Mechanical properties Key terms and concepts Stainless Steel History Composition Corrosion resistance Manufacturing process Types of Stainless Steel Mechanical consideration and clinical implications. References

1. METALLURGY
Presented by-
Vaibhav Ambashikar
1

2. Recap
• Metallurgy
• History
• Periodic table
• Evolution of metals in Orthodontics
• Importance in Orthodontics
• Molecular structure and Interatomic bonding
2

3. • Lattice structure and Crystal arrangements.
• Lattice Defects
• Physical metallurgy
• Chemical/ Extractive metallurgy
• Properties of metal
• Annealing
• Corrosion
3

4. Contents
• Mechanical properties
Key terms and concepts
• Stainless Steel
History
Composition
Corrosion resistance
4

5. Manufacturing process
Types of Stainless Steel
Mechanical consideration and clinical implications.
• References
5

6. Mechanical Properties?
Why is important?
6

7. MECHANICAL PROPERTIES
• Strength – Maximum stress that a structure can withstand
without sustaining a specific amount of plastic strain (yield
strength) or stress at the point of fracture.
• Stress – Force per unit area within a structure subjected to an
external force or pressure.
– Internal distribution of the load.
• Strain – Change in length per unit initial length.
– Internal distortion produced by the load,deflection per
unit length.
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013 7

8. Types of Stress/Strain
• Tensile stress – Ratio of tensile force to the original
cross-sectional area perpendicular to the direction of
applied force.
• Caused by a load that tends to stretch or elongate a body.
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013
8

9. • Compressive stress
• Ratio of compressive force to the original cross-sectional
area perpendicular to the axis of applied force.
• Caused by a load that tends to compress or shorten a
body.
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013 9

10. • Shear stress
• Ratio of force to the original cross-sectional area parallel
to the direction of force applied to a test specimen.
• Two non-linear forces act in opposite directions which
causes sliding of one part of the body over the other.
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013
10

11. Types of Strain
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013
Tensile Strain Compressive Strain Shear Strain
11

12. Key Terms
• Modulus of elasticity (Young’s modulus) – Describes
the relative stiffness or rigidity of a material, which is
measured by the slope of the elastic region of the stress-
strain graph.
• Proportional limit – Maximum stress at which stress is
proportional to strain and above which plastic deformation
occurs.
• Ultimate tensile strength – Tensile stress (in a tensile
test specimen) at the point of fracture.
• Yield strength – The stress at which a test specimen
exhibits a specific amount of plastic strain.
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013 12

13. Stress Strain Curve
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007 13

14. Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th
Ed, Elsevier, 2007
Yield Strength
Wire does not return back to its
original dimension
14

15. Springback & Stiffness
• Related to E (modulus of elasticity).
• Stiffness  E
(i.e., load/deflection).
• Springiness  1/E.
• Stiffness  1/Springiness.
• The more horizontal the slope, the springier the wire.
• The more vertical the slope, the stiffer the wire.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007 15

16. Range
• Proffit – The distance that the wire will bend elastically
before permanent deformation occurs.
• Kusy – The distance to which the archwire can be
activated.
• Thurow – A linear measure of how far a wire or any other
material can be deformed without exceeding the limits of
the material.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007
16

17. Springback
• Proffit – The portion of the loading curve b/w elastic limit
and ultimate tensile strength.
• Kusy – The extent to which the range recovers upon
deactivation.
• Ingram et al – A measure of how far a wire can be
deflected without causing permanent deformation.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007
17

18. Clinical Implication
• Relationship b/w strength, stiffness & range:
• Clinically optimal springback occurs when the wire is
bent b/w its elastic limit and ultimate strength.
• The greater the springback, the more the wire can be
activated.
Strength = Stiffness  Range
18

19. Resilience
• Area under the stress-strain curve till proportional limit.
• Maximum amount of energy a material can absorb
without undergoing permanent deformation.
• When a wire is stretched, the space between the atoms
increases. Within the elastic limit, there is an attractive
force between the atoms.
• Energy stored within the wire.
• Strength + Springiness.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007
19

20. 20

21. Formability
• Amount of permanent deformation that a wire can
withstand before failing.
• Indication of the ability of the wire to take the desired
shape.
• Also an indication of the amount of cold work that it can
withstand.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007 21

22. Flexibility
• Large deformation (or large strain) with minimal force,
within its elastic limit.
• Maximum flexibility is the flexural strain that occurs
when a wire is stressed to its proportional limit.
• Maximum flexibility = Proportional limit
Modulus of elasticity.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007 22

23. Ductility
• Relative ability to deform plastically under a tensile stress before it
fractures.
• Ability of a material to be drawn into wires.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007 23

24. Malleability
• Relative ability to deform plastically under a compressive stress before it
fractures.
• Ability to be hammered into thin sheets without fracturing.
Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007 24

25. Other Mechanical Properties
• Toughness – Ability of a material to absorb elastic energy
and to deform plastically before fracturing; measured as the
total area under a plot of tensile stress v/s tensile strain.
• Brittleness –Relative inability of a material to deform
plastically.
• Fatigue – Repeated cyclic stress of a magnitude below the
fracture point of a wire can result in fracture. Fatigue
behavior is determined by the number of cycles required to
produce fracture.
25

26. Stainless Steel
• Steel is an alloy of iron and carbon.
• When 12%- 13% chromium is added to steel, stainless steel is formed.
• Three forms:-
1. Ferritic
2. Austenitic
3. Martensitic
Anusavice K, Shen C, Rawls H. Phillips’ Science of Dental Materials: Elsevier,12th Ed 2013 26

27. History
• First recognised in 1821 by French metallurgist Pierre Berthier.
• Patented by Clark and Woods in 1872
Airedale S. The Discovery Of Stainless Steel 2015. 27

28. • 1912- Harry Bearley developed it in Sheffield, England
• 1919- Stainless steel was first introduced in dentistry
In Germany.
Airedale S. The Discovery Of Stainless Steel 2015.
28

29. • 1930- Edward Angle used it to make ligature wires.
• 1937- Stainless steel was widely used for making
Orthodontic wires.
Today…
Airedale S. The Discovery Of Stainless Steel 2015.
29

30. COMPOSITION
TYPES CHROMIUM NICKEL CARBON
1. Ferritic (BCC) 11.5-27% 0 0.2% max
2. Austenitic (FCC) 16-26% 7-22% 0.25%
3. Martensitic (BCT) 11.5-27% 0-2.5% 0.15-1.2%
Anusavice K, Shen C, Rawls H. Phillips’ Science of Dental Materials: Elsevier,12th Ed 2013 30

31. Functions:-
• Chromium
Passivating film effect
Increases resistance to tarnish and corrosion
• Nickel
Increases strength
• Manganese
Increases hardness during quenching.
Scavenger for sulphur.
31

32. • Cobalt
Decreases hardness
• Silicon
Deoxidiser and scavenger
• Titanium
Inhibits precipitation of chromium carbide
32

33. Corrosion Resistance
• Passivation process of stainless steel
>12% Chromium<12% Chromium
Anusavice K, Shen C, Rawls H. Phillips’ Science of Dental Materials: Elsevier,12th Ed 2013 33

34. Manufacturing of Stainless Steel
Modern steel manufacturing process is done in two stages:-
1. Primary stage
2. Secondary stage
Deo B, Boom R, Fundamentals of Steelmaking Metallurgy 1993
34

35. Mining of Iron Ore
Magnetic separation of iron ore
from debris
35

36. Handbook of Metals 1992
36

37. 37

38. Primary Stage
• It is done in a blast furnace with basic oxygen as burning fuel.
• Oxygen lowers the carbon content and low carbon steel is formed.
Deo B, Boom R, Fundamentals of Steelmaking Metallurgy 1993 38

39. Secondary Stage
• Most commonly performed in an electric arc furnace.
Deo B, Boom R, Fundamentals of Steelmaking Metallurgy 1993 39

40. Refining
• It consists of purifying an impure metal.
• Most commonly performed techniques:-
Pyrometallurigical
Hydrometallurgical
40

41. Types of Stainless Steel
• According to hardness
Soft
Spring hard or Half hard
Hard
• According to crystal structure
Ferritic
Austenitic
Martensitic
Brantley W, Eliades T. Orthodontic Materials 2001 41

42. •According to AISI
Brantley W, Eliades T. Orthodontic Materials 2001
TYPE AISI NO.
Ferritic 430
Austenitic 302, 304, 316L
Martensitic 400
42

43. Ferritic Stainless Steel
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013
• Body Centered Cubic (BCC) structure
• Low cost
• Good corrosion resistance but low strength
• Little application in dentistry.
43

44. Martensitic Stainless Steel
• Body Centered Tetragonal structure (BCT)
• High yield strength and hardness
• Limited application in dentistry as they
are brittle
• Used for surgical and cutting instruments.
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013 44

45. Austenitic Stainless Steel
• Face Centered Cubic (FCC) structure
• Highest corrosion resistance
• Nickel is added to stabilize the austenite
• Preferred over Ferrite as it is..
Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed, Elsevier,2013 45

46. Phase transition
• Austenisation
From 912 to 1,394 °C alpha iron undergoes a phase transition from body
centered cubic(BCC) to the face centered cubic(FCC) configuration of gamma
iron, also called austenite.
As austenite cools, the carbon diffuses out of the austenite and forms carbon
rich iron-carbide (cementite) and leaves behind carbon poor ferrite.
Reed-Hill R, Abbaschian R. Physical Metallurgy Principles Boston 1991
46

47. • Austenite is cooled very rapidly undergoes spontaneous transformation from FCC
to BCT structure and is called Martensite.
• Martensite is a metastable phase that transforms to ferrite and carbide when it is
heated to elevated temperatures. This process is called tempering.
Reed-Hill R, Abbaschian R. Physical Metallurgy Principles Boston 1991
47

48. Mechanical properties with clinical implications
• Springback
Higher springback values provide the ability to apply large activations with a
resultant increase in working time of the appliance.
This, in turn, implies that fewer arch wire changes
• Formability-
High formability provides the ability to bend a wire into desired
configurations such as loops, coils, and stops without fracturing the wire.

49. • Stiffness/Modulus of elasticity-
This is the force magnitude delivered by an appliance and is proportional to
the modulus of elasticity.
Low stiffness or load deflection rates provide:
(1) the ability to apply lower forces
(2) a more constant force over time as the appliance experiences deactivation, and
(3) greater ease and accuracy in applying a given force.

50. Why NITI over SS?
• The large modulus of elasticity of stainless steel and its associated
high stiffness necessitate the use of smaller wires for alignment of
moderately or severely displaced teeth.
• A reduction in wire size results in a poorer fit in the bracket and may
cause loss of control during tooth movement.

51. • The yield strength to elastic modulus ratio (YS /E) indicates a lower springback of
stainless steel than those of newer titanium-based alloys.
• The stored energy of activated stainless steel wires is substantially less than that of
beta-titanium and nitinol wires.
• This implies that stainless steel wires produce higher forces that dissipate over
shorter periods of time than either beta-titanium or nitinol wires, thus requiring
more frequent activations or arch wire changes.

52. References
• Anusavice K, Shen C,Rawls H. Phillips’ Science of Dental Materials, 12th Ed,
Elsevier,2013
• Proffit W, Fields H, Sarver D. Contemporary Orthodontics. 4th Ed, Elsevier, 2007
• Airedale S. The Discovery Of Stainless Steel 2015.
• Manappallil J. Basic Dental Materials, 3rd Ed, Jaypee, 2010

53. • Deo B, Boom R, Fundamentals of Steelmaking Metallurgy 1993
• Handbook of Metals 1992
• Reed-Hill R, Abbaschian R. Physical Metallurgy Principles Boston 1991
• Brantley W, Eliades T. Orthodontic Materials 2001
• Kapila S, Sachdeva R. Mechanical properties and clinical applications of
orthodontic wires. AM J ORTHOD DENTOFAC ORTHOP 1989;96:100-9.