In manual metal arc (MMA) welding, the electric arc reaches a temperature of about 6000ºC. This arc provides the heat necessary to melt the electrode, and fuse it with the parent metal.
Welding units are essentially sources of high, constant current (~100 A) at a relatively low voltage (<100 V). The constant current supply produces a stable arc. Manual metal arc welding machines have an adjustable current output that can be set to suit the job and the type of electrode.
The current should remain nearly constant during welding where the operator’s movements tend to vary the arc length. These movements may be accidental, or deliberate, to control the weld pool. An increase in arc length will increase the voltage across the arc; however the machine is designed to control the current close to the amperage set by the operator. Melting of the electrode is then uniform in spite of the normal variations when welding.
This is achieved as shown in figure 1. The welder makes the arc by striking the tip of the electrode on the work to cause a momentary short circuit. This is at the point on the graph where V = O. With current flowing, the electrode is drawn away by the welder to establish the arc. The amperage and voltage for a typical arc length are shown at point x. For a longer arc length, there is significant increase in arc voltage and a small decrease in welding current (point y). In this way, the welding machine is designed to avoid large changes in current output when the welder varies the arc length.
Figure 1: Output curve of welding machine
Welding machines operate with either alternating current or direct current. AC machines are more widely used because they are simpler in design and cost somewhat less. However the DC arc is much more stable with certain types of electrode, and is consequently better for working with sheet metal. There are three types of MMA welding machines commonly available:
Alternating current welding machines are transformers which step down line voltage (240 or 415 volts) to provide a low voltage, high current supply. The welding current supplied by the secondary circuit of the transformer is set by the operator to suit the type and size of electrode and its use.
Engine driven DC models provide welding power where there is no mains supply available, for example on site work.
A transformer/rectifier both transforms an AC input voltage to a lower voltage AC, and then rectifies that voltage to give DC. Transformer/rectifiers have no moving parts and are quiet to operate, convenient and cost less than motor generator units.
Transformer or rectifiers are often designed to provide either DC or AC outputs. DC is normally preferred because of its greater arc stability but AC may be required at higher currents to avoid arc blow.
Table 1: AC/DC comparison chart
|
AC transformer sets |
DC motor generator sets |
Portability |
These machines generally consist of static step-down transformers and they are considered as stationary types. |
Most modern types have features that allow portability (especially the self contained types). They have an undercarriage fitted with wheels. |
Power supply |
The use of these machines is restricted to the location of the nearest alternating current power point. |
Petrol or diesel engine machines can be used in any location (special applications may use generators driven by compressed air - for example in mining). |
Efficiency |
70-90 percent electrically efficient. Many multi operator sets give higher efficiency. |
40-60 per cent electrically efficient but some modern types compare with alternating current efficiency. |
Polarity |
No polarity |
A choice of polarity is obtained by a simple reversal or a switch (DC- or DC+) |
Arc blow |
Unaffected |
Arc blow occurs even in normal currents and they are difficult to control above 300 amperes. |
Maintenance |
As there are no moving parts to be considered, maintenance costs are very low. |
Revolving and wearing parts add to running costs. |
Initial costs |
Cheaper plant as less construction is involved |
More costly due to generator and motor construction. |
Electrodes |
Restricted to use of electrodes that are suitable for alternating current only. |
Suitable for all types of electrodes. |
Running costs |
Cheaper running costs due to the use of an installed power supply. |
Added costs due to the use of electric motors or internal combustion engines. |
Voltage control |
Constant open circuit voltage. |
A variation of open circuit voltage is possible allowing a selection of electrode type and welding technique. |
Arc length |
Limited arc length. |
Greater tolerance in arc length due to the characteristic of the machine. |
Australian Standard AS1966 rates the output (duty cycle) of electric arc welding power sources. The machines are classified according to the type of service for which they are designed, for example: continuous duty, heavy duty, light industrial or limited output cycles.
The standard defines each of the classes according to the output (load current, load voltage) needed for a nominated duty cycle. The duty cycle allows for the fact that in any five minute period, current for welding may be drawn for only part of that time. For example, if welding is for a maximum of three minutes in any five minute period, the machinery only operates up 60% duty cycle (3/5 of 5 minutes). A much lower current must be selected for continuous (100%) operation.
All power sources must display a name plate stating the equipment class and the rated output and duty cycle for its class (e.g. 300 amps, 23 volts, 605% duty cycle). The 100% duty cycle output current must also be noted.
A multiple-strand insulated flexible copper or aluminium lead conducts the welding current from the power source to the work. A return clamp is fastened to the work, and another cable completes the welding circuit between the work and the power source.
Electrode holders should suit the minimum output current being used. The holder should be relatively light, comfortable to hold, fully insulated and sturdy enough to withstand the wear and tear from constant use. The holder should be rated to withstand the maximum current required for the activity.
Notes regarding polarity:
Follow the manufacturer’s recommendations on the range of current for different types and sizes of electrodes. Some typical current ranges for different electrode types of shown in table 1.
You should use the recommended procedure and draw on experience to select the electrode type and size. The choice of current will also depend on such factors as:
Electrode |
Diameter = 4.0mm |
Diameter = 3.25mm |
E4110 |
130-160 |
75-125 |
E4112 |
130-190 |
90-140 |
E4114 |
140-200 |
95-150 |
E4814 |
130-170 |
100-130 |
E4818 |
140-200 |
105-150 |
E4824 |
185-235 |
130-170 |
The notes below are for general information only, they do not qualify your to perform arc welding!
1. Turn on the power source.
2. Lightly touch the end of the electrode on to the work and you will complete the circuit and current will flow.
3. The electrode end rapidly heats, melting sufficiently to momentarily weld the electrode on to the work.
4. Due to the low voltage current you are using, the arc will not jump an air gap (as in a spark plug). You must therefore establish an arc by first touching the end electrode onto the work and then immediately, increasing the distance between end of the electrode and the work allowing droplets of metal and flux to cross the arc gap (approximately 3 mm) to form a molten pool.
5. If you fail to do this, droplets will bridge the arc gap, causing a short circuit and a freezing of the electrode end onto the work.
6. If your action is incorrect and the electrode freezes onto the work (that is becomes welded or stuck to it) a sharp backward angling of the electrode should break it free. When this occurs, keep your head shield on, as an arc flash will occur as the electrode breaks contact.
1. Once the arc has been established, the arc length is reduced to about 3 mm
2. As the molten pool accumulates, the tip of the rod is moved along the weld slowly, so as to maintain a molten pool approximately 8mm wide behind the arc.
3. The electrode is fed in constantly as it burns off, maintaining a steady rate of consumption by using the correct arc length.
Hazards in the welding workshop include electric shock, fumes, heat, glare and harmful rays.
Electric shocks are possible on the secondary (low voltage) side of the welding circuit. They may be caused by:
High voltage shocks shouldn’t happen if precautions are taken such as ensuring welding machines are maintained by licensed electricians and that you never interfere with the inside parts of the welding machines.
Gases, dusts and vapours are given off during welding. They can cause:
Fumes can result from:
Welding should be carried out in well ventilated areas. When welding high fuming materials such as galvanized steel, use extraction systems to carry away the fumes.
If an extraction system is not available, an approved respirator should be used to filter out fumes. Normally, respirators should only be used as a secondary protection.
Molten droplets from welding spatter can get into boots and clothing. You can avoid this by wearing protective clothing and footwear. When welding out of position, wear spats over your boots and under overall legs.
Protective clothing will help to protect you from heat, hot metal and harmful rays.
Figure 6: Full protective clothing for arc welding
Welding arcs give off a broad spectrum of electromagnetic radiation, from ultraviolet, through visible and infrared wavelengths.
Ultraviolet radiation in particular can damage the skin. Ray burn is like very severe sunburn; your skin reddens and then peels. If the ray burn is very severe, there may be blisters and sores.
Rays will harm the eyes, causing a condition called a flash or arc eye. The first symptom of a flash is an itchy feeling in the eye. Afterwards, a throbbing pain (much like sand in the eye) may stop you sleeping. There are eye drops that relieve the pain. Continuous flashes may cause blindness.
The use of filter lenses is essential to protect your eyes from these rays.
These are specially designed glass lenses to filter out harmful rays and allow you to see what you’re welding without causing any changes in the pupil size or damage to your eyes. Filters come in different shade numbers, according to the current range or type of welding.
The following table indicates the recommended minimum protective filters:
Approximate welding current (Amperes) |
Filter recommended |
Up to 100 |
8 |
100 – 200 |
10 |
200 – 300 |
11 |
300 – 400 |
12 |
Over 400 |
13 |
Welding should be done in special welding bays. When this is not possible, use portable screens to shield others working in the areas from the rays generated from the arc. You should also put up signs to warn people that you are welding.
Figure 7: Swivel filter and clear glass type visors
Figure 8: Head shield and hand held shield
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Student exercise 1 |
1. How does an arc welder melt steel?
2. A welder power supply is basically a source of high, constant ________ at a relatively low _______
3. Why should a welding unit supply a relatively constant current?
4. Name three types of manual metal arc welding equipment.
5. Name three hazards that are presented by welding.
6. What personal protective equipment should be worn by a welder?
7. What hazards exist for bystanders near welding operations, and how can they be protected?
When acetylene is burnt in pure oxygen, the flame temperature is about 3100°C. No other combination of burning fuel gas and oxygen can reach this temperature. Acetylene (C2H2) is the only fuel gas suitable for fusion welding carbon steel because it:
Oxyacetylene equipment includes the gas cylinders, pressure tubing, welding torches and tips, and consumables such as the gases, and rods.
Figure 9: Oxyacetylene welding equipment
Oxygen is an odourless, colourless gas which makes combustion (burning) possible. About 20% of the atmosphere is oxygen, and the other 80% is mainly nitrogen.
Industrial oxygen is produced by fractional distillation of liquid air. Oxygen can be supplied as a liquid and takes up very little space compared to a gas. But more commonly, oxygen comes in cylinders as a compressed gas.
Industrial oxygen cylinders come in three sizes and are filled to a pressure of 15,000kPa. They are always painted black, and as with all non-flammable gas cylinders, have a right-hand thread on the regulator connecting outlet.
There is a bursting disc at the tip of the cylinder designed to blow out at a much lower pressure than the amount that would rupture the cylinder. Because of this there is less chance of cylinders exploding in fires.
In the illustration below, the letters G, E and D indicate the cylinder size. These are the typical cylinder sizes you will encounter with G typically being the largest size cylinder available.
Figure 10: Oxygen cylinders
Figure 11: Oxygen cylinder valve
Acetylene is highly flammable, colourless and has a strong, odour that is not pleasant.
Stored acetylene is dissolved under pressure in liquid acetone in special cylinders. Porous material inside the cylinders is saturated with acetone to further reduce the risk of free compressed acetylene building up There is no free gas present in acetylene cylinders. The gas is dissolved under pressure in acetone. The effective cylinder pressure is 1600 kPa.
Caution! |
The cylinders are painted crimson and have more square shoulders than oxygen cylinders. They have left-hand threads at the regulator outlet and fusible plugs which will release the gas and stop the cylinder exploding if the temperature reaches 100°C.
Figure 12: Acetylene cylinders
Figure 13: Acetylene cylinder valve
Regulators are fixed to the gas cylinders for welding and cutting. The function of regulators is to:
|
Caution! |
Figure 14: Acetylene regulator
The oxy-acetylene welding torch is made to mix oxygen and acetylene in the proportions needed, and to control the volume of gases burnt at the welding tip.
The welding tip screws into the mixer on the end of the torch. The tip directs the flame where it is wanted and the size of the orifice or hole decides how much gas is supplied.
Thicker material needs more heat, so a larger tip is used. The image and chart below show tip sizes used on various thickness of low carbon steel.
Figure 15: Welding torch and tips
By adjusting the mixture of oxygen to acetylene, three basic flame types can be achieved, which are used for different purposes. There are called neutral, carburising and oxidising.
The neutral flame results when the amount of oxygen is just sufficient to oxidise the acetylene. The neutral flame is therefore ‘balanced’.
A neutral flame is used for fusion welding steel and cast iron. The flame is relatively harmless to steel, and will neither oxidise it, nor increase its carbon content. This flame produces an inactive weld pool without sparks.
When acetylene is in excess of the neutral flame condition, this produces a carburising flame.
The bright inner cone of the flame is surrounded by a longer white feather. This type of flame normally damages the weld area. However, a slight carburising flame is often set for pipe welding to guard against the more damaging oxidizing flame.
If a highly carburising flame is used, carbon is added to the weld area, welding is slower and there is sparking.
When oxygen is in excess of the neutral flame condition, this produces an oxidising flame.
The inner cone becomes shorter and pointed. The secondary stage of combustion is much shorter and there is often a hissing sound. If this flame is used to weld steel, the weld pool will boil and oxidise, destroying the properties of the steel.
A slightly oxidising flame is used for braze welding.
Figure 16: Oxyacetylene welding—flame adjustments
Figure 17: Neutral flame temperature zones
There are advantages as well as limitations of fusion welding low carbon steel over other forms of welding. The advantages include:
Disadvantages of include:
The safety notes are provided for general information, but note again that you must be specifically qualified and licensed to perform thermal welding and cutting at work.
When using oxyacetylene or other oxy-gas equipment, you must protect yourself from the radiated heat and light, from fumes and from hot objects and spatter. You also need to handle oxygen, acetylene and your equipment carefully. Personal protective equipment includes:
Oxygen by itself is not flammable or explosive, but it will support combustion and cause material to burn rapidly.
.
|
Student exercise 2 |
1 List four essential component parts required to operate an oxyacetylene welding plant.
2 Cylinders are identified by shape and colour. What colour are:
• oxygen cylinders? ____________________________________________________
• acetylene cylinders? __________________________________________________
3 Name two industrial uses for oxyacetylene welding.
4 List three types of flame settings.
5 Briefly describe how the atmosphere is excluded from the weld pool during the oxyacetylene process.
6 List three items of clothing you need to protect you when oxy-acetylene welding.
Check your answers with those given at the end of the section.
Oxyacetylene welding is a manual process which combines oxygen and acetylene gases to give a high temperature flame for welding.
The edges of the parts to be joined are melted and fused together to form a high-strength joint. Additional weld metal can be added from a hand-held filler rod.
Oxyacetylene welding is used for:
Oxyacetylene welding is often limited by its comparatively slow welding speed, but because of its ability to weld most metals, this versatile process is a valuable maintenance tool.
Two main techniques are used, known as forehand and backhand welding.
Filler rods come in several sizes and different types, made to provide a range of weld metal properties. Rods must be selected to suit the type of metal and the service requirements of the job. Manufacturers’ information sheets give guidance to selection.
Filler rods for low carbon steel are classified in AS1167 Parts 1 and 2:
A flux is not needed for fusion welding low carbon steel. The oxides that form on low carbon steel have a low melting point and are prevented from forming by de-oxidisers in the filler rod.
These deoxidisers, sometimes called reducing agents, are usually manganese, silicon and aluminium all of which join harmlessly with oxygen at temperatures lower than iron does.
Figure 18: Fusion welding
Brazing is similar to soldering because capillary action draws the molten filler metal between two close-fitting surfaces and joins them. Lap joints and prepared fillet joints are recommended. However, brazing is much stronger and more expensive than soft soldering and is done at temperatures above 450°C.
Brazing is commonly used for:
Most ferrous and non-ferrous metals used in industry can be joined with either copper or silver alloys. You can braze different metals together—such as copper to steel.
Examples of metals commonly brazed are
Fluxes are used for most brazing operations to:
Fluxes are available in a number of forms. The most common fluxes are powder, liquid or paste. Manufacturers’ information leaflets will help you to select the most suitable flux for a particular job.
Brazing alloys are available in many combinations. Filler metals come in two main groups:
You must choose the most suitable alloy. Points to consider include:
Silver alloys are free flowing. They can enter small openings and provide strong neat-looking joints. Silver brazing is also called silver soldering or hard soldering, and is used for joining steel, copper, bronze and brass.
For silver brazing you need a filler metal made of silver and copper alloy and a commercially prepared flux. Silver brazing alloy melts at a lower temperature than other alloys used for brazing. Common heating gas mixtures are oxygen and acetylene, and oxygen and LP gas.
Figure 19: Protective clothing
Soldering is a similar process to brazing, except that soldering alloys have a melting point of less than 450°C. Soldered joints are not as strong as with brazing. Soldering is used where the lower melting point or the excellent flowing and wetting ability of solder are important. Applications include electrical and plumbing work.
In electrical work, soldering is used where a permanent, low-resistance connection is required, for example in printed circuit boards, and for making earth connections. The eutectic alloy of 63% tin and 37% lead is used, which has very specific properties:
Electrical solder is supplied is wire form, which can easily be fed into a joint. The wire incorporates about 1% of rosin flux at its core, which helps to remove oxide from the parent metal, and protects it from rapid oxidation at soldering temperatures.
To obtain a good soldered joint for earth leads:
Fume from rosin flux is both an allergen and an irritant. Exposure over an extended period can lead to sensitivity and respiratory problems. Make sure that soldering is done with good ventilation.
Soldering stations for electronic work should be equipped with extractors or air filters to remove the soldering fumes.
Note that lead-free solders have replaced lead solder for all plumbing work, due to the toxicity of lead.
|
Student exercise 3 |
1 List three metals which can be joined using the brazing process.
2 Name two common heating gas mixtures used for brazing.
3 Name a type of weld joint recommended for brazing.
4 What distinguishes soldering from brazing?
5 Why is it important for the parent metal to be clean and free of oxide?
6 What is a eutectic mixture?
Check your answers with those given at the end of this section.
Braze welding is different from brazing and uses different joint preparation, filler metal and welding technique. For braze welding you use weld preparations like the ones used for fusion welding. The weld preparation allows for a full thickness weld and gives a strong joint.
Braze welding uses an oxyacetylene flame, a flux to clean the surface and a filler rod made from a copper/zinc alloy (bronze). The filler rod melts at a much lower temperature than the parts to be joined and the parent metal is not melted.
Molten bronze will flow onto properly heated and fluxed surfaces of metals with higher melting points. You need to adjust the flame to contain a slight excess of oxygen. The flux is added to the heated end of the filler rod and applied to the joint during brazing.
Braze welding is able to join a greater range of dissimilar metals, including non-ferrous and ferrous (eg copper tube) steel. It also requires much less heat input and causes less distortion.
Braze welding has greater consumable costs, and there is a loss of strength at moderately high temperatures (above 260°C). There is also the change of a corrosive attack if the weld comes into contact with ammonia.
Braze welding is used for repairing machinery and for fittings in maintenance work.
Other uses include leak proof joints on tanks and the construction of hollow section components such as those used for furniture manufacture.
Precautions are the same as for brazing:
Use the right equipment for the work you have to do. For example for bench work, use:
 |
Student exercise 4 |
1 Braze welding is different from brazing because:
2 Name the two main metals that make up the alloy filler rod for braze welding.
3 You use a flux in braze welding to give:
4 List three metals suitable for welding with the braze-welding process.
5 Name one use of braze welding.
6 (a) List two advantages of braze welding:
(b) List two disadvantages of braze welding:
Check your answers with those given at the end of this section.
The flame, or thermal, cutting process used for cutting iron and steel depends on a chemical reaction between heated iron and oxygen.
When a piece of iron or steel is heated to a temperature of 815°C (called the ignition temperature) the material will burn to form a lower melting-point substance, called iron oxide.
This chemical reaction generates a great deal of heat, which enables the cutting to continue. Once the metal begins to burn, the heat generated will lead to a progressive spread of oxidisation through the material. This important property of flame cutting allows you to cut and pierce thick steel without all of the metal heating up.
The ignition temperature is the temperature at which the reaction begins. The ignition temperature for low carbon steel is 815°C. This is well below its melting temperature, which is about 1450°C.
The cutting action is a function of the high-speed jet of oxygen. The purpose of the flame is to heat the metal to its ignition temperature so that the cutting can begin. Theoretically, once the cutting begins, the flame shouldn’t be necessary. However heat is lost from the work through conduction so it’s necessary to keep the pre-heat flame going while you’re working.
The most common gas combinations used for pre-heat are oxyacetylene, oxy-LPG (liquefied petroleum gas) and oxy-natural gas.
1 Use the flame to raise a small section of the metal to ignition temperature.
2 Release a high-speed jet of oxygen onto the heated section.
3 By controlling the direction of the blowpipe nozzle and combining it with the oxidising action, you can cut through the metal. The width of the cut through the steel is called the kerf.
Figure 20: Flame cutting
Flame cutting relies on a chemical reaction between heated iron and oxygen, so you can normally only use it on iron and steel iron. Non-ferrous metals (copper, brass, aluminium etc) can’t be flame cut.
If a metal is to be readily flame cut it needs:
This method will cut carbon steels and most low-alloy steels commonly used for construction work. Some high-alloy steels are difficult to cut, mainly because of the high melting point of the surface oxides that form under the oxygen jet. Non-ferrous metals (copper, brass, aluminium, etc) cannot be cut because of the absence of iron content and the formation of high melting-point oxides.
You use the same portable flame-cutting plant as you would for oxyacetylene welding except for the cutting torch or cutting attachment connected to the hoses.
The plant consists of:
Types of manual flame cutting torches are:
Figure 21: Blowpipe attachment with handle
The torch can be guided by hand (freehand) for normal use. However, there are aids to steady the cutting torch movement to improve the quality of the cut (eg a roller guide attachment as shown in Figure 14).
Figure 22: Roller guide attachment
Cutting nozzles are designed for different thicknesses and applications. Each nozzle is stamped to indicate the size, type of fuel gas and process.
Figure 23: Various oxyacetylene cutting nozzles
Every cutting nozzle or tip is stamped for easy identification. The stampings indicate the type of nozzle, the size of nozzle and special process identification.
The first part of the type number gives the form of nozzle connection. The 30 series nozzles are screw-in type with a threaded inlet connection. The 40 series nozzles are the taper seat type.
The second part of the type number indicates the fuel gas used. If the type number ends in a ‘_1’ the nozzle is used with acetylene, if it ends in ‘_4’ it is used with LPG (liquefied petroleum gas).
1 Acetylene
2 Low pressure acetylene
3 Coal gas
4 LPG
5 Hydrogen
The size number tells you the diameter of the main bore and is stamped underneath the type number. The size number is a tenth of a millimetre, for example a size 12 nozzle has a main bore diameter of 1.2 mm. Typical nozzle sizes are 6, 8, 12, 15 and 20.
Be careful when you’re flame cutting as the cutting stream can shower hot metal sparks over great distances. The protective clothing you need for flame cutting is the same as for welding:
Factors which will affect the quality of the cut are:
Figure 24: Effects of variations in flame-cutting procedures
 |
Student exercise 5 |
1 Flame cutting requires low carbon steel to be heated to an ignition temperature of:
2 List two additional basic steps for flame cutting:
(a) The flame is used to raise a small section of metal to ignition temperature.
(b)
(c)
3 Name three suitable fuel gases used for oxy-fuel gas cutting.
4 The gas used to support combustion when flame cutting is:
5 Symbols are used on cutting nozzles. What do the following numbers on a cutting nozzle indicate?
Type 41:
No. 12:
6 List the factors which will affect a flame cut finish:
(a) Material’s surface should be clean and free from paint, oil, grease, etc.
(b)
(c)
(d)
(e)
Check your answers at the end of this section.
You should now have a basic understanding of the equipment, procedures and processes involved in thermal welding and cutting. Safety is particularly important for thermal cutting and joining processes, and we remind you that specific qualifications and licensing are required for welding and cutting.
Safety area |
Safety considerations |
Protective clothing |
• shade 5 or 6 goggles |
Oxygen |
• don’t operate pneumatic tools with oxygen |
Acetylene |
• don’t try to transfer from one container to another |
Equipment |
• ensure the work area is well ventilated |
Brazing and flame cutting |
• check hoses regularly for leaks |
acetone |
a flammable and volatile liquid used as a solvent to dissolve and stabilise acetylene under pressure. |
acetylene |
a highly combustible gas composed of carbon and hydrogen (C2H2) and used as a fuel gas in oxyacetylene welding and cutting. When burnt with oxygen in the correct proportions, it produces a flame temperature of about 3100°C. |
alloy |
a mixture of two or more metals in solid solution. |
arc |
an electric current crossing the gap between an electrode and the work. |
automatic welding |
welding in which the means of making the weld are controlled by machine. |
backfire |
a loud snapping or popping noise caused when the blowpipe flame goes out suddenly or momentarily. |
backhand welding |
(rightward, backward) welding with the blowpipe flame pointing in the opposite direction to that in which the weld progresses—the opposite of forehand welding. |
backing strip |
material (metal, carbon etc) used to back up the root of the weld and retain the molten metal. |
blowhole |
a cavity in the weld metal caused by a bubble of gas becoming trapped in the solidifying metal. |
blowpipe |
an instrument for bringing together and properly mixing fuel gas and oxygen in such a manner that the mixture, when ignited, will produce a controlled flame. |
bonding |
the joining of two or more metals. |
braze welding |
unlike brazing, does not depend on capillary attraction. The parent metal is not melted, but the joint design is similar to that which would be used in fusion welding. The filler metal is a non-ferrous alloy, with a melting point lower than that of the metal being joined. |
brazing |
a joining process in which the molten filler metal is drawn by capillary action between two closely adjacent surfaces to be joined. The filler metal is a non-ferrous metal or alloy with a melting point lower than that of the metal being joined. A process that is more like soldering than welding. |
capillary action |
the phenomenon by which the molten filler metal flows between the properly fitted surfaces of the joint. |
Celsius (C) |
a temperature scale that registers the freezing point of water as 0°C and the boiling point as 100°C under normal atmospheric pressure. |
combustion |
the process of burning. |
conduction |
the transmission of an electric charge or heat through a medium. |
consumable |
material that is actually consumed during the welding process (eg electrodes, filler metals, fluxes, gases). |
contamination |
the oxidisation of the weld pool or the generation of gas from organic materials which will cause weakening of the weld. |
cover glass |
a clear glass used to protect the filter in goggles. |
cutting attachment |
an attachment to a welding blowpipe handle to convert it to a cutting blowpipe. |
cutting blowpipe |
equipment used in oxygen cutting. It is designed to control the gases for pre-heating, as well as the oxygen used for cutting. |
cutting oxygen |
the jet of oxygen from the central opening of the cutting nozzle which oxidises the preheated metal, allowing the cutting action to take place. It shouldn’t be confused with the oxygen that mixes with the acetylene for the pre-heat flames. |
deoxidising |
the process of removing oxygen. |
deposition rate |
the weight of metal deposited in a unit of time. |
diameter |
a straight line passing through the centre of a circle and touching the circumference on both sides. |
downhand welding |
see flat position. |
ductility |
the ability to be easily moulded or shaped. |
ferrous |
a metal that contains iron. |
filter (lens) |
a filter, usually made of glass, designed to protect a welder’s eyes from glare and harmful radiation. |
flashback |
the burning back of the flame into the blowpipe or the ignition of an explosive mixture in one of the gas lines. |
flat position |
the position in which welding is performed from the upper side of the joint when the weld face is nearly horizontal. |
flux |
a chemical powder or paste which dissolves oxides, cleans metal and prevents oxidisation during welding. |
flux inclusion |
a cavity in the weld metal containing flux caused by a quantity of flux becoming trapped as the metal hardens. |
forehand welding |
(leftward, forward) welding with the blowpipe flame pointing in the direction of the weld—that is, towards the unfinished seam. The opposite of backhand welding. |
freehand flame cutting |
the cutting process in which the operator both holds and guides the hand-cutting torch. |
fusion |
the joining of two metals during the welding process. |
gas thermal cutting |
the parting or shaping of materials by the application of heat with a stream of cutting oxygen. |
horizontal welding |
welding in a position in which the line of the weld is horizontal but the surface of the work is vertical. |
ignition temperature |
the temperature at which a material will ignite (eg 815°C for iron). |
kerf |
the space left during flame cutting by the removal of metal. |
manual |
working by hand. |
non-ferrous |
a metal that does not contain iron. |
overhead welding |
welding in which the filler metal is deposited from the underside of the joint and the face of the weld is approximately horizontal. |
oxide |
a compound of oxygen with another element or substance. Rust and mill scale are examples of iron oxides. |
oxidisation |
the process of forming an oxide (eg a section of steel rusting). |
oxygen |
a colourless and odourless gas which supports combustion and is present in the atmosphere to the extent of approximately |
parent metal |
the metal of the part to be welded as (base metal) opposed to the metal that is added from the filler rod. |
portability |
able to be moved easily. |
preheat |
heat applied before welding or cutting operations begin, used to prevent distortion, cracking and unwanted hardening. |
radiation |
the transfer of heat through space by wave motion. All bodies that are at a higher temperature than their surroundings radiate heat. |
semi-automatic welding |
welding in which some of the welding variables are automatically controlled, but manual guidance is necessary. |
shielding |
a process in which gases produced are added during the welding to exclude the harmful elements contained in the atmosphere. |
slag |
a fused, non-metallic residue produced from some welding processes. |
slag inclusion |
non-metallic material trapped in a weld. |
tack weld |
a short weld used for assembly purposes only. |
tip |
the generally detachable part of a blowpipe from which gas or gases emerge for welding. |
toxic |
poisonous. |
vertical welding |
welding in a position in which the axis of the weld is almost vertical. |
volatile |
evaporating rapidly. |
weld |
a union between pieces of metal at faces rendered plastic or liquid by heat pressure or both. Filler metal may be used. |
welding hose |
reinforced rubber hose strongly built to resist the pressure of the gases and to withstand constant bending and twisting. |
weld metal |
metal in a welded joint which has been melted in making the weld. The weld metal includes the filler and parent metals. |
weld pool |
the metal pool which is deposited while making the weld. It can consist of filler metal, plate material or a mixture of both. |
|
Check your progress |
1 State the identifying colours for:
• oxygen cylinders _____________________________________________________
• acetylene cylinders____________________________________________________
2 What are the pressure gauge settings required when using a size 15 tip?
• oxygen ____________________________________________________________
• acetylene ___________________________________________________________
3 List five important personal safety items that must be worn at all times while operating oxyacetylene welding equipment.
4 List four hazards associated with oxyacetylene welding equipment.
5 What would you do to correct a flame that was producing soot (a carburising flame)?
6 Why is it necessary to protect the molten puddle with the flame?
7 What are the two functions of a welding torch mixer?
8 Explain how brazing with oxyacetylene differs from welding with oxyacetylene.
9 State three reasons for using flux in brazing.
10 Name two metals used for filler metals in brazing.
11 What is the parent metal melted during the braze welding process?
12 Can ferrous and non-ferrous metals be joined using braze welding?
13 What substance is formed when steel is heated to ignition temperature?
14 Is the melting point of this new substance higher or lower than that of the parent metal?
15 What property of the chemical reaction between heated iron and oxygen allows flame cutting to proceed?
16 Describe the three basic steps of the flame-cutting operation.
(a)
(b)
(c)
17 Can non-ferrous metals be successfully flame cut?
18 Cutting nozzles are stamped with identifying numbers and size numbers. What does the identifying number tell you?
• First part ___________________________________________________________
• Second part _________________________________________________________
Answers to Check your progress are at the end of the module.
1 The identifying colours are:
2 The pressure gauge settings required when using a size 15 tip are:
3 The five important personal safety items are:
4 The four hazards are:
5 Increase oxygen flow
6 Stop atmospheric contamination
7 The two functions of a welding torch mixer are:
8 Brazing with oxyacetylene differs from welding with oxyacetylene in that welding is a fusion process (molten parent metal)
9 The three reasons for using flux in brazing are:
10 Two metals used for filler metals in brazing are:
11 Is the parent metal melted during the braze welding process? No
12 Can ferrous and non-ferrous metals be joined using braze welding? Yes
13 The substance formed when steel is heated to ignition temperature is iron oxide.
14 The melting point of this new substance is lower.
15 The property of the chemical reaction between heated iron and oxygen which allows flame cutting to proceeds is rapid oxidation.
16 The three basic steps of the flame-cutting operation are:
17 Can non-ferrous metals be successfully flame cut? No
18 The identifying numbers and size numbers tell you:
1. The arc is at a very high temperature (~6000 C), and the heat from this reaches the metal by radiation and conduction.
2. A welder power supply is basically a source of high, constant current at a relatively low voltage.
3. The constant current helps to establish and maintain a stable arc, allowing weld metal to be deposited evenly.
4. AC transformers, DC generators, and transformer/rectifier units.
5. Hazards include exposure of the eyes and skin to UV light and heat radiation from the arc, exposure to fumes, hot objects and weld spatter. Electric shocks from the welder supply can be fatal, particularly if gloves or clothing are wet.
6. PPI includes welding gloves, visor with glass for arc welding (shade 8 or more, depending upon current), leather apron, long sleeved clothing, boots, spats. Respirators should only be used in combination with forced-air ventilation or extraction.
7. The UV light can cause damage to the eyes including welding flash and blindness. Bystanders should be protected from the welding light by use of screens.
1 Any four component parts from the following list:
gas bottles
regulators
valves
valve key
hoses
torch
mixer
tip
2 (a) black
(b) crimson
3 Any two from the following industries:
light fabrication such as sheet metal
repairs to automotive parts
repairs to farm machinery
general ferrous metal joining
4 (a) carburising
(b) neutral
(c) oxidising
5 By keeping the weld pool covered by the flame
6 Any three items of clothing from the following list:
shade 5 or 6 goggles—Australian Standard approved
a shirt and trousers of non-flammable material that is tough and wear resistant
firm-fitting leather shoes or boots, leather gloves and head covering
1 (a) steel and stainless steel
(b) copper
(c) brass
2 (a) oxygen and acetylene
(b) oxygen and LP gas
3 Lap joint or prepared fillet joint.
4 Soldering alloys melt at a lower temperature, typically less than 450°C
5 The parent metal needs to make a continuous metallurgical joint with the soldering alloy. If oxide or other contaminants are present, they will for a layer which separates the solder from the parent metal and prevents the ‘wetting’ of the parent metal. This is known as a ‘dry’ joint, and will lead to unreliable electrical connections.
1 Different joint preparation, filler metal and techniques.
2 Copper and zinc.
3 A clean surface.
4 Any ferrous and non-ferrous metals including copper, steel, brass, cast iron etc.
5 Any one of the following typical uses:
leakproof joints
repair of cast iron and light metal parts
joining hollow section furniture parts
6 Advantages and disadvantages:
(a) Two advantages—less distortion, can join a wide range of dissimilar metals.
(b) Two disadvantages—cost, corrodes in contact with ammonia or loss of strength above 260°C.
1 815°C
2 (a) The flame is used to raise a small section of metal to ignition temperature.
(b) High-speed jet of oxygen.
(c) Moving nozzle and maintenance of heat.
3 (a) acetylene
(b) liquefied petroleum (LPG)
(c) natural gas
4 Oxygen
5 Type 41: taper seat for LP gas.
No. 12: bore diameter of 1.2 mm.
6 (a) material’s surface—it should be clean and free from paint, oil, grease, etc
(b) metal composition
(c) cutting nozzle size and condition
(d) gas pressures
(e) size of preheat flame
(f) cutting speed
Source: http://lrr.cli.det.nsw.edu.au/LRRDownloads/5113/1/5113_1.doc
Web site to visit: http://lrr.cli.det.nsw.edu.au
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