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BDS3 Dental Materials Science > Stainless Steel > Flashcards

Stainless Steel Flashcards

1
Q

what category does stainless steel come under

A

it is a wrought alloy

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2
Q

what is a wrought alloy

A

can be manipulated / shaped by cold working i.e. it can be drawn into a wire

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3
Q

uses of stainless steel

A

orthodontic wires and partial denture clasps

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4
Q

components of steel

A

> 98% iron
<2% carbon
0.5-1% chromium (to increase tarnish resistance)

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5
Q

what is cast / pig iron

A

when the carbon content is above 2%

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6
Q

uses of stainless steel

A

cutting instruments >0.8% C
medical instruments e.g. forceps <0.8% C

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7
Q

iron is allotropic, what does this mean

A

in a solid state it can exist in 2 crystalline forms (2 phases) depending on the temperature

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8
Q

examples of iron acting as an allotrope

A

900 < temp < 1400C = face centred cubic lattice (FCC) with a higher carbon solubility (2%)
temp <900 or >1400C = body centred cubic lattice (BCC) with a low carbon solubility (0.05%)

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9
Q

what 4 things are found in a Fe-C diagram

A
  1. austenite
  2. ferrite
  3. cementite
  4. pearlite
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10
Q

austenite

A

interstitial solution, FCC; exists as high temp i.e. >720 C

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11
Q

ferrite

A

very dilute solid solution, exists at low temp

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12
Q

cementite

A

Fe3C exists at low temp

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13
Q

pearlite

A

eutectoid mixture of ferrite & cementite

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14
Q

what is an alloy

A

2 metals that form a common lattice structure, are soluble in one another and form a solid solution

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15
Q

what are 2 types of solid solution

A

substitutional & interstitial

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16
Q

2 types of substitutional solid solution

A

a) random - where both types of atoms in the lattice structure are arranged in a random fashion
b) ordered - can predict the type of atom based on its location

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17
Q

interstitial solid solution

A

2 atoms are markedly different in size e.g. iron occupies all the lattice sites and the carbon fits in the spaces in a random fashion

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18
Q

rapidly cooling FeC gives

A

grain structure locked in to make austenite

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19
Q

slowly cooling molten alloy FeC gives

A

ferrite & cementite but also large grains with poor mechanical properties which is not ideal

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20
Q

what happens when you quench FeC

A

should give austenite but in reality gives martensite

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21
Q

what is martensite

A

has a distorted lattice structure as C is unable to diffuse normally within the array of Fe atoms in each grain meaning it forms a hard & brittle material - don’t want

22
Q

what is tempering

A

heating followed by quenching which allows martensite to become a versatile alloy but not for dental application

23
Q

slow cooling austenite gives

A

pearlite (ferrite & cementite)

24
Q

fast cooling austenite gives

A

martensite (distorted lattice)

25
Q

tempering martensite gives

A

pearlite

26
Q

what components make up stainless steel

A

Fe
C
Cr (>12%)
Ni

27
Q

Cr function in stainless steel

A

only stainless if >12% Cr
lowers austenite to martensite temp and conversion
decreases % C at which eutectoid forms

28
Q

Ni function in stainless steel

A

to improve fracture strength UTS and corrosion resistance
lowers austenite to martensite temperatures

29
Q

what 2 forms does stainless steel come in

A
  1. martensitic SS
  2. austenitic SS
30
Q

martensitic SS

A
  • 12-13% Cr & little C
  • used to make dental instruments
  • tempered to produce very hard materials
31
Q

austenitic SS

A

contains sufficient Cr & Ni to suppress austenite to martensite transition e.g. 18% Cr + 8% Ni

32
Q

uses of austenitic SS

A
  • dental equipment & instruments - to be sterilised (not cutting edge) corrosion resistance important
  • wires e.g. orthodontics, readily cold worked, corrosion resistant
  • sheet forms e.g. for denture bases
33
Q

characteristics of 18-8 SS

A

doesn’t heat harden unlike martensitic version, it is soft & malleable when cast but work hardens rapidly so it cannot be repeatedly manipulated to form desired shape

34
Q

what does cold working only apply to

A

alloys

35
Q

what is cold working

A

work done on alloy at low temperature i.e. below recrystallisation temp. work done inc bending, rolling, swaging which causes slip (dislocations in grain being moved to grain boundaries where they stay). alloy then has fewer defects within lattice making up each grain so it is harder and stronger

36
Q

how are wrought alloys shaped

A

by cold working and drawn into wires; diameter of which depends on how this is done

37
Q

what is spring back ability

A

(EM/YM) ability of material to undergo large deflections (to form an arc) without permanent deformation i.e. returns to its original shape

38
Q

requirements for wires

A
  • high springiness
  • stiffness (for force for tooth movement)
  • high ductility
  • easily joined
  • corrosion resistance
39
Q

properties of SS

A

high stiffness
good springback ability
okay ductility
reasonable ease of joining

40
Q

properties of gold alloy

A

medium stiffness
okay springback ability
okay ductility
ease of joining good, solder

41
Q

properties of CoCr

A

high stiffness (heat treated)
okay springback ability
good ductility
ease of joining difficult

42
Q

properties of NiTi

A

low stiffness
great springback ability
poor ductility
ease of joining is difficult

43
Q

properties of beta Ti

A

medium stiffness
good springback ability
okay ductility
ease of joining - weld

44
Q

what is a risk when welding SS

A

weld decay

45
Q

what is weld decay

A

occurs between 500 - 900C. chromium carbides precipitate at grain boundaries forming CrC thats causes SS to become brittle so it is able to undergo only a small amount of deformation before fracturing thus limiting manipulation of the wire & makes it more likely to corrode

46
Q

how to minimise weld decay

A
  • low carbon content steels but this is expensive
  • stabilised SS which contains small amounts of titanium/niobium to form carbides preferentially so limiting prospects of CrC forming at grain boundaries
47
Q

why stress relief anneal SS

A

to ensure configuration of metal atoms (Ni, Cr, Fe) in each of the alloy grains to settle into an equilibrium

48
Q

process of stress relief annealing of SS

A

requires temp of SS to be held at 450C for 1-2mins. crucial that it doesn’t exceed this value as it will adversely affect properties either through a change in grain structure or due to metal carbides forming at grain boundary

49
Q

what is swaging

A

a metal forming technique where the metal of one part is deformed to fit around another part by hammering or forcing the material through a die. SS denture base requires swaging

50
Q

advantages of SS denture base

A

thin compared to acrylic
light
fracture resistant
corrosion resistant
high polish attainable
high impact strength
high abrasion resistance

51
Q

disadvantages of SS denture base

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 detail during many stages
  • difficult to ensure uniform thickness
  • uneven pressure on die & counter die - wrinkling of steel

BDS3 Dental Materials Science (9 decks)

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