Welding

Welding:

Welding is a process of joining two metal pieces by the application of heat. Welding is the least expensive process and widely used now a days in fabrication. Welding joints different metals with the help of a number of processes in which heat is supplied either electrically or by mean of a gas torch. Different welding processes are used in the manufacturing of Auto mobiles bodies, structural work, tanks, and general machine repair work. In the industries , welding is used in refineries and pipe line fabrication. It may be called a secondary manufacturing process.

Classification of welding processes:

There are about 35 different welding and brazing process and several soldering methods, in use by the industry today. There are various ways of classifying the welding for example, they may be classified on the basis of source of heat(flames ,arc etc.)

In general various welding processes are classified as follows.

1: Gas Welding

(a): Air Acetylene

(b): Oxy Acetylene

(c): Oxy Hydrogen Welding

2: Arc Welding

(a): Carbon Arc welding

(b); Plasma Arc welding

(c): Shield Metal Arc Welding

(d): T.I.G. ( Tungsten Inert Gas Welding)

(e): M.I.G. ( Metal Inert Gas Welding)

3: Resistance Welding:

(a): Spot welding

(b): Seam welding

(c): Projection welding

(d): Resistance Butt welding

(e): Flash Butt welding

4: SolidState Welding:

(a): Cold welding

(b): Diffusion welding

(c): Forge welding

(d): Fabrication welding

(e): Hot pressure welding

(f): Roll welding

5: Thermo Chemical Welding

(a): Thermit welding

(b): Atomic welding

6: Radiant Energy Welding

(a): Electric Beam Welding

(b): Laser Beam Welding

Welding Joints

Different types of welding joints are classified as Butt, Lap , Corner, Tee and edge joints which are shown in figure

GAS WELDING - OXYACETYLENE

Oxyacetylene gas welding is commonly used to permanently join mild steel. A mixture of oxygen and acetylene, burns as an intense / focussed flame, at approximately 3,500 degrees centigrade. When the flame comes in contact with steel, it melts the surface forming a molten pool, allowing welding to take place. Oxyacetylene can also be used for brazing, bronze welding, forging / shaping metal and cutting.

This type of welding is suitable for the prefabrication of steel sheet, tubes and plates.

PREPARING THE CYLINDERS FOR WELDING

Slowly open the main valve of the acetylene tank and adjust the acetylene pressure regulator to 5PSI (pounds to square inch).

Open the needle valve on the torch and adjust the pressure on the acetylene regulators to show 5PSI. Close the needle valve.

Go through the same procedure for the oxygen cylinder.

IGNITING THE ACETYLENE / OXYGEN MIXTURE

TURNING ON: Acetylene slowly turned on (quarter/half turn of the needle valve) and ignited, producing a small flame. At this stage, a small amount of soot/smoke is given off the end of the flame.

Acetylene increased and oxygen turned on slowly.

Acetylene increased slowly and oxygen more rapidly, to produce an intense, localised flame, capable of precise welding.

TURNING OFF: Turn off the oxygen first, followed by the acetylene

Welding inevitably means exposure to extremely high temperatures. For this reason, a leather apron and leather gloves are essential.

Special welding goggles protect the eyes from the potential ‘splatter’ of molten metal during the welding process. They also protect the welders eyes from the dangerous ultraviolet and infrared light waves, produced by light emitted by the intense flame of the torch. The goggles usually have ‘flip up’ lenses. These allow the welder to look through normal lenses when arranging the metals to be welded and the ‘flip down’ tinted lenses when welding.

All welding must be carried out in a well ventilated area, as breathing the fumes that are generated during welding, can be dangerous.

The welding area should be clear of any potential fire and trip hazards.

A welding bench, although not always essential, helps keep welding in a safe area. This type of bench is safe because it will not catch fire and the fire brick area is ideal for welding small items.

STEEL WELDING PROCEDURE - FLAT PLATES
OXYACETYLENE

The two steel plates are ‘tacked’ together. This involves welding in two or three places, to hold the plates together.

The entire joint is welded, slowly and accurately. The flame is focus at one end of the joint, forming a small pool of molten steel. The welding rod / filler rod is introduced to the flame, as the pool is slowly ‘pushed’ down the entire length of the joint.

The joint is allowed to cool slowly. When cool, the ‘slag’, which forms during the welding process, is tapped away using a welding hammer. The joint can now be inspected for accuracy and strength.

WELDING STEEL PLATES AT RIGHT ANGLES

When steel plates are welded together, at an angle of 90 degrees, the edges that meet are ground at 45 degrees. This gives a much stronger joint.

After grinding, the joint is positioned and ‘tacked’ to ensure there is no movement during the welding procedure.

The entire joint is welded. It is allowed to cool slowly. Rapid cooling, such as dipping in cold water will damage the joint and possibly weaken it.

BRAZING WITH OXYACETYLENE

1. Oxyacetylene is often used to produce precise brazed joints. The two pieces of steel to be brazed, are cleaned with emery cloth, removing grease and dirt. Flux is applied to the joint. This protects the area to be brazed, preventing oxidation (oxidation will prevent the ‘solder’ from flowing along the joint).

2. The area is preheated by holding the torch a small distance from the steel. This slowly warms up the two surfaces to be joined and evaporates the water in the flux.

3. The flame is focussed at one end of the joint, raising the temperature of the steel to ‘red heat’. Brazing rod is fed into the joint . The molten brazing rod follows the heat of the flame, as the torch proceeds along the joint, until the brazing is complete.

4.The steel is allowed to cool slowly. When cold, a wire brush is used to clean the joint, revealing a ‘bronzed’ line of solder, which holds the two pieces of steel permanently together.

BRAZE WELDING (BRONZE WELDING) AND BRAZING
OXYACETYLENE

Oxyacetylene can be used to braze and to braze weld (bronze weld). These techniques are often confused, as they involve the use of the same equipment.

Bronze filler rod is an alloy composed of copper and tin. Brass filler rod is also an alloy, composed of copper and zinc. Either rod can be used.

The steel tube and steel plate have been heated to a dull red heat. The ‘bronze’ filler rod (coated with flux) is then introduced to the joint. It flows onto the two surfaces, forming a ‘fillet’ around the joint, joining the two surfaces.

Braze / bronze welding provides a strong joint and is ideal of steel frames, where a certain amount of flexibility is required.

The filler rod is broader than that used in brazing.

When brazing, flux is applied to the joint, to prevent oxidation of the surfaces. The steel tube and plate are heated to bright red heat and the brazing rod (filler rod) applied to the joint. The end of the filler rod melts and flows around the joint, through capillary attraction.

This gives a joint that looks like a ‘line’ of bronze.

Brazing is ideal for manufacture of bicycle frames, as it does not ‘fracture’ as easily as steel welded joints.

Brazing and braze welding (bronze welding), can be used to join similar and also dissimilar metals such as:

Mild Steel - Galvanized Steel
Stainless Steel - Copper
Mild Steel - Stainless Steel
Stainless Steel - Copper-Nickel
Mild Steel - Cast Iron

BRAZING USING A BRAZING HEARTH

Steel can be joined by using a technique called brazing. A high temperature is needed for this and so a brazing hearth is normally used. Brazing gives a permanent joint that is ideal for most metalworking projects in schools and colleges. In industry this technique is used on products such as bicycle frames where there is a need for a certain amount of flexibility in the joint. In simply terms, two steel parts are joined by heating them to ‘red’ heat and then applying a brazing rod to the joint. The brazing rods melts at a lower temperature than the steel and so it melts to form a molten liquid. This liquid brazing rod then flows along the joint between the two steel parts. The equipment needed is shown below.
	THE EQUIPMENT
	The brazing Hearth contains a compressor which pressurises air and gas so that it is forced out the nozzle of a gas-air torch. As it comes out the nozzle it can be ignited so that it burns fiercely. If the torch is used correctly it can heat up metals to a high temperature. This is exactly the type of equipment that is needed for a brazing joint between two pieces of steel. Above the brazing hearth is an extractor. This extracts any fumes that collect during the heating process. The hearth normally has fire bricks arranged on its surface. These reflect heat back into the heating area so that high temperatures can be reached. A small rotating table is sometimes used to position the metals to be heated.

BRAZING A JOINT

	1. Two pieces of steel sheet are to be brazed. The steel must first be cleaned so that grease and dirt is removed. Wire wool or emery cloth are the most suitable abrasives. A borax flux (powder) is mixed with water to produce a paste which is brushed along the joint. Flux prevents oxidation taking place on the metal surfaces as this would prevent brazing being successful.

	2. The compressor is turned on and this pressurises the gas and air. As the gas-air control is slowly turned on, gas is fed through the nozzle and this is ignited by a pilot light on the nozzle or by a match.

	3. Once alight the gas-air control is turned to allow more air/gas through the nozzle which gives a longer, more fierce flame. The length of the flame can be adjusted with the control until the desired type of flame is achieved. As a rough guide, a blue section of flame will appear near the nozzle, the end of this is the hottest part of the flame.



4. The two pieces of steel should have already been placed on the rotating table. Fire bricks are used to raise both pieces slightly off the surface of the table so that heat can flow all the way round it. Fire bricks should also be placed at the back and sides of the metal so that heat does not escape and is reflected back.


The steel is given a gentle overall heating first which raises the temperature slowly. This allows the steel to expand slowly and for the water in the flux to evaporate without moving the steel out of position. The flame is moved around the joint fairly quickly throughout this first stage of heating.

	5. The flame is then moved forward with the blue tip of the flame nearly touching the steel. The focus of heat should now be on the joint, as the flame is slowly moved backwards and forwards along it. The joint will eventually become so hot that it becomes red in colour.

	6. A brazing rod (copper-zinc alloy) is then pushed gently against the joint and if the temperature is right the end of the rod will melt and begin to run along the joint. The rod is fed into the joint until a brazed joint is complete. The steel is allowed to cool slowly. If cooled quickly, such as quenching in water, the joint can crack or become distorted.

	The diagram opposite shows how the copper-zinc alloy (brazing rod) forms a joint between the two pieces of steel sheet.
EXTRA SAFETY: Using a brazing hearth and soldering/brazing is potentially dangerous as high temperatures are reached. Serious burns can result if safety procedures are ignored. Leather aprons and gloves will offer good protection if you accidentally touch hot metal. Goggles are essential as ‘splashes’ of hot flux or brazing rod (solder) could damage eyes permanently. Furthermore, if there is a need to pick up materials that have been heated on the hearth, always use steel tongs and place the hot material on a steel plate/block. The steel block/plate will conduct the heat away from the material without cooling it too quickly.

THE BRAZING HEARTH - HEALTH AND SAFETY

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	The brazing hearth is a very dangerous piece of equipment if misused because it is possible to raise materials to high temperatures. Furthermore, the temperatures are reached through the use of a powerful flame. Some metals can even be heated to such high temperatures that they become molten. Before anyone uses the brazing hearth - safety training is essential. The brazing hearth should be well maintained and left in a safe condition after use. The fume extractor must always be used so that any harmful fumes are carried away safely. It is always advisable, when using this equipment, to assume that anything material in the hearth is hot. Never assume that the surface of the brazing hearth is cold as it retains its heat for along time after being used. It is very important to wear appropriate safety clothing. This will protect the user if an accident occurs. Our cartoon character Ed the Handyman shows how dangerous the brazing hearth can be if misused.

SAFETY INSTRUCTIONS:

When using the brazing hearth always;
Face the brazing hearth.
Turn on the torch slowly and carefully.
Never turn round with the torch, this is highly dangerous.
Concentrate fully on the heating of the material.
When finished heating the material place the torch back onto its rest
Never pick up materials without using tongs and wearing leather gloves.
Allow the material to cool before moving it elsewhere in the workshop.

HARDENING AND TEMPERING

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Steel can be treated by intense heat to give it different properties of hardness and softness. This depends on the amount of carbon in the steel (only high carbon steel can be hardened and tempered). CARBON CONTENT OF COMMON STEELS: Mild steel: 0.4% carbon, Medium carbon steel approximately 0.8% carbon, High Carbon Steel approximately 1.2% carbon (this steel is also known as Tool Steel and includes Silver Steel and Gauge Plate). Mild steel and medium carbon steel do not have enough carbon to change their crystalline structure and consequently cannot be hardened and tempered. Medium carbon steel may become slightly tougher although it cannot be harden to the point where it cannot be filed or cut with a hacksaw (the classic test of whether steel has been hardened). If steel is heated until it glows red and is quenched in clean water immediately, it becomes very hard but also brittle. This means it is likely to break or snap if put under great pressure. On the other hand, if the red hot steel is allowed to cool slowly, the resulting steel will be easier to cut, shape and file as it will be relatively soft. However, the industrial heat treatment of steel is a very complex and precise science.
In a school workshop most heat treatment of metals takes place on a brazing hearth. A rotating table and fire bricks are essential. The fire bricks reflect the intense heat back on to the metal being heated. This is achieved by arranging the bricks in a semi-circle behind the metal being heated. Without the bricks, heat would escape and this would limit the temperature that could be reached.


HARDENING AND TEMPERING

Heat treatment of steel in a school workshop is normally a two stage process. For example, if a high carbon steel or silver steel screw driver blade has been manufactured, at some point it will have to be ‘’hardened’ to prevent it wearing down when used. On the other hand it will have to be ‘tempered’. This second heating process reduces the hardness a little but toughens the steel. It also significantly reduces the brittleness of the steel so that it does not break easily. The whole process is called ‘hardening and tempering’.
STAGE ONE: The screw driver blade is heated, slowly at first, warming up the whole blade. Then the heat is concentrated on the area at the end of the blade. This gradually becomes ‘red’ hot.
STAGE TWO: The screw driver blade is removed quickly from the brazing heart, with blacksmiths tongs and plunged into clean, cold water. Steam boils off from the water as the steel cools rapidly. At this stage the blade is very hard but brittle and will break easily.

STAGE THREE:

The screw driver blade is cleaned with emery cloth and heated again on the brazing hearth. Heat is concentrated at the end of the steel blade. The steel must be watched very carefully as it changes colour quite quickly. A blue line of heat will appear near the end of the blade and it travels towards the tip as the temperature rises along the blade. When the line of blue reaches the tip the brazing torch is turned off. The blue indicates the correct temperature of ‘tempering’.

STAGE FOUR:

The screw driver blade is placed on a steel surface, such as an anvil face. This conducts the heat away and allows slow cooling of the screw driver blade. When cold, the blade should be tough and hard wearing and unlikely to break or snap. This is due to the tempering process.

USEFUL COLOUR INDICATORS OF TEMPERATURE

When heating steel on the brazing hearth, colour changes take place. These can be used to indicate the temperature of the metal. The table opposite is a rough guide.

The table opposite shows the temperatures and the associated colours required when tempering steel for particular uses. For instance, when making wood turning tools, they must be heated to a brown colour, whilst tempering.

CASE HARDENING OF MILD STEEL

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Case hardening is a simple method of hardening steel. It is less complex than hardening and tempering. This techniques is used for steels with a low carbon content. Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm. One advantage of this method of hardening steel is that the inner core is left untouched and so still processes properties such as flexibility and is still relatively soft.
STAGE ONE: The steel is heated to red heat. It may only be necessary to harden one part of the steel and so heat can be concentrated in this area.

STAGE TWO: The steel is removed from the brazing hearth with blacksmiths tongs and plunged into case hardening compound and allowed to cool a little. The case hardening compound is high in carbon.

STAGE THREE:

The steel is heated again to a red colour, removed from the brazing hearth and plunged into cold, clean water.

The steel rod should now have a hardened outer surface and a flexible, soft interior. The process can be repeated to increase the depth of the hardened surface.

ANNEALING METALS

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Annealing is a heat process whereby a metal is heated to a specific temperature /colour and then allowed to cool slowly. This softens the metal which means it can be cut and shaped more easily. Mild steel, is heated to a red heat and allowed to cool slowly. However, metals such as aluminium will melt if heated for too long.
Aluminium can be annealed but care must be taken whilst heating. The flame should be held at a distance to the aluminium so that it gives a generalised heating to the metal. A ‘trick of the trade’ is to rub soap on to the surface of the aluminium and then heat it on the brazing hearth. It takes only a short time for the soap to turn black. The brazing torch should be turned off immediately and the aluminium allowed to cool slowly. It is now annealed and should be very soft and malleable.

PHYSICAL PROPERTIES: Annealed metals are relatively soft and can be cut and shaped more easily. They bend easily when pressure is applied. As a rule they are heated and allowed to cool slowly.

The animation above shows that an annealed metal is usually softer and can be deformed more easily than metals that are not annealed.

PHYSICAL PROPERTIES: Hardened metals are difficult to cut and shape. They are very difficult if not impossible to bend. As a rule they are heated and cooled very quickly by quenching in clean, cold water.

The animation above shows that metals that have not been annealed are very difficult to deform.

The arc welding is a fusion welding process in which the welding heat is obtained from an electric arc struck between the work(or base metal) and an electrode. The temperature of the heat produced by the electric arc is of the order of 6000°C to 7000°C. Both the direct current (D.C) and alternating current(A.C) may be used for arc welding, but the direct current is preferred for most purposes. When the work is connected to the positive terminal of the D.C welding machine and the negative terminal to an electrode holder, the welding set up is said to have straight polarity. On the other hand, when work is connected to negative and the electrode to a positive terminal, then the welding set up is said to have reversed polarity. The straight polarity is preferable for some welds while for other welds reversed polarity should be used.

Following are the two types of arc welding depending upon the type of electrode:

(A): Un shielded arc welding:

When a large electrode or filler rod is used for welding, it is said to be un- shielded arc welding.

(B): Shielded arc welding:

When the welding rods coated with fluxing material are used, then it is called shielded arc welding.

Arc welding Processes:

The following are the various welding processes commonly used in engineering practice.

1: Carbon arc welding

In carbon arc welding, the welding heat is obtained from an electric arc between a carbon electrode and the work. In welding heavy plants, the additional metal is deposited in the weld from a filler rod.

2: Metal arc welding

In metal arc welding , the arc is produced between the metal electrode(also called filler rod) and the work piece. During the welding process, the metal electrode is melted by the heat of the arc and fused with the work piece. The temperature produced by the heat is about 2400° C to 2700° C.

3: Metallic inert gas (MIG) Arc welding:

In MIG welding , the electrode is consumable, the filler metal is deposited by the arc which is completely surrounded by an inert gas.

4: Tungsten inert gas(TIG) arc welding

In TIG welding, the heat is produced from an arc between the non consumable tungsten electrode and the work piece. The welding zone is shielded by an atmosphere of inert gas(such as helium or argon) supplied from a suitable source. The direct current with a straight polarity is used for welding copper alloys and stainless steel. Whereas the reversed polarity is used for magnesium. The alternating current is more versatile in welding for steel, cast iron, aluminum and magnesium.

5: Atomic hydrogen welding

In atomic hydrogen welding, the arc is obtained between two tungsten electrodes (non consumable) while a stream of hydrogen passes by the arc and envelopes the welding zone.

6: Stud arc welding

It is a direct current arc welding process, and is used for welding metal studs to the flat metal surfaces.

7: Submerged arc welding

In submerged welding, the arc is produced between a bare metal electrode and the work piece. The submerged arc welding is mostly done on low carbon and alloy steels, but it may be used on many of the non-ferrous metals.

8: Thermit welding

In this welding, a mixture of iron oxide and aluminum known as thermit, is used. The mixture is ignited only at a temperature of about 1500°C. A major advantage of the thermit welding is that all parts of the weld section are molten at the same time and the weld cools almost uniformly. This results in a minimum problem with internal residual stresses. The thermit welding is often used in joining iron and steel parts that are too large to be manufactured, such as rails, trucks frames, locomotive frames, other large sections used on steam and rail roads, for stern frames, rubber frames etc. In steel mills, Thermit electric welding is employed to replace broken gear teeth, to weld new necks on rolls and pinions and to repair broken shears.

Electric Resistance Welding

It is a type of pressure welding. It is used for joining pieces of sheet metal or wire. The welding heat is obtained at the location of the desired weld by the electrical resistance through the metal pieces to a relatively short duration, low voltage, high ampere electric current. The amount of current can be regulated by changing the primary turns of the transformer. When the area to be welded is sufficiently heated, the pressure varying from 25MPa to 55MPa is applied to the joining area by suitable electrodes until the weld is solid. The various types of electric resistance welding are as follows:

(1) Spot welding

It is used for welding lap joints, joining components made from plate material having 0.025 to 1.25 mm in thickness. The plate to be joined together are places between the two electrode tips of copper or copper alloy.

(2) Roll spot and seam welding

When the spot welds on two over lapping pieces of metal are spaced, the process of welding is known as roll spot welding. If the spot welds are sufficiently made close, then the process is called seam welding. This process is best for metal thickness ranging from 0.0.25 to 3 mm.

(3) Projection welding

It is similar to spot welding except that one of the metal pieces to be welded has projections on its surface at the points, Where the welds are to be made. In other words it is a multi spot welding process.

(4) Butt welding

The butt welding is of two type :

· Upset butt welding

· Flash butt welding

The upset butt welding is especially adopted to rods, pipes and many other components of uniform sections. The flash butt welding is extensively used in the manufacture of steel containers and in the welding of mild steel shanks to high speed drills and reamers.