Manual metal arc welding

MANUAL METAL ARC WELDING INTRODUCTION

The terms commonly used when referring to Manual Metal Arc welding are M.M.A, Arc welding or Stick welding.

Manual metal arc welding was first invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield.

The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK.

It is worth noting that coated electrodes were slow to be adopted because of their high cost.

However, it was inevitable that as the demand for sound welds grew, manual metal arc became synonymous with coated electrodes.

When an arc is struck between the metal rod (electrode) and the workpiece, both the rod and workpiece surface melt to form a weld pool.

The types of electrodes used will affect arc stability, depth of penetration, metal deposition rate and positional capability and are greatly influenced by the chemical composition of the flux coating on the electrode.

Electrodes can be divided into three main groups: Cellulosic, Rutile, Basic.

MANUAL METAL ARC WELDING INTRODUCTIONCellulosic electrodes contain a high proportion of cellulose in the coating and are characterized by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.

Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Basic electrodes contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

M.M.A welding up until recently was the widest used welding process, with most manufactured products being welded by M.M.A., this is no longer the case with M.I.G. /M.A.G. taking over from it.

UNDERSTANDING WELDING HAZARDS

Welding operators use highly technical welding processes that can create

dangerous situations and conditions in the workplace.

Both Oxy-Acetylene and Electric arc welding can pose Safety and Health

Hazards for Welders and workers near by.

Exposure to these Hazards can result in discomfort, temporary or permanent

physical injury or even death.

But welding can be safe, if you protect yourself from the dangers.

You owe it to yourself and your fellow workers to find out what the hazards are.

Evaluate there presence in the work environment and learn how to control them.

RADIANT ENERGY

The intense light rays emitted from an electric arc are Ultra violet and Infra red

(heat rays).

Excessive exposure to Ultra violet rays from welding flashes can cause

permanent damage to the eyes and in extreme cases blindness.

Excessive exposure to Infra red rays can cause burning of the skin.

Arc Flash also known as ‘Welders flash’ or ‘arc eye’ is one of the most common

and serious risk to a welder. It is a painful ocular condition sometimes

experienced by welders who have failed to use adequate eye protection.

The protection of the eyes is such an important matter that it merits individual

consideration.

RADIANT ENERGYPersonal Screening

For most operations a hand held screen made of lightweight, insulating, and non

reflecting material can be used.

It must have an approved ‘filter glass’. Goggles alone do not give adequate protection

for arc-welding and cutting operations.

Make sure that the screen is of a size and shape to shield the face, throat, wrist and

hand.

Where it is necessary to protect the head or to have both hands free a Helmet type

screen fitted with an approved filter should be used.

The filter glass plate provided in helmets and shields should be of reputablemanufacture conforming to the latest British/European Standards.

Cracked or ill fitting glasses should be promptly replaced.

The clear cover glass should always be maintained in front of the filter glass for theprotection thereof.

Frequent renewal of these cover glasses is necessary since they soon becomecovered with spatter which reduces visibility.

In all types of arc welding, ultra -violet and infra-red radiation from the arc is emitted.

It requires constant care to avoid arc flashes both when initiating the arc and alsofrom nearby operators.

Eye burns from the arc are not generally permanent injuries, although they areexceedingly painful.

Such burns are frequently referred to as ‘flashes’ and feel like hot sand in the eye.

For eye burns consult your first aid or Doctor.

N.B. DO NOT UNDER ANY CIRCUMSTANCES LOOK AT AN ELECTRIC ARC WITH UNPROTECTED EYES

RADIANT ENERGY

PROTECTIVE FILTERS

Filter glasses are expensive. Protect them from damage when not in use. Recommended Filters for Electric Welding

FACE PROTECTION

The helmet, hand shield or face shield should be maintained in good condition. If cracks occur in the fibre material the shield should be immediately replaced since the leakage of arc rays may cause serious burns.

N.B. DO NOT USE CRACKED OR DEFECTIVE HELMETS OR SHIELDS

SHIELDING Screening – General

Screen arc-welding and cutting operations so that

persons who work in the vicinity are protected from

‘flashes’

Arc eye

It is not unusual for irritation and watering of the eyes to

start some hours after exposure to arc rays.

When symptoms occur:

1. Use an eye bath to wash the eyes with an approved eye lotion.

2. Repeat at about four-hourly intervals

3. In the meantime, cold compresses, made by soaking cotton wool in cold water (which has been boiled previously), may be applied.

4. If going into bright light, dark glasses should be worn.

5. Report to the first aid room as soon as possible

If recovery from ‘Arc eye’ is not complete in 36 to 48 hours, medical advice must be sought.

TEMPERATURE EXTREMES

Due to the welding process, welding materials, the work environment and the weather.

Excessive heat can cause :-



Excessive heat can cause :- Fainting

Heat Stress

Exhaustion

Excessive cold can cause :-




Heat Stress

Exhaustion

Excessive cold can cause :- Frostbite

Hypothermia

Fatigue




Heat Stress

Exhaustion

Excessive cold can cause :- Frostbite

Hypothermia

Fatigue

Follow an appropriate work rest routine and ensure availability of water.

FUMES AND GASES

Fumes and gases are prime examples of silent and often unseen dangers

These clouds of particles can cause metal fume fever when inhaled.

Welding fumes are made up of two components:

Visible fume

This appears as smoke rising from the weld; these are small particles of metal or metal oxide. This is called the ‘particulate fume.

Invisible fume

This type of fume cannot be seen and is a mixture of a number of gases. This is called the ‘gaseous fume’.

Both visible fume and invisible fume are present in any welding and must be removed to avoid possible hazards.

Fume extraction

FUMES AND GASES

Fumes and gases are prime examples of silent and often unseen dangers

These clouds of particles can cause metal fume fever when inhaled.

Welding fumes are made up of two components:

Visible fume

This appears as smoke rising from the weld; these are small particles of metal or metal oxide. This is called the ‘particulate fume.

Invisible fume

This type of fume cannot be seen and is a mixture of a number of gases. This is called the ‘gaseous fume’.

Both visible fume and invisible fume are present in any welding and must be removed to avoid possible hazards.

Fume extraction

When you are welding within a workshop, extraction must be used to dispose of the welding fumes harmlessly

FUMES AND GASES

The need for good ventilation and fume extraction when working in confined spaces applies equally to all arc welding processes, but with inert gas welding additionalfactors have to be considered, such as:

Argon is heavier than air and will accumulate at a low level in confined spaces,reducing the oxygen content and raising the danger of suffocation.

There is no practical way of detecting the presence of Argon or Nitrogen, the gasesare odourless and so give no warning to people.

Degreasing operations using TRICHLOROETHYLENE, CARBON TETRACHLORIDEor PERCHLOROETHYLENE, all give off vapours if brought in contact with the arczone, rays from the arc (15 metres), decompose to form irritating fumes containing thepoisonous gas PHOSGENE.

NOISE

Excessive noise can cause:-

Tiredness,

Irritability,

Headaches,

a rise in blood pressure,

Loss of concentration,

a drop in productivity,

Accidents

and long term hearing problems resulting in industrial deafness

SPARKS AND SLAG

Molten metal in the form of sparks and spatter can cause damage to the ears and eyes

Slag resulting from some arc welding operations can cause burns to the eyes and exposed parts of the skin if incorrectly removed.

Protective clothing - Normal dress

Outer clothing should be free from oil, grease or flammable substances.Protect the forearms from exposure to arc rays, sparks and slag; do not roll up sleeves. Cuffs on overalls, turn-ups on trousers, exposed long hair and low-cut

shoes are likely lodging places for sparks or globules of hot metal and slag.

Special protection

Protect the front of the body from the throat to theknees with a suitable leather cape and apron.If only an apron is worn this must provide fullprotection. Wear suitable leather gloves to protect thewrists.

ELECTRICITY

Is so much an integral part of a welders work, that familiarity may cause the welder to lose respect for this powerful energy source. Yet all power sources are potentially dangerous and improper handling may result in a violent jolt or death.

In arc welding operations, where certain energised parts are exposed, an operator must be especially careful to observe certain safety rules in order to ensure a maximum amount of personal safety and incidentally to protect those working near him.

In most cases safety is a matter of common sense and the rules apply to both A.C. and D.C. welding generators, A.C. transformers, inverters and A.C., D.C. rectifiers.

LIVE CIRCUITS

In most welding shops the power supply used for welding machines will be 440 volts and in some cases lower at 240 volts.

The operators should be impressed with the fact that these are high voltages, capable of inflicting severe and often fatal injuries. It is equally important to avoid the open circuit voltage, which can be harmful under abnormal conditions.

It is therefore important that no work be done on the wiring, switches, controls etc. without first ascertaining that the main power switch is open. All checking of power sources must be performed on a dead circuit.

ELECTRICITY

WELDING CABLES

Welding cables must not be operated at currents in excess of their rated capacity since this is uneconomical and will cause overheating and rapid deterioration of the insulation.

The cables should be inspected frequently and all breaks in insulation repaired with rubber and friction tapes. Any joints not made with properly insulated connectors should be adequately insulated.

In addition to the potential hazard to personnel a hazard occurs when an exposed section of the cable comes in contact with metallic objects grounded in the welding circuit causing an arc. If flammable materials, such as oil or grease, happen to be in the vicinity a fire may result.

Elimination of these accidental grounds will improve the efficiency and quality of welding. For the same reasons all the cable connections should be maintained tight.

N.B. DO NOT PERMIT CABLES TO BE OPERATED HOTDO NOT OPERATE WITH WORN OR POORLY CONNECTED CABLES

ELECTRICITY

DAMP WORKING CONDITIONS

Dampness between the body and an energised part lowers the resistance of the passage of current to the operator’s body and may produce a harmful electric shock.

High temperature and humidity causing body perspiration as well as dampness to metal, material and welding equipment can be hazardous. To prevent a harmful body shock keep hands and clothing dry. The operator should never stand or lie in puddles of water or on damp ground when welding. Always find a dry board or rubber mat to stand or lie on.

ELECTRIC SHOCK

Ensure that all contact surfaces for connections are clean and that the connections are kept tight.In water cooled systems attend to or report any leakages.In T.I.G. welding switch off the welding current when adjusting or changing the tungsten electrode or in some instances the polarity.When using filler wire ensure contact with the parent plate when feeding into the welding pool.Avoid the use of ac in wet or damp conditions. (peak voltage — 1.4 x O.C.V.)Extra care is required when using ac with high frequency (voltage 450 to 10,000 volts).

Exposure to a wide range of chemicals is routine for a welder, heat and light applied to:

fluxes,

anti-spatter compounds,

glues,

rust inhibitors,

paints,

Degreasers

and other such compounds may cause chemical reactions.

The resulting toxic substances can be absorbed through

inhalation,

or the skin.

Damage to vital organs, the nervous system, and the muscle system may result.

CHEMICALS

The threat of fire is always present in the welding environment.

Fire hazards include sparks, heat and molten metal.

Don’t weld near flammable materials, move to a safe area. If this isn’t possible, remove all flammable materials from the area, or protect them with sheet metal or other flameproof covering.

FIRES




Keep fire extinguishing equipment nearby, for example, fire buckets, hoses and fire extinguishers.

FIRES




Keep fire extinguishing equipment nearby, for example, fire buckets, hoses and fire extinguishers.

Keep floors free of rubbish, and only work on wooden floors if they are covered with metal or other fireproof material.

Welding Helmets, Cutting Shields, Goggles and other safety equipment may prevent the Welder from seeing the sparks fly

These sparks can travel up to 20 meters and go through small spaces like cracks, or under doors.

Have fire watchers nearby to spot possible hazards

Don’t weld or cut metal touching combustible walls, ceilings, partitions and coverings. Do not weld ductwork without cleaning it first.

FIRES

The welding operator uses gases which are flammable, explosive and stored under pressure.

In the process of welding, explosions may occur if;

EXPLOSIONS


In the process of welding, explosions may occur if; equipment is used incorrectly, damaged equipment is used, the worker welds on a pressurized container, welding in the presence of explosive materials or pressure is increased in a container due to the application of heat.

Welding on containers that have held flammable or combustible materials can be an extremely dangerous operation.

EXPLOSIONS




Do not weld containers which have held combustible or flammable materials which when heated give off flammable or toxic vapours without proper cleaning, purging or inerting

Never connect an earth/return lead to electrical circuits or to a pipe containing flammable gases or liquids.

EXPLOSIONS




Do not weld containers which have held combustible or flammable materials which when heated give off flammable or toxic vapours without proper cleaning, purging or inerting

Never connect an earth/return lead to electrical circuits or to a pipe containing flammable gases or liquids.

Handle compressed gases safely, in line with recommended procedures. Make sure you keep any cylinders you transport or use upright. Always secure them to prevent them moving and falling.

Contact your supervisor or manager if you smell propane, acetylene or other flammable vapours.

EXPLOSIONS

Welding operators often work as part of a team. It’s very important that in this situation all machines, tools and support equipment are used for their specific functions and in a safe manner.

Your ability to identify the hazards is the first step toward safe welding.

WORK AREA

Welding operators often work as part of a team. It’s very important that in this situation all machines, tools and support equipment are used for their specific functions and in a safe manner.

Your ability to identify the hazards is the first step toward safe welding. Check that:

• All flammable materials are located away from welding activity.

• Fire resistant curtains or shields are used to separate welding from other activities

• Warning signs are posted.

• Matches, lighters and other ignition sources are kept away from the area.

• Drums, barrels, tanks and other containers have been properly cleaned.

• There is sufficient ventilation.

• Electric arc processes have been adequately grounded.

• Electricity and shielding gases are readily available.

• Lighting is adequate and lights are cleaned regularly, and

• The correct type of fire extinguishers are readily available.

Controlling hazards at the site is more difficult than in the welding shop

WORK AREA

Welders may sometimes have to assume an awkward position due to limited space; the resulting fatigue and muscle strain could lead to an accident.

The welder may be required to enter a confined space where hazardous vapours and gases could accumulate.

Prior to entering a confined space to commence welding and cutting operations, permission must be obtained from the responsible officer who must be satisfied that:

WORK AREA




• Vessels, or other confined spaces have been adequately cleaned or rendered inert.

• All mechanical equipment has been locked out.

• Pipes supplying chemicals have been blanked off.

WORK AREA




• Vessels, or other confined spaces have been adequately cleaned or rendered inert.

• All mechanical equipment has been locked out.

• Pipes supplying chemicals have been blanked off.

• Electrical switches are locked off.

• Entry and exit routes have been considered, so the worker can exit without help.

• The air in the confined space has been tested, and that there is enough Oxygen.

Once it has been evaluated for potential hazards in the work environment, appropriate control measures can be implemented to minimize the dangers

WORK AREA

Manual Metal Arc Welding

ElectrodesThe filler metal is provided by the melting of an electrode. The grip end, which is inserted in the electrode holder, is bare of covering from a distance of approximately 30 mm.The other end, known as the striking end, is sometimes pointed to ensure good electrical contact when striking the arc.

Purposes of the electrode covering are:To give stability to the arcTo provide good arc transfer conditionsTo control the reactions occurring during weldingTo protect the molten metal during transferTo provide good welding characteristicsTo provide a suitable slag protection for the cooling weldTo ensure that the deposited metal has satisfactory

chemical, physical and mechanical properties.

IMPORTANT! Electrodes should be stored and in dry conditions

Carefully segregate different types and sizes of electrode.

Sizes of electrodes is designated by the diameter of the core wire. The range of sizes is: Metric Imperial Metric Imperial Metric Imperial

1.6 mm 16 s.w.g. 4.0 mm 8 s.w.g. 8.0 mm 5/16 in2.0 mm 14 s.w.g. 5.0 mm 6 s.w.g. 10.0 mm ⅞ in2.5 mm 12 s.w.g. 6.0 mm 4 s.w.g.3.25 mm 10 s.w.g. 6.3 mm ¼ in

The initial installation and connection of the power source to the appropriate mains supply should be carried out by acompetent person.Ensure that the equipment is adequately earthed.

Connections for secondary sideUse secondary cables of a suitable size for the maximum

welding current.Connect one end of the welding lead to the electrode holder

then connect the other end to the output terminal on the power source.

Connect one end of the return lead to a clamp or terminal on the work piece, then connect the other end to the return terminal on the power source.

Ensure that the work has a direct earth connection.

Joints Loose joints or bad contacts cause over-heating of cable, electrode-holder handle etc and adversely affect the welding arc.

Make any necessary joints in cables, using properly designed cable connectors

Make sure that good electrical contact is always obtained when connecting cables to power source, electrode holder, and return clamp Pay particular attention to mechanical

joints that may work loose, especially when using A.C.

Assembly of equipment

General ProcedureThe following instructions (which are not repeated later in

the text) apply to manual metal-arc welding.Always:

1. Comply with the prescribed safety precautions and fire prevention procedures.

2. Check that the return and earth leads are firmly connected to the bench or workpiece and to power source.

3. Check that the welding lead is connected to power source and that the connection to the electrode holder is tight and sound.

4. Check that the power source is switched5. Use effective protective equipment and wear the

necessary protective clothing.6. Concentrate on watching the welding operation.7. Have full control of the movements of the electrode and

hold it steady.8. Hold the electrode holder with just sufficient grip to give

control – tight gripping will cause muscle fatigue.9. Position yourself to avoid stretching and the risk of

overbalancing.10. Support the arm holding the electrode holder by keeping

it near the body, but do not restrict freedom of movement.

11. Check that portable screens are in place. Warn unscreened observers before the arc is struck. 12. Ensure that the welding screen is in front of the eyes before striking the arc; keep it there until the arc is broken.13.Place the electrode holder in a safe place when not in use. 14.Use goggles when chipping off hot slag.15.Ensure that all slag and spatter is cleaned off fusion faces and previously deposited metal before starting the next run. 16. Switch off power source when not in use. 17. Switch off mains supply to power source at end of workperiod. 18. Leave the work area in a tidy and orderly manner and ensure that equipment is properly stowed.

WELDING PROCEDUREBefore welding commences several factors should be checked which are likely to affect the quality of the weld.1. Set-up

1.1 Preparation should be suitable for required joint. 1.2 Edges should be smooth and free from oxide, paint, scale grease etc. 1.3 Where a gap is required, as for butt welds, this should be even throughout with proper root faces

and where necessary set with spacers. Plates should be held in alignment when tacking. 1.4 Unless otherwise specified no gaps should be allowed on fillet weld set-ups. Any unspecified

gaps will increase the possibility of distortion and may severely reduce the strength of the joint.2. Equipment The welding plant should be capable of providing the correct current and voltage without over-heating. Ensure that all electrical connections are good.3. Electrodes Electrodes of the correct type and size for the job in hand should be available. Ensure that they are in a dry, undamaged and generally good condition.4. Tacking Tacks should be sound and penetrating as they may eventually form part of the completed weld. Higher current is generally required than is used for the run. The length and pitch of the tacks will vary from single spots at 40 mm pitch on sheet metal up to 40 – 50 mm in length at 200-300 mm pitch on heavy

plate.5. Current setting The table below is a general guide. Current settings will vary according to the type of electrode, set-up and welding position.

Size of Electrode (mm)

1.6 2.0 2.5 3.25 4.0 5.0 6.0 8.0

Current setting(amps)

24 – 45 50 – 75 70 – 95 95 – 130 135 – 180 155 – 200 190 – 315 300 – 500

The welding arc

When the current is switched on and an electrode is placed in the holder and the electrode end is placed in firm contact with the workpiece (i.e. short circuited). Current will flow through the circuit.This flow is interrupted by breaking the contact with the workpiece. When the gap is narrow, and if the open-circuit voltage of the power source is high enough, the current leaps across the gap andcreates an arc.

When the arc is struck the voltage falls to what isknown as the arc voltage.

The arc voltage is between 20 and 25 volts for mosttypes of electrodes at normal arc length.

The arc voltage depends upon:

• The type of electrode used

• The length of the arc.

When the arc is made, the end of the electrode and the local area of the workpiece rapidly reach fusion temperature. The electrode core wire melts. The molten metal is transferred across the arc gap to fuse with the workpiece.

Example Procedures

Striking the arc

Example procedure

Material One piece of mild steel

Preparation Clean surface

Electrode Size 4.0 mm

Current 150 Amps

1. Ensure that the electrical contact between plate and bench is good. The plate should be in the flat position.2. Use gloved hand to insert the grip end of electrode in the electrode holder .3. Point electrode downwards, and away from the

body at an angle of 65º – 75 º to the plate surface.4. Lower the electrode holder until the electrode

striking end is about 25 mm away from the plate at the point where deposition is to start.5. With welding screen in position, lower the electrode until the striking end touches the plate (use an action similar to that of gently striking a match)6. Contact of the electrode end with the plate closes the electrical circuit and current flows. Immediately withdraw the electrode a slight distance from the plate to establish the welding arc.7. After a few seconds break the arc by withdrawing the electrode end smartly from the plate.8. Repeat this procedure until skill in establishing the arc is attained.

Example Procedures

Note: Difficulty is often experienced in making the arc. The

electrode end may be withdrawn too far or allowed to fuse

(or freeze), to the plate, because it is not withdrawn quickly

enough. If freezing occurs give the electrode holder a sharp

twist to wrench the electrode free. Keep the welding screen

in front of the eyes. If the electrode is not freed, immediately

open the electrode holder jaws and move the holder away

(or switch off the welding current), remove the electrode

with a chisel.Breaking the arc

A simple withdrawal of the electrode end will break the arc.

Before breaking the arc:

1. Pause with the electrode held in position long enough

to build up the weld pool.

2. Move the electrode quickly sideways and away from the plate surface.

The procedure overleaf will avoid the formation of:

a. Cavities

b. blisters

c. porosity or fine cracks in the crater, which result from premature or hasty breaking of the arc.

The trainee will learn later how to strengthen the end of the weld run.

DEPOSITING STRAIGHT RUNS

CORNER JOINT BUTT JOINT

LAP JOINT FILLET JOINT

Technology

Manual metal arc welding