Stainless Steel And Wrought Alloys

Question 1

What is the definition of a wrought alloy?

Answer 1

An alloy that can be manipulated by cold working

It can be drawn into a wire

Question 2

What is the composition of steel? Any significance of the numbers? What are some uses of steel?

Answer 2

> 98% iron

<2% carbon

More than 2% carbon and its called cast iron or pig iron

Also small .5-1% chromium - improve tarnish finish

Manganese - sulphur scavenger

Forceps or cutting insturments

Question 3

What word can describe irons structure? What does this mean?

Answer 3

Allotropic

In a solid state, it ca exist in two forms.

Below 900C or above 1400C it has a body centred cubic crystalline structure

In between it has a face centred cubic lattice structure

Question 4

What are 3 types of iron and when are they formed?

Answer 4

Austentite - interstitial solid solution existing at high temp >720C

Ferrite - very dilute solid solution existing at low temp

Clementine - Fe3C, existing at low temp

Question 5

What is meant by a solid solution?

Question 6

What types of substitutional solutions are there?

Answer 6

Random - atoms are randomly assorted

Ordered - in an ordered fashion, can predict the type of atom based on location

Question 7

Other than substitutional solutions, what other type is there? What is it?

Answer 7

Interstitial

Different sized atoms. E.g. Fe and C

Fe takes up all the lattice atom spots and Carbon is interspersed between the lattice.

Question 8

How is austentite produced? Why is it superior to cementite and ferrite? What actually usually happens?

Answer 8

Austentite is produced when the alloy is cooled rapidly

Ferrite and cementite are formed when the alloy is cooled very slowly.
- this gives large grains and poor mechanical properties

However quenching usually provides martensite

Question 9

What are the components of stainless steel? What makes it stainless?

Answer 9

Fe, C, Cr, Ni

At least 12% Cr to make it stainless

Question 10

What is the function of chromium in stainless steel?

Answer 10

Forms a chromium oxide layer on its surface, forming a barrier to corrosion.

Essential in dentistry due to saliva and acidic liquids in the oral environment

Question 11

What are the two types of stainless steel? And their differences and uses?

Answer 11

Austenitic or martensitic

Martensitic
- 12-13% chromium + little carbon
- heat hardenable through tempering process
- used for dental instruments

Austenitic
- sufficient chromium and nickel to suppress austenite to martensite transition

E.g. 18% cr and 8% ni or 12%/12%

• used for dental equipment and instruments, not cutting edge
• wires or denture bases

Question 12

What is 18-8 stainless steel and its properties?

Answer 12

18% chromium, 8% nickel, 74% iron and 0.1% carbon

Does not heat harden, soft and malleable when cast but work hardens rapidly

Question 13

What is cold working?

Answer 13

Work done on a metal or alloy at low temperature
- below recrystallisation temp

E.g. bending rolling, swaging

Causes slip - dislocations collet at grain boundaries hence making a harder and stronger material

Question 14

What are some uses for 18-8 steel?

Answer 14

Orthodontic appliances - springs ands clasps

Partial dentures - clasp arms, wrought rests

Question 15

What requirements of properties are needed for a wire?

Answer 15

High springiness

Stiffness - depending on the force for tooth movement

High ductility - bending without fracture

Easily joined - without impairing properties

Corrosion resistant

Question 16

What properties does stainless steel have for wires?

Answer 16

Stiffness - high

Springiness - good

Ductility - ok

Joining ease - reasonable

Question 17

What properties does Gold have for wires?

Answer 17

Stiffness - medium

Springiness - ok

Ductility - ok

Joining ease - easy, solder

Question 18

What properties does CoCr have for wires?

Answer 18

Stiffness - high (heat treated)

Springiness - ok

Ductility - good

Joining ease - difficult

Question 19

What properties does NiTi have for wires?

Answer 19

Stiffness - low

Springiness - excellent

Ductility - poor

Joining ease - difficult

Question 20

What properties does Beta-Ti have for wires?

Answer 20

Stiffness - medium

Springiness - good

Ductility - ok

Joining ease - weld

Question 21

What risk is there with soldering and welding S/STEEL?

Answer 21

Soldering can raise the temperature close to the melting point of the steel. This can cause grains to recrystallise, adversely affecting mechanical properties.

Welding can cause weld decay. This is when the steels temp is raised between 500-900C
- this can push Cr and c atoms to grain boundaries allowing CrC to precipitate there, causing the steel to become brittle and more susceptible to corrosion.

Question 22

How can weld decay be reduced?

Answer 22

Low carbon content steels (expensive)

Stabilised stainless steel - containing titanium or niobium

Question 23

What stress relieving procedure is needed for s/steel wires?

Answer 23

Annealing.

This is to ensure all the grains settle in equilibrium (Fe, Ni, Cr)

Temp held around 450C for a minute or two, value must not exceed this.

Question 24

What is swaging of a denture base?

Answer 24

Sheet steel placed between die and counter die, and takes on the shape formed.

Question 25

What properties of steel make it ideal for a denture base?

Answer 25

Thin, 1/10 of acrylic resin

Light

Fracture and corrosion resistant

Polishable

Conducts heat rapidly

Withstands large impact forces

Resists abrasion

Question 26

What are some drawbacks of steel as a denture base?

Answer 26

Sources of inaccuracy

Risk of damage due to high pressured during swaging

Difficulty ensuring thickness is uniform

Wrinkling of steel if hydraulic pressure is applied unevenly