Stainless steel alloys

Stainless Steel Alloys

1. STAINLESS STEEL ALLOYS

2. Introduction Steel is an alloy of iron and carbon . Carbon content should not exceed 0.2% max. When it contains 12 – 30% chromium by weight it is called stainless steel.

3. History First developed by accident by Harry Brearley in Sheffield, England. Stainless steel entered dentistry in 1919, introduced at Krupp’s dental poly clinic in Germany by F. Haupt Meyer . In 1930 Angle used it to make ligature wires. By 1937 the value of stainless steel as an orthodontic wire has been confirmed. Stainless steel today is used to make arch wires, ligature wires, band material, brackets and buccal tubes.

4. Classification Stainless steel can be classified on the basis of crystal structure : Ferritic (bcc) Austenitic (fcc) Martensitic (bct)

5. Composition (Percent by Weight) of Three Types of Stainless Steel* by Crystal Structure of Iron Types of Stainless Steel Chromium Nickel Carbon Ferritic (bcc) 11.5 – 27.0 0.20 max Austenitic (fcc) 16.0 – 26.0 7.0 – 22.0 0.25 max Martensitic (bct) 11.5 – 17.0 0 – 2.5 0.15 – 1.20 *Silicone, phosphorus, sulfur, manganese, tantalum, and niobium may also be present in small amounts. The balance is iron.

6. Functions of each component Chromium : - Increases tarnish and corrosion resistance . - Increases hardness, tensile strength and proportional limit. Nickel : - Increases strength. - Increases tarnish and corrosion resistance.

7. Carbon : - Enhances corrosion resistance to certain acids. - Decreases work hardening. - Improves formability. Silicon : - Deoxidiser and scavenger. Phosphorus : - Improves machinability. - Increases strength in austenitic stainless steel. - Decreases weldability.

8. Sulphur : - Helps in easy machinig of wrought parts. - Detrimental effect on corrosion and weldability. Manganese : - Increases strength, toughness and hardenability. - Improves hot working properties. Titanium : - It is added for carbide stabilization.

9. Niobium : - Increases strength. - Stabilizes carbon. Cobalt : - Increases hardness and tempering resistance.

10. Types of Crystal Lattice Ferritic (bcc) Stable between room temperature and 912*C. Carbon has very low solubility in this structure and reaches a maximum of 0.02% at 723*C. Interstices in bcc are very small. Good corrosion resistace at low cost provided increased strength is not required. This alloy is not hardenable by heat treatment. Littile application in Dentistry.

11. bcc cubic structure

12. Austenitic (fcc) Above 723*C, a solid solution of carbon in an fcc iron matrix called austenite is formed. Most corrosion resistant of all types of stainless steel. Used for orthodontic wires, endodontic instruments and crowns in pediatric dentistry. The addition of nickel to the iron-chromium-carbon composition stabilizes the austenite phase on cooling.

13. f cc cubic structure

14. The type 18-8 stainless steel, which contains 18% chromium and 8% nickel by weight, is the most commonly used alloy for orthodontic stainless steel wires and bands.

15. Austenite stainless steel is preferred to Ferritic stainless steel for dental applications because it has the following properties: Greater ductility and ability to undergo more cold work without fracturing Substantial strengthening during cold working (some transformation to martensite) Greater ease of welding Ability to overcome sensitization Less critical grain growth Comparative ease of forming.

16. When a plain carbon steel containing 0.8% carbon is cooled slowly in the austenite phase to 723*C, it undergoes a solid state eutectoid transformation to yield a microstructural constituent called pearlite . This consists of alternating fine-scale lamellae of ferrite and iron carbide (Fe3C), referred to as cementite , or simply, carbide.

17. Austenite cooled slowly quenched Martensite Ferrite + cementite (Fe3C) Annealed

18. Martensitic (bct) If austenite is cooled very rapidly (quenched), it will undergo spontaneous transformation to a body-centered tetragonal (bct) structure called martensite . It is very hard, strong, brittle alloy. The high hardness of this structure allows the grinding of a sharp edge, which will be retained in extended use.

19. b ct cubic structure

20. Martensite is a metastable phase that transforms to ferrite and carbide when it is heated to elevated temperatures. This process is called tempering ; it reduces hardness of alloy but increases its toughness.

21. Passivation, Sensitization and Stabilization Passivation : Chromium is added to stainless steel as passivating agent. It forms a very thin, transparent, adherent layer of Cr2O3 which provides a barrier to diffusion of oxygen thus preventing corrosion.

22. Sensitization : When austenitic stainless steel is heated to between approximately 400 and 900*C, iron-chromium carbides precipitate along the grain boundaries and chromium is depleted near the grain boundaries below concentrations necessary for protection. Thus, the stainless steel becomes susceptible to intergranular corrosion, and partial disintegration of the weakened alloy may result. This phenomenon is called sensitization .

23. Stabilization : Elements such as titanium and tantalum, which preferentially form carbides, can be added to the stainless steel to preserve the level of chromium when the metal is exposed to elevated temperatures. This process is called stabilization .

24. Mechanical Properties of Stainless Steel Used in Orthodontic Wires Elastic Modulus : 179 GPa Yield Strength : 1.6 GPa Ultimate Tensile Strength : 2.1 GPa Number of 90* Cold Bends Without Fracture : 5

25. Heat Treatment of Stainless Steel Definition – It is the process of subjecting a metal to given controlled heat followed by sudden or gradual cooling to develop desired qualities in metal. This process is of 2 types : Softening heat treatment – annealing Hardening heat treatment – tempering

26. Annealing Effect of cold working like low ductility and distorted grains can be reversed by heat treatment called annealing. It consists of 3 stages : Recovery Recrystalization Grain Growth

27. 1. Recovery – In this stage cold work properties begins to disappear. Slight decrease in tensile strength and no change in ductility is seen. Internal stresses incorporated in cold working are revealed but no change in microstructure is seen. Cold worked orthodontic appliances are subjected to this stage treatment to reduce warpage and distortion called as STESS RELIEF ANNEALING .

28. Recrystalization – Here old set of distorted grains are replaced by new set of distorted free grains. Material attains original ductility and strength. Grain Growth – Here grains start growing. Bigger the grains poorer the properties.

29. SOLDERING AND WELDING OF STAINLESS STEEL Soldering Stainless steel components are often joined by silver solders, which are alloys of silver, copper and zinc to which elements such as tin and indium may be added to lower the fusion temperature.

30. Technical considerations for soldering A needle-like, nonluminous, gas-air flame is used to minimize annealing of metal surrounding the joint. The reducing zone of the flame should be used. The soldering should be observed in a shadow, against a black background, so that the temperature can be judged by the color of the work piece. The color should never exceed a dull red.

31. Procedure Prior to soldering, the parts are tack-welded for alignment during soldering procedure Flux is applied, the heavier-gauge part is heated first As soon as the flux fuses, the solder alloy is added Continue heating, until the solder flows around and within the joint Remove work immediately from heat source and quench in water

32. 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 such as bands and brackets.

33. 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.

34. The welded area becomes susceptible to corrosion due to chromium carbide precipitation and loss of passivation. The grain structure is not affected. Increased weld increases the strength.

35. SUBSCRIBE TO MY CHANNEL