Stainless Steel & Wrought Alloys Flashcards
What are wrought alloys?
alloys that are manipulated and shaped by cold working , often drawn into wire
What are wrought alloys used for?
- orthodontic wires
- partical denture clasps
What is the composition of steel?
Iron - >98%
Carbon - <2%
Chromium - 0.5-1%
Additionally
- manganese
- molybdenum
- silicon
- nickel
- cobalt
What is iron with more than 2% carbon called?
cast iron or pig iron
What is the function of chromium in alloys?
improved tarnish resistance
What is the function of manganese in alloys?
sulphur scavenging
What are the dental functions of steel?
- cutting instruments (>0.8% carbon)
- medical instruments e.g. forceps (<0.8% carbon)
Describe the allotropic nature of iron
Iron undergoes two solid state phase changes with temperature
1 - below 900 degrees
- body centred cubic (BCC) lattice structure
- low carbon solubility (0.05%)
2 - between 900 and 1400 degrees
- face centred cubic (FCC) lattice
- higher carbon solubility (2%)
3 - above 1400 degrees
- same as , below 900 degrees
- body centred cubic (BCC) lattice structure
- low carbon solubility (0.05%)
What happens to the lattice volume of iron between 900-1400 degrees?
- lattice volume decreases
- exists as a face centred cubic (FCC) lattice
What are the four phases of Fe-C
- austenite
- ferrite
- cementite
- pearlite
Describe austenite
- interstitial solid solution (FCC)
- lattice contains iron in rows and columns with carbon interspersed
- exists at high temperatures (>720 degrees)
Describe ferrite
- very dilute solid solution
- exists at low temperatures
- soft and ductile
- large grains with poor mechanical properties
- not relevant to dentistry
Describe cementite
- Fe3C
- exists at low temperatures
- hard and brittle
- large grains with poor mechanical properties
- not relevant to dentistry
Describe pearlite
- eutectoid mixture of ferrite and cementite
What is a solid solution?
two metals that are soluble in one another forming a common lattice structure
Describe a random substitutional solid solution
the atoms of two metals are arranged in a lattice structure with a random order
Describe an ordered substitutional solid solution
the atoms of two metals are arranged in a lattice structure and the type of atom can be predicted based on its location
Describe an interstitial solid solution
formed by two different types of atoms which are markedly different in size, the smaller atoms occupy the space between the larger atoms in a lattice in a random manner
What does quenching steel create?
martensite
- not austenite as the Fe-C phase diagram would suggest
Why is martensite created by quenching steel?
- there is no time for carbon to diffuse normally through the iron atoms
- distorted lattice created
What are the properties of martensite?
- hard
- brittle
- not desirable in dentistry
By what process can martensite be made into other forms?
tempering
Describe the process of tempering
- martensite is heated to 450 degrees then quenched
- depending on temperature, time heated and quenching, either ferrite or cementite can be produced.
- mechanical properties can be controlled through heat treatment
What is the composition of stainless steel?
Iron - 74%
Carbon - 0.1%
Chromium - 12-18%
Nickel - 8-12%
What percentage chromium is required for steel to be considered stainless?
> 12%
What is the function of chromium in stainless steel?
- lowers the transition temperature and conversion rate of austenite to martensite
- decreases % carbon at which eutectoid forms
Why is stainless steel relevant to dentistry?
- corrosion resistant
- can withstand saliva and an acidic pH which usually provoke electrochemical reactions
How does chromium make stainless steel corrosion resistant?
it creates a chromium oxide layer
- however it can be attacked by chlorides
What is the function of nickel in stainless steel?
- lowers the austenite to martensite transition temperature
- improves fracture strength
- improves corrosion resistance
Describe martensitic stainless steel and its role in dentistry
- 12-13% chromium
- small quantity of carbon
- heat hardenable through tempering
- used for dental instruments
Describe austenitic stainless steel
- contains sufficient chromium and nickel to suppress austenite to martensite transition
What ratios of chromium to nickel can be used for austenitic stainless steel?
- 18:8
- 12:12
What are the uses of austenitic stainless steel in dentistry?
Dental equipment and instruments
- requiring sterilisation
- not used for cutting edges
- corrosion resistant rather than strong and hard
Wires
- orthodontics
- readily cold worked (can be bent)
- corrosion resistant
Denture Bases
- sheet form
- swaged (adapted to a die)
What are the components of 18-8 stainless steel wire?
What are its properties?
Chromium - 18%
Nickel - 8%
Carbon - 0.1%
Iron - 74%
- high stiffness
- good spring back ability
- ok ductility
- reasonably easy joining
What are the properties of 18-8 stainless steel wire?
- does not heat harden
- malleable when cast
- work hardens rapidly (cannot be repeatedly manipulated)
What is cold working?
Work done on a metal/alloy at a low temperature below the recrystallisation temperature
- can involve bending, rolling, swaging
- cause dislocations to collect at grain boundaries (slip)
- dislocations cannot transfer to other grains
- stronger, harder material produced
Describe the use of 18-8 stainless steel wires
orthodontic appliances
- springs
- clasps
partial dentures
- clasp arm
- wrought rests
What is the grade of stainless steel wire? Give examples
the grade depends on how much bending and manipulation is required
- soft
- hard
- half hard
- spring temper
What alloys can be used for wires?
- stainless steel
- cobalt-chromium
- gold
- nickel titanium
- beta titanium
What is the composition of cobalt chromium wire?
What are its properties?
Cobalt - 40%
Chromium - 20%
Nickel - 15%
Iron - 16%
- high stiffness
- ok spring back ability
- good ductility
- difficult to join
What is the composition of gold wire?
What are its properties?
Gold - 60%
Silver - 15%
Copper - 15%
Pt/Pd - 10%
- medium stiffness
- ok springback ability
- ok ductility
- easy joining, can be soldered
- used for slow, restrained tooth movements
What is the composition of nickel titanium wire?
What are its properties?
Nickel - 55%
Titanium - 45%
+ some cobalt
- low stiffness
- excellent spring back ability
- poor ductility
- difficult to join
- used for slow movements
What is the composition of beta titanium wire?
What are its properties?
Mostly titanium with some molybdenum
- medium stiffness
- good spring back ability
- ok ductility
- required welding
What is springiness?
the ability of a material to undergo large deflections (to form an arc) without permanent deformation
- term specific to wires
- calculated as elastic limit/Young’s modulus
What are the desired properties of a wire?
- high springiness
- undergo large deflections
- no permanent deformation
- stiffness
- depends on the required force for tooth movement
- high ductility
- ability to bend without fracture
- allows for manipulation to desired configuration
- easily joined
- joining by soldering or welding
- characteristics should not be compromised
- corrosion resistant
- must not rust in the oral cavity
Describe the process of soldering and its risks?
- gold solder or silver solder available
- must be careful with silver solder as it must be heated to 700 degrees which is close to the melting point of stainless steel
- increased risk of recrystallisation which must be avoided
- quench rapidly to maintain ultimate tensile strength
What is weld decay?
precipitation of chromium carbides at grain boundaries when heated to between 600-900 degrees.
- results in brittle alloy
- less chromium located in central region of solid solution
- more susceptible to corrosion
- limits manipulation possible before fracture
What is weld decay?
precipitation of chromium carbides at grain boundaries when heated to between 600-900 degrees.
- results in brittle alloy
- less chromium located in central region of solid solution
- more susceptible to corrosion
- limits manipulation possible before fracture
How can weld decay be minimised?
- low carbon content steels
- very expensive
- stabilised stainless steel
- contain small quantities titanium or niobium
- preferential carbide formation
- reduced formation at grain boundaries
Describe the process of stress relief annealing for stainless steel wires
- wire heated to 450 degrees for 1-2 minutes
- atoms in each grain settle into equilibrium
- allows different grade of wire to be created
- temperature must not exceed this
- precipitation of carbides occurs above 500 degrees
- grain structure is affected above 650 degrees
How are stainless steel denture bases formed?
- sheet of stainless steel placed between die and counter-die
- considerable force applied
- sheet of alloy swaged
- alloy takes on shape of denture base
What are the advantages of a stainless steel denture base?
- considerably thinner than acrylic
- light
- fracture resistant
- corrosion resistant
- high polish obtainable (very smooth)
- high thermal conductivity
- high impact strength
- high abrasion resistance
What are the disadvantages of a stainless steel denture base?
- potential for dimensional inaccuracy
- elastic recovery of steel resulting in inaccuracy
- damage caused to die under hydraulic press
- loss of fine detail
- difficult to ensure uniform thickness
- wrinkling of steel due to uneven pressure on die and counter die
