5.3. Gas Welding

Gas welding is one of the chemical welding processes in which the required heat energy is produced as a result of combustion of gases. The main materials available for gas welding are known to be acetylene C₂H₂, hydrogen H₂, natural gas C_mH_n, kerosene. Oxygen O₂ is used as an oxidizer.

Acetylene, possessing the highest heat-producing ability, provides in combustion the highest flame temperature (~3200°C) and gain wide application in gas welding. Calcium carbide CaC₂ interacts with water to produce acetylene in the acetylene generators:

(5.5)

Three types of generators are distinguished:

- contact-type (water recession);

- water-to-carbide;

- carbide-to-water.

Figure 5.13 represents the most widely applied generator of contact type.

Fig. 5.13. Acetylene generator of contact (water recession) type:

1 – tank; 2 – water; 3 – bell; 4 – calcium carbide; 5 – acetylene

A tank 1 contains water 2. Calcium carbide in net 4 is hang up in the bell 3 and reacts with water producing acetylene. When tap is closed, acetylene pressure increases, water is expelled and its level in the tank drops. The reaction is retarded. When acetylene is consumed, pressure is lowered, water returns to the bell, the reaction surface increases and acetylene yield correspondingly rises.

Hence, acetylene is usually produced in acetylene generators, or is taken from cylinders, charged at special station. Acetylene is explosive gas and requires careful treatment. Cylinders are filled with charcoal powder and acetone under a pressure of 1.6 MPa. Cylinders are white painted.

Oxygen is obtained from the air using a selective evaporation at special shops. Blue cylinder is filled by oxygen under a pressure of 15 MPa.

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

Welding torches are used to produce acetylene-oxygen mixture in subsequent combustion of which the welding flame is obtained. Welding torches of injector type are normally employed nowadays (Fig. 5.14). The oxygen pressure before an injector 4 is about 0.3...0.4 MPa. Running out with high speed into mixture chamber 3 it produces significant vacuum, by which acetylene is sucked into the chamber (its pressure within the hoseline may be rather low, from 0.001 to 0.015 MPa). There is replaced tip (head) 2 with calibrated orifice at the end of the gas torch. It serves for regulation of the flame power.

Fig. 5.14. Gas welding torch: 1 – fuel mixture; 2 – tip; 3 – mixture chamber; 4 – injector; 5 – valves

Pressure regulators are employed to reduce the gas pressure within the cylinder to the working value and to maintain the value at a constant level automatically.

The ratio between C₂H₂ and O₂ is adjusted by corresponding valves 5 on the torch.

Three particular zones are differed in welding flame (Fig. 5.15):

- core (I)

- welding zone (II)

- tongue, or jet (III)

The highest temperature is achieved in welding zone (3100...3200°C). Hence, welding process is carried out in this zone.

Three types of flame are distinguished according to the ratio between oxygen and acetylene in the mixture:

- balanced (normal) - O₂/C₂H₂1

- oxidizing - O₂/C₂H₂ > 1

- reducing (carbonizing) - O₂/C₂H₂ < 1

Different flame types are employed in welding of various alloys. For instance, in welding the high carbon steel or cast iron reducing flame is needed; in welding the brasses oxidizing one is required. In the majority of cases balanced flame is employed.

Gas or oxy-acetylene welding provides gradual (smooth) and slow (regulated) heating to be achieved. It is the main peculiarity and advantage of the process. That is why gas welding is used for thin steel parts (0.2…0.5 mm in thickness), non-ferrous alloys, cast iron and a number of alloy steels inclined to cracking (crackness).

Fig. 5.15. Peculiar zones of welding flame: 1 – core; 2 – welding zone; 3 – tongue (jet)