Stainless Steel and Wrought Alloys

Question 1

What are some examples of what stainless steel can be used for in dentistry?

Answer 1

Orthodontic appliances and denture base material.

Question 2

What is a wrought alloy?

Answer 2

It’s an alloy which can be manipulated/ shaped by cold working
So it can be drawn into a wire.
This lends itself to application as an orthodontic appliance.

Question 3

What is steel made up of?

Answer 3

Iron (at least 98% iron)
Carbon
Chromium
Manganese
and others including nickel and cobalt etc.

Question 4

What is “steel” called when it has more than 2% carbon in it?

Answer 4

Cast iron or pig iron.

Question 5

What does allotropic mean?

Answer 5

So, in a solid state, it can exist in TWO crystalline forms – two phases – depending on its temperature.
Below 900C or above 1400C it has a BODY CENTRED CUBIC crystalline structure.
In between it forms a FACE CENTRED CUBIC lattice structure.

Question 6

Is steel an allotropic material?

Answer 6

Yes. Below 900C or above 1400C it has a BODY CENTRED CUBIC crystalline structure.
In between it forms a FACE CENTRED CUBIC lattice structure. The iron lattice will expand between these two temperatures.

Question 7

What are the key phases that can exist in the iron and carbon phase diagrams?

Answer 7

Austenite= interstitial solid solution (iron in rows with carbon atoms interspersed), exists at high temperatures over 720 degrees celcius.

Ferrite= very dilute solid solution that exists at a low temperature.

Cementite= Fe3C that exists at a low temperature.

Pearlite= Eutectoid mixture of ferrite and cementite.

Question 8

What is a solid solution?

Answer 8

Two metals form a common lattice structure- they are soluble in one another, they form a solid solution.

Question 9

What are the two types of a substitutional solid solution?

Answer 9

Random (both types of atoms are in the lattice structure and are arranged in a random fashion)

Ordered (we can predict the type of atom based on its location).

Question 10

What is an interstitial solid solution?

Answer 10

Two atoms are markedly different in size – that’s a prerequisite. One atom fits the lattice structure and the other occupies the spaces in a random fashion.

Question 11

Why is cooling a molten alloy down slowly unfavourable?

Answer 11

It generates large grains with poor mechanical properties.

Question 12

In a Fe-C phase diagram what SHOULD quenching give us? and what does it actually give us in practice?

Answer 12

It should give us austenite but it gives us martensite instead (we don’t want this).

Question 13

What is martensite?

Answer 13

MARTENSITE has a distorted lattice structure, as a result of carbon being unable to diffuse normally within the array of iron atoms in each grain.
This means it forms a hard and brittle material-
We don’t want this.

Question 14

What is martensite useful in?

Answer 14

Martensite is very useful in non-dental applications.
It can be used to produce materials that are soft or hard – or somewhere in between.
And that’s achieved by TEMPERING the material.

Question 15

What can you do to martensite to produce more useful materials?

Answer 15

TEMPERING.

Question 16

What are the steps in tempering martensite?

Answer 16

Altering its temperature, and the duration you maintain it at a specific temperature and then quenching it. This will determine the proportion of ferrite (soft, ductile) and cementite (hard, brittle) produced- different properties from different proportions.

Question 17

What must stainless steel have it it to be regarded as “stainless”?

Answer 17

At least 12% chromium.

Question 18

What is the most important property in dentistry that stainless steel possesses?

Answer 18

Corrosion resistance- Stainless steel forms a chromium oxide layer on its surface, which protects it from corrosion.

Question 19

What does nickel do in stainless steel?

Answer 19

Improves its fracture strength and corrosion resistance.

Question 20

What are the two forms of stainless steel

Answer 20

Martensitic and austenitic.

Question 21

What is martensitic stainless steel?

Answer 21

Used to make dental instruments
Made up of 12-13% chromium
Can be tempered to produce hard materials.

Question 22

How can you suppress the conversion of austenite to martensite?

Answer 22

By having the right proportions of Cr and Ni - specifically, either 18:8 or 12:12 ratio.

Question 23

What are the uses of austenitic stainless steel?

Answer 23

Dental equipments and instruments (not cutting edge)

Corrosion resistance makes sure it withstands the autoclave process in the dental practice

WIRES*- can be cold worked, ot resists corrosion.

Question 24

What is swaging?

Answer 24

Adapted to a die.

Question 25

What is the composition of 18-8 stainless steel and what are its properties?

Answer 25

18-8 S/steel consists of 18% Cr, 8% Ni, 74% Fe and just 0.1% Carbon

18-8 S/steel doesn’t heat harden – unlike the martensitic version,
It’s soft and malleable when cast. But it work hardens rapidly – so it can’t be repeatedly manipulated to form the desired shape.

Question 26

What is cold working?

Answer 26

*Work done on metal /alloy at LOW TEMPERATURE
- below recrystallisation temperature:
(eg bending, rolling, swaging)

• causes SLIP - dislocations collect at grain boundaries
• hence stronger, harder material
• aka WORK or STRAIN HARDENING.

Question 27

What are the uses of 18-8 stainless steel wires?

Answer 27

Uses- orthodontic appliances (springs/clasps), partial dentures (clasp arms/wrought rests).

Question 28

What are the grades of 18-8 stainless steel wires and what does it depend on?

Answer 28

Depends on the degree of bending required
Soft- half hard
Hard- spring temper.

Question 29

What is the composition of CoCr in wire form?

Answer 29

40% Co ; 20% Cr. And 15% Ni and Fe.

Question 30

What is the composition of gold in wire form?

Answer 30

60% gold and 15% each for Ag and Cu.

Question 31

What is the composition of Ni titanium alloy wires?

Answer 31

55:45 split for Ni Ti.

Question 32

What is meant by the “springiness” of a material (EL/YM)?

Answer 32

Ability of a material to undergo large deflections (to form arc) without permanent deformation (ie it returns to its original shape).

Question 33

What other characteristics are of interest in wires?

Answer 33

high springiness ( EL / YM)
(ie undergo large deflections without permanent deformation)

stiffness (YM) - depends on required force for tooth movement

high ductility - bending without fracture

easily joined without impairing properties - soldered, welded

corrosion resistant.

Question 34

There is a table linking each materials properties. The main ones mentioned are…

Answer 34

S/Steel is satisfactory across the board; CoCr is pretty similar except for the ease of joining several wire components.
Gold is satisfactory too, though its rigidity is less – this makes it suited to scenarios where a more restrained rate of movement of the dentition is needed.
NiTi is excellent for its springiness, and is ideal for moving teeth slowly. The main challenge here is in joining NiTi wires together.

Question 35

What can be used to solder stainless steel wires?

Answer 35

S/steel can be soldered using gold or silver.
Care has to be taken though as the temperature rise created is close to the melting point of s/steel.
So there is a risks the s/steel grains may recrystallize – which adversely affects its mechanical properties.
Quenching the alloy would avoid this.

Question 36

How would you prevent stainless steel from recrystallising during soldering?

Answer 36

By quenching.

Question 37

What is “weld decay”?

Answer 37

This phenomenon occurs when s/steel’s temperature is raised to between 500 to 900C. This can push the Cr and C atoms to the grain boundaries – allowing CrC to “precipitate” there. If CrC forms at the grain boundaries it causes the S/steel to become brittle – so its able to undergo just a small amount of deformation before fracturing. This limits the amount of manipulation of the wire to match the desired configuration. That’s a severe restriction.
It also makes s/steel more likely to corrode.

Question 38

How do you overcome weld decay?

Answer 38

Minimise by :

1. Low carbon content steels - expensive
2. Stabilised stainless steel
   - contain small quantities of
   TITANIUM or NIOBIUM
   - forms carbides preferentially
   - not at grain boundaries.

Question 39

Why does stainless steel need to undergo “stress relief annealing?

Answer 39

This is to ensure the configuration of the metal atoms - Ni, Cr and Fe - in each of the alloy grains settle into an equilibrium.

Question 40

How do you stress relief anneal stainless steel?

Answer 40

This process requires the temperature of s/steel needs to be held at around 450C for a minute or two. It’s crucial that the temperature of the s/steel DOES NOT exceed this value – otherwise it may be adversely affected :- either through a change in the grain structure; OR due to metal carbides forming at the grain boundaries. SO care is required at each stage.

Question 41

How is stainless steel “swaged” to make a denture base material?

Answer 41

Shown here is a S/steel sheet positioned between a die and counter-die.
When these are pressed together the sheet of alloy is SWAGED – taking on board the shape of the denture base.

Question 42

What are the benefits of steel as a denture base material?

Answer 42

1. Thin 0.11mm - acrylic
   1.52mm
   2. Light
   3. Fracture resistant
   4. Corrosion resistant
   5. High polish
   obtainable
   6. High thermal
   conductivity
   7. High impact strength
   8. High abrasion
   resistance.

Question 43

What are the disadvantages of stainless steel as a denture base material?

Answer 43

Possible dimensional inaccuracy (contraction of die not matched by model expansion)
Elastic recovery of steel – inaccuracy
Damage of die under hydraulic pressure
Loss of fine detail during the many stages
Difficult to ensure uniform thickness
Uneven hydraulic pressure on die and counter die
> wrinkling of steel.