Stainless Steel in Dentistry

Stainless steel is one of the most widely used materials in dentistry for the production of dental instruments, e.g. scalpel blades and forceps, orthodontic wires, denture bases and partial denture clasps, endodontic posts and as stainless steel crowns for the treatment of severely decayed primary molars.

1. DEPARTMENT OF ORTHODONTICS TOPIC : STAINLESS STEEL PRESENTED BY: Danish Hamid 3 RD Prof. ROLL NO. : 02

2. INTRODUCTION Steel is an alloy of iron and carbon. Carbon content should not exceed 0.2% max. When it contains 12 to 13% chromium it is called stainless steel. Steel exists in three forms; Austenitic, Martensitic and Ferritic.

3. History First developed accidently by Harry Brearley in Sheffield, England. Stainless steel was introduced in dentistry in 1919 by F. Haupt Meyer. In 1930, angle used it to make ligature wires. By 1937, the value of stainless steel as orthodontic wire had been confirmed. Stainless steel now a days is used to make arch wires, ligature wires, band material, brackets and buccal tubes.

4. Composition Types Chromium Nickel Carbon Ferritic 11.5 – 27 % 0.2 % max. Austenitic 16 – 26 % 7 – 22 % 0.25 % Martensitic 11.5 – 27 % 0 – 2.5 % 0.15 – 1.2 % Silicon, Phosphorus, Sulphur, Manganese, Tantalum, Niobium may be present in small amounts.

5. Function of constituent metals Chromium : A thin transparent, tough, impervious oxide layer of Chromium oxide forms on the surface of the alloy when subjected to room air - “passivating film effect”. Increases hardness, tensile strength and proportional limit. Nickel : Increases strength. Increases tarnish and corrosion resistance. Cobalt : Increases tarnish and corrosion resistance. Decreases hardness.

6. Manganese: Scavenger for sulphur. Increases hardness during quenching. Silicon: Deoxidiser and scavenger. Titanium: Inhibits the precipitation of Chromium carbide

7. Types Of Stainless Steel FERRITIC (BCC) AISI-400 Stable between room temperature and 912 C. Carbon has low solubility in this structure. Interstices in BCC are very small. Good corrosion resistance at low cost provided increased strength is not required. Temperature change does not induce phase change in solid state. The alloy can’t be hardened by heat treatment. Little application in Dentistry.

8. AUSTENITIC (FCC) AISI-302,304 Most corrosion resistant of all types of stainless steel. Formed between 912 – 1394C 18-8 stainless steel – 18% Chromium, 8% Nickel and 0.15%(302) 0r 0.08%(304) Carbon – 18-8. Austenite is preferred to Ferritic because of greater ductility, ability to undergo more cold work without fracture. Increased strength during cold working, ease of welding, readily overcomes sensitisation, less critical grain growth and ease of forming. When austenite is allowed to cool slowly to room temp. it forms Fe3C and ferrite. The iron carbide compound is called Cementite and the solid solution of ferrite along with cementite is called Pearlite.

9. MARTENSITIC (BCT) AISI-400 If austenite is cooled rapidly (Quenched), it will undergo spontaneous diffusion-less transformation to a Body Centered Tetragonal. The lattice is highly distorted, strained resulting in a hard strong brittle alloy. Martensite decomposes into ferrite and carbide. Decomposition is accelerated by appropriate heat treatment to reduce hardness but this is counter balanced by increased toughness – “Tempering”. Increased strength and hardness – used for surgical and cutting instruments.

10. MARTENSITIC (BCT) AISI-400 Yield strength of 492 MPa (annealed). Hardened – 1898 MPa Brinell’s hardness range- 230 – 600. Elongation – less than 2%. Reduced ductility. Corrosion resistance is the least. Reduced further with Hardening heat treatment.

11. MECHANICAL PROPERTIES Modulus of Elasticity: This is a measure of stiffness of the material. Gives the flexibility of the wire component. 179 GPa Strength: Capacity of a material to resist a deforming load without exceeding the limits of plastic deformation. Strength is proportional to the resiliency of the material. Yield strength: The stress at which increase in strain is disproportionate to stress. 1579 MPa 0.2% plastic deformation. Ultimate strength: The strength at which the material fractures. 2117 MPa Tensile strength – 200 MPa Resilience: Total energy storage capacity. The amount of energy absorbed by a structure when it is stressed within it’s proportional limit. Knoop’s hardness number: 600

12. GENERAL PROPERTIES SENSITISATION: When heated between 400 and 900 C Austenitic stainless steel loses it’s resistance to tarnish and corrosion. Carbon atoms migrate to grain boundaries and combine with chromium to form chromium carbide where the energy is the highest. If the stainless steel is severely cold worked the carbide precipitate along slip planes, as a result the areas deficient in chromium are less localized and carbides are more uniformly distributed . Corrosion resistance is reduced in regions adjacent to grain boundaries in which the chromium level is depleted below that necessary for protection ( approx. 12%)

13. STABILIZATION This is the process by which carbon is made unavailable for the sensitizing reaction. This is done at the time the alloy is manufactured by either keeping the carbon content exceptionally low or by adding other metals such as Titanium that precipitates as carbide instead of chromium.

14. ANNEALING The effect associated with cold working such as strain hardening, low ductility and distorted grains can be reversed by simply heating the metal. The greater the amount of cold working the more rapidly the effects can be reserved by annealing. Stages of annealing: Recovery. Recrystallisation. Grain growth.

15. . Recovery Cold work properties begin to disappear. Slight decrease in tensile strength and no change in ductility. All the residual stress is relaxed. Recrystallisation Old grains disappear totally and are replaced with strain free grains. Occurs mostly in regions where defects have accumulated. It attains it’s soft and ductile condition at the end of this stage. Grain Growth The Grain size and number of the recrystallised structure depends on the amount of prior cold working. On repeated annealing larger grains consume smaller grains. At the end of annealing the number of grains decrease and size increases.

16. Cold Working Cold forming stainless steel is generally different to plain carbon (mild) steels, primarily because stainless steels are stronger, harder and more ductile, work harden more rapidly and must maintain their inherent corrosion resistance. When stainless steel is cold worked , carbide is precipitated along slip planes , so chromium is dispersed throughout rather than conc. at the boundaries. More protection from corrosion.

17. SOLDERING It is a process of joining two metals by the use of an intermediate alloy which has a lower melting point. Soldering temperature – 620 to 665 C. Ideally silver solders are used- alloy of silver, copper, zinc to which tin and indium are added to lower the fusion temperature and improve solderability. Needle like non luminous gas air flame is used. Thinner the diameter of the flame, less the metal surrounding the joint is annealed. The work is held 3mm beyond the tip of the blue cone in the reducing zone of the flame. Soldering should be observed in shadow against a black background so the temperature can be judged by the colour of the work. The colour should not exceed dull red. If possible the parts should be tag welded to hold them together.

18. The flux is applied and the heavier gauge is heated first. Flux should cover all the area and the metal should be allowed to flow around the joint. The work should be immediately quenched in water. The flux used for soldering stainless steel contains fluoride to dissolve the passivating film formed by the chromium. The solder does not wet the metal when such a film is present. Potassium Fluoride is one of the active chemicals in this respect. Flux : • Aids in removing the oxide coating so as to increase the flow . • Dissolves any surface impurities. • Reduces the melting point of the solder.

19. WELDING Joining of two or more metal pieces directly under pressure without introduction of an intermediary or a filler material. Spot welding is used to join various components in orthodontics. A large current is allowed to pass through a limited area on the overlapping metals to be welded. The resistance of the material to the flow of current produces intense localized heating and fusion of metals. The welded area becomes susceptible to corrosion due to Chromium carbide precipitation and loss of passivation. Increased weld area increases the strength.

20. Factors to be taken into account during soldering and welding As the annealing temperature of stainless steel falls within the soldering and welding temperature ranges, these procedure can lead to loss of working range and elasticity of the metal. Precautions : By using low fusing solders. Using low diameter needle like flame. Reducing the number of welding procedures and duration

21. CHARACTERISTICS OF CLINICAL RELEVENCE Spring back (maximum elastic deflection) The extent to which the range recovers upon deactivation of an activated arch wire. A measure of how far a wire can be deformed without causing permanent deformation or exceeding the limits of the material. Higher the spring back, grater the working range and lesser are the requirements of frequent activations. Stainless steel has a spring back lesser than Nickel-titanium or beta titanium . Working range and flexibility The distance a wire will bend elastically before permanent deformation occurs. Flexibility is the measure of the amount at which the wire can be strained without undergoing plastic deformation.

22. Load deflection rate For a given load the deflection observed within the elastic limit. The force magnitude delivered by an appliance and is proportional to the modulus of elasticity. Low load deflection rate provides ability to apply low forces, a more constant force over time while deactivation, greater ease and accuracy in applying a given force . Stress relaxation When a wire has been deformed and held in a fixed position the stress may diminish with time even though the total strain may remain constant. Resilience The capacity of a material to absorb energy when the material is elastically deformed. It is measured by the area under the stress strain curve. Stiffness Amount of force required to produce a specific amount of deformation.

23. SOURCES PHILIPS ’ SCIENCE OF DENTAL MATERIALS 12TH EDITION CONTEMPORARY ORTHODONTICS – W.R. PROFFIT 5TH EDITION SLIDESHARE.NET Thank you