Stainless steel by ihsan1 /certified fixed orthodontic courses by Indian dental academy

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com


HISTORY
 INTRODUCTION
 COMPOSITION
 CRYTAL SRUCTURE
 CLASSIFICATION
 FERRITIC
 MARTENSITIC
 AUSTENITIC



PROPERTIES
 MODULUS OF ELASTICITY : YOUNGS
MODULUS(ELASTIC MODULE)
 MECHANICAL PROPERTIES
 DUCTILITY
 MALLEABILITY
 STRAIN HARDENING OR WORK
HARDENING



HEAT TREATMENT
 CORROSION RESISTANCE




USES OF STAINLESS STEEL IN
ORTHODONTICS


The corrosion resistance of iron-chromium
alloys was first recognized in 1821 by the
French metallurgist Pierre Berthier
 First developed by accident by Harry
Brearley in Sheffield, England
 Stainless steel entered dentistry in the
year 1919 – krupp’s dental clinic, Germany
by Hauptmeyer
 In 1930 Angle used it as ligature wire



INTRODUCTION


Steels are iron based alloys that contain
less than 1.2% carbon



When 12-30% chromium is added to steel
the alloy is called stainless steel



Steel exists in three Ferritic, austenitic and
martensitic forms.

IRON
Fe - derived from Latin word ferrum.
Atomic number-26; melting point 1535°C.
Iron is the 4th common element in the
earth’s crust.
 The ores of iron are mainly Haematite,
Magnetite, Limonite and siderite. Blast
furnace is used for the primary reduction
of iron ore to iron.



CHROMIUM


Chromium is extracted by a process
known as Goldschmidt's alumino
thermic process.



The corrosion resistance of stainless steel
is largely due to the passivating effect of
chromium


SILICON



Improves resistance to oxidation at higher
temperature and to corrosion.


NICKEL
Its principal ores are
 Garnierite
 Nickel Glance
 Nickel is extracted by a process known as
Monds Process.
 Nickel stabilizes a homogenous mass and
corrosion resistant austenitic phase at low
temperature. Alloying with nickel improves
the corrosion resistance to oxidizing acids.



CARBON



Provides strength and hardness and it
increases corrosion.


MANGANESE AND
MOLYBDENUM



Increases strength and retains ductility
and malleability of iron


CORROSION RESISTANCE
PASSIVATION
The corrosion resistance of stainless steel
is largely due to passivating effect of
chromium.
 About 11% of chromium is needed to
produce corrosion resistance in pure iron.
 Chromium resists corrosion well because
of the formation of a strongly adherent
coating of Cr2O3 on the surface.



SENSITISATION


Carbon is an undesirable impurity in
stainless steel.



Carbon does not enter into the physical
structure of these steels.




But at temperature between 800 - 1200 ºc
carbon reacts with chromium to form
chromium carbide.



This is harmless in itself. But chromium
tied up as the carbide cannot contribute to
the corrosion resistance of the metal. This
is called sensitization.


STABILIZATION


Addition of small amounts (about 6 times
carbon) of niobium or titanium with
tantalum stabilizes stainless steel, these
arrest temporarily , the diffusion of carbon
atom



Stabilized steel becomes more expensive


PRECAUTIONS TO REDUCE
CORROSION RESISTANCE


Do not heat the article above 350 ºc for a
longtime



Brazing temperature should be low and
the time should be short



Select solder of very low fusion
temperature



Do not use carbon steel instruments for
manipulation of wires



Do not use chlorine or fluorine containing
cleansers



Welding or brazing is performed as quickly
as possible i.e. spot welding or laser
welding methods are suitable


CRYTAL SRUCTURE
BCC

BCT


FCC

CLASSIFICATION
Based on lattice arrangements

Ferritic (bcc)

Austenitic (fcc)

Martenistic (bct)
 Based on carbon content(0.8%)

Hypo-eutectoid

Hyper-eutectoid



According to American iron and steel
institute

300 – 502
 According to Anderson

Soft

Half-hard

Hard





FERRITIC



MARTENSITIC



AUSTENITIC



Ferrite is iron which contains practically no carbon. The light parts of
the micrograph are ferrite crystals. These areas are surrounded by
dark lines, which are slightly curved.



The dark points within the ferrite crystals are carbide particles which
give the steel its hardness. The carbide precipitation should be
situated inside the crystals and not along the boundaries, as this
causes a greater susceptibility to corrosion by chemicals,



They have b.c.c structure
Good corrosion resistance at low cost
Not very strong
They have very little application in dentistry













Martensitic steels have a higher carbon content than
ferritic steels. The carbide particles are closely packed in
a matrix of ferrite. The boundaries between the ferrite
crystals are seldom clearly defined. As shown by the
micrograph, the carbide particles are rather small,
rounded and evenly distributed in the steel.
It has got f.c.c lattice structure.
It has got high strength & hardness.
It is used for cutting and surgical instruments.
It has got less corrosion resistance compared to other
forms.









In austenitic steels the grain boundaries are
narrow straight lines. In this case also, we find
precipitation of carbide in the crystals, though to
a lesser degree than ferrite, and generally well
within the crystals.
It has got b.c.t lattice structure
It is the most corrosion resistant form
They are the most commonly usedby
orthodontists in the form of bands and wire
Ease of welding
Readily over comes sensitization

COMPOSITION OF THREE DIFFERENT
TYPES OF STAINLESS STEEL
Type of
space lattice

Chromium Nickel
%
%

Carbon
%

Ferritic
b.c.c

11.5 - 27

0

0.2 max

Austenitic
f.c.c

16 - 26

7 - 22

0.25 max

Martensitic
b.c.t

11.5 - 17

0 – 2.5

0.15 – 1.2


PROPERTIES
Bio compatibility
 High corrosion resistance
 Chemically stable in oral or implant
environment
 Good ductility and malleability
 Undergoes work hardening by large
amount ,that is why thinner wires have
higher mechanical properties



Less critical grain growth
 Fairly high formability factor
 Melting tempreture 1240 – 1260 ºc
 Respond to heat treatment
 Can be welded and soldered
 Easily available in various forms and not
expensive
 When heated above 400 ºc undergoes
sensitization which can be remedied to a
certain extend





Most of these properties are required for
selection for orthodontic appliances


MODULUS OF ELASTICITY : YOUNGS
MODULUS(ELASTIC MODULE)


Ratio of stress to strain.



The SI unit of modulus of elasticity is the Pascal. Given
the large values typical of many common materials,
figures are often quoted in megapascals or gigapascals.
Wire with low modulus of elasticity has increased
flexibility as compared to a wire of high modulus of
elasticity.




MECHANICAL PROPERTIES
Yield strength

1100 – 1750 Mpa

Ultimate tensile strength

2200 Mpa

Modulus of elasticity
Surface hardness

170,000 – 200,000
Mpa
250 – 400 KHN

Density

8.5 gm/cc

Percentage elongation
upto

35%


DUCTILITY


Stainless steel has the ability to be drawn
into wires and withstand permanent
deformation under tensile load without
fracture


MALLEABILITY
A malleable metal is capable of being
extended, shaped, or otherwise deformed
without cracking.
 Stainless steel because of its malleability
can be used as bands etc.
 This property is important in metalworking,
as materials that crack or break under
pressure cannot be hammered or rolled.





Stainless steel has the ability to withstand
permanent deformation



Malleable materials can be formed using
stamping or form pressing


STRAIN HARDENING OR WORK
HARDENING
If a metal is continuously stressed it
becomes stiffer and harder. Hardening of
a metal by cold working is called strain
hardening or work hardening
 It is a method to produce high tensile
strength wire



CONSEQUENCES OF WORK HARDENING

Increased surface hardness
 Greater yield and ultimate strength
 Decreased ductility
 Proportional limit is increased
 Decreased resistance to corrosion
 No change in elastic modulus



HEAT TREATMENT


It is a general process using thermal
energy to change the charachteristics of
metallic alloy as in tempering precipitation,
hardening or annealing


ANNEALING


It is a process that produces conditions by
heating and maintaining at a suitable
temperature, and then cooling very slowly.



It is used to induce softness, relieve
internal stresses, refine the structure and
improve cold working properties.


There are three stages in the annealing
process, with the first being the

Recovery phase

Recrystallization

Grain growth





The effect associated with cold working
such as strain hardening, low ductility &
distorted grain can be reversed by simply
heating the metal.



Greater the amount of cold working the
more rapidly the effect can be reversed by
annealing




The high temperature of annealing may
result in oxidation of the metal’s surface,
resulting in tarnish and corrosion.


USES OF STAINLESS STEEL IN
ORTHODONTICS
Orthodontic brackets
 Orthodontic bands
 Stainless steel is used for making
auxiliaries. Austenitic stainless steel is
used for these auxiliaries e.g. Lingual
button, Lingual Cleats; Lingual Sheath
 Ligature wires are made up of stainless
steel wires.



It can be used for making different
instruments like different types of pliers.
For this austenite stainless steel is used.
For making cutting pliers Martensitic
stainless steel is used.
 stainless steel wires can be used in fixed
orthodontic treatment with edgewise
appliances.





Stainless steel wires are used in making
Up righting springs and minisprings.



It is used for making instrument stands,
instrument trays, impression trays etc.



Different gauges of stainless steel round
wires are used for the fabrication of
different components of removable
appliances like clasps, springs, labial bows
etc.

Stainless steel by ihsan1 /certified fixed orthodontic courses by Indian dental academy

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