Stainless steel and their properties

Stainless steel and its properties as in Dentistry

1. Stainless steel & properties of alloys -KAVYA KALAPALA

2. Stainless steel Steel: - Steels are iron based alloys that contain less than 1.2% carbon. Stainless steel: -When approximately 12-30% chromium by weight is added to steel, the alloy is known as Stainless Steel.

3. History of stainless steel Before 1912,orthodontists used noble metals such as gold,platinum,iridium & silver, and their alloys. In 1912, Harry Brearley of Sheffield, UK, discovered it while searching for a corrosion resistant alloy for gun barrels.

4. In 1919,it was introduced to dentistry by F.Hauptmeyer of Germany who called it WIPLA,meaning “like platinum” In 1930, Angle used stainless steel as a ligature wire.

5. Manufacture of stainless steel Hematite, magnetite,limonite & siderite are the main ores of iron. These ores are reduced to iron using a blast furnace,from where it is transferred to other plants for the manufacture of steel. There are 3 main processes of making steel 1.Bessemer process: by Henry Bessemer in 1856 Molten iron + O 2 (air)》Steel + Heat + CO 2

6. 2.Open hearth process: By William Siemens in 1960 Pig iron + scrap + flux (in contact with hot flames ) 》》Steel 3.Heroult’s electric arc furnace : -by Paul Heroult in 19 th century - He used an electric furnace with a high temperature generated between an arc,which was placed between electrodes and the metal.

7. Composition of stainless steel Macroelements: iron-very strong,very corrosive,main component Of stainless steel Nickel-soft,provides corrosion resistance, increases ductility and toughness,adds luster Chromium-provides passivation effect, Micro elements : Carbon:increases mechanicalstrength & hardness Manganese;improves ductility, binds the steel alloys together,decreases brittleness Silicon:increases resistance to oxidation, increases strength Titanium:stabilises the sensitised steel

8. Types of stainless steel Based on the different crystallised configuration of the structure, stainless steel is of 3 types Ferritic stainless steel Martensitic stainless steel Austenitic stainless steel 1 Ferritic stainless steel : Pure iron at room temperature has BCC structure and is referred to as ferrite It is stable upto 912`C Advantages :-Good resistance to corrosion -Low cost Disadvantages :-Low strength -Cannot be heat hardened

9. 2.Martensitic stainless steel : Named after the Germans metallurgist , Adolf Martens. FCC of austenite (on heating &quenching) 》》BCT of martensite This transformation is spontaneous and diffusionless and there is subtle but rapid rearrangement of atomic positions. The rapid cooling traps the carbon atoms that do not have time to diffuse out of the crystal structure. Advantages : -Can be heat treated -Because of its high strength and hardness, it can be used for surgical and cutting instruments, but shanks & orthodontic pliers. Disadvantages : -It is highly brittle owing to the distorted and strained lattice structure. -Corrosion resistance is less than in other types.

10. 3.Austenite stainless steel : - It was invented by Dr.W .Hatfield in 1924 -This alloy is also called 18-8 stainless steel based on the percentage of chromium and nickel respectively. Advantages High Stiffness Highly ductile & malleable Can be cold worked to increase the strength without fracture Good formability Corrosion resistant Disadvantages Lower springback makes it a poor initial alignment arch wire More frequent activations are required due to high stiffness Delivers high force causing discomfort to the patient

11. Pearlite 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. Pearlite consists of alternating fine scale lamellae of ferrite and iron carbide, referred to as cementite or simply carbide The carbide phase is much harder and more rigid than austenite or ferrite

12. Duplex Steel having a 2 phase structure of almost equal properties of austenite & ferrite is known as Duplex Properties: High resistance to stress corrosion cracking High tensile strength than austenite/ ferrite steel Increased resistance to chloride ion attack

13. Uses of stainless steel in orthodontics Brackets and buccal tubes Archwire Bands Attachment (buttons,cleats,hooks,etc) Ligatures Springs Micro implants Wires used in removable appliances & retainers Orthodontic instruments & pliers

14. Passivation To provide resistance to tarnish & corrosion, chromium is added to stainless steel, which forms a thin,transparent but tough and impervious oxide layer on the surface of the alloy,which is called passivation It provides a barrier to diffusion oxygen and other corrosive species, also prevents further corrosion of the underlying alloy.

15. Sensitisation 18-8 stainless steel may loose it’s resistance to corrosion if it is heated between 400’C-900‘C,it is due to precipitation of chromium carbide at the grain boundaries at high temperature as passivation qualities are lost.

16. STaBILISATION It is a method to minimise sensitisation An element is introduced in place of chromium that precipitates as carbide Titanium is most commonly used

17. SHAPE MEMORY : Significant: during phase transition from austenite to martensite. Memory effect is achieved by first establishing a shape at temperature near 482’C;then on cooling,a second shape is formed; on subsequent heating through a lower transition temperature, the material returns to its original shape. SUPERELASTICITY / PSEUDOELASTICITY: It is produced by transition of austenite to martensite by stress due to volume change which results from change in crystal structure Initially stressing alloy results in standard stress –strain behaviour, at the areas of stress, where phase transition is induced,there is increase in strain reffered to as superelasticity. Unloading results in reverse transition & recovery

18. alloys & their properties Alloy: An alloy is a material composed of 2/more elements, at least one of which should be a metal It is made by fusing 2/more metals or a metal and a non metal

19. Base metal A metal which readily oxidises or dissolves to release ions Eg: Al,Sn,Pb,Ni,Cu

20. Noble metal A metal which is highly resistant to oxidation and dissolution in organic solutions Eg Pt, Pd,Rh, Ir, Os

21. Cast metal Solidification of pure metal produced cast metal

22. Wrought metal It is formed when the parent metal is subjected to various deformation processes like drawing, machining, beating,rolling,etc

23. Ingot An ingot is a piece of relatively pure metals that is cast into a shape suitable for further processing

24. What is stress……..???!!

25. Stress It is the internal resistance of the body to external force It is equal and opposite in direction to force applied

26. STRAIN It is expressed as change in dimension to its original dimension, when internal resistance is not sufficient to withstand external force

27. Hooke’s law With in the elastic limit stress is directly proportional to strain

28. Modulus of elasticity It is the relative stiffness or rigidity with in the elastic limit

29. What is resiliency…,?

30. resiliency It is the energy storage capacity of the wires which is the combination of strength and springiness

31. Load deflection rate Deflection observed for a given load within elastic limit

32. springiness It is the measure of how far a wire can be deflected without causing permanent deformation

33. formability It represents the amount of permanent bending the wire will tolerate before it breaks

34. elasticity It is the tendency of a solid material to return to the original shape after being deformed

35. Elastic limit Maximum stress a material will withstand without permanent deformation

36. poisson‘S ratio It is the ratio of axial strain to lateral strain

37. Yield strength Stress at which ,material exhibits a specified limiting deviation when greater than the proportionality of stress to strain

38. Fatigue Material weakening or break down when it is subjected to repeated series of stress

39. A point at which wire breaks Failure point

40. Spring back The amount of elastic strain that a metal can recover when loaded to unloaded from its yield strength

41. Stiffness Amount of force required to produce a specific deformation

42. Hardness Ability of a material to resist scratching

43. Malleability It is the ability of a material to withstand rupture under compression,as in rolling or hammering into a sheet

44. Ductility It is the ability of material to withstand permanent deformation under a tensile load without rupture

45. Bauschinger effect The property of materials where material stress or strain characteristics change as a result of microscopic stress distribution of material

46. Coefficient of thermal expansion Fraction change in size per degree change in temperature at constant pressure

47. Coefficient of thermal conductivity It is the quantity of heat in calories per second that passes through a specimen, 1cm thick and a cross sectional area of 1 cm2, when the temperature difference between the surfaces perpendicular to the heat flow of specimen is 1 degree K

48. Cold working /work hardening The process of strengthening a metal by changing it’s shape without the use of heat and subjecting to mechanical stress to cause a permanent change

49. Heat treatment 1.Softening heat treatment / solution heat treatment / annealing : Heating metal to specific temperature and then cooling at slow rate which produces refined microstructure, forms single phase solid solution via quenching Removes internal stresses due to cold working, welding,casting,rapid cooling

50. Stages of annealing RECOVERY : Most beneficial changes occur Maximum residual stresses get relieved 2.RECRYSTALLISATION : Deformed grains begin to recrystallisation New stress free grains are formed 3.GRAIN GROWTH: Recrystallised grains continue to grow with larger grains consuming smaller grains

51. 2.Hardening heat treatment /precipitation heat treatment /age hardening: Alloys must be kept at elevated temperatures for hours to allow precipitation to take place ,called Ageing Used to increase the yield strength of malleable metals It involves the addition of impure particles to increase material strength

52. Soldering It is a process where two or more metals are joined together by melting and flowing a filler metal into the joint Types:1.Soft solderin: MP of filler metal below 350’C 2.Hard soldering:MP of filler metal below 450’C 3.Brazing:MP of filler metal more than 450’C

53. Welding It is the process by which two pieces of similar metals are joined together without the addition of another metal

54. Biocompatibility It is the ability of a material to induce an appropriate and advantageous host response during its intended clinical usage

55. Biohostability The ease with which a material will culture or accumulate bacteria,spores and viruses