DMS-wrought alloys Flashcards
What is stainless steel composed of?
72% Iron
18% chromium
8% Nickel
- 7% titanium
- 3% Carbon
What is allotropic?
This means the alloy undergoes two solid state changes with temperature.
Describe iron as an allotropic alloy?
At temperatures >1400C- the lattice is a body centred cubic lattice.
At temperatures >900 and <1400- the lattice reorganises to a Face centred lattice (more atoms and more packed).
This results in the lattice expanding.
Compare the different phases of the FE-C diagram.
Austenite- the key phase where iron is oragnised into rows and columns with carbon atoms interspersed.
Ferrite- only exists at low temperatures
Cementite- Where 3 iron atoms and 1 carbon atom combine.
How do you produce ferrite and cementite and why don’t we.
When you cool down iron very slowly.
We don’t do this because it produces large grains with reduced properties.
What is martensite and how do we form it.
This is a distorted lattice because carbon cannot diffuse normally within the array of iron atoms. (it is hard and brittle)
From quenching (rapidly cooling) Austenite.
What is austenitic stainless steel?
Where there are the right proportions of chronium and nickel the conversion of austentite into martensite is suppressed.
What is 18-8 stainless steel?
This is a type of austenitic stainless steel.
It does not heat harden
It is soft when cast and work hardens rapidly.
What is the spriginess of orthodontic wires?
This is the ability of a material to undergo large deflections (form an arc) without permanent deformation.
How do we join wires together?
We use gold or silver to soulder the wires together.
What is weld decay and how do we avoid it?
The temperature (500-900*C) causes chromium carbide to precipitate at the grain boundary, making the stainless steel brittle and limiting the manipulation of the wire.
We minimise weld decay by using stabilised stainless steel (which contains small quantities of titanium and niobium) . This limits the chromium carbide formation at the grain boundaries.
What is swaging?
How we shape an alloy sheet.
This is done by pressing a die and counter die togethre under significant pressure, causing the alloy sheet to take the shape of the denture base
Discuss the advantages of Swaging?
The denture base is:
thin and light
It is fracture corrosion and abrasion resistant
Has high impact strength
High thermal conductivity.
Discuss the disadvantages of swaging?
It is inaccurate (steel has elastic recovery and the die contracts)
The die can also be damaged during swaging.
It is also difficult to ensure uniform thickness.
The steel can also be wrinkled if there is uneven pressure between the die and counter die.
