3. INTRODUCTION
Stainless steel are the major alloys used in
dentistry and especially orthodontics because of its
superior performance .
It is related to the binary iron-carbon system, which
yields the primary phase of carbon steel.
4. HISTORY
In 1912 stainless steel developed accidently by Harry
Brearley in Sheffield, England for the rifiling of gun
barrels in the world war I.
Stainless steel use in orthodontics starts back in 1920s.
By that time gold alloys were the first choice for the
fabrication of wires , bands etc .
Gold alloys were very expensive , so development of new
alloys was benefited.
5. However stainless steel was not completely accepted until
a few decades later .
Archie Brusse (founder of Rocky mountain metal products
) presented a table clinic on the first complete stainless
steel system at the American Society of Orthodontics by
1933.
By 1950 , the 300 series type stainless steel were used
by most orthodontic material.
Around 1960s when gold was definitively abandoned.
6. IRON –CARBON SYSTEM
STEEL- It Is an alloy of iron and carbon .
- Carbon content not exceeding 2.1%. by weight
It exists in three forms -
1. Ferritic
2. Austenitic and
3. Martensitic
7. FERRITE
FERRITE : Carbon has low solubility in bcc structure and
reaches maximum of 0.02% at 7230C
It has body centered cubic structure (bcc ) lattice structure.
8. AUSTENITE
This material is formed above 7230C
It forms a face cubic centered (fcc) lattice structure.
This material has maximal solubility of carbon is 2.1
%
9. MARTENSITE
If austenite is cooled rapidly (Quenched) it will
undergo spontaneous diffusionless transformation
to a Body Centered Tetragonal (BCT) lattice
structure
.
The lattice is highly distorted, strained resulting in
a hard strong and brittle alloy.
10. 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 the hardness of the alloy and but
increases it toughness
11. STAINLESS STEEL
When approx. 12 % - 30% chromium by weight is
added to carbon steel , the alloy is commonly
called as stainless steel
Based on the previously described crystal
structures formed by iron and carbon atoms , there
are three major types of stainless steel .
1. FERRITIC STAINLESS STEEL
2. AUSTENTIC STAINLESS STEEL
3. MARTENSITIC STAINLESS STEEL
12. Composition
TYPES CHROMIUM NICKEL CARBON
FERRITIC 11.5-27% 0 0.2% MAX
AUSTENITIC 16-26% 7-22% 0.25%
MARTENSITIC 11.5-27% 0-2.5% 0.15-1.20%
12
Minor quantities of Silicon, phosphorous, sulphur , Manganese, titanium.
In addition to Iron
13. FUNCTION OF COMPOSITIONAL
COMPONENT
Chromium:
Increases tarnish and corrosion resistance.
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
14. 14
Manganese:
Scavenger for Sulphur
Increases hardness during quenching
Silicon:
Deoxidizer and scavenger.
Titanium:
Inhibits the precipitation of Chromium carbide.
16. FERRITIC STAINLESS STEEL (bcc)
PROPERTIES-
1. Good corrosion resistance at low cost provided
increased strength is not required.
2. Temperature change does not induce phase change
in solid state
3. The alloy is not hardenable by heat treatment.
4. Not readily work hardenable.
5.Little application in Dentistry.
17. MARTENSITIC STAINLESS STEEL (bct)
PROPERTIES-
1. Can be heat treated in the same manner as carbon
steel
2. High yield strength.
3. Increased hardness
4. It is used for manufacturing of surgical and cutting
instruments.
18. Austenitic stainless Steel (fcc)
Most corrosion resistant of all types of stainless steel
TYPE 18- 8 stainless steel is common type i.e. -
Chromium18%, Nickel 8% by weight.
AISI 302,304
It is preferrable to ferritic stainless steel for dental
applications.
19. PROPERTIES
1. Corrosion resistance
2. Greater ductility
3. Ability to undergo more cold work without
fracturing.
4. Substantial strengthening during cold working.
5. Greater ease of welding .
6. Ability to overcome sensitization
7. Less critical grain growth
8. Comparative ease of forming .
20. CORROSION RESISTANCE
-With the presence of chromium in the alloy.
- Chromium forms a very thin , transparent ,
adherent layer of Cr2O3 , through the process
called passivation .
-A coherent oxide layer formed that passivated the
surface, thereby rendering the alloy
“stainless.”
21. STAINLESS STEEL CORROSION CAUSES
.
1. Any site of surface roughness on metal
2. Chlorine containing cleansers should not be used
to clean removable appliance made from stainless
steel
3. Incorporation of bits of carbon or other metal in its
surface.
4. Inter –granular corrosion .
5. Organic and inorganic deposits
22. SENSITISATION:
When austentic stainless steel is heated between approx. 400
and 900 0C
Iron – chromium carbides precipitate along the grain
boundaries and chromium is depleted near grain boundaries
below concentrations necessary for protection .
Thus it becomes suspectible to intergranular corrosion and
partial disintegrations of the weekened alloy may result .
This phenomenon is called as sensitization.
23. STABLIZATION
Introduction of any element i.e. titanium and tantalum which
preferentially form carbides.
Can be added to stainless steel to preserve the level of
chromium when the metal is exposed to elevated
temperatures this process is called stabilization.
24. SOLDERING
A group of process of fusing two similar or dissimilar
metals by heating them to a suitable temperature below
the solidous of the substrate metals and applying filler
metals having a liquidous not exceeding 4500C that
melts and flows by capillary attraction between the parts
without appreciably affecting the dimension of joined
structure.
Soldering temperature – 620 – 6650C
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.
25. WELDING
25
Joining of two or more similar metal pieces by applying heat,
pressure without introduction of an intermediary or a filler
material to produce localized union across the interface through
fusion or diffusion
Spot welding is used to join various components in orthodontics.
i.e. flat brackets , bands etc .
A heavy 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
Chromium carbide precipitation and loss of passivation
The grain structure is not affected
Increased weld area increases the strength.
26. OTHER TYPES OF STAINLESS STEEL
1. DUPLEX STAINLESS STEEL
2. PRECIPITATION – HARDENING STAINLESS
STEEL
3. SUPER STAINLESS STEEL
27. DUPLEX STAINLESS STEEL
This alloy microstructure is composed of a mixture
of austentic and delta – ferritic phases .
Chromium content is high :18% to 26%
Nickel content is low : 4% to 7%
Molybdenum 2% to 3%
Commonest grade is AISI 2205 .
28. PROPERTIES:
1.High weldability
2.High yield strength
3.High tensile strength
4. High toughness
5. Corrosion resistance
It has better properties than austentic and ferritic
stainless steel.
29. PRECIPITATION – HARDENING
STAINLESS STEEL
This alloy developed since 1946.
It is a precipitation hardenable martensitic alloy.
Wide range of application in dental and medical fields
GRADE – AISI 630
Composed of –
Chromium -15.50% to 17.50%
Nickel -3% to 5%
Carbon – 0.07%
Copper – 3% to 5%
And lower amount of magnese , silicon , phosphorus and
sulfur.
30. PROPERTIES
1. Highly Ductile
2. High strength and hardness
3. Good corrosion resistance
31. SUPER STAINLESS STEEL
Despite the fact that austentic stainless steel are
most widely use in orthodontic appliance.
But there are concerns related to allergic reactions
caused by nickel.
In order for the need of better physical property
alloys , a super austentic stainless steel known as
SR- 50A have been reported as having localized
corrosion resistance similar to titanium alloys .
32. Because the passive film is enhanced by the
synergistic effect of high concentration of nitrogen
and molybdenum .
This alloy has been used experimentally for
manufacturing the brackets and wires with very
promising result.
