Stainless Steel and Wrought Alloys

Question 1

What are wrought alloys?

Answer 1

An alloy that can be manipulated or shaped by cold working and drawn into wire

Question 2

What are common uses of wrought alloys?

Answer 2

Orthodontic wires
Partial denture clasps

Question 3

What is the composition of steel?

Answer 3

> 98% iron
<2% carbon

Question 4

What are the other constituents of steel?

Answer 4

Chromium - 0.5-1% - improves tarnish resistance
Manganese - sulfur scavenger
Molybdenum
Silicon
Nickel
Cobalt

Question 5

What are the dental uses of steel?

Answer 5

Cutting instruments - >0.8% C
Forceps - <0.8% C

Question 6

What does allotropic mean?

Answer 6

Undergoes two solid state phase changes with temperature so can exist in 1 of 2 crystalline structures
eg - iron

Question 7

Describe the structure of iron >1400C

Answer 7

Body centred cubic (BCC) lattice structure - low carbon solubility (0.05%)

Question 8

Describe the structure of iron between 900 and 1400C

Answer 8

Face centred cubic (FCC) lattice - higher carbon solubility (2%)

Question 9

Describe the structure of iron below 900C

Answer 9

BCC lattice structure - low carbon solubility (0.05%)

Question 10

What different solutions are seen on the Fe-C phase diagram?

Answer 10

Austenite
Ferrite
Cementite
Pearlite

Question 11

What is an alloy?

Answer 11

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

Question 12

What are the different types of solid solution?

Answer 12

Substitutional - random or ordered
Interstitial - atoms markedly different in size

Question 13

What grain structure is found on the Fe-C phase diagram on rapid cooling?

Answer 13

Austenite

Question 14

When are ferrite and cementite found on the Fe-C phase diagram?

Answer 14

If molten alloy is cooled slowly - gives bad mechanical properties

Question 15

What happens when steel is quenched?

Answer 15

It actually gives martensite, not austenite

Question 16

Describe martensite

Answer 16

Forms a distorted lattice as carbon is unable to diffuse normally within the iron atoms in each grain
Makes a hard, brittle material - we don’t want this

Question 17

What happens on cooling of austenite?

Answer 17

Slow cooling gives pearlite, ferrite or cementite
Fast cooling gives martensite

Question 18

What happens when martensite is tempered?

Answer 18

It gives pearlite, ferrite or cementite

Question 19

Describe tempering of martensite

Answer 19

Heating to 450C followed by quenching
Temperature and duration can cause conversion to ferrite (soft and ductile) or cementite (hard and brittle)
Control over heat treatment allows control over mechanical properties

Question 20

What are the main components of stainless steel?

Answer 20

Iron
Carbon
Chromium
Nickel

Question 21

Describe the role chromium plays in stainless steel

Answer 21

Must have at least 12% Cr to be stainless steel
Lowers austenite to martensite temperature
Lowers austenite to martensite rate
Causes corrosion resistance due to chromium oxide layer
Can be attacked by chlorides

Question 22

Describe the role Nickel plays in stainless steel

Answer 22

Lowers austenite to martensite transition temperature
Improves ultimate tensile strength
Improves corrosion resistance

Question 23

What are the two types of stainless steel?

Answer 23

Martensitic
Austenitic

Question 24

Describe martensitic stainless steel

Answer 24

12-13% chromium and little carbon
Heat hardenable (tempering process)
Can be tempered to produce dental hand instruments

Question 25

Describe austenitic stainless steel

Answer 25

Contains sufficient chromium and nickel to suppress the austenite to martensite transition eg - 18% Cr and 8% Ni or 12% Cr and 12% Ni

Question 26

What are the uses of austenitic stainless steel?

Answer 26

Dental equiptment and instruments - not cutting edge Orthodontic wires Sheet forms for denture bases

Question 27

What is the composition of 18-8 stainless steel?

Answer 27

18% chromium 8% nickel 0.1% carbon 74% iron

Question 28

Describe 18-8 stainless steel

Answer 28

Does not heat harden Soft and malleable when cast Work hardens rapidly - can’t be repeatedly manipulated to form desired shape

Question 29

What is cold working?

Answer 29

Work done on a metal/alloy at low temperature eg - bending, rolling swaging Causes slip - dislocations collect at grain boundaries Makes a stronger, harder material

Question 30

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

Answer 30

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

Question 31

What are the grades of 18-8 stainless steel wires and what do they depend on?

Answer 31

Soft Half hard Hard Spring temper Depends on the degree of bending required

Question 32

What are the different type of alloy wires?

Answer 32

Stainless steel Cobalt chromium Gold Nickel titanium β-titanium

Question 33

What is the composition of cobalt chrome wires?

Answer 33

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

Question 34

What is the composition of gold wires?

Answer 34

Au - 60% Ag - 15% CU - 15% Pt/Pd - 10%

Question 35

What is the composition of Ni-Ti wires?

Answer 35

Ni - 55% Tn - 45% Some cobalt

Question 36

What is the composition of β-Ti wires?

Answer 36

Titanium Some molybdenum

Question 37

What is springiness?

Answer 37

EL/YM The ability of a material to undergo large deflections (form an arc) without permanent deformation

Question 38

What are the requirements of wires?

Answer 38

High springiness Stiffness High ductility Easily joined Corrosion resistant

Question 39

What can be used for soldering stainless steel wires?

Answer 39

Gold solder Silver solder

Question 40

How are stainless steel wires soldered?

Answer 40

Temperature rise is close to stainless steel melting point Quenched rapidly to maintain UTS

Question 41

What is wild decay?

Answer 41

A risk of soldering Occurs between 500-900C Chromium carbides precipitate at grain boundaries Allow becomes brittle and more susceptible to corrosion

Question 42

How is wild decay minimised?

Answer 42

Low carbon content steels - expensive Stainless steel with small quantities of titanium or niobium

Question 43

Describe stress relief annealing in stainless steel

Answer 43

Temperature of stainless steel at 450C for 1-2 minutes Ensures configuration of metal atoms in grains settles into an equilibrium

Question 44

Describe swaging of stainless steel to make a denture base

Answer 44

A stainless steel sheet is placed between a die and counter-die Pressure is applied and the sheet takes the shape of the counter-die as a denture base

Question 45

What are the advantages of stainless steel denture bases?

Answer 45

Thin Light Fracture resistant Corrosion resistant High polish obtainable High thermal conductivity High impact strength High abrasion resistance

Question 46

What are the disadvantages of stainless steel denture bases?

Answer 46

Possible dimensional inaccuracy Elastic recovery of steel may make it innacurate Damage of die under hydraulic pressure during swaging Loss of fine detail during many stages Difficult to ensure uniform thickness Uneven pressure on die and counter-die during swaging causes wrinkling of steel