Wrought alloys in dentistry

Wrought Metal Alloys

1. WROUGHT METAL ALLOYS

2. GOOD MORNING PRESENTER:- Dr.ZAKIR HUSSAIN DEPT OF PROSTHODONTICS

3. CONTENTS KEY TERMS DEFINITION USES DEFORMATION OF METALS ANNEALING EFFECTS ON WROUGHT ALLOYS CLASSIFICATION OF WROUGHT ALLOYS STAINLESS STEEL COBALT-CHROMIUM-NICKEL ALLOYS NICKEL-TITANIUM ALLOYS BETA-TITANIUM ALLOYS CONCLUSION

4. KEY TERMS ANNEALING - Its is the process of controlled heating and cooling designed to produce desired properties in a metal. T his process increases the ductility, stabilizes shape. COLD WORKING - The process of plastically deforming metal at room temperature. DUCTILITY - The ability of a solid to be elongated plastically without fracture. MALLEABILITY - The ability of a metal to be hammered into thin sheets without fracture.

5. DEFINITION WROUGHT :- Beaten to shape. ALLOYS :- A metal made by combining two or more metallic elements to give greater strength or resistance to corrosion. SO WHAT IS WROUGHT METAL ALLOYS? These are cold worked metals that are plastically deformed to bring about a change in shape of structure and their mechanical properties . WHAT IS COLD WORKING? It refers to a process of strengthening a metal by changing its shape without the use of heat. Its also called strain hardening or work hardening and it involves subjecting the metal to mechanical stress so as to cause a permanent change to the metals crystalline structure. Major cold working methods include:- Rolling, bending, shearing and drawing . Rolling involves metal being passed through pair of rollers to reduce its thickness

6. PROCESS OF COLD WORKING

7. USES OF WROUGHT ALLOYS CLASP ORTHO WIRES PREFORMED CROWNS SURGICAL INSTRUMENTS

8. DEFORMATION OF METALS When a tensile force is applied on a metal below the propotional limit, there is small increase in separation between metal atoms. Plastic or permanent deformation occurs when the applied stress is above the elastic limit. Crystal imperfections are seen within the metal and are of two types 1)Point defects 2)Line defects

9. POINT DEFECTS :- A vacancy or vacant atom site in a crystalline lattice may occur at a single site in the atomic arrangement and two vacancies may occur as a divacancy . Point defects are equilibrium defects, and a crystalline material that is in equilibrium will contain a certain no of these defects at a given temperature.

10. Line defects :- The simplest type of line defect is known as edge dislocation . If sufficient shear stress across the top and bottom faces of a metal crystal, the bonds in the row of atoms a djacent to the dislocation will be broken and new bonds with the new row will be established.

11. Wrought alloys contain low percentages   of elements ; that is, alloying elements total less than about 4 pct . Casting alloys contain the same elements as wrought, but in  greater amounts ; for example, the silicon content in cast alloys can range up to 22 pct . The   principal difference between wrought and cast alloys  is as follows:  wrought alloys  are ductile enough so as to be hot or cold worked during fabrication, whereas cast alloys  are brittle to the degree that shaping by deformation is not possible and they must be fabricated by  casting THE GRAIN SIZE IN WROUGHT ALLOYS IS LESSER THAN 100 MICRONS AND IN CAST ALLOYS ITS MORE THAN 100 MICRONS.

12. EFFECTS OF ANNEALING OF WROUGHT ALLOYS Annealing  is a heat treatment process in which the alloy is heated up to the prescribed annealing temperature for a prescribed time followed by controlled cooling in order to soften the alloy. The effects of cold working which results in metals with greater strength and hardness but lesser ductility can be reversed by simply annealing it. It takes place in 3 successive stages 1 ) Recovery stage 2 ) Recrystallization 3 ) Grain growth Higher the melting point of metal, higher is the temperature needed for annealing.

13. RECOVERY :- At this stage the results of cold working begin to disappear, but not significantly. There is slight decrease in tensile strength and no change in ductility during recovery stage. During this stage there is relieving of internal strain energy resulting from cold working. RECRYSTALLIZATION :-This stage occurs after recovery stage. The atoms at this stage are rearranged into a lower energy configuration. The old and deformed grains disappear and new structure of strain free grains appear. After completion of recrystallization the metal regains the microstructure resembling that before cold working.

14. GRAIN GROWTH :- W hen the cold worked metal is annealed at an elevated temperature, the grain size increases and its called grain growth. This grain growth ceases after a relatively coarse grain structure is produced.

15. WROUGHT BASE METAL ALLOYS A no of wrought base metal alloys are used in dentistry, mainly as wires in orthodontic treatment. The wrought alloys commonly used are:- a) Stainless steel (Fe- Ch -Ni) b) cobalt- chromium –nickel c) nickel-titanium d) beta -titanium

16. STAINLESS STEEL Steel is iron based alloy with less than 1.2% carbon. When chromium(12-30%) is added to steel ,the alloy is called stainless steel St steel is resistant to tarnish and corrosion because of PASSIVATING EFFECT of Chromium. What is P assivating effect? I t is a thin, transparent but tough and impervious oxide layer(chromium oxide)formed on the surface of the alloy, when it subjected to air, which protects against corrosion.

17. TYPES OF STAINLESS STEEL BASED ON LATTICE ARRANGMENT OF IRON:- Ferritic stainless steel Austenitic stainless steel Martensitic stainless steel Ferritic steel :- pure iron at room temp has bcc( body centered cubic ) and is referred as ferritic which is stable upto 912 Celsius. They have good corrosion resistance but less strength and hardness.

18. AUSTENITIC STAINLESS STEEL :- At temperature between 912and 1394 Celsius, the stable form of iron is a face centered cubic structure( fcc ) called austenitic . These austenitic stainless steel are the most corrosion resistant of the stainless steel. MARTENSITIC STAINLESS STEEL :- when FCC is cooled very rapidly(quenched) it transforms to a body centered tetragonal ( BCT) structure called martensitic. This steel has high strength and hardness and so they are used to make surgical and cutting instruments but they have the least corrosion resistance when compared to other types of steel.

19. COMPOSITION IRON IS THE MAJOR COMPONENT. CHROMIUM--18% NICKEL– 8% CARBON– 0.08-0.15% THIS TYPE OF ALLOY IS CALLED 18-8 STAINLESS STEEL . It is commonly used to make bands and wires. NOTE:- Austenitic stainless steel is preferred over others because of:- 1)greater ductility and undergoes cold work 2)ease of welding 3)overcome sensitization 4)ease in forming

20. SOLDERING :- Metal parts are joined together by melting a filler metal between them at a temp below the solidus temperature of the metal being joined and below 450 celsius . EG:- GOLD AND SILVER SOLDERS . WELDING :- Is when two pieces of similar metal are joined together using heat or high temperature without the addition of another metal.

21. Wrought cobalt-chromium-nickel alloys :- It is also called as ELGILOY . The hardness ,tensile strength and yield are same as 18-8 steel . Tarnish and corrosion resistance are excellent . Composition include cobalt, nickel, chromium, manganese, carbon, beryllium, iron.

22. NICKEL-TITANIUM ALLOYS THEY ARE ALSO CALLED AS NITINOL . They have large working range or elastic deflection because of the property of SHAPE MEMORY AND SUPERELASTICITY . The “memory” effect is achieved first by establishing a shape at 482 celsius and then when cooled it forms into another shape and followed by subsequent heating through a lower transition temp causes the wire to return to its original shape.

23. The cobalt content is used to control the transition temperature range, which can be near mouth temperature. CLINICAL IMPORTANCE :-The wires are shaped at high temperatures into a ready made arch form shape. After adaptation by clinician into brackets bonded to mal-posed teeth, the wire returns to its original form which promotes tooth movement. USES OF NITINOL :- 1)Good for orthodontic wires when low force and large working range is needed. 2)Good for endodontic files in curved root canals to avoid perforation.

24. DISADVANTAGES OF NITINOL :- Higher friction than stainless steel . Difficulty in soldering . Expensive . Nickel has got some hazardous and allergic effect.

25. USES OF NICKEL TITANIUM ALLOYS

26. BETA-TITANIUM ALLOYS PURE TITANIUM has different crystallographic forms at high and low temperatures. At temperatures below 885 Celsius the hexagonal closed packed (hcp) or alpha lattice is stable. At higher temperatures the metal rearranges to body centered cubic(bcc) or beta crystal. The beta form is use commonly in orthodontic applications The beta form can be highly cold worked and both alpha and beta forms have excellent corrosion resistance and environmental stability. Titanium alloys are highly reactive with oxygen at high temperature and so the metal joining procedures is done in vaccum or argon environment

27. USES OF TITANIUM ALLOYS

28. CONCLUSION In last few decades, a variety of new alloys have been introduced into dentistry. Appropriate use of alloy types enhances patient comfort and reduces chair side time and duration of treatment. It is beneficial to exploit the desirable qualities of a particular wire type to satisfy the demands of the presenting clinical situation. This in turn would provide the most optimal and efficient treatment results.

29. Thank you