Stainless Steel And Wrought Alloys Flashcards
What is the definition of a wrought alloy?
An alloy that can be manipulated by cold working
It can be drawn into a wire
What is the composition of steel? Any significance of the numbers? What are some uses of steel?
> 98% iron
<2% carbon
More than 2% carbon and its called cast iron or pig iron
Also small .5-1% chromium - improve tarnish finish
Manganese - sulphur scavenger
Forceps or cutting insturments
What word can describe irons structure? What does this mean?
Allotropic
In a solid state, it ca exist in two forms.
Below 900C or above 1400C it has a body centred cubic crystalline structure
In between it has a face centred cubic lattice structure
What are 3 types of iron and when are they formed?
Austentite - interstitial solid solution existing at high temp >720C
Ferrite - very dilute solid solution existing at low temp
Clementine - Fe3C, existing at low temp
What is meant by a solid solution?
What types of substitutional solutions are there?
Random - atoms are randomly assorted
Ordered - in an ordered fashion, can predict the type of atom based on location
Other than substitutional solutions, what other type is there? What is it?
Interstitial
Different sized atoms. E.g. Fe and C
Fe takes up all the lattice atom spots and Carbon is interspersed between the lattice.
How is austentite produced? Why is it superior to cementite and ferrite? What actually usually happens?
Austentite is produced when the alloy is cooled rapidly
Ferrite and cementite are formed when the alloy is cooled very slowly.
- this gives large grains and poor mechanical properties
However quenching usually provides martensite
What are the components of stainless steel? What makes it stainless?
Fe, C, Cr, Ni
At least 12% Cr to make it stainless
What is the function of chromium in stainless steel?
Forms a chromium oxide layer on its surface, forming a barrier to corrosion.
Essential in dentistry due to saliva and acidic liquids in the oral environment
What are the two types of stainless steel? And their differences and uses?
Austenitic or martensitic
Martensitic
- 12-13% chromium + little carbon
- heat hardenable through tempering process
- used for dental instruments
Austenitic
- sufficient chromium and nickel to suppress austenite to martensite transition
E.g. 18% cr and 8% ni or 12%/12%
- used for dental equipment and instruments, not cutting edge
- wires or denture bases
What is 18-8 stainless steel and its properties?
18% chromium, 8% nickel, 74% iron and 0.1% carbon
Does not heat harden, soft and malleable when cast but work hardens rapidly
What is cold working?
Work done on a metal or alloy at low temperature
- below recrystallisation temp
E.g. bending rolling, swaging
Causes slip - dislocations collet at grain boundaries hence making a harder and stronger material
What are some uses for 18-8 steel?
Orthodontic appliances - springs ands clasps
Partial dentures - clasp arms, wrought rests
What requirements of properties are needed for a wire?
High springiness
Stiffness - depending on the force for tooth movement
High ductility - bending without fracture
Easily joined - without impairing properties
Corrosion resistant
What properties does stainless steel have for wires?
Stiffness - high
Springiness - good
Ductility - ok
Joining ease - reasonable
What properties does Gold have for wires?
Stiffness - medium
Springiness - ok
Ductility - ok
Joining ease - easy, solder
What properties does CoCr have for wires?
Stiffness - high (heat treated)
Springiness - ok
Ductility - good
Joining ease - difficult
What properties does NiTi have for wires?
Stiffness - low
Springiness - excellent
Ductility - poor
Joining ease - difficult
What properties does Beta-Ti have for wires?
Stiffness - medium
Springiness - good
Ductility - ok
Joining ease - weld
What risk is there with soldering and welding S/STEEL?
Soldering can raise the temperature close to the melting point of the steel. This can cause grains to recrystallise, adversely affecting mechanical properties.
Welding can cause weld decay. This is when the steels temp is raised between 500-900C
- this can push Cr and c atoms to grain boundaries allowing CrC to precipitate there, causing the steel to become brittle and more susceptible to corrosion.
How can weld decay be reduced?
Low carbon content steels (expensive)
Stabilised stainless steel - containing titanium or niobium
What stress relieving procedure is needed for s/steel wires?
Annealing.
This is to ensure all the grains settle in equilibrium (Fe, Ni, Cr)
Temp held around 450C for a minute or two, value must not exceed this.
What is swaging of a denture base?
Sheet steel placed between die and counter die, and takes on the shape formed.
What properties of steel make it ideal for a denture base?
Thin, 1/10 of acrylic resin
Light
Fracture and corrosion resistant
Polishable
Conducts heat rapidly
Withstands large impact forces
Resists abrasion
What are some drawbacks of steel as a denture base?
Sources of inaccuracy
Risk of damage due to high pressured during swaging
Difficulty ensuring thickness is uniform
Wrinkling of steel if hydraulic pressure is applied unevenly
