Handbook-Manual Cutting

cutting or plasma arc cutting equipment is not available, it is also possible to cut high-alloy steels such as 18-8

stainless steel. To do this, use preheat flames with a slight excess of acetylene. Be sure to thoroughly preheat the line of cut along the starting edge. Once the cut has been successfully started move the nozzle back and forth along the line of cut (not, as in the case of cast iron, across the line of cut) and keep the preheat flames a bit farther from the surface than you would in cutting carbon steel. Cutting will be noisy; the sound will resemble that produced by rapid sawing of hot wood. The slag formed will be incandescent, and will crackle and spark violently. Cutting will be slow, and if the cut is lost it will be extremely difficult to restart. Cutting Heavy Sections If you are called on to cut steel 150 mm (6 in.) or more thickness, keep these points in mind: 1. Oxygen cutting is a chemical reaction; the speed at which you can cut is not directly proportional to metal thickness. If you can cut steel 10-20 mm thick at a speed of 375 mm per minute, you can probably cut 50 mm steel at a rate of about 125 mm per minute. 2. The flow of oxygen required to cut 150 mm (6 in.) steel is much greater than the flow needed for 12 mm steel. One reason why cutting charts provided by apparatus makers usually call for higher oxygen pressures in cutting thick steel is because loss of pressure as the oxygen passes through the torch at those higher flow rates is greater than at lower flow rates. However, never fall into the trap of thinking that using oxygen pressures substantially higher than those recommended for your nozzle will make the work easier. It will make it harder. In cutting 20 mm steel, you can get away with using pressures well above those recommended; in doing so, you waste oxygen and reduce cut quality, but you don’t lose the cut. In cutting heavy sections, excessive pressure can give you real trouble. The excessive pressure will create turbulence in the cutting oxygen stream as it leaves the nozzle, and you may wind up with a big cavity in the middle of the steel without getting the cut all the way through it. 3. Oxygen hose size must be adequate to carry the required high flow of oxygen without excessive pressure drop. The oxygen pressure recommended by the nozzle manufacturer for a large nozzle to be used in hand cutting will usually be that required at the oxygen regulator when 25 feet (8 m) of 3/8-in. (9 mm) hose is used between Continued on next page...