DMS - Steel & Wrought Alloys Flashcards

1
Q

define alloy. (3)

A

Two metals that form a common lattice structure

The metals are soluble in one another

They form a solid solution

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

name the two types of solid solutions formed by alloys and their subtypes (3).

A
  1. Substitutional solid solution:
    Two subtypes;
    Random - Both types of atoms within the structure are arranged in a random fashion
    Ordered - You can predict the type of atom based on its location.
  2. Interstitial solid solution - Two atoms marked different in size
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what are wrought alloys?

A

Alloys which can be manipulated by cold working - can be drawn into a wire.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Name the metals within a steel alloy. (4)

A

Iron
Carbon
Nickel
Chromium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

iron is describe as allotropic - define this.

A

in a solid state it can exist in 2 crystalline forms/phases depending on the temperature.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

at what temperatures is iron a body centred cubic crystalline structure?

A

< 900 degrees

> 1400 degrees

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

at what temperatures is iron a face centred cubic lattice structure?

A

between 900 and 1400

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

list the phases of iron. (3)

A

Austenite (high temp) - key phase
lattice containing iron in rows and columns with carbon atoms interspersed.

Ferrite:
Very dilute solid solution
Exists when cooled slowly

Cementite
Exists when cooled slowly
3 iron to 1 carbon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what phases of iron are undesired and why is this the case? (2)

A

Ferrite and cementite

only formed if you cool the alloy extremely slowly = generate large grains with poor mechanical properties = not desired.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what does quenching iron produce?

A

martensite (type of stainless steel)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

how do we prevent quenched iron producing martensite instead of austenite?

A

This can be suppressed by having the correct proportions of chromium and nickel.

18% chromium & 8% Ni
12% chromium & 12% Ni

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what’s the difference between steel and stainless steel?

A

stainless steel > 12% chromium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what is the key attribute of chromium?

A

creates an oxide which increases corrosion resistance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what are the advantages of nickel? (3)

A

Lowers austenite to martensite temperature
Improves strength
Improves corrosion resistance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

name the 2 types of stainless steel

A

austenitic

martensitic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what are the uses of austenitic stainless steel? (3)

A

Dental instruments (can withstand the autoclave during sterilisation)

Dental wires - readily cold worked and corrosion resistant

Sheet form for denture bases - via swaging

17
Q

what are the advantages of cold working an alloy?

A

allows dislocations to be pushed to the grain boundaries (SLIP)
= stronger and harder material

18
Q

what materials can be cold worked?

A

alloys/metals

19
Q

list the properties of stainless steel. (2)

A

Doesn’t heat harden (martensitic does)
Malleable when cast
Can’t be repeatedly manipulated into the desired shape since it work hardens rapidly.

20
Q

what types of alloy wires can you use instead of austenitic stainless steel? (4)

A

Co-Cr - NOT the same as denture material
Gold
Ni-Ti
Titanium

21
Q

what properties are required for wires? (5)

A

High springiness
Stiffness/young’s modulus - depends on the rewired force for tooth movement
High ductility - bend without fracture
Easy joined (without impairing properties)
Corrosion resistant

22
Q

define springiness.

A

Ability of a material to undergo large deflections i.e. form an arc without permeant deformation - returns to its original shape.

23
Q

what is the problem with welding stainless steel wires? (2)

A

welding decay could occur = brittle and increases susceptibility to corrosion

24
Q

what are the problems associated with welding decay in stainless steel? (2)

A

brittle alloy

More susceptible to corrosion since there is less chromium in the central region

25
Q

at what temperature does welding decay occur at?

A

500-900 degrees

26
Q

how do we avoid weld decay in stainless steel? (2)

A

Use stainless steel with;
Low carbon content - expensive
Low quantities of Ti or niobium can be used.

27
Q

describe stress relieving annealing in stainless steel.

A

Required after cold working Stainless steel wires

wires are held at 450 degrees for 1-2 mins to ensure that the configuration of the metal atoms in each of the alloy grains settle into an equilibrium.

28
Q

how is stress relieving annealing carried out in SS?

A

held at 450 degrees C for 1 - 2 m

29
Q

what are the advantages of using stainless steel as a denture base? (8)

A
Thin - 0.11mm 
Light 
Fracture resistant 
Corrosion resistant 
Can be polished to create a smooth surface  
High thermal conductivity 
High impact strength - can withstand large forces 
High abrasion resistance
30
Q

what are the disadvantages of using stainless steel as a denture base? (6)

A

Dimensionally inaccuracy
Elastic recovery of steel = inaccuracy
Risk of damaging under the high hydraulic pressure of swaging
Loss of fine detail
Difficult to ensure uniform thickness
Uneven hydraulic pressure on die = wrinkling