In oxyacetylene welding, the two gases used for producing flame are oxygen and acetylene. Oxygen is used to support and intensify combustion. It is generally prepared in factories by liquefying air and then separating it into its component parts by rectification. From factories, it can be obtained under high pressure in cylinders which are fitted with pressure regulators to get oxygen at desired pressure for welding.
Each cylinder is connected to the blow pipe by flexible hoses. The exact pressure used depends on the blowpipe nozzle size and thickness of the plate to be welded. Oxygen cylinders are painted black and acetylene cylinders are painted maroon. Similarly acetylene is also available commercially in cylinders. However, still it is being produced at the welding place for many applications.
Acetylene can be easily produced by the chemical reaction between water and calcium carbide (CaC2). It has to be produced and used at only low pressures as at high pressures explosion might be there. Generally it is prepared in two ways i.e. either by dropping CaC2 on water or by dropping water on CaC2. The pressure is kept same by placing constant weight on the cover of the container in which it is being produced and stored.
The reaction between water and CaC2 is given as below:
Flame:
This is the phenomenon produced at the surface of the nozzle tip where two gases meet and undergo combustion with the evolution of heat and some light.
The chemical reaction for complete combustion of oxygen and acetylene is as follows:
Thus for complete combustion, ratio of oxygen to acetylene is 21/2 to 1. The temperature of flame is dependent upon the relative proportion of the two gases. But temperature and complete combustion are not the only factors for welding because for different purposes, different ratios of gases have given best results e.g., for normal welding most suitable mixture is generally obtained by having equal proportions of oxygen and acetylene.
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This mixture produces a neutral flame which is neither oxidising nor carburising and is very suitable for welding, under this condition the following chemical actions take place:
From the above equations it is obvious that complete combustion takes place in two phases. From the oxygen and acetylene as obtained from torch, incomplete combustion takes place producing carbon monoxide.
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This reaction takes place at the inner core of the flame where the highest temperature is developed and it can be clearly seen as well- defined white inner cone. Further reactions take place in outer cone and get their oxygen from surrounding atmosphere. The above form of flame with temperature distribution is shown in Fig. 9.24.
Neutral flame obtained by mixing equal quantities of acetylene and oxygen is used for welding of all metals like ferrous materials, copper and aluminium alloys except brass.
Deviating from this ratio, carburising and oxidising flames can be obtained. In carburising flame (obtained by more quantity of acetylene) the inner zone is surrounded by a secondary luminous zone and extends into the outer envelope. In oxidising flame, the inner cone is very much shortened and pointed and also the luminosity is reduced.
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It is used mainly for welding brass. Carburising flame is very suitable for welding steel as rate of welding is faster by this flame than with neutral flame. It is also used for depositing stellite. Oxidising flame is very suitable for cutting operations due to very high temperatures and also for welding non- ferrous metals (brasses and bronzes) due to formation of a tenuous oxide film over the molten metal which prevents vaporisation of zinc.
Thus oxy-acetylene welding enables accurate control of both the size and type of flame which is very important for joining non-ferrous materials and metals of small section. Correct flame adjustment is essential for the production of sound welded joints by the oxy-acetylene process.
The two important considerations are:
(i) Flame size,
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(ii) Type of flame.
Oxy-acetylene welding blowpipes are supplied with a set of interchangeable nozzles, each of which has a different bore size. The bore determines the flame size, and nozzle selection is based on the thickness of the metal to be welded.
To bring the blowpipe (torch) into operation, the acetylene is first turned on and ignited. The oxygen valve is then gradually opened and regulated to produce the required flame.
The initial cost of gas welding equipment is less compared to arc welding, but running cost is high.
Principles of Operation and Welding Technique:
For forming the welded joint, a filler metal is added in the form of welding rod when the surfaces to be welded are just near melting stage. For obtaining satisfactory bond and for floating out impurities, generally some flux is also used. In some instances the joints are formed simply be fusing the parts to be joined, without the application of filler metal or flux.
There are two general methods of oxyacetylene welding known as forehand welding and backhand welding, (also known as rightward and leftward welding). In the first type, the rod follows the nozzle tip in the direction in which the weld is being made. The torch in moved in straight line from left to right and filler rod is moved in a series of loops along the weld preparation. In the latter type, the nozzle tip follows the rod in the direction of travel.
The filler rod is moved in straight line and the blowpipe is moved backwards and forwards in a series of loops as the weld proceeds. Backhand type welding is very suitable for pipe and plate welding because of combined economy and weld quality obtained by its application.
Backhand welding is normally used for welding metals of thickness upto 5 mm, but for heavier sections, the forehand method is used. The inclination of welding rod in either case is about 30°—40°, and that of torch is 40°—50° in case of forehand type welding and 60°— 70° in case of backhand type welding.
Rightward (Forehand) method has the following advantages over other method:
(i) As the flame is always directed towards the solidified weld, it results in annealing effect and better mechanical properties are obtained.
(ii) Very little agitation is produced because torch moves in a straight line. Agitation leads to excessive oxidation within the molten weld pool.
(iii) Smaller V angles are permissible due to better accessibility of the flame.
(iv) Less filler metal is required, gas consumption, welding time and cost are low.
(v) Operators get a better vision of welding zone.
Backfire (Popping):
Backfire or popping usually occurs due to pre-ignition of gases and results in small explosion at the flame.
Various causes are:
(i) The gas is flowing out too slowly and the pressure is too low for the tip size or orifice used. This results in fast flame propagation (burning) than the speed of flow of gas at the tip. This condition can be overcome by slightly increasing the pressure of both the gases.
(ii) If the tip is too close to weld or operating in a hot corner or by over use, it may be overheated and result into popping. The tip must be cooled under such a condition.
(iii) Sometimes carbon deposits or hot metal particles get lodged inside the orifice of the tip and act as igniters when they get overheated and result in backfire. The tip should therefore be cleaned to avoid backfire.
(iv) Popping also occurs due to keeping the inner core of the flame submerged in the puddle.
Flashback:
A flashback is a condition where the gas burns back to the regulator in which case hose, torch and regulator may get damaged. Flashback may occur if oxygen feeds back into the acetylene hose to form a combustible mixture which may result in a violent explosion.
This trouble may be caused by a clogged barrel, or mixture passage along with an excessive oxygen pressure. In some cases, organic oxides may form inside the oxygen hose and if the hose be raised to ignition temperature, an explosion may occur.
Low-Pressure and High-Pressure Oxyacetylene Welding Processes:
The terms ‘low-pressure’ and ‘high-pressure’ refer to the form in which the acetylene is supplied. In the low-pressure process the acetylene is generated in a low-pressure generator by the action of water on calcium carbide, and is supplied to the blowpipe at low pressure from a gas-holder incorporated in the generator.
The gas has to be purified, and a special back-pressure valve must be inserted between the gas-holder and the blowpipe. Special storage facilities must be provided for the calcium carbide, and the generator must be cleaned and recharged periodically.
The equipment for the high-pressure welding process is compact and portable. It comprises two cylinders of compressed gas, each fitted with a pressure regulator and a high-pressure blowpipe. One cylinder contains dissolved acetylene, and the other contains oxygen. The supply of gas flow from cylinders to the blowpipe in rubber canvas hoses is controlled by the pressure regulators. The gas is very pure, but its cost is relatively high.