2 - Steel alloys Flashcards
Define a wrought alloy.
Manipulated by cold working (ie drawn into a wire)
Define steel.
- iron >98%
- carbon <2%
What are some common materials added to steel?
- chromium (improve tarnish resistance)
- manganese (sulphur scavenger)
- molybdenum
- silicon
- nickel
- cobalt
Define allotropic.
Undergoes 2 solid states, phase changes with temperature.
Describe iron’s structure at > 1400 degrees.
- body centric cubic lattice
- low carbon solubility
Describe iron’s structure at < 1400 degrees.
- face centred cubic lattice
- high carbon solubility
Define austenite.
- interstitial solid solution (FCC)
- exists at high temperature
- iron in lattice, carbon interspersed
Define ferrite.
- very dilute solid solution
- exists at low temperature
Define cementite.
- Fe3C
- exists at low temperature
Define pearlite.
Eutectoid mixture of ferrite and cementite
Define eutectoid.
The minimum temperature at which a material exists as a single solid solution phase
Define martensite.
- when austenite is quenched
- C atoms aren’t able to diffuse into lattice
- distorted lattice
- hard and brittle
How does the cooling process affect austenite?
Fast cooling = martensite
Slow cooling = pearlite
How does martensite turn into pearlite?
- tempering
- heated to 450 degrees then quenched
What is the composition of stainless steel?
- Fe
- C
- Cr
- Ni
Describe chromium’s role in stainless steel.
> 12% Cr
- lowers austenite to martensite temperature
- lowers austenite to martensite rate
- decreases the % of C at which the eutectoid is formed
- chromium oxide prevents corrosion
Describe nickel’s role in stainless steel.
- lowers austenite to martensite transition temperature
- improves fracture strength
- improves corrosion resistance
Define martensitic stainless steel.
- 12-13% chromium, little carbon
- heat hardenable
- used in dental instruments
Define austenitic stainless steel.
- contains enough chromium and nickel to suppress austenite to martensite transition
- 18% Cr, 8% Ni
- 12% Cr, 12% Ni
What are the uses of austenitic stainless steel?
- dental equipment and instruments (not cutting)
- orthodontic wires
- denture bases (swaged into a sheet)
What is the composition of 18-8 stainless steel?
18% chromium
8% nickel
0.1% carbon
74% iron
What are the properties of 18-8 stainless steel?
- does not heat harden
- malleable when cast but work hardens rapidly, cannot be repeated manipulated to get desired shape
What are the uses of 18-8 stainless steel?
- orthodontic springs and clasps
- RPD clasp arms and rests
What are the grades of 18-8 stainless steel?
Chosen based on degree of bending required
- soft
- hard
- half hard
- spring temper
What are examples of alloys used in wires?
- stainless steel
- cobalt chromium
- gold
- nickel titanium
- beta-titanium
What is the composition of cobalt chromium (wire)?
40% cobalt
20% chromium
15% nickel
16% iron
What is the composition of gold alloy (wire)?
60% gold
15% silver
15% copper
10% palladium
What is the composition of nickel titanium?
55% nickel
45% titanium
+ some cobalt
What is the composition of beta-titanium?
- titanium
- molybdenum
Define springiness.
Ability of materials to undergo large deflections (form an arc) without permanent deformation
What is the best properties for a wire?
- high springiness
- high ductility
- easily joined
- corrosion resistant
- stiffness depends on function
What is used to solder stainless steel wires?
- gold solder
- silver solder
What is weld decay?
- occurs between 500 - 900 degrees
- chromium carbines precipitate at grain boundaries
- alloy becomes brittle
- more susceptible to corrosion
How can weld decay be avoided?
- low carbon content steel (expensive)
- stabilised stainless steel (by titanium or niobium)
Define swaging.
Thin sheet of metal is shaped by a die and counter-die
What are the advantages of stainless steel as a denture base?
- thin
- light
- fracture resistant
- corrosion resistant
- high polish obtainable
- high thermal conductivity
- high impact strength
- high abrasion resistance
