Stainless Steel and Wrought Alloys

Question 1

Give 2 uses of Wrought Alloys

Answer 1

wires (orthodontic)
partial denture clasps

Question 2

How are wrought alloys shaped/manipulated - and give an example

Answer 2

by cold working
(drawn into wire (orthodontic))

Question 3

Composition of Steel

Answer 3

> 98% IRON
<2% CARBON

Question 4

The traditional composition of steel is more than 98% iron and less than 2% carbon- if there is more than 2% carbon, what is this now called?

Answer 4

cast iron or pig iron

Question 5

Name some other constituents of Steel

Answer 5

Chromium (0.5-1%), manganese, silicon, nickel and more

Question 6

What is the purpose of chromium in steel and what is it’s percentage of composition ?

Answer 6

0.5-1%
improves tarnish resistance

Question 7

Give 2 uses of steel

Answer 7

Cutting Instruments
Forceps

Question 8

What does allotropic mean?

Answer 8

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 9

Is Steel an allotropic material?

Answer 9

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 10

What are the key phases which exist in the iron-carbon phase diagram?

Answer 10

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 11

What is a solid solution?

Answer 11

Two metals that form a common lattice structure and are soluble in one another- these form a solid solution

Question 12

Name and describe 2 types of substitional solid solution?

Answer 12

Random: both types of atoms are in a lattice structure and are arranged in a random fashion

Ordered: we can predict the type of atom based on its location

Question 13

What is an Interstitial Solid Solution?

Answer 13

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 14

Why is cooling a molten alloy down slowly unfavourable?

Answer 14

it generates large grains with unfavourable qualities

Question 15

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

Answer 15

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

Question 16

What is martensite?

Answer 16

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.