Stainless Steel & Wrought Alloys Flashcards
What is the definition of a ‘wrought’ alloy?
- It is an alloy which can be manipulated/shaped by cold working
- So it can be drawn into a wire
What are 2 uses of wrought alloys?
- Wires (orthodontic)
- Partial Denture Clasps
What elements is a steel alloy composed of? (2)
- Iron
- Carbon
What % of iron is in a steel alloy?
> 98%
What % of carbon is in a steel alloy?
<2%
IF there is more than 2% carbon this would mean the material is no longer classed as steel. What would it be classed as?
- Cast iron or pig iron
What are the other minor constituents of steel? (6)
- Chromium (0.5-1%)
- Manganese
- Molybdenum, Silicon, Nickel, Cobalt
What is the function of Chromium in Steel?
- Improves tarnish resistance
What is the function of Manganese in Steel?
- Sulphur Scavenger
What are 2 uses of steel?
- Cutting instruments
- Forceps
Iron is ‘Allotropic’. What does this mean?
- IT undergoes TWO solid state phase changes with temperature
- In other words, in a solid state, it can exist in TWO crystalline forms - two phases - depending on its temperature
What crystalline structure is Iron in at a temperature of >1400 or <900 degrees Celsius?
- Body centred cubic (BCC) crystalline structure
- Low carbon solubility (0.05%)
What crystalline structure is Iron in at a temperature of >900 and <1400 degrees Celsius?
- Face centred cubic (FCC) crystalline structure
- Carbon solubility higher (2%)
What is austenite in the Fe-C Phase diagram?
- Interstitial solid solution (lattice has iron in rows and columns with come C atoms interspersed) , FCC
- Exists at a high temp (i.e. >720 degrees celsius)
What is ferrite in the Fe-C Phase diagram?
- Very dilute solid solution
- Exists at low temps
What is cementite in the Fe-C Phase diagram?
- Fe3C
- Exists at low temps
What is pearlite in the Fe-C Phase diagram?
- Eutectoid mixture of Ferrite and Cementite
What forms a solid solution?
- TWO metals that form a common lattice structure and are soluble in one another
What is a random substitutional solid solution?
- Where both types of atoms in the lattice structure are arranged in a random fashion
What is an ordered substitutional solid solution?
- We can predict the type of atom based on its location in the lattice structure
What is an interstitial solid solution?
- The 2 different atoms are markedly different in size - that’s a prerequisite
- The larger atom will occupy all the lattice sites and the smaller atom fits in the spaces, in random fashion
From the Fe-C phase diagram it shows us that quenching should give us austenite however what does it actually give us?
- Actually gives us Martensite, which behaves quite differently
- (martensite is NOT a supersaturated austenite solution)
What are the features of Martensite? (3)
- Martensite has a DISTORTED LATTICE structure, as a result of carbon being UNABLE to diffuse normally within the array of iron atoms in each grain
- This means it forms a hard and brittle material - we don’t want this
What does fast cooling of austenite produce?
- Martensite which can be turned into Pearlite by tempering
What does slow cooling of austenite produce?
Pearlite
- Ferrite
- Cementite
Martensite is very useful in non-dental applications. It can be used to produce materials that are soft or hard, or somewhere in between. How is this achieved?
- Achieved by tempering
What is tempering?
- Altering its temperature, and the duration you maintain it at a specific temperature and then quenching it
- Heating to 450 degrees Celsius followed by quenching
When tempering martensite, the temperature and duration affect the conversion. What can it convert to? (2)
- Ferrite (soft, ductile)
- Cementite (hard, brittle)
- Control over mechanical propertied through heat treatment
- Versatile alloy
What are the 4 main components of stainless steel?
- Iron (Fe)
- Carbon (C)
- Chromium (Cr)
- Nickel (Ni)
Stainless steel is only considered stainless steel if it has at least what % of Chromium?
- At least 12%
What are the properties of Chromium in stainless steel? (3)
- Lowers austenite to martensite temperature
- Lowers austenite to martensite rate
- Decreases % carbon at which eutectoid formed
What is one of stainless steels key attributes?
- Corrosion resistance due to chromium oxide layer
- There’s a 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.
Stainless steel is resistant to corrosion but what can it be attacked by?
- Can be attacked by chlorides
What are the properties of Nickel in stainless steel? (3)
- Lowers austenite to martensite transition temperature
- Improves fracture strength
- Improved corrosion resistance
What are the 2 different types of stainless steel?
- Martensitic
- Austenitic
What is the martensitic composition of stainless steel?
- 12-13% Chromium + little carbon
What is the property of martensitic stainless steel?
- Heat hardenable (tempering process)
What is martensitic stainless steel used to make?
- Dental instruments (but this is not the type of stainless steel that interests us)
How can we suppress the transition of austenite to martensite?
- There is a way that transition – austenite to martensite - can be suppressed.
- By having the right proportions of Cr and Ni - specifically, either 18:8 or 12:12 ratio.
What are the 3 uses of austenitic stainless steel?
- Dental equipment and instruments
- Wires (ortho)
- Sheet forms for denture bases
Why is austenitic stainless steel good for making dental equipment and instruments?
- Its corrosion resistance ensures it withstands the autoclave process
- Corrosion resistance more important than strength and hardness
- (NOT cutting edge instruments)
Why is austenitic stainless steel used to produce orthodontic wires? (2)
- IT is readily cold worked so can bend into shape
- It is corrosion resistant
Stainless steel can be used to from denture bases. What needs to be done to it for this to happen?
- This requires it to be swaged (adapted to a die)
What is swaging?
- Swaging is a forging process in which the dimensions of an item are altered using dies into which the item is forced. Swaging is usually a cold working process, but also may be hot worked.
What are the components of 18-8 Stainless steel? (4)
18% Chromium
8% Nickel
0.1% Carbon
74% Iron
Does 18-8 stainless steel heat harden?
- NO
When cast, what is 18-8 stainless steel like?
- IT is soft (malleable) when cast
BUT
- IT work hardens rapidly - so it can’t be repeatedly manipulated to form the desired shape
What is another name for cold working?
- Work or strain hardening
What is work hardening?
- It’s work done on metal/alloy at LOW TEMPERATURE - below recrystallisation temperature. Work done could be:- bending, rolling, swaging.
What does work hardening cause?
- These processes cause SLIP – that is defects/dislocations in a grain being moved along to the grain boundaries; where they stay, they can’t jump to another grain
- As a consequence of this, the alloy has fewer defects within the lattice structure making up each grain – this makes the alloy a stronger, harder material.
What are wrought alloys?
- Ones that can be manipulated/shaped by cold working
- The goal is to draw them into the form of a wire. The diameter of which depends on how this is done
What are the uses of wrought alloys? (2)
- Wires (orthodontic)
- Partial denture clasps
What are the uses of 18-8 stainless steel wires? (2)
- Orthodontic appliances - springs & clasps
- Partial dentures - Clasp arms, wrought rests
What are the different grades of 18-8 stainless steel wires? (4)
- Soft
- Half hard
- Hard
- Spring temper
- The grade you select depends on how much bending, shape manipulation you require for your dental appliance
As well as stainless steel wires, what other materials can be used? (4)
- Gold
- Cobalt Chromium
- Nickel-titanium
- beta-titanium
What is the composition of cobalt chromium used for wires?
Co 40%
Cr 20%
Ni 15%
Fe 16%
What is the composition of Gold used for wires?
Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%
What is the composition of NiTi used for wires?
Ni 55%
Ti 45%
+ some cobalt
What is the composition of Beta-Ti used for wires?
Ti
Some molybdenum
What is Springiness (EL/YM)?
- Ability of a material to undergo large deflections (to form arc) without permanent deformation (i.e. it returns to its original shape)
What are the properties required for a wire? (5)
- High Springiness (i.e. undergo large deflections without permanent deformation)
- Stiffness (YM) - depends on required force of tooth movement
- High ductility - bending without fracture
- Easily joined without impairing properties - soldered, welded
- Corrosion resistant
Look at the properties table in the lecture!
:)
What can stainless steel be soldered using? (2)
- Gold solder
- Silver solder (melting point <700 degrees celsius)
Why must care be taken when stainless steel is soldered?
- Care has to be taken as the temperature rise created is close to the melting point of s/steel.
- So there is a risks the s/steel grains may recrystallize – which adversely affects its mechanical properties.
- Quenching the alloy would avoid this and maintain UTS.
Between what temperatures does weld decay occur and what can this cause?
- Occurs between 500-900 degrees celsius
- This can push the Cr and C atoms to grain boundaries - allowing CrC to ‘precipitate’ there
- If CrC forms at the grain boundaries it causes the S/steel to become brittle – so its able to undergo just a small amount of deformation before fracturing.
- This limits the amount of manipulation of the wire to match the desired configuration. That’s a severe restriction.
- It also makes s/steel more likely to corrode.
How can we minimise the disadvantages of weld decay? (2)
- Low carbon content steels - EXPENSIVE
- Stabilised stainless steel - contain small quantities of TITANIUM or NIOBIM
- Forms carbides preferentially
- Not at grain boundaries
Why do stainless steel wires need stress relief annealing?
- As with other alloys, which have undergone various processes – such as cold working – a stress relief annealing process is needed for s/steel wires.
- This is to ensure the configuration of the metal atoms - Ni, Cr and Fe - in each of the alloy grains settle into an equilibrium
- This process requires the temperature of s/steel needs to be held at around 450C for a minute or two. It’s crucial that the temperature of the s/steel DOES NOT exceed this value – otherwise it may be adversely affected :- either through a change in the grain structure; OR due to metal carbides forming at the grain boundaries. SO care is required at each stage.
- Grain structure affected above 650 degrees
- Precipitation of carbides above 500 degrees
Look at swaging diagram slide
:)
What are the advantages of using stainless steel as a denture base? (8)
- Thin 0.11mm - acrylic is 1.52mm
- Light
- Fracture resistant
- Corrosion resistant
- High polish obtainable
- Hight thermal conductivity
- High impact strength
- High abrasion resistance
What are the drawbacks of using stainless steel as a denture base? (6)
- 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
