Chapter 15 Oxyfuel Gas Cutting 419 Copyright Goodheart-Willcox Co., Inc. 15.5 Cutting Steel with an Oxyfuel Gas Cutting Torch Steels that can be effi ciently cut using an oxyfuel gas cutting torch have oxides with a lower melting tem- perature than the base material. For these steels, cut- ting presents little diffi culty. When the cutting jet is turned on, the iron oxides that form melt at a lower temperature than the base metal. These oxides are easily blown away by the cutting oxygen jet, leaving a clean and straight cut. When an expert handles the cutting torch, or if an automatic cutting machine is used, the kerf formed has a smoothness similar to that of a saw cut. When oxyfuel gas cutting steel, the welder must carefully monitor and adjust the following variables: • Pressure of the oxygen fed to the cut. • Size of the oxygen jet orifi ce. • Speed of the cutting torch across the metal. • Distance of the preheat fl ame from the metal. • Size of the preheat fl ames or the amount of heat delivered to the base metal. • Torch tip position (angle) relative to the metal. • Alignment of the torch tip orifi ces with the kerf. The oxygen pressure determines the velocity of the oxygen jet. The orifi ce size determines the amount of oxygen delivered in cfh (cubic feet per hour) or lpm (liters per minute) at any particular pressure. The cut should proceed just fast enough to provide a slight amount of drag at the line of cutting. If the drag is too small, the oxygen consumption is too great. The cut widens out as the jet penetrates the thickness of the metal, leaving a bell-mouthed kerf on the side of the metal away from the torch. See Figure 15-14. If the drag is large, the cutting tip orifi ces may be too small for the job. If the torch is moved too rapidly across the work, the metal at the bottom (far side) of the cut is not burned away, Figure 15-15. The metal does not receive enough heating and oxygen to complete the cut, leav- ing a bell-mouthed kerf on the side of the metal near- est the torch. The large drag results in a turbulent action of the torch gases, which leaves a kerf that is very rough and irregular in shape. If the metal preheat temperature drops below the ignition temperature (metal darkens from cherry red to dark red or black), the oxygen to the cutting orifi ce should be closed off by releasing the cutting oxygen lever. The metal should again be preheated to the proper temperature by the preheating fl ames. The cutting oxygen should be turned on and the cut continued when a cherry red color is obtained. These starting and stopping actions can cause an irregular cut and should be avoided. Bell-mouthed kerf Steel plate Slag Goodheart-Willcox Publisher Figure 15-14. The effect of using too much oxygen when cutting steel. Note how the cut widens at the bottom of the plate to create a bell-mouthed kerf. Oxygen Preheating mixture Direction of travel Uncut portion Cut portion Drag Turbulence Goodheart-Willcox Publisher Figure 15-15. The effect of moving a cutting torch too rapidly across the work. The drag becomes too large.