DMS - Steel & Wrought Alloys

Question 1

define alloy. (3)

Answer 1

Two metals that form a common lattice structure

The metals are soluble in one another

They form a solid solution

Question 2

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

Answer 2

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

Question 3

what are wrought alloys?

Answer 3

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

Question 4

Name the metals within a steel alloy. (4)

Answer 4

Iron
Carbon
Nickel
Chromium

Question 5

iron is describe as allotropic - define this.

Answer 5

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

Question 6

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

Answer 6

< 900 degrees

> 1400 degrees

Question 7

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

Answer 7

between 900 and 1400

Question 8

list the phases of iron. (3)

Answer 8

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

Question 9

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

Answer 9

Ferrite and cementite

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

Question 10

what does quenching iron produce?

Answer 10

martensite (type of stainless steel)

Question 11

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

Answer 11

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

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

Question 12

what’s the difference between steel and stainless steel?

Answer 12

stainless steel > 12% chromium

Question 13

what is the key attribute of chromium?

Answer 13

creates an oxide which increases corrosion resistance

Question 14

what are the advantages of nickel? (3)

Answer 14

Lowers austenite to martensite temperature
Improves strength
Improves corrosion resistance

Question 15

name the 2 types of stainless steel

Answer 15

austenitic

martensitic

Question 16

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

Answer 16

Dental instruments (can withstand the autoclave during sterilisation)

Dental wires - readily cold worked and corrosion resistant

Sheet form for denture bases - via swaging

Question 17

what are the advantages of cold working an alloy?

Answer 17

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

Question 18

what materials can be cold worked?

Answer 18

alloys/metals

Question 19

list the properties of stainless steel. (2)

Answer 19

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

Question 20

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

Answer 20

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

Question 21

what properties are required for wires? (5)

Answer 21

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

Question 22

define springiness.

Answer 22

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

Question 23

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

Answer 23

welding decay could occur = brittle and increases susceptibility to corrosion

Question 24

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

Answer 24

brittle alloy

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

Question 25

at what temperature does welding decay occur at?

Answer 25

500-900 degrees

Question 26

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

Answer 26

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

Question 27

describe stress relieving annealing in stainless steel.

Answer 27

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.

Question 28

how is stress relieving annealing carried out in SS?

Answer 28

held at 450 degrees C for 1 - 2 m

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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