A distinction is made between thermal and mechanical gouging methods. The thermal methods are generally faster than the mechanical ones.
Thermal gouging is an essential part of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected - usually by blowing it away. Normal oxyfuel gas or arc processes can be used to produce rapid melting and metal removal.
Gouging operations can be carried out using the following thermal processes:
• oxyfuel process
• plasma arc
• manual metal arc
• air carbon arc
Flame or Oxyfuel Gouging
Flame gouging is a variant of conventional oxyfuel gas welding. Oxygen and a fuel gas are used to produce a high temperature flame for melting the steel. When gouging, the steel is locally heated to a temperature above the 'ignition' temperature (typically 900deg.C) and a jet of oxygen is used to melt the metal - a chemical reaction between pure oxygen and hot metal. This jet is also used to blow away molten metal and slag. It should be noted that compared with oxyfuel cutting, slag is not blown through the material, but remains on the top surface of the workpiece. The gouging nozzle is designed to supply a relatively large volume of oxygen through the gouging jet. In oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a near-neutral preheating flame. The oxygen jet flow rate determines the depth and width of the gouge.
Plasma Arc Gouging
The use of the plasma arc as a gouging tool dates back to the 1960s when the process was developed for welding. Compared with the alternative oxyfuel and MMA gouging techniques, plasma arc has a needle-like jet which can produce a very precise groove, suitable for application on almost all ferrous and non- ferrous materials. Plasma gas can be argon, helium, argon - H 2 , nitrogen or air.
Manual Metal Arc and Air Carbon Arc Gouging
In these processes a electrical arc is generated to melt the material. Other techniques like special electrodes or a jet of compressed air are used to blow away the molten material. No specific high purity or compressed gases are needed in these processes.