Stainless steel and wrought alloys Flashcards
(40 cards)
1
Q
What is the definition of Wrought alloy?
A
- An alloy which can be manipulated/shaped by cold working
- So it can be drawn into wire
2
Q
What are some uses of wrought alloys?
A
- Wires for orthodontics
- Partial denture clasps
3
Q
What is steel composed of?
A
- 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
4
Q
What are some uses of steel?
A
- Cutting instruments (<0.8% C)
- Forceps (<0.8% C)
5
Q
What does allotropic mean?
A
- Undergoes two solid state phases changes with temperature
6
Q
What temperature does Iron have a Body centred cubic crystalline structure?
A
- Temp >1400C (low carbon solubility 0.05%)
- Temp <900C
7
Q
What temperature does Iron have Face centred cubic lattice structure?
A
- Temp in between 900 and 1400C
- Has higher carbon solubility 2%
- During this temp the Iron lattice expands which is what you’d expect
8
Q
What is Austenite?
A
- 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
9
Q
What is Ferrite?
A
- Very dilute solid solution of Iron
- Exists at low temp
10
Q
What is Cementite?
A
- Fe3C
- Exists at low temp
11
Q
What is Pearlite?
A
- Eutectoid mixture of Ferrite and Cementite
12
Q
What is a solid solution?
A
- Two metals that form a common latice structure
- Are soluble in one another
- Two types of solid solution
13
Q
What are the two types of Substitional solid solution?
A
-Random ( both types of atoms in lattice structure arranged in random fashion)
- Ordered ( Can predict type of atom based on its location)
14
Q
What are grains of solid solution?
A
- Several lattices together
- Can be random or ordered
15
Q
What is an Interstitial solid solution?
A
- Atoms of different sizes (prerequisite)
- Solute atoms occupy the interstitial (holes between atoms) in crystal lattice of the solute
- E.g. Fe and C
16
Q
Findings of a Fe-C phases diagram?
A
- When Iron cools rapidly the grain structure is Austenite
- Ferrite and Cementite exist if you slow the molten alloy down extremely slowly (not interested)
17
Q
What is quenching?
A
- Rapid cooling of metal to adjust the mechanical properties of its original state
18
Q
What does quenching of steel give us?
A
- Should give us Austenite according to phase diagram
- But in practice gives us Martensite
19
Q
What is Martensite?
A
- 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
20
Q
What is the composition of stainless steel?
A
- Iron 72%
- Carbon <2%
- Chromium 17-19%
- Nickel 7-9%
21
Q
When is steel regarded as stainless steel?
A
- If there is at least 12% chromium
22
Q
What is the role of Chromium in stainless steel?
A
- 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
23
Q
What is the role of Nickel in stainless steel?
A
- Lowers Austenite to Martensite transition temp
- Improves fracture strength
- Improves corrosion resistance
24
Q
What two types of stainless steel are there?
A
- Martensitic
- Austenitic
25
What is Martensitic stainless steel?
- 12-13% Chromium and little carbon
- Can be hardened by heat during tempering process
- Useful for dental instruments
26
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
27
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)
28
What does swaged mean?
- Stainless steel sheet positioned between die and counter die
- When these are pressed together a sheet of alloy is swaged
29
What are the constituents of 18-8 stainless steel wire?
- Iron 74%
- Chromium 18%
- Nickel 8%
- Carbon 0.1%
30
What are the properties of 18-8 stainless steel wire?
- Does not heat harden
- Soft (malleable) when cast
- But work hardens rapidly
31
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
32
What are some uses of 18-8 stainless steel wires?
- Orthodontic appliances like springs and clasps
- Partial dentures like clasp arms and wrought rests
33
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
34
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
35
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)
36
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
37
How is weld decay minimised?
1. Low carbon content sheets but this is expensive
2. Stabilised stainless steel
- Contain small quantities of Titanium or Niobium
- Forms carbides preferentially not at grain boundaries
38
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
39
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
40
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
