Converts high voltage AC to a lower voltage (220V to 80V-100V). This means the output current is greater than the input current. This is due to the primary coil having more windings than the secondary coil.
Smoothing Capacitor
Produces a more uniform direct current by smoothing the surges produced by the rectifier.
Rectifier
Allows current to flow in one direction only.
Converts AC to DC.
Consists of four diodes arranged so to allow current to flow in one direction only.
Circuit for Manual Metal Arc Welding
2. Safety
Ensure equipment is well maintained.
Wear protective clothing.
Weld curtain/cubicle.
Ventilation
3. Hazards
Electric shock: Ensure conditions are dry, equipment is earthed and and insulated.
UV Light radiation: Suitable weld shield/goggles. Use of welding curtain.
Inhalation of fumes: Ventilation system.
4. Function of Electrode Coating
Generates Carbon Dioxide shield to protect from oxidation by nitrogen and oxygen.
Forms a slag.
Facilitates striking of Arc.
5. Functions of Slag
Fluxing action prevents contamination/oxidation of the weld by atmospheric gases.
Allows the weld to cool slower, producing a better weld.
Prevents cracks and brittleness in the weld.
Multi Run Welds
Produces a finished weld that is more refined in structure than single run welds.
Each run of welds has a post heating effect on the previous.
Oxidation is prevented by the envelope of products of combustion.
1. Acetylene
Acetylene would explode if directly compressed. Instead cylinders are filled with porous acetone which can absorb 25times the amount of acetylene. This is known as dissolved acetylene.
2. Safety
Colour Coding
The gas cylinders are colour coded for safety as follows:
Oxy-Acetylene welding safety colour coding
Flashback Arrestors
Flashback arrestors are fitted to the regulators to prevent the feedback of flame through the hose.
Gauges
There are 2 gauges on each cylinder.
The low pressure gauge shows supply in the torch.
The high pressure gauge shows pressure in cylinder.
The piece is heated to a cherry red colour and then cooled rapidly in water oils and brine.
At the cherry red colour the steel will have undergone structural changes (FCC to BCC, martensite) and the rapid cooling does not allow it to revert to its original condition.
Result
Very hard and Brittle.
Tempering
Process
It is done by heating the article to a suitable temperature (500°C) below the reddening point of steel and then cooling it in oil or water.
The higher the temperature the greater the reduction in hardness and brittleness.
The temperature is gauged by the colour of the oxide film exhibited on the surface of the piece.
Result
Allows the carbon to diffuse out of BCC Structure.
Removes some of the hardness but increases toughness in martensite
Annealing
Process
Steel is annealed by heating it to a cherry red and allowed to soak to achieve uniform heating. It is then cooled very slowly. This can be carried out in the furnace by stitching it off when the steel has reached the correct temperature.
Result
Annealing is done to soften metal and relieve internal stress in work hardened pieces.
Refines grain size.
Normalising
Process
Normalising is carried out to refine the structure of steel and remove inner hardness caused by hammering, bending and rolling.
The steel is heated to a cherry red colour, approximately 50°C above its upper critical point and allowed to cool naturally in still air.
Result
Produces harder steel than annealing.
Improves Ductility and Toughness.
Removes internal stresses.
Refines abnormal grain structures.
Case Hardening
Process
Mild steel does not contain a sufficient amount of carbon to be hardened in the same way as High Carbon Steel is.
It is possible to increase the carbon content at the skin which can then be hardened.
It is a two stage process: (1) Carburising (2) Heat Treatment.
Result
Resists wear and breakage.
Pack Carburising
Process
The piece is placed in a box of powdered carbon.
The box is placed in a furnace and heated above its upper critical temperature.
The longer it is left, the higher the carbon content of the steel.
Applied to steels containing less than .2% carbon.
Result
Increases carbon content of mild steel at the surface.
Spherodising
Process
Piece is heated to 700°C and soaked.
Iron Carbide forms as spheroids.
Re-crystalisation
Where new crystals begin to grow from the distorted or dislocated nuclei formed during cold working.
The component is heated and new crystals grow until they have completely replaced the original distorted structure.
Temperature Measurement
Optical Pyrometer
Compares the intensity of light from the filament of a lamp.
The current flow is adjusted using a variable resistor to match the light of the furnace.
When the filament disappears a temperature reading can be taken.
Thermo-electric Pyrometer
Two dissimilar metals are joined together with a galvanometer placed at the cold junction.
A rise in temperature at the hot junction creates a current.
Monomer: A monomer is a molecule of a compound which reacts with other monomers to form a polymer.
Mer: A mer is a repetitive unit in a polymer.
Addition Polymerisation of Ethylene (Thermoplastics)
Ethylene consists of 4 hydrogen atoms and 2 carbon atoms joined together by a double bond. (Primary and secondary bond)
A catalyst is added which introduces a free radical. A free radical is a reactive atom containing an unpaired electron.
The free radical joins onto one of the monomers causing the weak secondary bond between the carbon atoms to break.
The mer cannot exist on its own as it now has an unpaired electron.
It very rapidly links with other mers to form long chain like structures called polymers.
To stop this reaction an inhibitor is added. This results in a large number of interwoven chains.
Where these chains touch/overlap, secondary bonds called Van Der Waals forces are formed. These are weak bonds that will allow the polymer to soften whereby its shape can be altered by applying heat or pressure.
Co-Polymer: Two unlike monomers joined together in a polymer chain. E.g. Poly Vinyl Acetate
Elastomer: A group of polymers consisting of linear chains that are coiled, entangled and are subject to cross linking. This allows these materials to be very elastic at room temperature.
Lubricants: Make the polymer easier to mould. Various types of waxes are used in small amounts.
Pigments: Give the Polymer colour
Stabilisers: Stabilisers are substances which stop a polymer ageing. They improve resistance to heat and light.
Vulcanisation: Natural rubber is processed with sulphur to form cross links between chain molecules to improve wear resistance and life. It is less flexible that natural rubber which is soft.
Van Der Waals Forces: Weak secondary bonds. May be disrupted by heat.
Primary bonds held together by strong cross links.
Process: Condensation Polymerisation
Forms strong primary bonds between chains.
Two monomers react to form a new molecule with water or alcohol emitted as a bi-product.
The polymer cannot be resoftened.
E.g. Phenol Formaldehyde
Properties
High melting points.
High tensile strength.
Can withstand high temperatures without losing their rigidity.
Primary bonds between molecule chains.
Compression Moulding
Split formed mould.
Polymer can be in powder or slug form.
Combination of heat and pressure (coalescence) allows piece to be formed.
Triggers chemical reaction cross linking and the object is removed.
High quality finish
E.g. Electrical fittings, Bottle tops.
Transfer Moulding
The moulding powder is placed in a compartment above the mould where it is heated.
The plunger forces the molten polymer into a cooled cavity.
The polymer solidifies in the mould which is then opened and the product is removed.
Used to make complex products.
E.g. socket covers.
Compression moulding (above) & Transfer moulding (below)
Thermoplastics
Bonding
Bonded by covalent bonds.
Secondary bonds with weak Van Der Waals forces which can be broken down by heat.
Process: Addition Polymerisation
Monomers join up to form long chain like molecules called polymers.
These are arranged like spaghetti and when each polymer overlaps, weak temporary bonds called Van Der Waals forces are formed.
E.g. Polyethylene
Properties
Low melting points.
Easy to mould
Can be remoulded and are subject to disruption by heat.
Can be recycled .
Low tensile strength.
Secondary bonds between molecule chains.
Extrusion
The thermoplastic moulding powder is fed from a hopper into a heated chamber.
A large archimedian screw moves the softening plastic through the chamber.
This plastic is forced through a die at the end of the machine. The die gives the desired extruded shape which is then cooled by air or water and cut into lengths.
It may also be cooled in a vacuum chamber.
E.g. Piping.
Injection Moulding
The thermoplastic in granule form is fed into a heated compartment by a hopper.
A plunger forces the plastic along the machine barrel where they are melted by heaters.
A torpedo compacts the materials.
The softened materials are then forced into the mould by the torpedo where it cools and solidifies.
The mould is opened and the plastic product is ejected.
E.g. Lego
Calendering
Continuous lengths of sheets are produced by calendaring.
The material passes through a series of heated rollers to produce the desired thickness of the material.
Strength: The ability of a material to withstand forces of compression, tension, shear and torsion.
Tensile Strength: The maximum pulling stress a material can withstand without fracture.
Torsional Strength: The ability to resist twisting forces.
Hardness: The ability to resist abrasion, wear, indentation and scratching.
Ductility: Ability of a material to be stretched into a wire (e.g. copper).
Malleability: The ability to be flattened or shaped without rupture.
Toughness: Ability to withstand blows or impact.
Elasticity: Ability to return to original shape when freed from force distorting it.
Conductivity: Ability of a material to allow heat or electricity to flow though it.
General Definitions, Terms & Formulae
Young’s Modulus:A measure of stiffness in a material
Creep: Creep is the slow deformation of a metal over time, resulting from a constant load/force acting on the metal. Factors that contribute to creep include time, temperature and nature of the force. E.g. Lead is prone to creep.
Ultimate Tensile Strength (Max load)
Proof Stress
Fatigue: Fatigue is failure due to cyclic stressing (on/off loading). Failure begins as a crack and grows/spreads until failure due to stress. Factors that contribute to fatigue are sharp corners, time of exposure, nature of force and corrosion.
Factor of Safety: Is the degree of structural capacity beyond applied loads.
Non Destructive Testing (NDT)
X-ray Testing
Radiation is passed through the material by an X-ray tube.
If no faults are present, absorption is uniform on the negative photographic film.
If a fault is present in the weld, less radiation is absorbed, thus producing a variation in the intensity of the emergent beam.
This is detected on the negative photographic film.
Where the defect exists, a dark spot forms.
X-Ray Testing
Eddy Current Testing
A coil is charged with high frequency alternating current is placed close to a material.
This produces Eddy Currents on the material.
A magnetic field is created in the piece by the current.
A defect will alter this magnetic field which will then be located by a search coil.
This records and displays the faults.
Ultrasound/Ultrasonic testing
Based on reflection of signals.
High frequency vibrations are sent through the piece.
These are reflected back once they reach the opposite surface of piece.
If a flaw is present, the vibrations will be reflected back from it(the flaw). The amount of time for which it takes the vibration to return will be shorter than if no flaw were present.
Penetrant Dye test
A dye is applied to the piece being tested and allowed to soak in for a sufficient period of time.
If surface cracks are present, these will absorb the dye.
The piece is then wiped clean.
Chalk dust is then placed on the piece. This will absorb the dye in the crack and illustrate where the flaws are present.
Advantages of Non-Destructive Testing
More economical as piece can be used if no flaw is present, especially if piece to be tested is expensive to manufacture.
