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Manual Metal Arc Welding

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Manual Metal Arc Welding

• Most Versatile Welding Process• Suitable for almost all types of metals and all positions• Its operation is comparatively easy• It is a fusion welding• The heat being provided by electric arc• The arc has an average temperature of around 6,000 degree C• M.M.A welding is carried out using either a.c. or d.c.• In case of d.c. current + ve or – ve polarity may be used• A high open circuit voltage (o.c.v.) required is 65-90 volts• Lower welding voltage required is 20-40 volts• Reasonable range of current must be available; 30-350 amps

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Effect of Amperage too highExcessive penetration, burn through, porosity, spatter, deep craters, undercut, electrode overheats, high deposition (positional welding difficult).

Effect of Amperage too lowPoor penetration or fusion, unstable arc, irregular bead shape, slag inclusion, porosity, electrode freezes to the weld, possible stray arc strikes.

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Effect of Voltage too lowPoor penetration,Electrode freezes to workPossible stray arcsFusion defectsSlag inclusionsUnstable arcIrregular bead shape.

Effect of Voltage too highPorosity SpatterIrregular beadSlag inclusionVery fluid weld poolpositional welding difficult.

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Travel speed too slow• Excessive deposition• Cold laps• Slag inclusions• Irregular bead shape.

Travel speed too Fast• Narrow thin bead• Slag inclusion• Fast cooling • Undercut• Poor fusion and Penetration

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An electrode connected to the d.c.+ve pole will have two thirds of the available energy.The remaining one third of the energy in the parent materialIt will result in:wide and shallow weld poolBroad HAZSlow Rate of CoolingHydrogen Intrapment

An electrode connected to the d.c. –ve pole has One third of the energy develops at the electrode and two thirds of the energy in the parent material. This will result in Weld pool which is narrow, deep and fast freezing limited h.a.z.May lead to hydrogen entrapment and a brittle metallurgical structure

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In A.C. The polarity is reversing 100 times per second (50 c.p.s.).Effect of equalizing the heat distributionHeat at the electrode and half in the parent material.

Types of Consumables used in MMARutile: Titanium Dioxide, Clay , Sodium silicateCellulosic: Cellulose( wood pulp), Titanium Dioxide, Sodium silicateBasic: Lime stone ( Calcium carbonate ), Titanium Dioxide, Sodium silicate

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MIG/MAG Welding Process

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• This process uses a bare wire consumable electrode

• The wire, typically 0.8 -1.6 mm diameter

• Is continuously fed from a coil through a specially designed welding gun.

• The possibility of atmospheric contamination is eliminated by introducing a shielding gas.

MIG/MAG Welding Process

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MIG Welding Process

• When an inert gas is used for shielding the welding, the process is know as metal inert-gas (MIG) welding

• Argon is an efficient shielding gas, being inert

• Argon does not chemically react with the weld metal

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• When an active gas is used for shielding the welding process is know as metal active-gas (MAG) welding

• CO2 commonly used as Shielding Gas in MAG Welding • Therefore the MAG process is widely referred to as CO2 welding

• CO2 will react with Iron at high temperatures, to produce Iron Oxide

• Shielding Gases changes the electric properties of the weld

• This will influence on the metal transfer rate,heat input,penetration and weld profile characteristics.

MAG Welding Process

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• Shielding Gases changes the electric properties of the weld

• This will influence on the metal transfer rate, heat input, penetration and weld profile characteristics.

Example Gasses and applications for MIG/MAG Welding are

Pure Argon Aluminum , copper , 9% Nickel steelArgon + 1% to 5% Oxygen Stain less steelCO2 Carbon steel 0.4% C, Low alloy steel Argon + 5% to 25% CO2 Carbon steel, Low alloy steel Argon + 5% Hydrogen Nickel and its alloysArgon + 15% Nitrogen Copper and its alloys 75% Helium + 25% Argon Aluminum and Copper alloys75% Helium + 25% Argon Austenitic Stainless steel

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Metal Transfer Modes in MIG/MAG Welding

• Spray or free flight transfer,• Dip transfer (semi-short circuiting arc),• Globular transfer, Pulsed transfer.

Spray or free flight transfer• The weld metal transfers across the arc in the form of a fine spray.• This gives high deposition rates and deep penetration welds.• It is well suited to thick materials and for flat or horizontal welding positions

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Dip transfer (semi-short circuiting arc)To achieve this low amperage and low arc volts are required.This produces relatively cool arc Used on thinner sections for all positional welding, including Vertical down welding

Globular transferThis occurs in the intermediate range between Spray and DipThis has mechanized and automated set up

Pulsed TransferPulses of high powered current are super imposed over a constant Semi short circuiting background modeThis permits hotter welding with high deposition rates

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For MIG/MAG welding, usually electrode d.c. +ve of a flatCharacteristic is used

Advantages• Minimal wastage of consumable electrode,• No frequent changing of consumable electrode,• Little or no interpas cleaning required (no slag produced)• Heavier weld beads are produced.• Faster welding process,• Low hydrogen content welds • Preheat may not be required.

Disadvantages•Increased risk or porosity – due to displacement of the gas shield, more maintenance of plant involved

• High risk of lack or fusion.

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TIG Welding

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The TIG welding process uses a non-consumable tungsten electrode to provide an arc.

Filler metal, when required, is fed from a separate filler rod.

A shielding gas, e.g. argon, is fed through the welding gun to the weld area and provides a gas shield to prevent contamination by the atmospheric gases.

No fluxes are used with the process.

When the arc only is used to produce the weld, without the addition of separately fed filler wire, the process is known as autogenous TIG

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TIG Welding is used for deposition of high quality root runs on pipe work.

When high quality root runs are needed, a back purge is used to prevent oxidizing (coking) of the weld metal.

In most cases electrode d.c. –ve polarity is used

d.c. +ve needs higher voltage and overheat and melt the tip

The scavenging action achieved with electrode d.c + ve

A compromise is met using AC current so that for 50% of each current cycle the electrode is positive.

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The gas shield fulfils two main function

It provides a suitable ionizable atmosphere for the electric arc.

It protects the weld pool from atmospheric contamination. Excessive gas pressure can cause rippling of the weld pool.

Three gases may be considered for TIG welding: argon, helium, and nitrogen.

Argon Produces a smooth, quiet arc with low arc volts.

The addition of between 1% and 5% of the active gas hydrogen, will raise the arc voltage and give deeper penetration.

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The inert gas helium is lighter than argon, therefore requires higher.

Helium creates a higher arc voltage which is useful for welding thick sections. Helium is also more expensive than argon.

Nitrogen is inert at room temperature.At arc temperature Nitrogen combines with oxygen and becomes Active therefore not suitable in most of the cases

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A high OCV (open circuit voltage) of around 90 volts is required for TIG welding to ensure arc stability at all times.

The power sources used for TIG are generator, transformer , transformer-rectifier.

High frequency current is superimposed at the start of all d.c.welding operations.

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Plasma arc welding is basically a modification of the TIG process

The PAW process has the ability to perform welds by the keyhole technique on closed square butts on 1.5-10 mm thick plates

Plasma Arc Welding

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The welding torch consists of a non-consumable tungsten electrode

This electrode is fixed in a nozzle

Through this nozzle plasma gas flowsThis nozzle lies within another nozzle through which the shielding gas flows.

Gas is fed into the inner nozzle under low pressure and passes through the electric arc where it becomes ionized.

This increases the gas pressure and thus the temperature which is in the range of 10,000 – 17,000 oC.

This superheated ionized gas is referred to as plasma.

Shielding and plasma gases used are pure argon, helium or argon helium mixtures.

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• Submerged arc welding uses a continuously fed bare wire consumable electrode, 1.6 to 6.4 mm diameter.• The weld pool is protected by separately supplied shielding flux in fused or agglomerated form.

Submerged Arc Welding

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• It is possible to feed more than one consumable wire electrode into the weld pool at the same time. Single,twin,triple wire feed system is commonly used

• Submerged arc welding is normally fully mechanized.• The arc and molten weld metal are completely submerged beneath the layer of shielding flux.

• The flux also provides additives to the weld, removes impurities from the weld and provides a thermal blanket.

• Rapid deposition of heavy weld beads is possible without spatter.

• Both a.c. and d.c. power sources are used with SAW

• It is possible to weld 37 mm thick weld in one pass, using 5000 Amp current

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Deep penetration will be achieved because of the high current used.

Submerged arc welding is widely used in ship building, structural steel work, general engineering applications, and for the fabrication of pipes and pipelines.

Carbon steel, alloy steel and stainless steels are the main materials welded using this process.

Only possible to weld in the flat or horizontal vertical position.

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Fused fluxes• The ingredients are mixed and melted at a high temperature• The mixture is then poured onto large chill blocks or directed into a stream of water to produce granules which have a hard glassy appearance. • The materials is then crushed, sieved for size, and packaged.

Advantages of fused fluxes• Good chemical mix achieved.• They do not attract moisture.• The easy removal of impurities and fine particles etc. when recycling

Fluxes for SAW are divided into two types:• Fused – granulated.• Agglomerated – powdered.

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Agglomerated fluxes

• All the flux materials are dry mixed and then bonded with either potassium or sodium silicate.• Backed at a temperature below the melting point.• The powder is sieved for size and packaged

• Easy addition of deoxidants and Ferro-alloys.

Dis - Advantages of Agglomerated fluxes• Absorb moisture• Difficult redrying and recycling• Difficult removal of impurities

Advantages of Agglomerated fluxes

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Electro Slag Welding

1- Plate to be welded2- Shoes3- Molten Slag4- Electrode5- Molten Metal6-Finished Weld7- Pipe for cooling

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• Used to Join Metals 10 – 50 mm thick

• Plates upto 300 mm can be welded at 600-1200 mm /hour speed

• Welding is possible only when the plates are in vertical position

• Welding once started must be carried out till finish becouse restart produce defective areas

• Welds are mostly defect free

Electro Slag Welding