Submerged Arc Welding

• Uses bare wire electrode.
• Fed automatically from spool.
• Generates electric arc to heat metal/
• Flux in powder form.
• Fed using a hopper.
• Electric arc melts joint, flux and electrode.
• Excess flux is collected and re-used.
• Fully automated process.
• Used on Low carbon steels.
• E.g. Ship building and bridge construction.

Resistance Spot welding

• Components to be joined are placed between two non consumable electrodes and pressed together.
• The current is passed through the electrodes which generates the required heat.
• A nugget weld (circular mark) is formed.
• It is used on sheet metals.
• The process is energy efficient and... (More)

Lower Critical Point

• Occurs at 723°c.
• Temperature change occurs.
• The metal contracts dimensionally, glows less brightly and loses its magnetism.
• Known as Decalescence.

Upper Critical Point

• Varies depending on amount of carbon in the steel.
• Above this point, the metal exists as FCC austenite.

Eutectic Point

• Liquid to solid
• Occurs at 1140°C at 4.3% carbon
• Liquid steel changes to solid austenite and cementite.

Eutectoid Point

• Solid austenite changes to solid pearlite.
• Occurs at 723°C at .83% Carbon.

Ferrite

• Almost Pure Iron.
• Above a temperature of 1390°C or below 910°C, iron has a BCC lattice.
• Also known as alpha iron.
• Contains... (More)

Hardening

Process

• 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... (More)

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... (More)