Oxyfuel Gas Welding
Gas welding utilizes the heat produced by a gas flame to melt the filler metal, if used, and the base metal, thereby creating a weld. Gas welding is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes and repair work. A similar process, called oxyfuel cutting, is used to cut metals.
The most common gas welding process is oxyacetylene welding. The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material. The temperature at which it burns is a function of the amount of oxygen present in the gas mixture.
Gases for Oxyfuel Welding
Common gases are:
- Acetylene – Oxygen. Compared with other fuel gases, oxyacetylene can produce the hottest and most concentrated flame. The oxyacetylene flame also produces carbon dioxide, a shielding gas. The oxyacetylene flame burns about 3,773 K (3,500 °C; 6,332 °F). As a fuel, acetylene’s primary disadvantage, compared to other fuels, is its high cost.
- Stabilized methylacetylene-propadiene (MPS) has the storage and shipping characteristics of LPG and has a heat value a little lower than that of acetylene. MPS is recommended for cutting applications in particular rather than welding applications.
- Hydrogen – Oxygen. Hydrogen has a clean flame and is good for use on aluminium. It can be used at a higher pressure than acetylene and is, therefore, useful for underwater welding and cutting. It is a good type of flame when heating large amounts of material. Hydrogen is not used for welding steels and other ferrous materials because it causes hydrogen embrittlement. Oxyhydrogen flame burns at 3,073 K (2,800 °C; 5,072 °F).
Oxidizing – Neutral – Reducing Flames
The temperature at which it burns is a function of the amount of oxygen present in the gas mixture. The figure demonstrates three types of flames that can be produced with oxyacetylene mixtures. Welding is generally carried out using the neutral flame setting, which has equal quantities of oxygen and acetylene.
- Reducing flame. The reducing flame is the flame with low oxygen and an excess of acetylene. Flame has a secondary feather extending from the inner cone. This secondary feather is caused by excess acetylene in the flame mixture, which alters the chemical composition of the weld pool by reducing iron oxide (reducing effect) and adding carbon (carburizing effect). It has a yellow or yellowish color due to carbon or hydrocarbons.
- Neutral flame. The neutral flame is the flame in which the amount of oxygen is precisely enough for burning, and neither oxidation nor reduction occurs. The flame is considered neutral because it neither significantly adds nor subtracts any elements from the weld pool. A flame with a good balance of oxygen is clear blue.
- Oxidizing flame. The oxidizing flame is the flame produced with an excessive amount of oxygen. When the amount of oxygen increases, the flame shortens, its color darkens, and it hisses and roars. Since, as its name suggests, it oxidizes the metal’s surface, this flame has a harmful effect on the properties of ferrous alloys. With some exceptions (e.g., platinum soldering in the jewelry), the oxidizing flame is usually undesirable for welding and soldering.