GAS WELDING

Purpose of the work: to study the design and action principle of gas welding equipments, to master the adjustment of welding flame and welding regimes selection.

Theory. Gas welding is one of the chemical welding processes in which the necessary heat energy is produced as a result of oxidation (combustion) of a gas. The main substances available for gas welding are known to be the acetylene or another gas (hydrogen, natural gas) and oxygen used as an oxidizer.

Acetylene, possessing the high heat-producing ability, provides the highest flame temperature to be achieved (3100…3200°C) in combustion. The gas is usually produced in special installations – acetylene generators or taken from cylinders charged at special stations. Acetylene is explosive and requires careful treatment.

Oxygen is obtained from the air using selective evaporation at special shops and conveyed in cylinders.

Equipment necessary for gas welding is as follows: acetylene generators, water protective seals, oxygen cylinders, pressure regulators, welding torches.

Calcium carbide CaC₂ interacts with water to produce acetylene C₂H₂ in acetylene generators with a given productivity. Three types of generators are distinguished: contact-type (water recession), water-to-carbide and carbide-to-water.

In contact-type generators at the beginning the whole carbide amount interacts with water. As far as the gas is produced, water has to be removed from the work chamber. Thus, the interaction surface reduces and generator's productivity decreases. These generators provide the lowest acetylene yield to be obtained. Their productivity ranges from 0.8 to 1.25 m³/h.

In water-to-carbide generators water periodically drops on calcium carbide pieces. Such generators possess a simply design, relatively high acetylene yield (80…90%) and productivity (1.25…3 m³/h). They have obtained the most widespread.

Carbide-to-water generators (carbide in portions is fed into water) provide the highest acetylene yield (almost 95%) and productivity (10…35 m³/h) to be obtained. Simultaneously they are of the most complicated design.

Protective seal is used to prevent an explosion in case of so-called "reverse impact of the flame" occurred when the oxygen-acetylene mixture inflames within the torch and the flame spreads towards the generator. Such phenomenon happens if the torch is heated over the temperature of 500°C.

Oxygen and acetylene cylinders have spherical bottoms and necks at the top. Special pad at the bottom permits to set the cylinder upright. Normal capacity of the cylinder is 40 litres(liters). Oxygen is kept under the pressure of 15 MPa (6 m³ of the gas within the cylinder).

Acetylene is transported in the cylinders under the pressure no more than 1.6 MPa, because explosion may happen ifthe pressure exeeds this value. The gas should be dissolved in acetone and the cylinder's cavity should be filled with porous carbon mass (sizes ofcarbon grains range from 1 to 3.5 mm).

Pressure regulators are employed to reduce the gas pressure to the working value and to maintain the value at a constant level automatically. The thread of oxygen and acetylene regulators nuts differs that excludes the possibility to mount the regulators vice versa.

Welding torches are used to produce acetylene-oxygen mixture in subsequent combustion of which the welding -flame is obtained. Welding torches ofinjector type (Fig. 5.2) are normally employed nowadays. The oxygen pressure before the injector is about 0.3…0.4 MPa. Running out with the high speed into the mixture chamber it produces significant vacuum. Hence, acetylene is also sucked into the chamber (its pressure within the pipeline may be rather low – up to 0.001 MPa).

The flame power is determined by gas flow adjusted by replacement of tips. The torches of injector type are delivered as a set of a trunk and several tips. Technical data of the welding torches is given in the table 5.2.

The welding flame has three peculiar zones: core, welding zone and tongue (Fig. 5.3). The highest temperature is achieved in the welding (middle) zone. Hence, it is used in welding process.

 

Fig. 5.2. Gas welding torch: 1 –fuel mixture; 2 tip -; 3 – injector; 4 – valves

 

5.2. Technical data of the welding torchs

Type of the torch	N° of the tip	Thickness of work piece, mm	Gas consumption, 1/h	Oxygen working pressure MPa
Acetylene under the pressure of 0.001 MPa	Oxygen
ГС-2		0.3…0.6 0.5…1.5 1.0…2.5 2.5…4.0	25…60 50…125 120…240 230…400	28…70 55…135 130…260 260…440	0.8…0.4 0.1…0.4 0.15…0.4 0.2…0.4
ГС-3		0.5…1.5 1.0…2.5 2.5…4.0 4.0…7.0 7.0…11.0 10…18 17…30	50…125 120…240 230…400 400…700 660…1100 1050…1750 1700…2800	55…135 130…260 260…440 430…750 740…1200 1150…1950 1900…3100	0.1…0.4 0.15…0.4 0.15…0.4 0.2…0.4 0.2…0.4 0.2…0.4 0.2…0.4

 

Fig. 5.3. Welding flame: 1 – tongue; 2 – welding (middle) zone; 3 – core

 

Three types of the flame are distinguished according to the ratio between oxygen and acetylene in the mixture: balanced (normal), oxidizing and reducing (carbonizing) flame. Different flame types are employed in welding various alloys. In welding, for instance, of high-carbon steels the reducing flame is needed, in welding of brasses – the oxidizing one. In the majority of cases the balanced flame is employed. The ratio between acetylene and oxygen is adjusted by respective valves on the torch.

Gas welding provides fluent (smooth) and slow heating to be achieved. It is the main peculiarity of the process. That is why gas welding is used for thin steel parts (0.2…0.5 mm in thickness), non-ferrous metals, cast iron and a number of alloy steels inclined to crackness. If the thickness of welding parts rises, the heating time increases, but productivity falls abruptly. More than that slow heating causes deformation and strain that reduces the field of gas welding application.