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 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. fcc 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

19. 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.

20. bct cubic structure

21. • 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.

22. 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.

23. • 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.

24. • 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.

25. 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

26. 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

27. 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

28. 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.

29. • 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.

30. 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.

31. 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.

32. 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

34. 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.

36. • 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.

37. • 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.

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