Oxygen-acetylene is a commonly used combination of gases for fuel gas welding. The burning gases produce a high temperature flame which is suitable for welding a range of metals and gauges. Oxy-acetylene is a versatile welding method but its welding speed is slow compared with other welding methods.
Oxy-acetylene welding is used for fabrication of light sections such as tubular steel chair frames; repairs such as castings, automotive body panels and exhaust systems; general maintenance and farm use, because it is portable and can weld most metals.
In this section we’ll cover:
The illustration on the right shows a basic set of oxy-acetylene welding equipment which consists of:
Regulators control the delivery of gas at a chosen constant pressure regardless of any variation in the cylinder or pipeline pressure.
The table on the right shows gas pressures that are generally suitable for a range of tip sizes.
Acetylene is the fuel gas for the welding process. It is highly combustible and it can be very dangerous if mishandled.
Acetylene cylinders contain porous material which is impregnated with acetone. The acetylene is dissolved under pressure in this acetone.
Acetylene gas cylinders are easily distinguishable from the oxygen cylinders by their squat shape and maroon (reddish) colour. Acetylene cylinders are also fitted with left hand threads to prevent coupling with oxygen equipment.
Oxygen is not a combustible gas but supports the combustion of acetylene under pressure, producing a high temperature welding flame. Oxygen is compressed into black painted cylinders that are taller than acetylene cylinders and have right hand threads for coupling equipment.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ Which of the following statements are accurate? ][ Oxygen bottles are tall and black. ][ * Oxygen bottles are squat and black. ][ Acetylene bottles are squat and maroon. ][ * Acetylene bottles are tall and maroon. ][ Oxygen bottles are tall and black; acetylene are squat and maroon. ]]
[[ mc /f ][ What measure is used to ensure that gas welding equipment is correctly coupled to the gas bottles? ][ Opposite threads are used on oxygen and acetylene bottles. ][ * The bottles are different colours. ][ * The bottles are different heights. ][ * The bottle threads are different sizes. ][ Oxygen bottles have a right-hand thread; acetylene bottle threads are left-handed. ]]
Acetylene and oxygen burn together to form the heating flame required for the welding process. Temperatures vary in different parts of the flame. The hottest part being the tip of the inner cone which reaches around 3200°C. Accurate flame adjustment is very important in oxy-acetylene welding. There are three types of flame adjustment; neutral, oxidising (excess oxygen) and carburising (excess acetylene).
A neutral flame results from equal volumes of oxygen and acetylene being used. The inner cone of the flame is long and clearly defined within the outer envelope. The neutral flame is used for the majority of welding requirements with steel, cast iron and copper.
An oxidising flame is obtained by first setting a neutral flame, then reducing the acetylene to shorten the inner cone.
The oxidising flame is used when welding brass alloys as the chemical effect of the free oxygen tends to prevent loss of zinc from the parent metal.
A carburising flame is obtained by using an excess of acetylene. Through welding goggles the parts of the flame are clearly visible; an intense but feathery-edged inner cone surrounded by a luminous secondary cone and a bluish outer envelope forming a third zone.
A carburising flame is generally used for welding aluminium, bronze and nickel alloys.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mm /f ][ Match each flame type with the corresponding gas mix: ][ Neutral flame ~ Equal measures of both gases. ][ Oxidising flame ~ Excessive oxygen in the mix. ][ Carburising flame ~ Excessive acetylene in the mix. ][ Neutral = equal measures; Oxidising = excess oxygen; Carburising = excess acetylene. ]]
[[ mm /f ][ Match each flame type with the flame's characteristics: ][ Neutral flame ~ Inner cone long and well defined. ][ Oxidising flame ~ Short inner cone. ][ Carburising flame ~ Luminous secondary cone. ][ neutral = long inner cone; oxidising = short inner cone; carburising = secondary cone. ]]
[[ mm /f ][ Match each flame type with the metals it is suitable for welding: ][ Neutral flame ~ Copper and steel. ][ Oxidising flame ~ Brass alloys. ][ Carburising flame ~ Aluminium and bronze. ][ neutral = copper etc; oxidising = brass; carburising = aluminium etc. ]]
You should take every possible precaution to avoid accidents when operating oxy-acetylene equipment. Oxy-acetylene welding is quite safe if you use the equipment correctly and maintain it in good working condition.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /4 /f ][ Which of the following statements related to personal safety are sensible: ][ Cuffless trousers are preferred. ][ Wear leather gloves and aprons. ][ Wear clear goggles while preparing to weld. ][ * Always use matches to light a torch. ][ * Wear safety goggles if flashback is a risk. ][ * Use oxygen to dust off the job before welding it. ][ Don't use matches; always wear safety goggles; don't dust off using oxygen. ]]
[[ mc /r /f ][ How much clearance from flammable materials is required? ][ 10 metres ][ * 5 metres ][ * 3 metres ][ * 2 metres ][ 10 metres is recommended. ]]
[[ mr /f ][ Which of the following statements make fire-safety sense: ][ * Use wooden supports when welding. ][ Ensure an extinguisher is close by. ][ * Weld on an inert surface like concrete. ][ Watch out for hot metal sparks and globules. ][ Weld over (but not on) concrete floors. ][ Use metal, not wooden supports; don't weld on concrete - it may explode. ]]
[[ mr /f ][ Which of the following are sound operating precautions: ][ * Cylinder valves should be opened and closed quickly. ][ Acetylene cylinders must always be upright. ][ * The key should be removed from the acetlyene valve. ][ * Cylinder pressures should be lower than working pressure for safety. ][ Don't use higher than necessary gas pressures. ][ Open and close valves slowly; leave the key in the acetylene cylinder; cylinder pressures should be higher than working pressures for stability. ]]
[[ mr /f ][ Which of the following are unsound operating precautions: ][ Stabilise the flame using the regulator knob ][ Weld over the hoses in tight spaces. ][ Wash out petrol tanks before welding them. ][ * Keep sparks and flames away from welding equipment. ][ * Shut off oxygen, then acetylene, if a flashback occurs. ][ Never stabilise flames using the regulator; never weld over your hoses; never weld petrol tanks under any circumstances. ]]
Before you light up after assembling the equipment, or if it’s been half an hour or more since the last welding operation, the oxygen and acetylene lines should be purged to remove any air. Open each control valve on the torch to expel air and close again after a few seconds.
If the welding equipment is used frequently during the day, only the first two of the following steps need to be taken. Closing down for longer intervals or closing down at the end of the day will require the full procedure to be adopted.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ so /6 /f ][ Put the following setting-up steps in the correct order (be warned - some steps are missing): ][ Remove the oxygen cylinder cap. ][ Open the oxygen cylinder valve slightly, then close it. ][ Remove the acetylene cylinder cap. ][ Open the acetylene cylinder valve slightly, then close it. ][ Inspect the regulators then connect them both. ][ Inspect the hoses and torch then connect both hoses. ][ Fit a tip to the torch. ][ Check for leaks in the connections. ][ Oxygen cap, then valve; acetylene cap, then valve; regulators, then hoses; tip, then leaks. ]]
[[ so /f ][ Put the following steps for acetylene leak testing in the correct order: ][ Close the acetylene valve on the torch. ][ Set the acetylene regulator to 100kPa on the delivery gauge. ][ Close the acetylene cylinder valve. ][ Watch the delivery pressure gauge. ][ Locate leaks with a brush and soapy water. ][ Close the torch valve; set the regulator; close the cylinder valve; watch the gauge; locate leaks. ]]
[[ so /f ][ Put the following steps for adjusting oxygen gas pressures in the correct order: ][ Release the oxygen regulator adjusting knob. ][ Open the oxygen cylinder valve slowly. ][ Note the cylinder pressure on the high pressure gauge. ][ Open the oxygen control valve on the torch. ][ Screw in the oxygen regulator until the desired working pressure is indicated on the delivery gauge. ][ Close the oxygen control valve on the torch. ][ Release the regulator; open cylinder valve; note the pressure; open torch valve; screw in regulator; close torch valve. ]]
[[ mc /r /f ][ "Purging" means clearing the hoses of ... ][ Air. ][ * Oxygen. ][ * Acetylene. ][ * Water. ][ * Dust. ][ Air needs to be purged if the gear hasn't been used for a while. ]]
[[ mr /f ][ Which of these statements regarding lighting up are accurate: ][ Open the acetylene control valve on the torch first. ][ Don't light the acetylene flame until air has been purged from the hose. ][ * Open the oxygen control valve of the torch first. ][ * Adjust the acetylene flame so that it produces soot and flares noisily at the tip. ][ Adjust the oxygen control valve to get the desired welding flame. ][ Oxygen is opened after an acetylene flame is lit; you don't want a sooty flame or a flaring tip. ]]
[[ so /6 /f ][ Put the following closing-down steps in the correct order (be warned - some steps are missing): ][ Close the acetylene torch control valve. ][ Close the oxygen torch control valve. ][ Close both cylinder valves. ][ Open the oxygen torch control valve. ][ After oxygen has drained, close torch control valve. ][ Release the oxygen regulator knob. ][ Open the acetylene torch control valve. ][ After acetylene has drained, close torch control valve. ][ Release the acetylene regulator knob. ][ Close acetylene then oxygen on torch; Close both cylinder valves; Open, drain and close oxygen, then release regulator; open, drain and close acetylene, then release regulator. ]]
The surface of the metal that is to be welded should be free of rust, scale, paint, dirt, oil and grease. Cleaning can be carried out with a wire brush, old files or by grinding.
The type of joint to be used will depend on the strength of the parent metal, weld strength required and the eventual use of the part being constructed. Butt joints are suitable for oxy-acetylene welding because they are simple to set up and ensure fusion for the full thickness of the material.
Joints may be close butted or left with a small gap to assist penetration to the under side of the material.
Material that is 3mm thick or less should be left square on the edges.
Pieces that are over 3mm thick should be bevelled to an included angle between 80° and 90°. A narrower ‘V’ may be used with thicker material.
Material that is 13mm thick or more should be bevelled from both sides as shown in the illustration on the right.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ Which of the following statements about surface cleaning are accurate: ][ * You shouldn't clean surfaces with a grinder. ][ Paint and dirt needs to be removed before welding. ][ * You can weld through rust - it just burns off. ][ A wire brush is a good way to clean surfaces. ][ * Oil and grease aren't a problem - they'll just burn off. ][ Grinders are good surface cleaners; rust, oil and grease all need to be cleaned off before welding. ]]
[[ mm /f ][ Match the metal thickness with the correct edge preparation: ][ < 3mm ~ Square edges. ][ 3mm - 12mm ~ Bevelled on one side. ][ > 12mm ~ Bevelled on both sides. ][ < 3 = square; 3-12 = one bevel; > 13 = two bevels. ]]
In fusion 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.
Filler rods are available in several sizes (diameters). Different types of rods are manufactured to provide a range of weld metal properties. Filler rods should be selected to suit the type of metal and the requirements of the job. Always refer to the manufacturer’s specifications when you are selecting filler rods for a particular job.
For all mild steel welding operations, mild steel filler rods are used and no flux is necessary. The mild steel rods provide sufficient strength and ductility for the job. The size of the filler rod used will depend on the thickness of the parent metal.
For work up to 3mm thick a 1.5mm rod is satisfactory and a 3mm rod would be suitable for 5mm plate. If the rod is too small it will melt before the parent metal reaches welding temperature and if the rod is too big the parent metal may overheat before the rod can melt.
A neutral heating flame should be used and the gas mixture adjusted to produce the heat required for the job.
The welding method most suited to the types of projects undertaken in basic engineering courses is called forward welding or leftward welding.
When this welding technique is used you move the filler rod ahead of the torch which travels from right to left (for a right hand welder).
Point the flame in the direction that the weld is being made, hold the tip at about 60° to the surface of the plate and hold the filler rod at around 35° to the work.
The edges of the plate must be heated uniformly just ahead of the molten puddle. Move the tip and filler rod in half circular weaving movements as shown in the illustration above.
This technique helps you control the heat and enables the rod end and side walls of the plate to be melted into a uniformly distributed molten puddle. The envelope of the flame protects the molten weld pool from contamination by the atmosphere.
The illustration below shows a typical fusion welded butt joint. Maximum joint strength is achieved when the following weld requirements are met:
In practice, you may not always be able to achieve perfection in weld requirements which are usually set to given tolerances. You will be required to produce all welds on your job within the stated tolerances. This makes you personally responsible for the quality of each weld produced.
Example: The illustration on the right shows a fusion welded corner joint. The following is an example of weld requirements that could be stated for corner joints on a project:
Weld requirements will vary with the strength requirements of the job and the size of the material used.
The illustrations above and below show examples of other common fusion welded joints.
Weld requirements for these joints could be stated in much the same way as for the example.
The heat produced during the oxy-acetylene welding process causes expansion of the metal in the area of the joint. Contraction occurs as the metal cools down.
Various factors such as the thickness and quality of the parent metal, its melting point and the specific heat of the weld metal, will create varying conditions of expansion and contraction which cause forces or stresses to occur in the weld and parent metal.
There are two types of stresses which can occur in the oxy-acetylene welding process:
Residual stresses will cause distortion if the plates being welded are free to move. If the plates are rigid, the stresses remain as internal forces after the area has cooled.
Some examples of distortion caused by weld metal contraction are shown below.
The distortion shown in the illustrations on the right could be prevented using the following methods:
If the plates are clamped the parent metal must stretch or give in some way to prevent distortion. Residual internal stresses are likely to occur with this method.
The illustrations on the left show other effects of distortion caused by contraction of the weld area.
Flat plates can be tacked in position or wedged to overcome their tendency to pull together as shown in the illustrations below.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mc /r /f ][ Do you need a flux when doing fusion welding of mild steel? ][ * Yes. ][ No. ][ No - you don't need flux. ]]
[[ mr /f ][ Which of the following are pointers to a good fusion weld: ][ No slag or impurities in the weld. ][ * Slight indentation of the weld. ][ Good fusion over the whole of the side faces. ][ Penetration through the full thickness of the parent metal. ][ * Discoloured parent metal either side of the weld. ][ The weld should be slightly raised above the parent metal; discolouring doesn't indicate anything. ]]
[[ mr /f ][ Which of the following methods can prevent distorted weld joints? ][ Camping firmly to prevent movement. ][ Set the joint slightly larger so that the wels pulls the joint true. ][ * Cool the weld regularly while welding it. ][ * Cool the weld slowly after cleaning the slag off of it. ][ Cooling rates won't have any significant effect on the amount of distortion. ]]
The strength of butt welds depends mainly on the weld penetrating the full thickness of the parent metal. Causes of incomplete penetration include:
Corrective action could require any of the following:
The deposited weld metal should completely fuse with the side walls of the plate to form a consolidated joint of base metal and weld metal. Where this does not occur the joint can be weakened. Poor fusion could be caused by any of the following:
Corrective action could require any of the following:
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ Which of the following are valid causes of poor penetration with butt welds: ][ Poor preparation of the joint. ][ Using too large a filler rod. ][ Using too small a welding tip. ][ * Welding too slowly. ][ * Having too much heat on the job. ][ Slow welding and extra heat will not usually cause incomplete penetration. ]]
[[ mr /f ][ Which of the following are techniques for ensuring a good butt weld? ][ Use a filler rod that reaches the bottom of the weld. ][ Use sufficient heat to melt the filler rod. ][ Use sufficient heat to break down the welding surfaces. ][ * Use a linear motion to melt the bottom of the weld properly. ][ * Ensure the gap at the bottom of the weld is as small as possible. ][ You should use a weaving motion, and ensure the gap is wide enough to weld right to the bottom. ]]
Braze welding, or bronze welding, is a process of joining similar or dissimilar metals by using the heating flame and a copper-tin-zinc (bronze) filler rod. Braze welding is not a fusion welding process since the parent metals to be joined are not heated to their melting points. The bronze filler rod has a lower melting point than the parent metal or metals.
Bond strength of the weld depends on surface penetration of the molten bronze filler rod and the parent metal. Three distinct factors contribute to bond strength between the bronze and the parent metal:
Braze welding should not be used where the part is subject to temperatures above 340°C because of the decreased strength of the bronze at these temperatures.
Edges and surfaces are prepared in the same way as for a fusion weld exposing clean faces of the parent metal for the weld. An additional precaution is to clean back from the edge of the weld for a distance of about 12mm.
The tip should generally be a size smaller than for a similar fusion weld and the flame should be slightly oxidising. This will provide better control of the molten metal.
A special bronze flux is used with the filler rod so that oxides are removed as the weld proceeds. The rod is heated and dipped into the tin of flux before being applied to the job.
The constituents of bronze filler rods may vary. Different rods are used for different jobs. Tobin bronze, manganese bronze and phosphor bronze are among the types available. Always check the manufacturer’s recommendations when selecting a filler rod.
The parent metal must not be overheated and the rod must not be burned by contact with the luminous cone of the flame.
If the parent metal is at the correct temperature and the fluxed rod is applied, the bronze will flow smoothly, but if the parent metal is too cold, the rod metal will not spread. With suitable movement of the torch, you can control the temperature so there is sufficient time for the bronze to bond properly with the parent metal.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ Which of the following statement about braze welding are accurate? ][ It can be used to join dissimilar metals. ][ * The filler rod has a higher melting point than the parent metals. ][ * Braze welding is a fusion welding process. ][ The filler rod is made of bronze. ][ Braze welding is not a fusion welding process; the filler rod has a lower melting point. ]]
[[ mr /f ][ Which of the following factors improve bond strength? ][ Tinning of the surfaces to be welded. ][ Interalloying in the one of the join. ][ Intergranular penetration of the filler rod into the parent metal. ][ * Sufficient heat to fully melt the parent metals. ][ * Rapid cooling of the joint after the weld. ][ Parent metals should not melt during the weld; rapid cooling won't improve bond strength. ]]
Brazing is a process where molten filler metal is drawn by capillary action into the space between close adjacent surfaces of the parts to be joined. The base metal is not melted.
Absolute cleanliness of surfaces to be brazed is essential. Degreasing with sulphuric acid solution or other suitable preparations as well as mechanical cleaning methods of filing, grinding and rubbing with abrasive cloth or paper are generally used to prepare the surfaces.
Flux is applied in the region of the joint and the brazing alloy rod or strip should be coated with flux paste. The work is heated with the envelope of the flame until the flux begins to flow smoothly and rapidly along the joint.
The filler rod is applied and the molten filler alloy flows into the narrow space between the surfaces by capillary action.
Additional flux may need to be applied from time to time. Excessive temperatures should be avoided. You can check this by observing the under side of the joint. A disturbed oxide film indicates excessive temperature.
Brazing can be performed on many metals including steel, copper and aluminium. In all cases surface cleanliness is essential.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ so /f ][ Place these brazing steps into the correct order: ][ Clean the surfaces to be brazed thoroughly. ][ Apply flux to all surfaces and the brazing rod. ][ The flux is heated until it flows into the joint. ][ Molten filler rod flows into the narrow space. ][ Additional flux may be needed from time to time. ][ Clean => Apply flux => Flux heated => Filler rod flows => Extra flux. ]]