THERMAL CUTTING PROCESSES
steels and to prepare bevel and vee grooves. In this process, the metal is
heated to its ignition temperature, or kindling point, by a series of preheat
flames. After this temperature is attained, a high-velocity stream of
pure oxygen is introduced, which causes oxidation or ‘‘burning’’ to
occur. The force of the oxygen steam blows the oxides out of the joint,
resulting in a clean cut. The oxidation process also generates additional
thermal energy, which is radially conducted into the surrounding steel,
increasing the temperature of the steel ahead of the cut. The next portion
of the steel is raised to the kindling temperature, and the cut proceeds.
Carbon and low-alloy steels are easily cut by the oxyfuel process.
Alloy steels can be cut, but with greater difficulty than mild steel. The
level of difficulty is a function of the alloy content. When the alloy
content reaches the levels found in stainless steels, oxyfuel cutting cannot
be used unless the process is modified by injecting flux or iron-rich
powders into the oxygen stream. Aluminum cannot be cut with the
oxyfuel process. Oxyfuel cutting is commonly regarded as the most
economical way to cut steel plates greater than 1⁄2 in thick.
A variety of fuel gases may be used for oxyfuel cutting, with the
choice largely dependent on local economics; they include natural gas,
propane, acetylene, and a variety of proprietary gases offering unique
advantages. Because of its role in the primary cutting stream, oxygen is
always used as a second gas. In addition, some oxygen is mixed with the
fuel gas in proportions designed to ensure proper combustion.
Plasma Arc Cutting (PAC) The plasma arc cutting process (Fig.
13.3.13) was developed initially to cut materials that do not permit the
use of the oxyfuel process: stainless steel and aluminum. It was found,
however, that plasma arc cutting offered economic advantages when
applied to thinner sections of mild steel, especially those less than 1 in
thick. Higher travel speed is possible with plasma arc cutting, and the
volume of heated base material is reduced, minimizing metallurgical
changes as well as reducing distortion.
PAC is a thermal and mechanical process. To utilize PAC, the material
is heated until molten and expelled from the cut with a high-velocity
stream of compressed gas. Unlike oxyfuel cutting, the process does not
rely on oxidation. Because high amounts of energy are introduced through the arc, PAC is capable of extremely high-speed cutting. The
thermal energy generated during the oxidation process with oxyfuel
cutting is not present in plasma; hence, for thicker sections, PAC is not
economically justified. The use of PAC to cut thick sections usually is
restricted to materials that do not oxidize readily with oxyfuel.
Air Arc Gouging (AAG) The air carbon arc gouging system (Fig.
13.3.14) utilizes an electric arc to melt the base material; a high-velocity
jet of compressed air subsequently blows the molten material away. The
air carbon gouging torch looks much like a manual electrode holder, but
it uses a carbon electrode instead of a metallic electrode. Current is
conducted through the base material to heat it. A valve in the torch
handle permits compressed air to flow through two air ports. As the air
hits the molten material, a combination of oxidation and expulsion of
metal takes place, leaving a smooth cavity behind. The air carbon arc
gouging system is capable of removing metal at a much higher rate than
can be deposited by most welding processes. It is a powerful tool used to
remove metal at low cost.
Plasma Arc Gouging A relatively new development is the application
of plasma arc equipment for gouging. The process is identical to
plasma arc cutting, but the small-diameter orifice is replaced with a
larger one, resulting in a broader arc. More metal is heated, and a larger,
broader stream of hot, high-velocity plasma gas is directed toward the
workpiece. When the torch is inclined to the work surface, the metal can
be removed in a fashion similar to air carbon arc gouging. The applications
of the process are similar to those of air carbon arc gouging.

Additional comments :