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 hardHard - 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 expensive2. 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
