1.8 Basic Welding and Cutting

Welding is the technique used for joining metallic materials by fusion bonding process. A heat source generates sufficient heat to create and maintain a molten pool of metal of the required size and filler metal of same type may be added. The heat may be supplied by electricity or by a gas flame. Electric resistance welding can be considered fusion welding because some molten metal is formed. The welding operations performed on board the ship are:

• Manual Metal Arc Welding

• Oxy-acetylene welding

Metals are joined by welding because they are:

• good conductors of heat and electricity

• relatively hard, yet possess properties of malleability and ductility

• solid at normal temperatures - except Mercury

• dense

• low in specific heat

Manual Metal Arc Welding (MMAW) is the most commonly applied welding process. Electric arc welding employs a sustained luminous electrical discharge (arc) as a source of heat for melting the filler metal (welding rod) and the metals being welded.

The metals to be joined are heated and melted by an electric arc generated between the electrode and the base metal due to the flow of electric current.

Tiny globules of molten metal are transferred from the metal electrode to the weld joint. Arc welding can be done with either alternating or direct current.

In the MMAW process, the welder holds the electrode in the electrode holder with an insulated handle. The core of the electrode conducts the electric current to the arc and provides filler metal for the joint. The flux coating protects the molten metal from the atmosphere by forming a slag and a gaseous shield and stabilizes the arc. The slag helps to smooth and shape the weld bead, and has additional functions, depending on the type of electrode. Electrodes used for MMAW are usually 220-450 mm long with core wire diameters from 1.6 mm to 8.0 mm. The power supply can be alternating or direct current.

A set of MMAW equipment and Accessories includes:

• Welding Machine

• Welding lead (Welding Cable) attached with an electrode holder

• Welding return lead (Ground/ Earth Cable) attached with a ground clamp

Welding machine is normally equipped with operating switch (on/ off switch), current regulator (crank handle or control knob for regulating welding current), and terminals for connecting welding cable and ground cable to the machine.

Welding machines are available in different designs and models, in addition to the types, as portable or fixed, and should be selected properly according to the operational requirements such as location, working condition and desired capacity etc.

Electrode holder is a device to hold the electrode so that the welder can direct and maintain the arc on a localized spot until the desired weld is achieved. Electrode holders consist of copper jaws for clamping the electrode and a handle made of insulating material. The copper jaws are enclosed or covered by insulating material. There are many different types of electrode holders manufactured, and the welder should select the most comfortable one to meet his operational requirement. Whichever design is selected, the electrode holder should be fully insulated type so that the accidental electric shocks to the welder may be prevented during welding.

Ground clamp is the connector between the return cable and work or metal work bench to complete the welding circuit. Different types of ground clamps such as magnet type, screw-down type or clamp type are available for this purpose. Ground clamp is sometimes referred as return clamp. Ground clamp must be connected with the cable and to the work piece securely so that it cannot be disconnected by shaking or jerking of the cable.

Welding cable and ground cable are used for conducting the welding current in a complete circuit between the base material and the welding machine. Welding cable is sometimes referred as welding lead and ground cable is return lead. Cables consist of fine pure copper strands intertwined to form a number of wire bunches which are stranded to obtain the specific diameter of the cable.

Cable lugs are made of copper or alloy and may be covered with a fire insulation layer and used for connecting welding cable and ground cable to the welding machine at the respective terminals. The connections must be firmed and fixed securely to provide low resistance. A loose connection will cause resistance to the electric current flow and will result in overheating the contact point and burn the cable.

Cable connector is a fitting specially designed to join sections of welding cables. It consists of a male plug-end and female socket-end both of which are made of copper and insulated and protected by a rubber or neoprene jacket. 

Cable connectors are made to be easily connected or pulled apart. When joining the cables, female socket-end should be at the end of the cable connected to the welding supply unit and male plug-end should be at the end of the cable connected with the electrode holder.

Welding electrode is a stick of metal used for conducting the electric current to generate the arc between the electrode and the base metal and to melt them. Therefore, they are also termed as stick electrodes.

Uncovered electrode, also called bare electrode, may be distinguished as:

• Base electrode is cast or drawn solid metal rod.

• Flux cored electrode, which is tubular rod having a core of flux, and externally it looks like a base electrode.

Covered electrode is also known as coated electrode. Most metals, when heated, react with the atmosphere or other nearby metals. These reactions can be extremely detrimental to the welded joint. If the hot metal reacts with atmosphere, it rapidly oxidises when melted. As a result, an oxide layer is formed and bonding of the metals is prevented, and the oxidised molten metal droplets make the joint brittle. In addition, some valuable materials, added for specific properties. Therefore, metal core is covered under flux coating.

The functions of the electrode coating are many and varied but the most significant functions are as follows:

• To provide a gas to shield the arc and prevent excessive atmospheric contamination of the molten filler metal travelling across the arc

• To improve the smoothness and stability of the arc

• To produce a slag blanket to protect the hot weld metal from the air, to allow slower cooling and to enhance bead shape and surface cleanliness of the weld metal

• To provide fluxes, scavenger and deoxidizers to cleanse the weld and prevent excessive grain growth

• To allow alloying elements to be added to change the composition of the weld metal

Coated electrodes are distinguished based on the constituents. There are four main classes of electrode coating in MMAW process depending on the major constituents in the coating as:

• Acid coating

• Basic coating

• Cellulose coating

• Rutile coating

Size of Electrode and Amperage Size of an electrode is determined by the diameter of metal core covered by coating. Each electrode size has specified amperage.

1. The welding operator must wear protective clothing and non-conducting safety footwear, which are recommended for such job.

2. Clothing must be completely dry as some protection against electric shock.

3. It is particularly important that welding gloves used for electric welding must be dry because wet leather is a good conductor.

4. Protect other persons from glare using appropriate screening if and when necessary.

5. An assistant should be in continuous attendance during welding operation. He should be alerted:

• to the risk of accidental shock to the welder

• to be ready to cut off power supply instantly

• to apply artificial respiration without delay

6. In restricted area, where the welder is in close contact or is likely to be in contact with the structure’s structure by ordinary movements, protections should be provided by dry insulating mats or boards.

7. Ensure adequate ventilation in areas where welding is carried out.

8. Wear a face respirator when welding galvanized material that give rise to toxic fumes.

9. Keep welding cables clear of ladders, gangways, and doors.

10. Disconnect welding cables from power source before joining.

11. When welding operation is completed or temporarily suspended remove the electrode from the holder and switch off the main supply to power source.

12. Keep spare electrodes dry in their container until required for further use.

13. Eject hot electrodes ends into a suitable container.

1. Do not make electric welding operation under any circumstance while standing in water or any part of the welder's body is immersed.

2. Do not handle or touch hot electrode ends with bare hands.

3. Do not allow traffic to pass over the welding cables.

4. Never weld a vessel or drum that has contained flammable or toxic substance until it has been thoroughly cleaned and made safe by appropriate means.

5. Do not wrap welding return lead around components, worktable or, scaffolding or other metallic objects.

6. Do not attach welding return leads to manipulators or fixtures unless these are specially designed for this purpose.

It is a gas fusion welding process, in which the base metal is fused to form a molten pool by the application of localized heat generated by the gas flame, which is generated by combustion of oxygen and acetylene gas mixture.

Oxygen and acetylene are mixed in the blowpipe fitted with a nozzle and ignited to produce an intensely hot and metallurgically neutral flame. Then, the flame is applied to the edges of the joint to fuse the base metals. The filler metal of similar composition as the base metal may be added to the joint if necessary.

OAW is a manual operation that needs practice to achieve a certain level of skill. It can be done with or without the filler metal. The equipment used for gas welding process is inexpensive and portable; however, the process is slow and can sometimes cause distortion in the base metals more than other welding processes because it requires tremendous amount of heat to melt the metals. As a result, it is suitable to use on materials of thin to medium thickness and not economical for thicker base metals. OAW is applied in general engineering and in the light fabrication and vehicle industries.

Oxy-acetylene flame is the most efficient source of heat amongst the gas flames to be used for welding and allied process, due to the superior characteristics and advantages it possesses. An oxy-acetylene flame may consist of two to three of the following components:

• The inner tip or cone

• The beard or brush

• The outer flame or envelope

Carburizing flame is a reducing flame, in which amount of acetylene is exceeding that of oxygen. A carburizing flame can be easily distinguished by the secondary flame between the inner cone and the flame envelope. The secondary flame is less bright and whiter than the inner cone, but considerably brighter than the flame envelope. The carburizing flame adjusted with a secondary flame which length is approximately two times of the inner tip is suitable for non-ferrous metals. Carburizing flame is necessary for hardfacing. A carburizing flame has a temperature approximately 3000°C (5550°F).

Neutral flame may be produced when the acetylene and oxygen are mixed in the proportion of 1: 1.1 (one part of acetylene with a slight excess of oxygen). If a neutral flame is properly adjusted, the secondary flame becomes disappear, but the inner cone and the outer envelope appear distinctly. Since neutral flame is metallurgically neutral to the base metal (i.e.: the flame neither carburises nor oxidises the base metal), it is suitable for welding steels, stainless steels, cast iron, copper, aluminium etc. and for cutting and heating steels. A neutral flame has a temperature approximately 3200°C (5850°F).

Oxidising flame is ignited when the significantly excess amount oxygen is fed into the flame. Oxidising flame has no beard and both inner cone and flame envelope are shorter than the other flames. Oxidising flame is harmful to many metals. it is of limited use. However, oxidising flame is required for welding brasses and bronzes. An oxidising flame has a temperature approximately 3315°C (6000°F).

Typical oxy-acetylene welding equipment includes followings:

1. Gas cylinders

2. Gas pressure regulators

3. Welding Blowpipe

4. Welding Nozzle

5. Welding Hoses

6. Protective Devices

7. Hose Connectors

Gas cylinders are pressed or deep drawn steel containers used for storing and transporting gases. A gas cylinder is composed with a bronze valve, which is the essential component of the gas cylinder and attached on top of the cylinders.

A cylinder valve consists of a valve wheel or square valve spindle and female threaded outlet connection, to which pressure regulator with male threaded connection is attached. The cylinder valve is designed to withstand the high pressure. As for safety reason, never allow oil, grease or dirt to come into contact with gas cylinder valve or connections.

Oxygen cylinders are usually identified by a coat of blue paint. But, sometimes oxygen cylinders are painted in black, because oxygen is a non-flammable gas. Under any circumstances, oxygen cylinder must be used in upright position and fastened or safely secured to prevent form falling.

A full oxygen cylinder can explode if heated. Therefore, oxygen cylinder must be kept in the cool and dry place, and away from direct sunlight and sources of heat. In case an oxygen cylinder is heated, it must be cooled down with water immediately from a protected position until there is no sign of steam formation on the cylinder.

Acetylene cylinder is a deep drawn steel cylinder, but it is not a seamless cylinder. It is made up of two halves welded in the middle. Acetylene cylinder is composed with a cylinder valve on top of the cylinder. The cylinder valve has a left hand female thread connection to which acetylene pressure regulator is to be attached.

Acetylene is extremely unstable. It can decompose exothermally releasing the energy in the form of heat, and can cause cylinder explosion. For that reason, fusible plugs, which melt at 100°C, are installed at the top or bottom of the cylinder. If cylinder temperature rises above specified range in

Gas pressure regulator reduces the cylinder pressure of the gas to a proper working pressure and supplies at a constant rate during a welding operation. A gas pressure regulator is attached to the cylinder top valve connection. Pressure regulators are identified by the same type colours that identify the gases.

Welding blowpipe is also called welding torch, which is designed to premix acetylene with oxygen in a certain specified proportion and to deliver the premixed gas mixture to the nozzle attached to it. Then, the gas mixture is ignited to produce a flame for gas welding operation.

Welding blowpipe designs may differ from one manufacturer from another, but the major components are common in all blowpipes. The major components of a welding blowpipe are:

• The body or shank

• The mixer or mixing chamber

• The gas control valves

Nozzles used for gas welding may be goose neck extension fitting into mixer portion of the blow pipe or tip screws on to the head of the blowpipe. A welding nozzle contains an opening called orifice through which acetylene and oxygen gas mixture flow and feed the flame. Because welding nozzles are made of brass, they are subjected to wear. Therefore, they should be replaced from time to time. To replace welding nozzles, appropriate wrench should be used, but pliers must never be used. Welding nozzles are produced in a wide variety of designs and sizes. Generally, the size of a welding nozzle is determined by the diameter of the orifice.

Hoses are used for delivering acetylene and oxygen in gas welding process. The hoses are non-porous, flexible but strong and not likely to kinking. Welding hose are made out of pressure type rubber or neoprene reinforced with braided fibre. The outer layer should be resistant to oil and grease and tough to endure the most workshop conditions.

The hoses are available as single hoses of internal diameter 1/8” to 1/2” or twin double-barrelled hoses (one line for oxygen and the other for acetylene). For safety reasons, gas-welding hoses are distinguished by colours as black or blue or green for oxygen and red for acetylene. Hoses should not be used for purposes any other than its intended use.

Protective devices are safety apparatuses for preventing flashback or backfire. They may be distinguished into two kinds as:

• Flashback arrestors

• Backflow check valves

Flashback arrestors are usually installed on the outlet of the pressure regulators. The principle functions of a flashback arrestor are:

• To prevent backflow of gas in a line into the other line which is at lower pressure

• To extinguish the flashback if it occurs

• To stop further spreading after a flashback

Flashback arrestor for oxygen has right hand thread connection and for acetylene has left hand thread connection.

Hose fittings are nuts with tails (nipples). As the standard safety measure, hose connectors are standardised by the type of thread to distinguish for oxygen and acetylene.

Oxygen hose fitting has a plain hexagon nut with right hand threads.

Gas welding accessories are auxiliary tools used by the operator to facilitate the gas welding operation. The basic accessories are:

1. Igniter

2. Nozzle Cleaners

Igniter is a device to ignite the oxy-acetylene gas mixture. It is also known as a spark lighter. Using a common lighter or match can cause burns to the welder hand or other parts of the body because of the sudden flare-up ignition of the gas mixture. Thus, an igniter is specially designed to provide a certain degree of safety to the welder.

An igniter consists of flint and serrated friction-roller made of steel. Igniters are available in various designs. Some igniters are equipped with a pistol grip and others have a replaceable rotating flint holder with a long handle.

Dos

1. Store oxygen and fuel gas cylinder apart.

2. Handle and store gas cylinders carefully.

3. Ensure that there are no gas leaks.

4. Avoid oil or grease in contact with cylinder valves or fittings.

5. Take care that they are not dropped or allowed to fall from a height.

6. Cylinder should be used in an upright position and fastened to prevent them from falling or being knocked over.

7. Keep valve key readily available on the fuel gas cylinder to shut off the gas supply immediately in case of emergency.

8. Close cylinder valves when not in use.

9. If fire breaks out, remove all gas cylinders to a safe place.

1. Do not allow any flame near cylinders.

2. Do not allow any electric arc welding to be undertaken in the immediate vicinity of gas cylinders.

Acetylene Safety

1. Store acetylene cylinders in a cool and dry place; and keep away from heat and direct sunlight.

2. Use flameproof lamps and switches in the acetylene cylinder store.

3. Close cylinder valve when not in use.

4. If gas leak when valve is closed:

• Move the cylinder into the open air space, away from electric motors and sources of sparks or heat.

• Forbid smoking and naked light.

• Advise the suppliers immediately.

1. Do not inhale oxygen from the cylinders.

2. Do not use it as a method of ventilation.

3. Do not allow it to leak.

4. Do not use oxygen as a substitute for compressed air.

5. Never allow oil or grease to be in contact with cylinder valves or cylinder fittings, because oxygen reacts explosively with oil or grease.

In the oxy-acetylene flame cutting, the surface of the material is locally heated to ignition temperature by the combustion of oxy-fuel gas flame and subjected to oxidation by the oxygen in the surrounding air or by the stream of cutting oxygen or both. The oxidising reaction increases the temperature of operation and burned away the oxide resulting to form a gash in the metal. As a result, the metal is cut.

Generally, the equipment used for gas welding can be used for gas cutting except the blowpipe and nozzle.

The blowpipes used for gas flame cutting may be exclusively constructed for heavy duty cutting or gouging purpose or may be combined torch, which is designed to be fitted with cutting attachment or welding attachment and used for welding and light duty cutting purposes.

Cutting nozzles are distinctive according to the type of blowpipe, type of fuel gas and application. Acetylene nozzle is made in a single-piece design.

Brazing is a gas flame process that joins metals by the use of oxy-acetylene flame with non-ferrous metal or alloy filler metals which melts above 400°C but below the melting point of the base metal.

Brazing process offers certain advantages over the other joining processes. Most of these advantages mainly originate in use of the lower operating temperature. The advantages include:

• Elimination of warping

• Fast metal bonding speed

• Reduction of distortion

• Less risk of lock-in stress

• Low cost

Brazing is highly suitable method for joining dissimilar metals of widely different melting points, repairing grey iron castings, and joining pipes, thin sheet metals, and small assemblies. Brazing is extensively used in refrigeration, air-conditioning, electronics and automotive industries.

Flux is a substance added to the metal surface to remove oxide and similar contaminants, and to protect the metal surface against contamination from the gases and other contaminants in the surrounding atmosphere, thereby a stronger bond between the surfaces to be joined.

The principal functions of flux are:

1. To provide protective coating against oxidation

2. To contribute to the capillary attraction of the filler metal so as to flow more freely

3. To dissolve and remove any oxide that may formed during the operation

Brazing fluxes are commercially available in powder, paste or liquid form. Some manufacturers produce flux-coated brazing rods.

1. Clean the parts to be joined mechanically and chemically to remove all dirt, grease, oil, and other surface contaminants thoroughly.

2. Select a suitable flux for the desired brazing operation, and apply it on the surfaces of the parts to be joined.

3. Align the parts to be joined.

4. Secure in position by clamping or other suitable means. The parts must be held together until the filler metal solidifies.

5. Preheat the surface to the required temperature. The correct brazing temperature will be indicated first by a gradual drying of the flux and finally by its becoming fluid.

6. As soon as the flux becomes fluid, add the filler metal to the joint.

7. If all conditions such as cleaning, aligning, securing and preheating etc. in the brazing procedure have been properly met, the filler metal will spread over the metal surface and into the joint by capillary attraction.

8. Stop heating as soon as the filler has completely covered the surface of the joint.

9. Never overheat when brazing as this will tend to increase the porosity of the filler and weaken the bond.

10. Allow the parts to cool naturally and the filler solidifies completely before removing the clamp or other securing devices.

11. Remove any excess flux because residues of all fluxes interfere with paint or any other surface coating and some fluxes tend to corrode the surface. Scrubbing with hot water is generally accepted for post-cleaning operation.