WELDING Q 5
INTRODUCTION • Welding is the joining of parts by FUSING them together • Fusion involves HEAT and PRESSURE between the parts being joined • Some techniques require pressure • Other techniques just require very high temperatures and no pressure
IDEAL CONDITIONS FOR WELDING • SMOOTH joint surfaces that match each other • CLEAN joint surfaces, free from oxides, grease and dirt • Metals to be joined must have the same MICROSTRUCTURE (example: steel to steel) • Metals must be of good quality no internal impurities
PREPARING JOINT EDGES • Large plates may have to be bevelled. • Cleaning is very important, sometimes it is cleaned chemically or mechanically. • OXIDATION – is when metals are heated to high temperatures their surfaces are more easily affected by the oxygen in the atmosphere • To prevent oxidation surfaces must be shielded from the atmosphere during the welding operation
MANY WELDING TECHNIQUES YOU CAN TELL DIFFERENCE BETWEEN THEM BY: • The way the metal is heated • The way additional filler metal is fed into weld IMPORTANT TYPES OF WELDING • Gas Welding • Electric Arc Welding • Electric Resistance Welding
GAS WELDING • Gas welding involves the mixture of a fuel gas and oxygen • Fuel gas usually ACTEYLENE • ACETYLENE mixed with OXYGEN is - OXYACETYLENE WELDING
Oxy-Acetylene Welding and Cutting • EQUIPMENT • Oxygen • Acetylene • Regulators • Welding Hoses – oxygen BLUE, acetylene RED • Torch handle
OXY-ACETYLENE WELDING • Oxy-acetylene welding is a common gas welding method • It is a fusion weldingprocess • Gas is fed from two separate cylinders • The acetylene and oxygen mix with each other in torch where it will escape and be ignited and burn as a very hot flame - 3500ºC. • Intense heat is capable of melting the metal
Heat from the flame is capable of melting the joint edges until the metal melts and starts to flow • When molten metal from both edges meet, it fuses. • As the metal cools it becomes solid and the two parts will be permanently joined • Filler metal in rod form can be fed by hand into weld pool • During welding it is dipped constantly at regular intervals into weld pool
TYPES OF OXY-ACETYLENE FLAMES • NEUTRAL FLAME – has equal proportions of oxygen and acetylene. • Ratio 1:1 • Used for welding, steel, stainless steel and cast iron
TYPES OF OXY-ACETYLENE FLAMES Cont. • OXIDISING FLAME – this flame contains excess oxygen, • Ratio Oxygen to Acetylene = 1.5:1 • Working temperature of up to 3,500ºc • It is used to weld copper and brass but not steel as it would oxidise and contaminate the joint
TYPES OF OXY-ACETYLENE FLAMES Cont. • CARBURISING FLAME – contains excess acetylene • Ratio Oxygen to Acetylene = 0.9:1 • Working temperature of approximately 3150ºc • It is used to weld aluminium, special alloy steel and gives excellent protection against Oxidisation
3 Welding Flames CARBURISING OXIDISING NEUTRAL
SAFETY FACTORS OXY-WELDING • Oxygen should never be used instead of compressed air • Special care when using pressurised flammable gases • Use goggles when welding • Ventilation
Safety features integrated into oxyacetylene equipment include: • Gas cylinders are colour-coded with acetylene having a maroon cylinder and oxygen having a black cylinder • Regulators cannot be interchanged with colour coding • Acetylene has left handed thread fittings • Flashback arrestors are placed on gas lines • Hoses are colour-coded • Regulators are fitted to cylinders with an over-ride valve • Adequate ventilation systems
Colour Coding In Oxyacetylene Welding Equipment • Oxygen cylinder is black and acetylene is maroon, hoses and regulators are colour coded with oxygen as blue and acetylene as red.
Safety precautions in preparation of equipment and materials for oxyacetylene welding • Oil and grease needs to be removed to minimise reaction with welding process • Torch must be thoroughly cleaned before starting weld Flashback arrestors need to be in place on fuel and oxygen line • Correct colour coding of cylinders and hoses must be observed
DISSOLVED ACETYLENE • Acetylene is highly explosive when compressed, for this reason acetylene cylinders are packed with a porous material, which is filled with acetone • Acetone can absorb up to 25 time its own volume of acetylene for each atmosphere of pressure it receives
Eletric Arc Welding 4 Types • Manual metal arc (MMA) welding. • Metal inert gas (MIG) welding. • Tungsten inert gas (TIG) welding. • Submerged arc welding (SAW) All work on the same principal of electric arc.
Shielded Metal Arc Welding Stick Welding • Definition: • Consumable electrode coated with chemicals that provide flux and shielding • The filler metal (here the consumable electrode) is usually very close in composition to the metal being welded.
Manual Metal Arc Wedling Principle of Operation • Electricity is passed through an electrode which jumps between the electrode and the work piece. • This causes an arc which produces great heat melting the consumable electrode and the work piece causing the edges to fuse together. • The weld pool is protected from oxidation by the gasses produced by melting the chemicals on the electrode coating. This wire electrode also acts as a filler
material to fill the gap between the two parts being joined. A slag is formed which protects the weld area from oxidation and minimises cracking of the weld as it allows the joint cool slowly. • MMA welding is made more effective with the use of an adjustable transformer to allow for different thicknesses of steel to be welded. • MMA welding has many operational uses such as repair work on construction steel
FUNTIONS OF TRANSFORMER • A TRANSFOREMR IS USED TO STEP DOWN THE MAINS VOLTAGE TO A SUITABLE LEVEL FOR ARC WELDING
Transformer • A step-down transformer is used to change the mains voltage from 220V to a suitable level (80-100V) for welding. • This will provide the high current needed for welding. • This type of transformer has more turns on the primary coil than the secondary coil and will induce alternating current (AC) at a lower voltage.
FUNCTION BRIDGE RECTIFIER • A bridge rectifier permits current to flow in one direction only • For MMA WELDING it converts AC to DC. • This is used to supply Direct Current when Arc Welding • The Voltage is rectified by using the rectifier which consists of 4 Diodes- (A Diode allows current to flow in one direction only)
Rectifier • The rectifier changes alternating current (AC) to direct current (DC). It • consists of four diodes which allows two of the diodes to conduct on each • half-cycle of the AC supply
Single Phase Bridge Converter • During one half –cycle of the AC supply, diodes D1 and D3 are conducting. • During the next half cycle, diodes D2 and D4 are conducting. • The Diodes are arranged to allow current to flow across the arc in the same direction regardless of the polarity of the AC supply
Capacitor • The capacitor is employed to provide a smooth supply of low voltage DC.
Tungsten Inert Gas Welding (TIG) • Definition: • TIG welding is an arc that is formed between a non-consumable tungsten electrode and the metal being welded. • Gas is fed through the torch to shield the electrode and molten weld pool. • Benefits: • Welds with or without filler metal • Precise control of welding variables (heat) • Low distortion • Shielding Gases: • Argon
Tungsten Inert Gas Welding (TIG) Cont. • Applications • Most commonly used for aluminum and stainless steel • For steel • Slower and more costly than consumable welding • Except for thin sections or where very high quality is needed
Why use Tungsten Inert Gas When Welding Aluminium? • Aluminium oxidises very quickly when heated. This tenacious oxide layer is overcome with the use of an inert gas, such as argon, and the cathodic action of the arc on the work-piece. Aluminium can be welded successfully in this way by TIG welding.