DMS - Steel & Wrought Alloys Flashcards
define alloy. (3)
Two metals that form a common lattice structure
The metals are soluble in one another
They form a solid solution
name the two types of solid solutions formed by alloys and their subtypes (3).
- 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. - Interstitial solid solution - Two atoms marked different in size
what are wrought alloys?
Alloys which can be manipulated by cold working - can be drawn into a wire.
Name the metals within a steel alloy. (4)
Iron
Carbon
Nickel
Chromium
iron is describe as allotropic - define this.
in a solid state it can exist in 2 crystalline forms/phases depending on the temperature.
at what temperatures is iron a body centred cubic crystalline structure?
< 900 degrees
> 1400 degrees
at what temperatures is iron a face centred cubic lattice structure?
between 900 and 1400
list the phases of iron. (3)
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
what phases of iron are undesired and why is this the case? (2)
Ferrite and cementite
only formed if you cool the alloy extremely slowly = generate large grains with poor mechanical properties = not desired.
what does quenching iron produce?
martensite (type of stainless steel)
how do we prevent quenched iron producing martensite instead of austenite?
This can be suppressed by having the correct proportions of chromium and nickel.
18% chromium & 8% Ni
12% chromium & 12% Ni
what’s the difference between steel and stainless steel?
stainless steel > 12% chromium
what is the key attribute of chromium?
creates an oxide which increases corrosion resistance
what are the advantages of nickel? (3)
Lowers austenite to martensite temperature
Improves strength
Improves corrosion resistance
name the 2 types of stainless steel
austenitic
martensitic
what are the uses of austenitic stainless steel? (3)
Dental instruments (can withstand the autoclave during sterilisation)
Dental wires - readily cold worked and corrosion resistant
Sheet form for denture bases - via swaging
what are the advantages of cold working an alloy?
allows dislocations to be pushed to the grain boundaries (SLIP)
= stronger and harder material
what materials can be cold worked?
alloys/metals
list the properties of stainless steel. (2)
Doesn’t heat harden (martensitic does)
Malleable when cast
Can’t be repeatedly manipulated into the desired shape since it work hardens rapidly.
what types of alloy wires can you use instead of austenitic stainless steel? (4)
Co-Cr - NOT the same as denture material
Gold
Ni-Ti
Titanium
what properties are required for wires? (5)
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
define springiness.
Ability of a material to undergo large deflections i.e. form an arc without permeant deformation - returns to its original shape.
what is the problem with welding stainless steel wires? (2)
welding decay could occur = brittle and increases susceptibility to corrosion
what are the problems associated with welding decay in stainless steel? (2)
brittle alloy
More susceptible to corrosion since there is less chromium in the central region
at what temperature does welding decay occur at?
500-900 degrees
how do we avoid weld decay in stainless steel? (2)
Use stainless steel with;
Low carbon content - expensive
Low quantities of Ti or niobium can be used.
describe stress relieving annealing in stainless steel.
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.
how is stress relieving annealing carried out in SS?
held at 450 degrees C for 1 - 2 m
what are the advantages of using stainless steel as a denture base? (8)
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
what are the disadvantages of using stainless steel as a denture base? (6)
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
