Stainless Steel & Wrought Alloys

Question 1

What are wrought alloys and where are they used?

Answer 1

manipulated/ shaped by cold working eg drawn into wire
Use : wires (orthodontic) partial denture clasps

Question 2

What is the composition of steel?

2 main

Answer 2

iron >98%
carbon <2%

Question 3

What are the other constituents of steel?

Answer 3

Chromium: tarnish resistance

Manganese: removes sulphur

Nickel, Molybdenum, Silicon: mechanical & corrosion properties

Question 4

What are the uses for steel?

Answer 4

cutting instruments
forceps

Question 5

What is allotropic?

Answer 5

undergoes two solid state phase changes with temperature

Question 6

What are the two solid state phases that iron has and at what temperatures?

what are they called

Answer 6

Above 1400°C (Body-Centered Cubic - BCC structure):
Iron has a BCC lattice structure, which is less accommodating for carbon atoms. Hence, iron has low carbon solubility at this temperature, holding only about 0.05% carbon.

Between 900°C and 1400°C (Face-Centered Cubic - FCC structure):
Iron transitions to an FCC lattice structure, which has higher carbon solubility due to more space within the lattice. This structure can hold up to 2% carbon. (austenite)

Below 900°C (BCC structure):
Iron reverts back to a BCC structure similar to the one above 1400°C, again with low carbon solubility (0.05%). (ferrite)

Question 7

What are the different phases of Fe-C?

Answer 7

Austenite
FCC, high temp (>720°C), moderate carbon solubility

Ferrite
BCC, low temp, very low carbon solubility, soft

Cementite (Fe₃C)
Iron carbide, low temp, hard and brittle

Pearlite
Eutectoid mix of ferrite and cementite, forms <727°C, balanced strength

Question 8

What is an alloy?

Answer 8

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

Question 9

What are the types of substitional solid solution?

Answer 9

random
ordered

Question 10

What is the interstitial solid phase of Fe C?

Why is it important?

Answer 10

Carbon atoms occupy interstitial spaces within the iron lattice.

Iron Phase: Occurs in the austenite (FCC) phase, allowing carbon to dissolve in iron.

Effect: Increases hardness and strength of iron-based alloys (like steel) by distorting the lattice.

Question 11

What does quenching of auestenite form?

Answer 11

martensite

(not supersaturated austenite solution?

Question 12

What is quenching?

Answer 12

rapid cooling process
Increases hardness and strength by “locking in” high-energy, unstable structures.
Reduces ductility, making the metal more brittle.

Question 13

What is the structure of martensite?

Answer 13

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

Question 14

What does slow cooling of austenite form?

Answer 14

pearlite
(ferrite, cementite)

Question 15

How can martensite be formed into pearlite?

Answer 15

tempering

Question 16

What is tempering?

Answer 16

• heating (450) followed by quenching
• temperature and duration affect conversion to ferrite (soft, ductile) and cementite (hard, brittle)
• control over mechanical properties through heat treatment
• versatile alloy

Question 17

What is the composition of stainless steel?

Answer 17

Fe, C, Cr, Ni,

Question 18

What consitutes steel as stainless?

Answer 18

if Cr is >12%

Question 19

What is does chromium do the alloy?

Answer 19

lowers Austenite to Martensite temperature

lowers Austenite to Martensite rate

decreases % carbon at which Eutectoid formed

corrosion resistance due to chromium oxide layer

Question 20

What can penetrate the protective oxide layer?

Answer 20

can be attacked by chlorides

Question 21

What is the role of the nickel in stainless steel?

Answer 21

lowers Austenite to Martensite transition temperature

improves fracture strength

improves corrosion resistance

Question 22

What are the two types of stainless steel?

Answer 22

martensitic

austenitic

Question 23

What is martensitic stainless steel composition and what is it used for?

Answer 23

12 - 13% chromium + little carbon

dental instruments

Question 24

What does austenitic stainless steel contain to prevent martensite transition?

Answer 24

contain sufficient Chromium & Nickel
to suppress Austenite to Martensite transition

Question 25

What are the uses for austenitic stainless steel?

Answer 25

1. Dental equipment and instruments - to be sterilised 2. Wires 3. Sheet forms for denture bases

Question 26

What are the consituents of stainless steel (18-8)?

Answer 26

18% Chromium 8% Nickel 0.3% Carbon 74% Iron

Question 27

What are the qualities of 18-8 steel?

Answer 27

does NOT heat harden soft (malleable) when cast BUT work hardens rapidly

Question 28

What is heat hardening?

Answer 28

Making a metal harder and stronger by heating then rapidly cooling (quenching) — it changes its internal structure. | marsentite

Question 29

What is cold working? | aka work or strain hardening

Answer 29

 work done on metal/alloy at LOW TEMPERATURE - below recrystallisation temperature: (eg bending, rolling, swaging)  causes SLIP-dislocations collect at grain boundaries  hence stronger, harder material

Question 30

How are wrought alloys manipulated/shaped? | what are their uses

Answer 30

cold working wires (ortho), partial denture clasps

Question 31

What are the uses of 18-8 steel?

Answer 31

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

Question 32

What are the grades of 18-8 steel? | What does it depend on?

Answer 32

depends on bending or deformation the material needs to endure in its intended application. soft half hard hard spring temper

Question 33

What are the alloys used for wires?

Answer 33

stainless steel (austenitic) CoCr gold (similar to type iv) Ni-Ti beta-Ti

Question 34

What is springiness (EL/YM)?

Answer 34

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

Question 35

What are the ideal properties for wires?

Answer 35

* high springiness ( EL / YM) (ie 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 36

What is used to solder stainless steel wires? | Use gold/silver solder to prevent grain growth

Answer 36

gold silver (with MP <700c)

Question 37

What must be done after soldering?

Answer 37

- avoid recrystallisation - quench rapidly to maintain UTS

Question 38

What is weld decay and what temperatures does it occur at?

Answer 38

If the welded stainless steel is held at elevated temperatures for too long (for instance, during the welding process), it increases the likelihood of chromium carbide formation, particularly in the range of 450°C to 850°C. | chromium carbides precipitate at grade boundaires

Question 39

What is the result of weld decay?

Answer 39

* alloy becomes brittle * less chromium in central region of solid solution * more susceptible to corrosion

Question 40

How can weld decay be minimised?

Answer 40

**Low Carbon Content Steels:** Use low carbon steels to reduce the risk of chromium carbide formation, but this option can be more expensive. **Stabilized Stainless Steel:** Employ stabilized stainless steels with small quantities of titanium or niobium, which preferentially form carbides without depleting chromium at grain boundaries, thus preserving corrosion resistance.

Question 41

What is stress relief anneal?

Answer 41

heat treatment process designed to reduce internal stresses in metals that can occur during manufacturing processes like welding, drawing, or forming. gentle heating to “relax” the metal without ruining its properties.

Question 42

How is stress relief annealing done in stainless steel wires?

Answer 42

it is possibe but it needs control especially with temperatures around 450°C for a duration of 1-2 minutes. the short time at temperature helps to relieve stresses without significantly affecting the microstructure of the stainless steel.

Question 43

What happens if annealing temperature exceeds 650c?

Answer 43

grain structure of the stainless steel can be adversely affected. At these temperatures, grains may grow larger, which can lead to a reduction in mechanical properties such as strength and toughness.

Question 44

What happens if annealing temperature exceeds 500c?

Answer 44

risk of carbides precipitating out of the solution, especially if the steel has a higher carbon content. This can lead to a reduction in corrosion resistance and mechanical properties due to chromium depletion at the grain boundaries.

Question 45

What is swaging?

Answer 45

involves deforming a metal by applying compressive forces, usually through dies used for denture bases

Question 46

What are properties/disadvantages of stainless steel denture bases?

Answer 46

STAINLESS STEEL DENTURE BASE PROPERTIES/ADVANTAGES 1. Thin 0.11mm compared to acrylic 1.52mm 2. Lightweight 3. Fracture resistant 4. Corrosion resistant 5. High polish obtainable 6. High thermal conductivity 7. High impact strength 8. High abrasion resistance

Question 47

What are the disadvantages of stainless steel as a denture base?

Answer 47

1. Possible dimensional inaccuracy (contraction of die not matched by model expansion) 2. Elastic recovery of steel – inaccuracy 3. Damage of die under hydraulic pressure 4. Loss of fine detail during the many stages 5. Difficult to ensure uniform thickness 6. Uneven pressure on die and counter die > wrinking of steel