the parts during the procedure. C) Preparing and fluxing the gap surfaces between the parts. soldering can be : 1.Free hand soldering 3.Investment soldering 2.Based on technique used.Infra red soldering . Investment soldering Recommended for precise arrangement of parts for bridge work or partial denture with wrought wire clasp arm. Procedure involves embedding of the metallic parts in an investment leaving a gap of about 0. Used when area of contact between the metallic parts being joined is large and whenever precision is needed in joining the metals.13mm . Is done without use of an investment. 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. Solder is generally melted onto one of the parts.Free hand soldering Most commonly used in orthodontics. .Infra red soldering Instead of using a torch to provide heat. 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. an infra red heating unit is available specifically for dental soldering. Material to be soldered is placed at the reflectors secondary focal point. . The main problem in the use of this unit is locating the focal centre of the light on the spot to be soldered. at which the reflected infra red energy of the tungsten source is focused. Failure to focus at the right spot can result in cold joints that are porous. Joint design Whenever possible. When soldering wires. the joint should be encased completely in solder. The excess bulk formed can used to advantage as in the stop lock for intermaxillary and extraoral traction. . wires should be joined by turning one wire around the other and soldering the joint. The solder will be found to have a bright smooth surface which is perfectly clean and hygienic. by picking it away with a probe. Flux should be removed from soldered joints when the joint has barely cooled.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. . needle flame (1cm long). Jet of blow lamp should be small enough to produce a fine. blue flame melts the solder adequately as well as gives the operator time to observe the flow . quiet.Heat control Most convenient method of melting solder for stainless steel is by means of miniature butane blow lamp. A soft. Remelting a joint to add more solder and make adjustments increase the rise of burning the solder and wire. . If possible soldering procedure should be done in one heating. Localization of heat to site of soldering is important to avoid annealing of a large section of wire.Even slight overheating of the joint produces burning of the wire and solder resulting in weak joint and rough pitted surface of the solder. wires or cubes. .Availability of solders Dental solders are supplied in variety of shapes and forms such as strips. Choice of solder depends largely on the operation to be performed and each form is available in range of fineness. rods. Fine wire forms are most desirable for orthodontic applications. Thin strips represent the conventional form for general applications. Rod forms are often notched along two sides.Small cubes. 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. . approximately 1mm square are convenient for soldering a contact area on an inlay or for other small soldering procedures. General considerations Gap Should be neither too great nor too small. If the gap is too great. strength will probably be limited by flux inclusions. joint strength will be strength of the filler metal. . porosity caused by incomplete flow of the filler metal or both. If it is too narrow. Flame application to the joint should be continuous and not to be removed until the brazing is complete. Temperature Should be the minimum required to complete the brazing operations. . Flame gives protection from oxidation especially at the brazing temperature.Flame Should be neutral or slightly reducing portion of the flame. Time Longer time increases the possibility of diffusion between parent metal and filler metal. Both conditions result in weaker joints. . Shorter time increases possibility of incomplete filling of joint and possibility of flux inclusion in the joint. 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 . Defective soldering Overheating of wires during soldering can lead to diffusion between solder and wire.Therefore to prevent changes in microstructure the heating operation should be kept at minimum to achieve a successful operation. . recrystallization. surface pitting. internal porosity and microstructure changes. Oxidation from improperly adjusted torch or oxidation caused by removing the reduced portion of the flame from the joint before the solder flows. Contamination owing to improper cleaning or sulfur released from overheated investment. . Flux insufficient to cover the joint.Failure to flow is generally due to one or more of following : Parts were too cool when solder was applied. Soldering application in orthodontics: Quad helix with spring Hooks for arch wires Labial bow soldered to adam’s clasp Lingual arches Retention appliances . Welding . Each of them achieve metal to metal contact differently. Pressure welding Laser welding Spot welding . Methods of welding There are 3 methods of welding used in dentistry.Definition It is joining two pieces of metal without the use of an intermediatory alloy. 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. . eg gold foil (foil. In pressure welding the problems of surface roughness are overcome by large compressive forces. mat or powdered pure gold) restorations are pressure welded by hand or mechanical condenses. pressure welding results.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. . high intensity pulse of light that can be focused. 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.Laser welding A laser generates a coherent. . By selecting the duration and intensity of the pulse. This means there is very little heating of the total appliance. . This procedure can be performed on the master cast.Owing to the expansion form the locally high temperature. two liquid surfaces contact and form a weld on solidification. except at the point of application. Lasers can be directed at small regions and can apply high energy to these regions in a very short amount of time. and change of state. Spot welding Is a convenient method for uniting pieces of metal of the same kind. . Most metals may be spot welded. Method is clean and quick and produces joints which are strong and reliable. 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. .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. The pressure of the electrodes then. Since the current is constant. .In small bench machines. when current is passed from one electrode to the other through the metal. so more heat will be generated at the contact areas than in the interior parts. so creating the weld. Spring pressure is usually employed. therefore. the metals will become plastic first at the contact point. forces the metal parts together. heat is generated in and between the metal parts which is sufficient to make them plastic. Small welds are generally considered better since bonding is achieved with a minimum of change in the original grain structure. . very short time.Orthodontic welder design Pioneer work on the design of a welder for orthodontic purpose was done by Friel (1993) and Mckeag (1939). Spot welding is carried out without the aid of flux or any other protecting material. Welding machines from orthodontic purposes are designed to deliver heavy currents for accurately predetermined. . . Longer the time allowed for a weld.Heat required for spot welding is generated at the interface of the workpieces. 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 electrodes which make electrodes quickly available. Electrode design is important feature in orthodontic welders. the electrodes together so free. welder include turret various shapes of a feature which holds that both hands are Timing switch may be automatic or controlled electronically or by capacitor discharge. latches and attachments in fixed appliance construction which are usually provided on rotating turrets so that any pair of u & l may be selected. . variety of electrodes tips are required for welding of wires. . Light tapes can be welded to heavy wires using flat electrodes. only a skin at the surface making contact with the light part becomes plastic. 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. .When welding a light part to a heavy part the bulk of the heavy part is not raised to welding temp. 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. so avoiding actually welding the fine wire. A properly welded joint does not need any reinforcement. This tape can be welded to the arch wire making a strong joint. where the results have been . 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.Another method to overcome the problem is to make an attachment with a strap or loop of tape. Thank you .