Stainless Steel and Wrought Alloys

Question 1

wrought alloys

Answer 1

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

Question 2

wrought alloys uses

Answer 2

• wires (orthodontic)

- partial denture clasps

Question 3

composition of steel

Answer 3

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

Question 4

chromium role in SSteel

Answer 4

improve tarnish resistance

Question 5

maganese role in SSteel

Answer 5

sulphur scavenger

Question 6

uses of SSteel

Answer 6

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

Question 7

iron

Answer 7

key component in steel

Question 8

iron is

Answer 8

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.

Question 9

austenite

Answer 9

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

exists at high temp (ie >720 C)

Question 10

ferrite

Answer 10

very dilute solid solution; e

exists at low temp

Question 11

cemenite

Answer 11

Fe3C ;

exists at low temp

Question 12

pearlite

Answer 12

Eutectoid mixture of Ferrite and Cementite

Question 13

alloy is

Answer 13

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

form a SOLID SOLUTION

Question 14

substitutional solid solution types (2)

Answer 14

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.

Question 15

interstitial solid solution

Answer 15

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

Question 16

cooling of Fe-C rapidly

Answer 16

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.

Question 17

quenching of austentite ->

Answer 17

MARTENSITE

NOT supersaturated austenite solution

Question 18

martensite

Answer 18

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

Question 19

tempering of steel

Answer 19

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

Question 20

martensite uses

Answer 20

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.

Question 21

4 constituents of StSteel

Answer 21

Fe
C
Cr
Ni

Question 22

chromium in StSteel

Answer 22

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

Question 23

key role of chromium in StSteel dental appliances

Answer 23

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

Question 24

nickel in StSteel

Answer 24

lowers Austenite to Martensite transition temperature

improves fracture strength

improves corrosion resistance

Question 25

martensitic StSteel

Answer 25

12 - 13% chromium + little carbon heat hardenable (tempering process) dental instruments (not relevant to this lecture)

Question 26

austenitic StSteel

Answer 26

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)

Question 27

3 dental uses of austenitic

Answer 27

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)

Question 28

stainless steel in wires

Answer 28

18-8 Stainless Steel 18% Chromium 8% Nickel 0.1% Carbon 74% Iron

Question 29

18:8 stainless steel in wires properties

Answer 29

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

Question 30

cold working

Answer 30

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

Question 31

wrought alloys

Answer 31

manipulated/ shaped by cold working e.g. drawn into wire Use : - wires (orthodontic) - partial denture clasps

Question 32

18:8 stainless steel wires uses

Answer 32

orthodontic appliances - springs & clasps partial dentures - clasp arms, wrought rests

Question 33

18:8 stainless steel wires grades

Answer 33

depends on degree of bending required (manipulation etc) - Soft Half Hard - Hard Spring Temper

Question 34

5 alloys - wires

Answer 34

Stainless Steel Cobalt-Chromium Gold Nickel-Titanium (similar to Type IV) beta-Titanium

Question 35

stainless steel (austenitic) components

Answer 35

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

Question 36

cobalt chromium (not as partial denture) components

Answer 36

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

Question 37

gold (similar to Type IV) components

Answer 37

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

Question 38

Ni-Ti components

Answer 38

Ni 55% Ti 45% + some Cobalt

Question 39

beta- titanium components

Answer 39

Ti, some molybdenum

Question 40

springiness (EL/YM)

Answer 40

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

Question 41

wires - requirements (5)

Answer 41

``` 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

Question 42

alloy properties stainless steel

Answer 42

``` stiffness - high spring back ability - good ductility - ok ease of joining - reasonable ```

Question 43

alloy properties gold

Answer 43

``` stiffness - medium spring back ability - ok ductility - ok ease of joining - easy, solder ```

Question 44

alloy properties Ni Ti

Answer 44

``` stiffness - low spring back ability - excellent ductility - good ease of joining - difficult ```

Question 45

alloy properties Beta - Ti

Answer 45

``` stiffness - medium spring back ability - good ductility - ok ease of joining - weld ```

Question 46

alloy properties CoCr

Answer 46

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

Question 47

comparisons of alloys

Answer 47

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.

Question 48

S/Steel soldering

Answer 48

``` 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.

Question 49

weld decay

Answer 49

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

Question 50

minimise weld decay by (2)

Answer 50

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

Question 51

stress relieved anneal - stainless steel wires

Answer 51

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

Question 52

swaging of S/STeel denture base

Answer 52

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.

Question 53

advantages of swaged SSteel denture base (8)

Answer 53

Thin 0.11mm - acrylic 1.52mm Light Fracture resistant Corrosion resistant High polish obtainable High thermal conductivity High impact strength High abrasion resistance

Question 54

disadvantages of Swages S/Steel (6)

Answer 54

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