Oxy–fuel welding and cutting
Oxy–fuel welding and oxy–fuel cutting are processes that use fuel gases and oxygen to weld or cut metals. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen-acetylene welding in 1903. Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material in a room environment.
A common propane/air flame burns at about, a propane/oxygen flame burns at about, an oxyhydrogen flame burns at and an acetylene/oxygen flame burns at about.
During the early 20th century, before the development and availability of coated arc welding electrodes in the late 1920s that were capable of making sound welds in steel, oxy–acetylene welding was the only process capable of making welds of exceptionally high quality in virtually all metals in commercial use at the time. These included not only carbon steel but also alloy steels, cast iron, aluminium, and magnesium. In recent decades it has been superseded in almost all industrial uses by various arc welding methods offering greater speed and, in the case of gas tungsten arc welding, the capability of welding very reactive metals such as titanium.
Oxy–acetylene welding is still used for metal-based artwork and in smaller home-based shops, as well as situations where accessing electricity would present difficulties. The oxy–acetylene welding torch remains a mainstay heat source for manual brazing, as well as metal forming, preparation, and localized heat treating. In addition, oxy–fuel cutting is still widely used, both in heavy industry and light industrial and repair operations.
In oxy–fuel welding, a welding torch is used to weld metals. Welding metal results when two pieces are heated to a temperature that produces a shared pool of molten metal. The molten pool is generally supplied with additional metal called filler. Filler material selection depends upon the metals to be welded.
In oxy–fuel cutting, a torch is used to heat metal to its kindling temperature. A stream of oxygen is then trained on the metal, burning it into a metal oxide that flows out of the kerf as dross.
Torches that do not mix fuel with oxygen are not considered oxy–fuel torches and can typically be identified by a single tank. Most metals cannot be melted with a single-tank torch. Consequently, single-tank torches are typically suitable for soldering and brazing but not for welding.
Uses
Oxy–fuel torches are or have been used for:- Heating metal: in automotive and other industries for the purposes of loosening seized fasteners.
- Neutral flame is used for joining and cutting of all ferrous and non-ferrous metals except brass.
- Depositing metal to build up a surface, as in hardfacing.
- Also, oxy-hydrogen flames are used:
- *In stone working for "flaming" where the stone is heated and a top layer crackles and breaks. A steel circular brush is attached to an angle grinder and used to remove the first layer leaving behind a bumpy surface similar to hammered bronze.
- *In the glass industry for "fire polishing".
- *In jewelry production for "water welding" using a water torch.
- *In automotive repair, removing a seized bolt.
- *Formerly, to heat lumps of quicklime to obtain a bright white light called limelight, in theatres or optical lanterns.
- *Formerly, in platinum works, as platinum is fusible only in the oxyhydrogen flame and in an electric furnace.
Apparatus
The apparatus used in gas welding consists basically of an oxygen source and a fuel gas source, two pressure regulators and two flexible hoses, and a torch. This sort of torch can also be used for soldering and brazing. The cylinders are often carried in a special wheeled trolley.There have been examples of oxyhydrogen cutting sets with small gas cylinders worn on the user's back in a backpack harness, for rescue work, and similar.
There are also examples of both non-pressurized and pressurized liquid fuel cutting torches, usually using gasoline. These are used for their increased cutting power over gaseous fuel systems and also greater portability compared to systems requiring two high pressure tanks.
Regulator
The regulator ensures that pressure of the gas from the tanks matches the required pressure in the hose. The flow rate is then adjusted by the operator using needle valves on the torch. Accurate flow control with a needle valve relies on a constant inlet pressure.Most regulators have two stages. The first stage is a fixed-pressure regulator, which releases gas from the cylinder at a constant intermediate pressure, despite the pressure in the cylinder falling as the gas in it is consumed. This is similar to the first stage of a scuba-diving regulator. The adjustable second stage of the regulator controls the pressure reduction from the intermediate pressure to the low outlet pressure. The regulator has two pressure gauges, one indicating cylinder pressure, the other indicating hose pressure. The adjustment knob of the regulator is sometimes roughly calibrated for pressure, but an accurate setting requires observation of the gauge.
Some simpler or cheaper oxygen–fuel regulators have only a single-stage regulator, or only a single gauge. A single-stage regulator will tend to allow a reduction in outlet pressure as the cylinder is emptied, requiring manual readjustment. For low-volume users, this is an acceptable simplification. Welding regulators, unlike simpler LPG heating regulators, retain their outlet pressure gauge and do not rely on the calibration of the adjustment knob. The cheaper single-stage regulators may sometimes omit the cylinder contents gauge, or replace the accurate dial gauge with a cheaper and less precise "rising button" gauge.
Gas hoses
The hoses are designed for use in welding and cutting metal. A double-hose or twinned design can be used, meaning that the oxygen and fuel hoses are joined. If separate hoses are used, they should be clipped together at intervals approximately apart, although that is not recommended for cutting applications, because beads of molten metal given off by the process can become lodged between the hoses where they are held together, and burn through, releasing the pressurized gas inside, which in the case of fuel gas usually ignites.The hoses are color-coded for visual identification. The color of the hoses varies between countries. In the United States, the oxygen hose is green and the fuel hose is red. In the UK and other countries, the oxygen hose is blue, and the acetylene hose is red. If liquefied petroleum gas fuel, such as propane, is used, the fuel hose should be orange, indicating that it is compatible with LPG. LPG will damage an incompatible hose, including most acetylene hoses.
The threaded connectors on the hoses are handed to avoid accidental mis-connection: the thread on the oxygen hose is right-handed, while the fuel gas hose has a left-handed thread. The left-handed threads also have an identifying groove cut into their nuts.
Gas-tight connections between the flexible hoses and rigid fittings are made by using crimped hose clips or ferrules, often referred to as 'O' clips, over barbed spigots. The use of worm-drive hose clips or Jubilee Clips is specifically forbidden in the UK and other countries.
Non-return valve
Acetylene is not just flammable; in certain conditions it is explosive. Although it has an upper flammability limit in air of 81%, acetylene's explosive decomposition behaviour makes this irrelevant. If a detonation wave enters the acetylene tank, the tank will be blown apart by the decomposition. Ordinary check valves that normally prevent backflow cannot stop a detonation wave because they are not capable of closing before the wave passes around the gate. For that reason a flashback arrestor is needed. It is designed to operate before the detonation wave makes it from the hose side to the supply side.Between the regulator and hose, and ideally between hose and torch on both oxygen and fuel lines, a flashback arrestor and/or non-return valve should be installed to prevent flame or oxygen–fuel mixture being pushed back into either cylinder and damaging the equipment or causing a cylinder to explode.
European practice is to fit flashback arrestors at the regulator and check valves at the torch. US practice is to fit both at the regulator.
The flashback arrestor prevents shock waves from downstream coming back up the hoses and entering the cylinder, possibly rupturing it, as there are quantities of fuel/oxygen mixtures inside parts of the equipment that may explode if the equipment is incorrectly shut down, and acetylene decomposes at excessive pressures or temperatures. In case the pressure wave has created a leak downstream of the flashback arrestor, it will remain switched off until someone resets it.