418 Modern Welding Practically all steels fall under the fi rst classifi ca- tion and, therefore, cutting 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 auto- matic cutting machine is used, the kerf formed has a smoothness similar to that of a saw cut. The second class includes cast iron, some alloy steels, stainless steel, and nonferrous metals. These metals are more diffi cult to cut because their oxides have a higher melting temperature than the metal. It is almost impossible to cut an even kerf. It is very important that these oxides, called refractory oxides, be reduced by chemical action or be prevented from forming. See Chapter 23 for special cutting processes used with refractory oxides. To perform oxyfuel gas cutting on metals from this second classifi cation, 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. ● The alignment of the torch tip orifi ces with the kerf. Note that 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 L/min (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. If the drag is large, the cutting tip orifi ces may be too small for the job. Figure 14-17 shows the result if too much oxygen is fed to the steel being cut. The cut widens out as the jet penetrates the thickness of the metal. This leaves a bell-mouthed kerf on the side of the metal away from the torch. Figure 14-18 shows the result if the torch is moved too rapidly across the work. The metal at the bottom (far side) of the cut is not burned away if the torch is moved too rapidly. This is because it does not receive enough heating and oxygen to complete the cut. 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 Figure 14-17. The effect of using too much oxygen when cutting steel. Note how the kerf 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 Figure 14-18. The effect of moving a cutting torch too rapidly across the work. The drag becomes too large.