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BDS3 DMS > Stainless Steel and Wrought Alloys > Flashcards

Stainless Steel and Wrought Alloys Flashcards

1
Q

wrought alloys

A

manipulated/ shaped by cold working e.g. drawn into wire

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2
Q

wrought alloys uses

A
  • wires (orthodontic)

- partial denture clasps

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3
Q

composition of steel

A

Iron >98%
Carbon <2%
- Above 2% carbon–> cast iron, pig iron

Other constituents
- Chromium	 (0.5 - 1%)
improve tarnish resistance
- Manganese                
sulphur scavenger
- Molybdenum, Silicon, Nickel, Cobalt etc
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4
Q

chromium role in SSteel

A

improve tarnish resistance

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5
Q

maganese role in SSteel

A

sulphur scavenger

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6
Q

uses of SSteel

A 
Cutting Instruments (>0.8%  C) (medical instruments)
Forceps etc. (<0.8% C)
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7
Q

iron

A

key component in steel

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8
Q

iron is

A

Allotropic - undergoes TWO solid state phase changes with temperature.
- In a solid state is can exist as 2 crystalline forms/phases depending on temperature

  1. Temp. > 1400C
    BCC lattice structure; low Carbon solubility (0.05%)
  2. 900 < Temp. < 1400C
    FCC lattice; higher Carbon solubility (2%)
  3. Temp < 900C:
    BCC lattice structure; low Carbon solubility (0.05%)

drop in the lattice volume between 900 and 140C – as the lattice re-configures to an FCC form – from a BCC form.
- Note that between these two temperatures the IRON lattice will expand, which you’d expect.

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9
Q

austenite

A

interstitial solid solution, FCC;
lattice has iron inrows and columns

exists at high temp (ie >720 C)

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10
Q

ferrite

A

very dilute solid solution; e

exists at low temp

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11
Q

cemenite

A

Fe3C ;

exists at low temp

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12
Q

pearlite

A

Eutectoid mixture of Ferrite and Cementite

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13
Q

alloy is

A

TWO metals that form a COMMON LATTICE structure
- are SOLUBLE in one another

form a SOLID SOLUTION

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14
Q

substitutional solid solution types (2)

A

RANDOM
- where both types of atoms in the lattice structure – are arranged in random fashion

ORDERED
- Here we can predict the type of atom based on its location.

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15
Q

interstitial solid solution

A

two atoms are markedly different in size (requirement )

Here IRON occupies all the lattice sites – and the CARBON fits in the spaces, in random fashion

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16
Q

cooling of Fe-C rapidly

A

grain structure that is locked in is that of AUSTENITE.

So QUENCHING should give us Austenite – according to the phase diagram.
BUT in practice we get MARTENSITE, which behaves quite differently.

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17
Q

quenching of austentite ->

A

MARTENSITE

NOT supersaturated austenite solution

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18
Q

martensite

A
  • No time for diffusion of Carbon
  • Distorted Lattice
  • Hard, Brittle
    Undesired for dentistry
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19
Q

tempering of steel

A

heating (450 C) followed by quenching

temperature and duration affect conversion to:

  • ferrite (soft, ductile)
  • & cementite (hard, brittle)

control over mechanical properties through heat treatment

VERSATILE ALLOY

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20
Q

martensite uses

A

very useful – in non-dental applications.
- produce materials that are soft or hard – or somewhere in between.

achieved by TEMPERING the material.

  • Altering its temperature, and the duration you maintain it at a specific temperature and then quenching it
  • will determine the proportion of ferrite and cementite produced.
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21
Q

4 constituents of StSteel

A

Fe
C
Cr
Ni

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22
Q

chromium in StSteel

A

STAINLESS if > 12% Cr

  • lowers Austenite to Martensite temperature
  • lowers Austenite to Martensite rate
  • decreases % carbon at which Eutectoid formed

Corrosion resistance
- very relevant in dentistry.
risk of any metal in the oral environment experiencing corrosion – the presence of saliva, liquids with acidic pH levels – all ingredients designed to provoke an electrochemical reaction.

CRUCIALLY S/Steel forms a chromium oxide layer on its surface, which protects it from corrosion. It’s vital BUT can be attacked by chlorides

23
Q

key role of chromium in StSteel dental appliances

A

Corrosion resistance
- very relevant in dentistry.
risk of any metal in the oral environment experiencing corrosion – the presence of saliva, liquids with acidic pH levels – all ingredients designed to provoke an electrochemical reaction.

CRUCIALLY S/Steel forms a chromium oxide layer on its surface, which protects it from corrosion. It’s vital BUT can be attacked by chlorides

24
Q

nickel in StSteel

A

lowers Austenite to Martensite transition temperature

improves fracture strength

improves corrosion resistance

25
Q

martensitic StSteel

A

12 - 13% chromium + little carbon

heat hardenable (tempering process)

dental instruments (not relevant to this lecture)

26
Q

austenitic StSteel

A

Earlier in the IRON-CARBON phase diagram, on quenching the alloy, contrary to expectations MARTENSITE not AUSTENITE is produced, away that transition can be suppressed.
- the right proportions of Cr and Ni - specifically, either 18:8 or 12:12 ratio.

Uses

Dental equipment and - instruments -to be sterilised (NOT cutting edge)

  • corrosion resistance more important than strength and hardness
  • withstands autoclaving

wires e.g. orthodontics
- readily cold worked and corrosion resistant

sheet forms for denture bases
- swaged (adapted to a die)

27
Q

3 dental uses of austenitic

A

Dental equipment and - instruments -to be sterilised (NOT cutting edge)

  • corrosion resistance more important than strength and hardness
  • withstands autoclaving

wires e.g. orthodontics
- readily cold worked and corrosion resistant

sheet forms for denture bases
- swaged (adapted to a die)

28
Q

stainless steel in wires

A

18-8 Stainless Steel

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

29
Q

18:8 stainless steel in wires properties

A

does NOT heat harden unlike martensitic version
- soft (malleable) when cast
BUT work hardens rapidly

Cold Working
work done on metal /alloy at LOW TEMPERATURE
- below recrystallisation temperature:
e.g. bending, rolling, swaging
- causes SLIP - dislocations collect at grain boundaries
- hence stronger, harder material
aka WORK or STRAIN HARDENING

30
Q

cold working

A

work done on metal /alloy at LOW TEMPERATURE
- below recrystallisation temperature:
e.g. bending, rolling, swaging
- causes SLIP - dislocations collect at grain boundaries
- hence stronger, harder material
aka WORK or STRAIN HARDENING

31
Q

wrought alloys

A

manipulated/ shaped by cold working e.g. drawn into wire

Use :

  • wires (orthodontic)
  • partial denture clasps
32
Q

18:8 stainless steel wires uses

A

orthodontic appliances - springs & clasps

partial dentures - clasp arms, wrought rests

33
Q

18:8 stainless steel wires grades

A

depends on degree of bending required (manipulation etc)

  • Soft Half Hard
  • Hard Spring Temper
34
Q

5 alloys - wires

A

Stainless Steel

Cobalt-Chromium

Gold

Nickel-Titanium (similar to Type IV)

beta-Titanium

35
Q

stainless steel (austenitic) components

A

Cr 18%
Ni 8%
C 0.1%
Fe 74%

36
Q

cobalt chromium (not as partial denture) components

A

Co 40%
Cr 20%
Ni 15%
Fe 16%

37
Q

gold (similar to Type IV) components

A

Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%

38
Q

Ni-Ti components

A

Ni 55%
Ti 45%
+ some Cobalt

39
Q

beta- titanium components

A

Ti,

some molybdenum

40
Q

springiness (EL/YM)

A

Ability of a material to undergo large deflections (to form arc) without permanent deformation (i.e. it returns to its original shape)

Diagram (a) shows we have a straight wire, an alloy.
Diagram (b) shows force applied at each end of the wire to make it form an arc shape – the sort of shape that fits a patient’s dentition
c) And in c) note that when the force at each end is released – the wire rebounds, it springs back to being straight – there’s no deformation
SPRINGINESS is calculated as the ratio ( EL / YM).
We’ll see shortly what the springiness of various alloys are

41
Q

wires - requirements (5)

A 
high springiness ( EL / YM)
- i.e. undergo large deflections without permanent deformation

stiffness (YM)
- depends on required force for tooth movement

high ductility
- bending without fracture

easily joined without impairing properties
- soldered, welded

corrosion resistant

42
Q

alloy properties

stainless steel

A 
stiffness
- high
spring back ability
- good
ductility 
- ok
ease of joining 
- reasonable
43
Q

alloy properties

gold

A 
stiffness
- medium
spring back ability
- ok
ductility 
- ok
ease of joining 
- easy, solder
44
Q

alloy properties

Ni Ti

A 
stiffness
- low 
spring back ability
- excellent
ductility 
- good
ease of joining
- difficult
45
Q

alloy properties

Beta - Ti

A 
stiffness
- medium
spring back ability
- good 
ductility 
- ok
ease of joining 
- weld
46
Q

alloy properties

CoCr

A 
stiffness
- high (heat treated)
spring back ability
- OK
ductility 
- GOOD
ease of joining 
- difficult
47
Q

comparisons of alloys

A

S/Steel is satisfactory across the board; CoCr is pretty similar except for the ease of joining several wire components.
Gold is satisfactory too, though its rigidity is less – this makes it suited to scenarios where a more restrained rate of movement of the dentition is needed.
NiTi is excellent for its springiness, and is ideal for moving teeth slowly. The main challenge here is in joining NiTi wires together.

48
Q

S/Steel soldering

A 
By
- Gold solder
- Silver solder (Melting point < 700 0C)
avoid recrystallisation – adversely affect mechanical properties
quench rapidly to maintain UTS

Care has to be taken though as the temperature rise created is close to the melting point of s/steel.

49
Q

weld decay

A

Occurs between 500 - 900 C

Chromium carbides precipitate at grain boundaries

  • alloy becomes brittle - limits the amount of manipulation of the wire to match the desired configuration
  • more susceptible to corrosion

less chromium in central region of solid solution

50
Q

minimise weld decay by (2)

A
  1. Low carbon content steels - expensive
  2. Stabilised stainless steel
    - contain small quantities of TITANIUM or NIOBIUM
    - forms carbides preferentially
    - not at grain boundaries
51
Q

stress relieved anneal - stainless steel wires

A

Possible (need care)
- 450 C, 1 - 2 min – does not exceed – change in grain structure or metal carbides at grain boundaries

Grain structure affected above 650 C
Precipitation of carbides above 500 C
Hence different grades
Care needed

52
Q

swaging of S/STeel denture base

A

S/steel sheet positioned between a die and counter-die.

When these are pressed together the sheet of alloy is SWAGED – taking on board the shape of the denture base.

53
Q

advantages of swaged SSteel denture base (8)

A

Thin 0.11mm - acrylic 1.52mm

Light

Fracture resistant

Corrosion resistant

High polish obtainable

High thermal conductivity

High impact strength

High abrasion resistance

54
Q

disadvantages of Swages S/Steel (6)

A

Possible dimensional inaccuracy (contraction of die not matched by model expansion)

Elastic recovery of steel – inaccuracy

Damage of die under hydraulic pressure

Loss of fine detail during the many stages

Difficult to ensure uniform thickness

Uneven pressure on die and counter die
~~~> wrinkling of steel

BDS3 DMS (9 decks)

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