CEMP-E
TI 809-26
1 March 2000
a. Oxyfuel Cutting. With oxyfuel gas cutting (OFC), the steel is heated with a torch to its ignition
temperature, then exposed to a stream of oxygen from the same torch. The oxygen causes rapid
oxidation, or "burning" to occur, which itself creates additional heat to allow the process to continue. The
force of the oxygen stream blows away the molten steel, leaving a cut edge. The fuel gas used in oxyfuel
cutting may be natural gas, propane, acetylene, propylene, MPS, or other proprietary fuel gases.
b. Plasma Arc Cutting. Plasma arc cutting (PAC) is sometimes used in shop fabrication, and is
generally limited to steels 25 mm (1 in.) thick or less. Similar to oxyfuel cutting, the steel is heated to the
point of melting, only this function is performed using an electric arc. The molten steel is then removed
by the high velocity stream of plasma (ionized gas) created by the arc itself, within the cutting torch.
Gases used for PAC include nitrogen, argon, air, oxygen, and mixtures of nitrogen/oxygen and
argon/hydrogen. With plasma arc cutting, the area of steel heated by the process is less, resulting in less
steel metallurgically affected by the heat of cutting, as well as less distortion. PAC generates
considerable fume and noise, and therefore a water table and water shroud is typically used to minimize
these undesirable environmental effects.
c. Edge Quality. The quality of thermally cut edges is governed by AWS D1.1 Section 5.15.4. Limits
are placed on surface roughness, as measured using ANSI/ASME B46.1, Surface Texture (Surface
Roughness, Waviness and Lay). A plastic sample, AWS C4.1-G, Oxygen Cutting Surface Roughness
Gauge, is typically used for visual comparison in lieu of physical measurement of surface roughness.
Limitations are also placed on the depth and sharpness of gouges and notches. AISC, in Section M2.2,
takes a minor exception to AWS D1.1 quality criteria.
5. AIR CARBON ARC GOUGING. Air carbon arc gouging (ACAG) is commonly used to perform edge
preparation for groove joints (especially J- and U-grooves), to remove unacceptable discontinuities from
weld deposits, and to remove temporary attachments such as backing bars or lifting lugs. It may also be
used to remove entire welds when structural repairs or modifications are necessary.
a. Process. The process appears similar to SMAW, with an electrode holder and a single electrode,
and is usually performed manually, however, the electrode is a carbon electrode covered with a copper
sheath. The electrode creates a controlled arc, melting the steel, which is quickly followed by the focused
application of compressed air from the electrode holder. The air provides continued rapid oxidation, as
well as removes the molten steel from the area. For complete information, see ANSI/AWS C5.3,
Recommended Practices for Air Carbon Arc Gouging and Cutting.
b. Surface Finishing. Following ACAG, the joint should be thoroughly cleaned by wire brushing.
Grinding of surfaces prior to welding is not required. If not welded, light grinding of the ACAG surface is
suggested.
3-7
Previous Page
