Stainless steel and Wrought Alloys

Question 1

What are the constituents of stainless steel?

Answer 1

Iron - 72%
Chromium - 18%
Nickel - 8%
Titanium - 1.7%
Carbon - 0.3%

Question 2

What is work hardening?

Answer 2

The hard wire is made by drawing the metal in a cold state through a series of discs of successively smaller diameter.
This makes it stronger and gives it spring properties

Question 3

What is the definition of springiness?

Answer 3

Can undergo large deflections without permanent deformation

Question 4

What is stress relief annealing?

Answer 4

Holding SS wires at a temperature of around 450C for a minute or two
Required for stainless steel wires to ensure the correct configuration of metal atoms.
Ensures dimensional stability and enhances mechanical properties.

Question 5

What is weld decay?

Answer 5

Occurs between 500-900C
Can occur in stainless steel when welded
Causes it to become brittle and makes it more likely to corrode due to CrC precipitating at grain boundaries.
Severe restriction

Question 6

What is a wrought alloy?

Answer 6

An alloy that can be shaped/ manipulated through cold working

Question 7

What is meant by a solid solution wrought alloy and what are the 2 types?

Answer 7

2 metals that form a common lattice structure

Random - where both types of atoms in the lattice structure are arranged in random fashion

Ordered - Can predict the type of atom based on its location

Question 8

What is an interstitial solid solution?

Answer 8

2 atoms are markedly different in size

Question 9

What happens if you cool an alloy down extremely slowly?

Answer 9

Generates large grains with poor mechanical properties

Question 10

What are the 2 forms of stainless steel and which is more desirable?

Answer 10

Austentitic and martensitic

Austenite is more desirable.

Question 11

How do we ensure production of austentitic SS rather than martensitic SS when cooling?

Answer 11

Changing the ratio of chromium and nickel
Making ratio 18:8 produces AUSTENITE

Question 12

What are the disadvantages of martensitic SS?

Answer 12

Has a distorted lattice structure as a result of carbon being unable to diffuse normally within the array of iron atoms in each grain
Forms HARD and BRITTLE material

Question 13

How does corrosion resistance occur in SS?

Answer 13

Forms chromium oxide layer on the surface protecting from corrosion.

Question 14

Apart from orthodontic wires, what is another use of stainless steel?

Answer 14

Dental instruments
Denture base

Question 15

What are the benefits and drawbacks of a stainless steel denture base?

Answer 15

BENEFITS
Lightweight
Thin
Fracture, abrasion and corrosion resistant
Withstands large impact forces

DRAWBACKS
Risk of damage due to high pressures during swaging
Difficulty in ensuring uniformity in denture base thickness

Question 16

What are the requirements of an orthodontic wire?

Answer 16

High springiness
Stiffness (YM) - depends on force reqd. for tooth movement
High ductility
Easily joined - soldered
Corrosion resistant

Question 17

Roughly describe what happens at an atomic level in an ortho wire when bent

Answer 17

Dislocations are generated.
As the wire is bent to a new shape and HOLDS (without springing back to original shape) you cannot bend back to original shape without risking fracture, because the new dislocations are pinned in place.