Stainless Steel and Wrought Alloys

Question 1

What is the definition of Wrought alloy?

Answer 1

• An alloy which can be manipulated/shaped by cold working- So it can be drawn into wire

Question 2

What are some uses of wrought alloys?

Answer 2

• Wires for orthodontics- Partial denture clasps

Question 3

What is steel composed of?

Answer 3

• 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

Question 4

What are some uses of steel?

Answer 4

• Cutting instruments (<0.8% C)- Forceps (<0.8% C)

Question 5

What does allotropic mean?

Answer 5

• Undergoes two solid state phases changes with temperature

Question 6

What temperature does Iron have a Body centred cubic crystalline structure?

Answer 6

• Temp >1400C (low carbon solubility 0.05%)- Temp <900C

Question 7

What temperature does Iron have Face centred cubic lattice structure?

Answer 7

• Temp in between 900 and 1400C- Has higher carbon solubility 2%- During this temp the Iron lattice expands which is what you’d expect

Question 8

What is Austenite?

Answer 8

• 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

Question 9

What is Ferrite?

Answer 9

• Very dilute solid solution of Iron- Exists at low temp

Question 10

What is Cementite?

Answer 10

• Fe3C - Exists at low temp

Question 11

What is Pearlite?

Answer 11

• Eutectoid mixture of Ferrite and Cementite

Question 12

What is a solid solution?

Answer 12

• Two metals that form a common latice structure- Are soluble in one another- Two types of solid solution

Question 13

What are the two types of Substitional solid solution?

Answer 13

-Random ( both types of atoms in lattice structure arranged in random fashion)- Ordered ( Can predict type of atom based on its location)

Question 14

What are grains of solid solution?

Answer 14

• Several lattices together- Can be random or ordered

Question 15

What is an Interstitial solid solution?

Answer 15

• Atoms of different sizes (prerequisite)- Solute atoms occupy the interstitial (holes between atoms) in crystal lattice of the solute- E.g. Fe and C

Question 16

Findings of a Fe-C phases diagram?

Answer 16

• When Iron cools rapidly the grain structure is Austenite- Ferrite and Cementite exist if you slow the molten alloy down extremely slowly (not interested)

Question 17

What is quenching?

Answer 17

• Rapid cooling of metal to adjust the mechanical properties of its original state

Question 18

What does quenching of steel give us?

Answer 18

• Should give us Austenite according to phase diagram- But in practice gives us Martensite

Question 19

What is Martensite?

Answer 19

• 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

Question 20

What is the composition of stainless steel?

Answer 20

• Iron 72%- Carbon <2%- Chromium 17-19%- Nickel 7-9%

Question 21

When is steel regarded as stainless steel?

Answer 21

• If there is at least 12% chromium

Question 22

What is the role of Chromium in stainless steel?

Answer 22

• 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

Question 23

What is the role of Nickel in stainless steel?

Answer 23

• Lowers Austenite to Martensite transition temp- Improves fracture strength- Improves corrosion resistance

Question 24

What two types of stainless steel are there?

Answer 24

• Martensitic- Austenitic

Question 25

What is Martensitic stainless steel?

Answer 25

• 12-13% Chromium and little carbon- Can be hardened by heat during tempering process- Useful for dental instruments

Question 26

How can Austenitic stainless steel be made during the process of quenching instead of Martensite?

Answer 26

• 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

Question 27

What are some Austenitic stainless steel uses?

Answer 27

• 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)

Question 28

What does swaged mean?

Answer 28

• Stainless steel sheet positioned between die and counter die- When these are pressed together a sheet of alloy is swaged

Question 29

What are the constituents of 18-8 stainless steel wire?

Answer 29

• Iron 74%- Chromium 18%- Nickel 8%- Carbon 0.1%

Question 30

What are the properties of 18-8 stainless steel wire?

Answer 30

• Does not heat harden- Soft (malleable) when cast- But work hardens rapidly

Question 31

What is cold working?

Answer 31

• 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

Question 32

What are some uses of 18-8 stainless steel wires?

Answer 32

• Orthodontic appliances like springs and clasps- Partial dentures like clasp arms and wrought rests

Question 33

What are the grades of stainless steel wire?

Answer 33

Soft - Half hardHard - Spring temper- Grade you select depends on how much bending, shape and manipulation required for dental appliance

Question 34

What is springiness/ Springback ability?

Answer 34

• Ability of material to undergo large deflections (to form an arc) without permanent deformation (returns to original shape)- Calculated as ratio EL/YM

Question 35

How can stainless steel wires be soldered?

Answer 35

• 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)

Question 36

What is Weld decay?

Answer 36

• Occurs between 500-900C- Chromium carbides precipitate at grain boundaries particularly where soldering occurred - Alloy becomes brittle and more prone to fracture

Question 37

How is weld decay minimised?

Answer 37

1. 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

Question 38

Why do stainless steel wires need a Stress relief annealing process?

Answer 38

• 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

Question 39

What are the properties of stainless steel denture base?

Answer 39

• Thin 0.11mm- Light- Fracture resistant- Corrosion resistant- High polish obtainable- High thermal conductivity- High impact strength- High abrasion resistance

Question 40

What are some disadvantages of stainless steel denture base?

Answer 40

• 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