Arc Welding
Course Programme (2-5 days):
Gas safety
Basic principles
Selection of nozzles
Gas pressures
Welding various joint types in position Oxy-Gas Welding
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Description of the Process
Oxy-Acetylene welding relies on the heat of a flame to melt the material being welded. Fusion can either be autogenous or with the addition of a filler material.
Safety
The high flame temperature required for oxy-fuel welding and cutting processes is obtained by combining oxygen with a fuel gas to produce flame temperatures of approximately 2500°C to 3000°C. There are certain precautions to be taken when using gases and these are described below:-
Fig.1 OXYGEN (black, right hand thread) should be handled carefully and must not be stored in warm areas. The pressure of oxygen in cylinders when full is 2500lbs.per square inch (200 bar) at normal temperature and any rise in surrounding temperature will cause an increase in cylinder pressure above 2500lbs.Oil or grease should never be allowed to come into contact with oxygen cylinders as an inflammable mixture will be formed, which may ignite spontaneously.
Fig. 2 ACETYLENE (maroon, left hand thread) should be stored in a separate fuel compound as acetylene and air form an explosive mixture, these cylinders should also be chained when either in use or in storage. No alloy containing more than 70% copper should be used on any part of the equipment as a highly explosive compound is formed. Cylinders must always be stored upright and away from high temperatures.
Fig 3. PROPANE (red, left hand thread). Storage as for acetylene, a fuel gas. Highly inflammable and can form explosive mixtures with air or oxygen (2% to 10%). With propane being heavier than air it can flow easily in still air some considerable distances therefore care must be exercised when using this gas.
Equipment
Pressure Regulators
Regulators should always be treated as delicate, precision instruments and must not be subject to sudden pressures or knocks. Not only are regulators identified by colour, they also have right hand or left hand threads depending on the type of gas being used. The type of thread can be identified by a notch cut on the equipment connections, showing the gas to be a fuel gas.
Single or multi stage regulators are available depending on working conditions. Single stage are suitable for welding with maximum outlet pressure of 2.1 bar, or scrap and heavy duty cutting with pressures of 8.3 to 14 bar.
Multi stage reduces pressure twice, giving a more stable output pressure, suitable for quality welding and cutting applications.
Blowpipes
Various types available, low pressure and high pressure, high pressure where gas is mixed in either the head or the shank, while a low pressure blowpipe with an injector mixing can be used with low pressure. Cutting torches must be kept clean and free from oil, grease etc. and periodically inspected for wear and damage.
Gas Hose
Should be maintained in good condition at all times and fitted with the proper connections. Most hoses are fitted with hose check valves to prevent damage to the regulators.
Flashback Arrestors
These are safety devices on the outlet of the oxygen and fuel gas regulators. A highly sensitive cut off mechanism operates at the slightest back pressure, whether the pressure wave is slow or sudden. An effective non-return valve, which also prevents flashback.
Oxy-Acetylene Welding Techniques
There are two techniques used to weld flat joints:
Leftward Welding
This method is used on low carbon steel sheet and plate in thicknesses up to 5 mm and also on cast iron and certain non-ferrous metals. As the name implies, the weld is started at the right hand side and progresses towards the left. The filler rod precedes the blowpipe and is held at an angle of 30° - 40° to the work surface. The blowpipe is held at an angle of 60° - 70° to the work surface and is given a slight side to side movement to ensure side fusion as the filler rod is fed into the molten pool.
Fig. 4
Rightward Welding
This method is used on steel plate over 5 mm thick. The weld is started at the left hand side of the joint and progresses towards the right. The blowpipe is held at an angle of 40° - 50° to the work surface and travels in a straight line. The filler rod, which is held at an angle of 30° - 40° to the work surface, follows the blowpipe and is fed into the molten pool with a circular action. A considerable amount of practice is required to perfect this technique.
The advantages of rightward welding over the leftward technique are higher speed, less distortion and more economical use of gas and filler rod. This is due to the fact that thicknesses up to and including 8 mm need not be bevelled and for butt joints in plate over 8 mm the included angle of the vee preparation is only 60°.
Fig. 5
The Oxy-Acetylene Flame
The heat source for this process is a chemical reaction resulting from the combustion of acetylene with oxygen. This is an exothermic reaction in which equal volumes of acetylene and oxygen supplied by the blowpipe react to produce carbon monoxide and hydrogen as products of the first stage of combustion. The reaction is as follows:-
Fig. 6 The Oxy-Acetylene flame
Theoretically, equal volumes of oxygen and acetylene are supplied to the blowpipe.
Chemical reactions are as follows:-
Stage 1
Acetylene + Oxygen = Carbon Monoxide + Hydrogen
C2H2 + O2 = 2CO + H2
Stage 2
Carbon Monoxide + Hydrogen + Oxygen = Carbon Dioxide + Water
CO + H2 + O2 = CO2 + H2O
In Stage 2, the carbon monoxide burns and forms carbon dioxide, while the hydrogen which is formed from the action in Stage 1, combines with oxygen to form water. The combustion is therefore complete and carbon dioxide and water (turned to steam) are the chief products of combustion.
This produces a flame temperature of approximately 32000°C.
Oxy-Acetylene Flame Types
Neutral Flame
As the supply of oxygen to the blowpipe is further increased, the flame contracts and the white cone becomes clearly defined, assuming a definite rounded shape. At this stage approximately equal quantities of acetylene and oxygen are being used and combustion is complete, all the carbon supplied by the acetylene is being consumed and the maximum heat given out. The flame is now neutral, and this type of flame is the one most extensively used by the welder, who should make himself thoroughly familiar with its appearance and characteristics.
Carburising Flame
This is a flame in which an excess of acetylene is burning, i.e. combustion is incomplete and unconsumed carbon is present. When lighting the blowpipe the acetylene is turned on first and ignited, giving a very smoky yellow flame of abnormal size, showing two cones of flame in addition to an outer envelope; this is an exaggerated form of the carburising flame, but gives out comparatively little heat and is of little use for welding. When the oxygen is turned on and the supply is gradually increased, the flame, though still of abnormal size contracts towards the blowpipe tip where an inner white cone of great luminosity commences to make its appearance. If the increase in the supply of oxygen is stopped before the cone becomes clearly defined and while it is still an inch or so long, the result is a carburising flame which is mainly used for hard surfacing and should not be employed for welding steel as unconsumed carbon may be introduced into the weld and produce a hard, brittle, deposit.
Oxidising Flame
A further increase in the oxygen supply will produce an oxidising flame in which there is more oxygen than is required for complete combustion. The inner cone will become shorter and sharper, the flame will turn a deeper purple colour and emit a characteristic slight "hiss", while the molten metal will be less fluid and tranquil during welding and excessive sparking will occur. An oxidising flame is only used for special applications, and should never be used for welding.
Oxy-Fuel Gas Equipment Safety Test
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