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REPRODUCED - COURTESY TWI-UK

Oxygen-fuel Gas Flame Gouging Oxygen-fuel (oxyfuel) flame gouging offers fabricators a quick and efficient method of removing metal. It can be at least four times quicker than cold chipping operations. The process is particularly attractive because of its low noise, ease of handling, and ability to be used in all positions. Process description 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. This can be as much as 300 litre/min through a 6mm orifice nozzle. In oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a neutral preheating flame. The oxygen jet flow rate determines the depth and width of the gouge. Typical operating parameters (gas pressures and flow rates) for achieving a range of gouge sizes (depth and width) can be seen in the Table . Typical operating data for manual oxyacetylene flame gouging Nozzle orifice dia.(mm) Gouge dimensions Gas pressure Gas consumption Travel speed (mm/min) Width (mm) Depth (mm) Acetylene (Bar) Oxygen (Bar) Acetylene (Litre/min) Preheat (Litre/min) Oxygen (Litre/min) 3 6-8 3-9 0.48 4.2 15 22 62 600 5 8-10 6-12 0.48 5.2 29 31 158 1000 6.5 10-13 10-13 0.55 5.5 36 43 276 1200 When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its surfaces are smooth with a dull blue colour. Operating techniques The depth of the gouge is determined principally by the speed and angle of the torch. To cut a deep groove the angle of the torch is stepped up (this increases the impingement angle of the oxygen jet) and gouging speed is reduced. To produce a shallow groove, the torch is less steeply angled, see above, and speed is increased. Wide grooves can be produced by weaving the torch. The contour of the groove is dependent upon the size of the nozzle and the operating parameters. If the cutting oxygen pressure is too low, gouging progresses with a washing action, leaving smooth ripples in the bottom of the groove. If the cutting oxygen pressure is too high, the cut advances ahead of the molten pool - this will disrupt the gouging operation especially when making shallow grooves. There are four basic flame gouging techniques which are used in the following types of application. Progressive gouging This technique is used to produce uniform grooves. Gouging is conducted in either a continuous or progressive manner. Applications include removal of an unfused root area on the reverse side of a welded joint, part-shaping a steel forging, complete removal of a weld deposit and preparing plate edges for welding. Spot gouging Spot gouging produces a deep narrow U-shaped groove over a relatively short length. The process is ideally suited to removal of localised areas such as isolated weld imperfections. Experienced operators are able to observe any imperfections during gouging. These appear as dark or light spots/streaks within the molten pool (reaction zone). Back-step gouging Once the material has reached ignition temperature, the oxygen stream is introduced and the torch moved in a backward movement for a distance of 15-20mm. The oxygen is shut off and the torch moved forward a distance of 25-30mm before restarting the gouging operation. This technique is favoured for removal of local imperfections which may be deeply embedded in the base plate. Deep gouging It is sometimes necessary to produce a long deep gouge. Such operations are completed using the deep gouging technique, which is basically a combination of progressive and spot gouging.