Trials Study Flashcards
Precipitation Hardening
Mostly for Al alloys,
1. Heat and soak to make a homogenous structure
2. Quench to retain uniform structure
3. Heat to slightly elevated temp, disperse precipitate throughout structure
Heat Treatment Types (6)
- Annealing (Process and full)
- Precipitation hardening
- Normalising
- Tempering
- Spheroidising
- Austempering
Normalising
Heat to austenitic region (above UCT), then cooled in still air
- finer grain structure then annealing, thus stronger
Process Annealing
Less that 0.3%C, heated to 550-650 (below UCT). Cooled in still air
Releaves stress from cold working or deformation
Unstressed ferrite, stressed pearlite
Full annealing
Heat to above UCT, (hypo-eutectoid or eutectoid), then cool slowly in furnace. Large, equaxied grains of unstressed pearlite and ferrite.
Spheroidising
Medium to high carbon steels
Heat steel to around 650-700, soak (slightly below UCT). Slowly cool
- forms spheres of cementite (easier to machine then lamellar pearlite)
- improved ductility, malleability
Hardening Process
Heating transforms to austenite (BCC-FCC). Quenching does not give it enough time to react, and so: Cannot transform to BCC, remains trapped in between as BCT (MARTENSITE) (>0.3%C)
- hard yet brittle
Tempering
Used to make tempered martensite from acicular martensite. (DIAGRAM: needles to needles with smudges)
- reduced hardness slightly, increases toughness.
Heat below UCT. Harder then annealed or normalised steels.
Austempering
Forms Banite, small cementite particles in a ferrite matrix.
- similar properties to tempered marten., yet more resiliant
Austenitic steel quenched to 400, soaked till uniform temp, then quenched to room temp.
Banite
Forms when quenching slightly slower to form martensite, some cementite forms. Even slower = pearlite.
Particles of cementite in a ferrite matrix.
Testing Types
Hardness Tests (4)
- Brinell
- Vickers
- Rockwell
- Shore Scleroscope
- Brinell
Manufacturing Processes:
Wire Drawing
Drawn through successively smaller diameter dies
OR
Put in tension and stetched by running it around progessively faster winding drums
- usually process annealed during to relieve stressed from cold working
Surface hardening types
- Case Hardening (carburising)
- Nitriding
- Flame hardening
- Induction hardening
Surface Hardening Pros and Cons
Hardness of outside with toughness of inside
Makes further heat treating difficult.
Case Hardening
Heating and soaking in a carbon rich atmosphere: carbon diffuses into surface
CONS: further heat treatment is difficult
Nitriding
Specific steel alloy is heated in furnace with gaseous nitrogen (from decomposition of material e.g. ammonia): soaked for long time.
- reacts with Al, Cr, of V
Nitriding Pros and cons
Pros: corrosion resistance, high hardness in core, keeps hardness up to 500.
BUT
More costly setup, only specific alloys, if heated beyond 500 properties lost.
Flame Hardening
Flame applied to steel above 0.4%C in localised area then quenched. Martensite formed on surface.
- Mechanised with a single assembly (waterjet follows flame holder)
Induction Hardening
Same idea as flame, but uses electromagnetic induction.
- uses induction coil, then current is turned off and quenched.
3 ways to alter steels
- Carbon Content
- Heat treating
- Alloying
Ferritic manufacturing processes
- Forging
- Rolling (hot and cold)
- Casting
- Extrusion
5.Powder forming
Manufacturing types
- Forming
- Casting
- Molding
- Joining
- Machining
- Additive
Forming Process and types
Mostly metals, done by applying force to PLASTICALLY DEFORM
1. Forging
2. Extruding
3. Drawing
4. Rolling
Forging
Applying force to plastically deform.
Grain flow follows the shape of the metal and is not interrupted: improves strength
Hot vs Cold Rolling
Hot: heated above recrystallisation temperature: structure will be unstressed, equiaxed grains.
LESS dimensionally accurate, poor surface finish
More ductile and malleable
Cold: below recryst: elongated grains
harder and increased strength with work hardening. Requires more force: stronger, more expensive machinery required.
Rolling: description and pros/cons
Passed through successive rollers that produce a smaller thickness.
Pros: automated, better mechanical properties
CONS:
Simple shapes, high tooling cost
Casting
Pouring of molten metal into a die, then cooing
- primary shaping process: often needs further manufacturing for good finish
Casting types
- Die Casting (pressure + gravity) - low melting point metals
- Sand Casting
- Investment/lost wax casting
Pressure Die Casting
Molten metal is poured into a chamber and forced into a reusable die using a ram. This is then cooled and removed.
Pressure Die Casting Pros and cons
Pros: good for low melting point metals
Good surface finish
Denser casting then with gravity
Reusable die
Good for high production runs
Cons: needs excess equiptment for ferritic metals
Expensive set up
Gravity Die casting + pros and cons
Molten metal poured into die using gravity
Pros: cheaper then sand casting in long production runs
Good surface finish
Tight tolerances
CONS:
- high set up cost
Sand Casting
Sand with a binder
Temporary mold is made by separating drag in cope in two halves, then imprinting with pattern.
They are rejoined, molten metal is added.
Once cooled, it is removed and sand is reused
- pins are used to make runner and riser: runner allows excess material to flow in as it contracts.
Investment/lost Wax Casting Pros and Cons
High tolerances, good surface finish
Expensive (single-use mold) and time consuming
Sand Casting pros and cons
Pros: low cost
Good final grain structure
Can do large and complex parts
Cons:
single-use mold: expensive for large production runs
Poor surface finish and dimensional accuracy: further machining required.
Investment/lost Wax Casting
- Temporary mold made out of wax: “tree”
- Dipped in ceramic slurry
- Wax melted out, molten metal poured in
- Once cooled, ceramic is shaken off
- Individual parts removed from tree.
Powder Forming/Power metallurgy
Metal is ground made into powder form, then blended with lubricants and mixed with desired compositions.
Then pressed into a mold: pressure compoacts particles together.
- then sintered together in a furnace to make a homogenous grain structure.
Casting Pros and Cons
Can have uneven grain structure due to cooling: increase stresses.
Extrusion
Metal is forced using a ram through a die: constant cross section. HOT WORKING PROCESS
Direct and indirect extrusion
Extrusion pros and cons
good surface finish
tight tolerances
BUT
Only good for 2D shapes
Powder metallurgy products (4)
- Poros metals: use deliberately larger powder sizes. Good for self-lubricating items e.g. bike suspension, electrical motor bearings
- Complex articles
- products difficult to machine
- composites
Powder Metallurgy pros and cons
Pros: good to make things otherwise to hard to make.
Cons:
Limited shapes
Expensive set-up
Final product less strong as normally produced
Machining processes
Drilling, turning, reaming, et.c
- removing material via a tool (usually secondary process)
Metals, plastics and composites
Joining types
- Welding
- Riveting
- Bolting
- Soldering
- Fastening
Welding process + Pros and cons
Using extremely high heat to fuse metal parts together: melts base metal.
Pros:
Less preparation
Stronger and more rigit
Less maintenance then riveted joints
Less corrosion (riveting, etc subject to galvanic corrosion)
Less equipment then riveting
Cons:
Subject to warping under heat and metal fatigue
Stress corrosion can occur
Some metals need special treatment before welding
Needs specialisation
Stress cracking can occur at HAZ
Bolting Pros and Cons
Pros:
Easy to use if holes made during construction
Less equipment needed
Cons:
drilling accuracy needed
Holes weaken beams
Differential aeration corrosion
Can loosen under vibrations
Riveting Pros and cons
Pros:
Not subject to vibration: thus more used for planes and transport craft.
Cons:
Time consuming to install and remove then bolting
Limited loads
Weaker then welding
Hooke’s law
Stress is proportional to strain
Bronze
Copper and tin
aluminium
Copper
Brass
Copper and zinc
Glass manufacturing processes
- Lamination
- Toughened
- Annealed
Toughened glass
Glass is heated, outer surfaces cooled at a greater rate and contracts
- outside in compression: must overcome compressive force first before applying tensile load.
Thermoset plastics and types
Undergo a chemical change when heat is applied: non-reversable
1. Epoxy resins
2. Silicone
3. Polyeutherane
4. Polyester
Thermoplastics and types
Weak Van Der Waal’s forces
1. Polyethylene
2. Polystyrene
3. PTFE
4. Acrylic
5. Polypropylene
6. PVC
7. ABS
Rubber
Natural polymer, can be vulcanised by adding sulpher and heating to 150: adds cross links
- more rigid but still flexible
Polymer Manufacturing
- Molding (injection, rotational)
- Extruding
- Thermoforming
- Calendaring
Injection molding
Done on polymers
Similar to investment casting but small pellets added into a hopper, forced through a barrel with a screw which heats up an melts until it is forced into mold.
Composite manufacturing
Guided media + pros and cons
Guided: transmission media that carries signals with a conductor e.g. fibre optic cables: has a PHYSICAL LINK
- short distance, but expensive for long distance/weird terrain
3 Guided media types
- Twisted pair cable
- Coaxial
- Fibre optical
Geotextiles
Fabric that lets through liquids but blocks solids e.g. road surfaces, retaining walls
True Stress
Final stress at failure: use FINAL diameter (vs. engineering stress, use original diameter)
