stainless steel and wrought alloys

Question 1

name two uses for stainless steel in dentistry

Answer 1

in orthodontic appliances
as a denture base material

Question 2

define a ‘wrought alloy’

Answer 2

an alloy which can be manipulated/shaped by cold working

Question 3

what are the components of steel?

Answer 3

98% iron
2% carbon
0.5-1% chromium
also contains manganese, molybdenum, silicon, nickel, cobalt

Question 4

what would steel have to be classed as if it contained any more than 2% carbon?

Answer 4

cast iron or PIG iron

Question 5

what is the role of chromium in steel?

Answer 5

improves tarnish resistance

Question 6

what is the role of manganese in steel?

Answer 6

sulphur scavenger

Question 7

name a use for steel in dentistry?

Answer 7

instruments e.g. forceps or cutting instruments

Question 8

explain how iron is allotropic

Answer 8

in a solid state, it can exist in two crystalline forms
<900C/>1400C= body centred cubic crystalline structure
>900C/<1400C= forms a face centred cubic lattice structure

Question 9

what causes the step change in the Fe-C phase diagram?

Answer 9

a drop in the lattice volume as it reconfigures to an FCC form from a BCC form

Question 10

what happens to the iron lattice during the step change in the Fe-C phase diagram?

Answer 10

it expands

Question 11

describe the structure of austenite

Answer 11

an interstitial solid solution
lattice has iron in rows and columns with some C atoms interspersed

Question 12

at what temperature does austenite exist?

Answer 12

> 720C

Question 13

what is pearlite a mixture of?

Answer 13

an eutectoid mixture of ferrite and cementite

Question 14

name 4 types of solid solution

Answer 14

1. random substitutional solid solution
2. ordered solid solution
3. grains of solid solution
4. interstitial solid solution

Question 15

describe a random substitutional solid solution

Answer 15

both types of atoms un the lattice structure are arranged in random fashion

Question 16

describe a grains of solid solution

Answer 16

lattices/grains made up of two types of atom
the grains are random substitutional solid solutions

Question 17

describe an interstitial solid solution

Answer 17

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

Question 18

when cooling steel rapidly, which grain structure is locked in?

Answer 18

austenite

Question 19

according to the Fe-C phase diagram, quenching steel should give us austenite but this is not what happens in practice- what is produced?

Answer 19

martensite

Question 20

why does martensite have a distorted lattice structure?

Answer 20

carbon is unable to diffuse normally within the array of iron atoms in each grain - forms a hard and brittle material

Question 21

describe the difference in mechanical properties of ferrite and cementite?

Answer 21

F- soft and ductile
C- hard and brittle

Question 22

what are the 4 components of stainless steel?

Answer 22

iron
carbon
chromium
nickel

Question 23

what are the roles of chromium in stainless steel?

Answer 23

• lowers the austenite to martensite temperature and the conversion rate
• decreases the percentage of carbon at which eutectoid is formed
• corrosion resistance

Question 24

what are the roles of nickel in stainless steel?

Answer 24

• lowers austenite to martensite transition temperature
• improves fracture strength
• improves corrosion resistance

Question 25

what are the two forms of stainless steel and their uses?

Answer 25

autenitic- ortho wire martensitic- dental instruments

Question 26

what are the advantages of using stainless steel as a denture base?

Answer 26

thin light fracture resistant corrosion resistant high polish obtainable- conducts heat readily high impact strength high abrasion resistance

Question 27

what are the disadvantages of using stainless steel as a denture base?

Answer 27

possible dimensional inaccuracy elastic recovery of steel - can lead to 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

Question 28

what is the purpose of stainless steel wire stress relief?

Answer 28

to ensure the configuration of the metal atoms in each of the alloy grains settle into an equilibrium

Question 29

what may happen if stainless steel exceeds 450C during stress relief?

Answer 29

grain structure may be changed or carbides may form at the grain boundaries

Question 30

what temperature does weld decay occur at?

Answer 30

500-900C

Question 31

what happens during weld decay?

Answer 31

chromium carbides precipitate at grain boundaries- alloy becomes brittle less chromium in central region of solid solution- more susceptible to corrosion

Question 32

what happens if cobalt chromium forms at the grain boundaries of stainless steel?

Answer 32

becomes brittle more likely to corrode

Question 33

how are the risks of weld decay minimised?

Answer 33

low carbon content steels stabilised s/steel with small amount of titanium/niobium forming carbides preferentially- limits prospects of cobalt chromium forming at grain boundaries

Question 34

what are the components of cobalt chromium wire?

Answer 34

40% cobalt 20% chromium 15% nickel 16% iron

Question 35

what are the components of gold wire?

Answer 35

60% gold 15% silver 15% copper

Question 36

what are the components of nickel-titanium wire?

Answer 36

55% nickel 45% titanium also some cobalt

Question 37

define 'spring back ability'

Answer 37

the ability of a material to undergo large deflections without permanent deformation

Question 38

how is springiness of a material calculated?

Answer 38

EL/YM

Question 39

what characteristics should the chosen wire (e.g. for ortho) have?

Answer 39

appropriate rigidity high level of ductility so it can be manipulated good strength and corrosion resistance

Question 40

at what ratio of chromium and nickel is the transition of austenite to martensite suppressed in austenitic s/steel ?

Answer 40

18:8 or 12:12%

Question 41

what % of martensitic stainless steel is chromium?

Answer 41

12%

Question 42

describe some of the features of 18:8 stainless steel

Answer 42

does not heat harden soft and malleable when cast work hardens quickly

Question 43

slip can sometimes occur during cold working. what does this mean?

Answer 43

defects/dislocations in a grain being moved along the grain boundaries grains cant jump to another grain meaning alloy has fewer defects in the lattice structure making up each grain alloy is now stronger and harder