Stainless steel and wrought alloys Flashcards
What is the definition of Wrought alloy?
- An alloy which can be manipulated/shaped by cold working
- So it can be drawn into wire
What are some uses of wrought alloys?
- Wires for orthodontics
- Partial denture clasps
What is steel composed of?
- Iron >98%
- Carbon <2%
*Above 2% carbon is regarded as cast iron or pig iron - Chromium 0.5-1% to improve tarnish resistance
- Manganese
- Nickel, cobalt
What are some uses of steel?
- Cutting instruments (<0.8% C)
- Forceps (<0.8% C)
What does allotropic mean?
- Undergoes two solid state phases changes with temperature
What temperature does Iron have a Body centred cubic crystalline structure?
- Temp >1400C (low carbon solubility 0.05%)
- Temp <900C
What temperature does Iron have Face centred cubic lattice structure?
- Temp in between 900 and 1400C
- Has higher carbon solubility 2%
- During this temp the Iron lattice expands which is what you’d expect
What is Austenite?
- Interstitial solid solution of Iron
- Face centred cubic lattice
- Lattice has iron in rows and columns and some C atoms interspersed
- Exists at high temp >720C
What is Ferrite?
- Very dilute solid solution of Iron
- Exists at low temp
What is Cementite?
- Fe3C
- Exists at low temp
What is Pearlite?
- Eutectoid mixture of Ferrite and Cementite
What is a solid solution?
- Two metals that form a common latice structure
- Are soluble in one another
- Two types of solid solution
What are the two types of Substitional solid solution?
-Random ( both types of atoms in lattice structure arranged in random fashion)
- Ordered ( Can predict type of atom based on its location)
What are grains of solid solution?
- Several lattices together
- Can be random or ordered
What is an Interstitial solid solution?
- Atoms of different sizes (prerequisite)
- Solute atoms occupy the interstitial (holes between atoms) in crystal lattice of the solute
- E.g. Fe and C
Findings of a Fe-C phases diagram?
- When Iron cools rapidly the grain structure is Austenite
- Ferrite and Cementite exist if you slow the molten alloy down extremely slowly (not interested)
What is quenching?
- Rapid cooling of metal to adjust the mechanical properties of its original state
What does quenching of steel give us?
- Should give us Austenite according to phase diagram
- But in practice gives us Martensite
What is Martensite?
- Has distorted lattice structure due to carbon being unable to diffuse normally within array of iron atoms in each grain
- This is hard and brittle
- Don’t want this
What is the composition of stainless steel?
- Iron 72%
- Carbon <2%
- Chromium 17-19%
- Nickel 7-9%
When is steel regarded as stainless steel?
- If there is at least 12% chromium
What is the role of Chromium in stainless steel?
- Corrosion resistance due to chromium oxide layer it forms on surface
- Vital as oral environment high risk for corrosion due to saliva, liquids with acidic pH levels
- But it can be attacked by chlorides
- Also lowers Austenite to Martensite temp and rate
- Decreases % C at which Eutectoid formed
What is the role of Nickel in stainless steel?
- Lowers Austenite to Martensite transition temp
- Improves fracture strength
- Improves corrosion resistance
What two types of stainless steel are there?
- Martensitic
- Austenitic
What is Martensitic stainless steel?
- 12-13% Chromium and little carbon
- Can be hardened by heat during tempering process
- Useful for dental instruments
How can Austenitic stainless steel be made during the process of quenching instead of Martensite?
- Need to have the right proportions of Chromium and Nickel to suppress Austenite to Martensite transition
- Either 18% Cr and 8% Ni
- Or 12% Cr and 12%Ni
What are some Austenitic stainless steel uses?
- Dental equipment and instruments to be sterilised
- No useful for cutting edge
- Corrosion resistance is more imp than strength and hardness
- Wires for orthodontics (readily cold worked and corrosion resistant)
- Sheet forms for denture bases (requires it to be swaged)
What does swaged mean?
- Stainless steel sheet positioned between die and counter die
- When these are pressed together a sheet of alloy is swaged
What are the constituents of 18-8 stainless steel wire?
- Iron 74%
- Chromium 18%
- Nickel 8%
- Carbon 0.1%
What are the properties of 18-8 stainless steel wire?
- Does not heat harden
- Soft (malleable) when cast
- But work hardens rapidly
What is cold working?
- Process of hardening and strengthening metals through plastic deformation at temps below their recrystallisation temp
- Metal subjected to mechanical stress that causes dislocations in crystal structure
- The dislocations hinder the motion of atoms and create barriers to slip
- Makes the metal harder and stronger
- Excessive cold working can make metal brittle and prone to fracture
What are some uses of 18-8 stainless steel wires?
- Orthodontic appliances like springs and clasps
- Partial dentures like clasp arms and wrought rests
What are the grades of stainless steel wire?
Soft - Half hard
Hard - Spring temper
- Grade you select depends on how much bending, shape and manipulation required for dental appliance
What is springiness/ Springback ability?
- Ability of material to undergo large deflections (to form an arc) without permanent deformation (returns to original shape)
- Calculated as ratio EL/YM
How can stainless steel wires be soldered?
- Use Gold or silver solder
- Careful as temp rise created is close to melting point of steel so risk of recrystalisation which adversely affects mechanical properties
- Quench rapidly to avoid this and maintain UTS (ultimate tensile strength)
What is Weld decay?
- Occurs between 500-900C
- Chromium carbides precipitate at grain boundaries particularly where soldering occurred
- Alloy becomes brittle and more prone to fracture
How is weld decay minimised?
- Low carbon content sheets but this is expensive
- Stabilised stainless steel
- Contain small quantities of Titanium or Niobium
- Forms carbides preferentially not at grain boundaries
Why do stainless steel wires need a Stress relief annealing process?
- Ensures configuration of metal atoms in each of the alloy grains settle into equilibrium
- Requires temp of stainless steel to be held around 450C for 1-2mins
- Crucial temp does not exceed this as
- Grain structure affected if above 650C
- Precipitation of carbides above 500C
- Which produce different grades
What are the properties of stainless steel denture base?
- Thin 0.11mm
- Light
- Fracture resistant
- Corrosion resistant
- High polish obtainable
- High thermal conductivity
- High impact strength
- High abrasion resistance
What are some disadvantages of stainless steel denture base?
- Poss dimensional inaccuracy as contraction of die not matched by model expansion
- Elastic recovery of steel leads to inaccuracy
- Difficult to ensure uniform thickness
- Uneven pressure on die and counter die leads to wrinkling of steel
