Concepts in OXYFUEL WELDING Several members of the

Concepts in OXYFUEL WELDING

Several members of the OFW group are based on gases other than acetylene. Most of the alternative fuels are listed in Table 23. 1, together with their burning temperatures and combustion heats. For comparison, acetylene is included in the list. Although oxyacetylene is the most common OFW fuel, each of the other gases can be used in certain applications—typically limited to welding of sheet metal and metals with low melting

Table 23. 1

Temperatures. In addition, some users prefer these alternative gases for safety reasons. The fuel that competes most closely with acetylene in burning temperature and heating value is methylacetylene-propadiene. It is a fuel developed by the Dow Chemical Company sold under the trade name MAPP (C 3 H 4) has heating characteristics similar to acetylene and can be stored under pressure as a liquid, thus avoiding the special storage problems associated with C 2 H 2. When hydrogen is burned with oxygen as the fuel, the process is called oxyhydrogen welding (OHW).

As shown in Table 23. 1, the welding temperature in OHW is below that possible in oxyacetylene welding. In addition, the color of the flame is not affected by differences in the mixture of hydrogen and oxygen, and therefore it is more difficult for the welder to adjust the torch. Other fuels used in OFW include propane and natural gas. Propane (C 3 H 8) is more closely associated with brazing, soldering, and cutting operations than with welding. Natural gas consists mostly of ethane (C 2 H 6) and methane (CH 4). When mixed with oxygen it achieves a high temperature flame and is becoming more common in small welding shops.

This is a special OFW process, distinguished by type of application rather than fuel gas. Pressure gas welding (PGW) is a fusion-welding process in which coalescence is obtained over the entire contact surfaces of the two parts by heating them with an appropriate fuel mixture (usually oxyacetylene gas) and then applying pressure to bond the surfaces. A typical application is illustrated in Figure 23. 1. Parts are heated until melting begins on the surfaces. The heating torch is then withdrawn, and the parts are pressed together and held at high pressure while solidification occurs. No filler metal is used in PGW.

Figure 23. 1

Filler metals are used to supply additional metal to the weld zone during welding. They are available as filler rods or wire , and may be bare or coated with flux. The purpose of the flux is to retard oxidation of the surfaces of the parts being welded by generating a gaseous shield around the weld zone. The flux also helps to dissolve and remove oxides and other substances from the weld zone, thus contributing to the formation of a stronger joint. The slag developed (compounds of oxides, fluxes, and electrode-coating materials) protects the molten puddle of metal against oxidation as it cools.