Welding
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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.

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Welding

WELDING

Q 5


Introduction

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

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

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

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

  • Gas welding involves the mixture of a fuel gas and oxygen

  • Fuel gas usually ACTEYLENE

  • ACETYLENE mixed with OXYGEN is - OXYACETYLENE WELDING


Welding

  • Oxy-Acetylene Welding and Cutting

    • EQUIPMENT

      • Oxygen

      • Acetylene

      • Regulators

      • Welding Hoses – oxygen BLUE, acetylene RED

      • Torch handle


Oxy acetylene welding

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


Welding

  • 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


Oxy acetylene equipment

Oxy-acetylene Equipment.


Weld run on a flat sheet

Weld Run on a flat sheet.


Weld run on a flat sheet1

Weld Run on a flat sheet.


Types of flames

Types of flames.


Types of oxy acetylene flames

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

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


Welding

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

3 Welding Flames

CARBURISING OXIDISING NEUTRAL


Safety factors oxy welding

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

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

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

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

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


Acetylene in tank

Acetylene in Tank


Eletric arc welding

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.


Basic welding circuit

Basic Welding Circuit.


Manual metal arc mma welding

Manual Metal Arc (MMA) Welding


Shielded metal arc welding stick welding

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

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


Welding

  • 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

FUNTIONS OF TRANSFORMER

  • A TRANSFOREMR IS USED TO STEP DOWN THE MAINS VOLTAGE TO A SUITABLE LEVEL FOR ARC WELDING


Transformer

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.


Transformer with air core

Transformer With Air Core


Transformer with iron core

Transformer With Iron Core


Function bridge rectifier

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

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


Transformer rectifier circuit

Transformer-Rectifier Circuit


Single phase bridge converter

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

Capacitor

  • The capacitor is employed to provide a smooth supply of low voltage DC.


Tungsten inert gas welding tig

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

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

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.


Gas metal arc welding mig

Gas Metal Arc Welding (MIG)

  • Definition:

    • The heat source is formed by creating an electric arc between the work piece and a wire, which is fed continuously into the weld pool.


Gas metal arc welding mig cont

Gas Metal Arc Welding (MIG) Cont.

  • Benefits:

    • Long welds can be made without starts and stops

    • Minimal skill required

    • Minimal cleaning of surface before weld

    • Allows welding in all positions


Gas metal arc welding mig1

Gas Metal Arc Welding (MIG)

  • Shielding Gases:

    • Inert

      • Argon, Helium

        • Used for aluminum alloys and stainless steels.

    • Active

      • 1 to 5% Oxygen, 3 to 25% CO2

        • Used for low and medium carbon steels

  • Applications

    • Gas Metal Arc Welding (MIG) is used to weld all commercially important metals, including steel, aluminum, copper, and stainless steel.


  • Mig versus tig welding

    MIG versus TIG WELDING

    • MIG (MAGS) welding is a very versatile process and can be used to weld

    • light sheet metal as well as heavy plate. It can also be automated for welding by robots on car and other assembly line products.


    Mig versus tig welding cont

    MIG versus TIG WELDING CONT.

    • TIG (TAGS) welding is a more specialised welding process, that features a

    • non-consumable electrode and demands the feeding of a filler metal, it has

    • made the welding of aluminium and stainless steel feasible.


    Why tig weld aluminium

    Why TIG Weld 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.


    Submerged arc welding

    Submerged Arc Welding.

    • Bare wire electrode is used.

    • Flux is in a powder form which is poured over and completely covered the weld pool.

    • Used in long, uninterrupted weld runs.

    • E.G. large steel reinforcing beams. (I beams)


    Submerged arc welding1

    Submerged Arc Welding


    Section of weld run

    Section of Weld Run


    Submerged arc welding2

    Submerged Arc Welding

    • In submerged arc welding, a bare wire electrode is used.

    • It is fed automatically from a spool and generates an electric arc to heat the metal.

    • The flux, in powder form, is fed from a hopper to completely cover the joint and the tip of the electrode.

    • The arc creates the heat to melt the joint, flux and electrode. A slag is formed to provide a protective coating for the weld. process.


    Submerged arc welding3

    Submerged Arc Welding

    • A slag is formed to provide a protective coating for the weld.

    • The excess flux powder can be collected and used again.

    • Submerged arc welding is a fully automated process.


    Submerged arc welding applications

    Submerged arc Welding Applications

    • used for large scale straight line welds such as steel reinforcing beams, shipbuilding and bridge construction


    Resistance welding spot welding

    RESISTANCE WELDING SPOT WELDING

    • PRESSURE, HEAT AND ELECTRICAL CURRENT

    • Components to be joined placed between 2 copper electrodes and the resistance of this current causes local rapid heating at interface, resulting in a nugget type spot weld


    Welding

    APPLICATIONSUse to weld overlapped sheet metal, commonly used in the motor industry or where small scale spot welds are needed ie orthodontics- re-sizing molar brands

    ADVANTAGES

    Energy efficient, limited work deformation, high production, easy automation, no filler metal requirements.


    Resistance welding seam welding

    RESISTANCE WELDING SEAM WELDING

    • Copper disc electrodes are used to provide a continuous run of overlapped spot welds as the current is activated at set intervals. E.G. Sewing machine

    • A time interval controlled so that these spots


    Functions of electrode coating

    FUNCTIONS OF ELECTRODE COATING

    • To generate a shield of carbon dioxide gas to protect weld joints from contamination by oxygen and nitrogen in the air

    • Forms a slag coating which shields the weld from oxidation and slows the cooling rate therefore preventing cracks and brittleness

    • Facilitates the striking of the arc between the work and the electrode


    Functions of slag

    FUNCTIONS OF SLAG

    • To form a coating which protects the weld from oxidation

    • It ensures a slow cooling rate for the weld

    • It prevents cracking and brittleness

    • It minimises impurities in the weld.


    Preventing atmospheric contamination of weld area

    Preventing atmospheric contamination of weld area:

    In manual metal arc welding (MMA), the flux electrode coating melts and gives off a gas to protect the weld from the surrounding air, protective slag coating forms on top of the weld.In oxy-acetylene welding the flame protects the weld area.

    MIG and TIG welding use a gaseous shield to protect the weld area.SAW covers the weld area with a powdered flux.


    Multi run welds

    Multi – Run Welds

    • A series of welds are run across the metals to be joined in multi-run welding. A superior weld is produced as each weld has a post heating effect on the previous run. The finished weld is stronger and more refined in structure than single run welds.


    Installing a safe school welding room

    Installing a safe school Welding Room

    • A ventilation system must be incorporated to efficiently remove the gases produced by welding.

      • The welding station should be shielded to protect others in the workshop from the intense light produced by welding.

    • Personal safety equipment (masks, apron, gloves, etc.) must be available.

      • The hazards of the area must be highlighted by signs.


    Hazards and remedies when welding

    HAZARDS AND REMEDIES WHEN WELDING

    • HAZARD = ELECTRIC SHOCK

    • REMEDY – Ensure conditions are DRY,

      and use well maintained machinery, where cables are properly secured, insulated and earthed.


    Welding

    • HAZARD = UV LIGHT RADIATION

    • REMEDIES – use correct facial equipment/visors with dark UV resistant face plates

      Use a welding curtain to protect fellow colleagues/classmates from exposure to UV light


    Welding

    • HAZARD – INHALATION OF FUMES AND GASES

    • REMEDY – Proper ventilation system


    Welding

    • HAZARD = EXPOSURE TO EXTREME HEAT AND TEMPS

    • REMEDY – Wear protective clothing, leather gloves, aprons, etc


    Brazing

    Brazing

    • Definition:

      • A process which a filler metal is placed at or between the faying surfaces, the temperature is raised high enough to melt the filler metal but not the base metal.

        • The molten metal fills the spaces by capillary attraction.

    • Torch Brazing

      • Oxy-fuel torch with a carburizing flame

      • First heat the joint then add the filler metal


    Brazing examples

    Brazing Examples


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