Stainless Steel & Wrought Alloys Flashcards
What are wrought alloys and where are they used?
manipulated/ shaped by cold working eg drawn into wire
Use : wires (orthodontic) partial denture clasps
What is the composition of steel?
2 main
iron >98%
carbon <2%
What are the other constituents of steel?
Chromium (0.5 - 1%) - improve tarnish resistance
Manganese - sulphur scavenger Molybdenum, Silicon, Nickel, Cobalt
What are the uses for steel?
cutting instruments
forceps
What is allotropic?
undergoes two solid state phase changes with temperature
What are the two solid state phases that iron has and at what temperatures?
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.
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%).
What are the different phases of Fe-C?
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
What is an alloy?
TWO metals that form a COMMON LATTICE structure
- are SOLUBLE in one another
- form a SOLID SOLUTION
What are the types of substitional solid solution?
random
ordered
What is the interstitial solid phase of Fe C?
Why is it important?
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.
What does quenching of auestenite form?
martensite
(not supersaturated austenite solution?
What is quenching?
rapid cooling process
Increases hardness and strength by “locking in” high-energy, unstable structures.
Reduces ductility, making the metal more brittle.
What is the structure of martensite?
- No time for diffusion of Carbon
- Distorted Lattice
- Hard, Brittle
What does slow cooling of austenite form?
pearlite
(ferrite, cementite)
How can martensite be formed into pearlite?
tempering
What is tempering?
- 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
What is the composition of stainless steel?
Fe, C, Cr, Ni
What consitutes is steel is stainless?
if Cr is >12%
What is does chromium do the alloy?
lowers Austenite to Martensite temperature
lowers Austenite to Martensite rate
decreases % carbon at which Eutectoid formed
corrosion resistance due to chromium oxide layer
What can penetrate the protective oxide layer?
can be attacked by chlorides
What is the role of the nickel in stainless steel?
lowers Austenite to Martensite transition temperature
improves fracture strength
improves corrosion resistance
What are the two types of stainless steel?
martensitic
austenitic
What is martensitic stainless steel composition and what is it used for?
12 - 13% chromium + little carbon
dental instruments
What does austenitic stainless steel contain to prevent martensite transition?
contain sufficient Chromium & Nickel
to suppress Austenite to Martensite transition
What are the uses for austenitic stainless steel?
- Dental equipment and instruments -
to be sterilised - Wires
- Sheet forms for denture bases
What are the consituents of stainless steel (18-8)?
18% Chromium
8% Nickel
0.1% Carbon
74% Iron
What are the qualities of 18-8 steel?
does NOT heat harden
soft (malleable) when cast
BUT
work hardens rapidly
What is cold working?
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
aka WORK or STRAIN HARDENING
How are wrought alloys manipulated/shaped?
cold working
What are the uses of 18-8 steel?
orthodontic appliances - springs & clasps
partial dentures - clasp arms, wrought rests
What are the grades of 18-8 steel?
What does it depend on?
depends on bending or deformation the material needs to endure in its intended application.
soft
half hard
hard
spring temper
What are the alloys used for wires?
stainless steel (austenitic)
CoCr
gold (similar to type iv)
Ni-Ti
beta-Ti
What is springiness (EL/YM)?
Ability of a material to undergo large deflections (to form arc) without permanent deformation (ie it returns to its original shape)
What are the ideal properties for wires?
- 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
What is used to solder stainless steel wires?
gold
silver (with MP <700c)
What must be done after soldering?
- avoid recrystallisation
- quench rapidly to maintain UTS
What is weld decay and what temperatures does it occur at?
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
What is the result of weld decay?
- alloy becomes brittle
- less chromium in central region of solid solution
- more susceptible to corrosion
How can weld decay be minimised?
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.
What is stress relief anneal?
heat treatment process designed to reduce internal stresses in metals that can occur during manufacturing processes like welding, drawing, or forming.
How is stress relief annealing done in stainless steel wires?
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.
What happens if annealing temperature exceeds 650c?
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.
What happens if annealing temperature exceeds 500c?
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.
What is swaging?
involves deforming a metal by applying compressive forces, usually through dies
used for denture bases
What are properties/disadvantages of stainless steel denture bases?
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
What are the disadvantages of stainless steel as a denture base?
- 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 > wrinking of steel
