Oxygas Cutting Operations
Before you begin a cutting operation with an oxygas cutting torch, make a thorough inspection of the area. Ensure that there are no combustible materials in the area that could be ignited by the sparks or slag produced by the cutting operation. If you are burning into a wall, inspect the opposite side of the wall, and post a fire watch as required.
Setting up the oxygas equipment and preparing for cutting must be done carefully and systematically to avoid costly mistakes. To ensure your own safety, as well as the safety of your coworkers and equipment, make sure the following steps are taken before any attempt is made to light the torch:
- Secure the cylinders so they cannot be accidently knocked over. A good way to do this is to either put them in a corner or next to a vertical column and then secure them with a piece of line. After securing the cylinders, remove the protective caps. Cylinders should never be secured to a structural member of a building that is a current conductor.
- Standing to one side, crack each cylinder valve slightly and then immediately close the valve again. This blows any dirt or other foreign matter out of the cylinder valve nozzle. Do not bleed fuel gas into a confined area because it may ignite. Ensure the valves are closed and wipe the connections with a clean cloth.
- Connect the fuel-gas regulator to the fuel-gas cylinder and the oxygen regulator to the oxygen cylinder. Using a gang wrench, snug the connection nuts sufficiently to avoid leaks.
- Back off the regulator screws to prevent damage to the regulators and gauges and open the cylinder valves slowly. Open the fuel-gas valve only one-half turn and the oxygen valve all the way. Some fuel-gas cylinders have a handwheel for opening the fuel-gas valve while others require the use of a gang wrench or T-handle wrench. Leave the wrench in place while the cylinder is in use so the fuel-gas bottle can be turned off quickly in an emergency. Read the high-pressure gauge to check the contents in each cylinder.
- Connect the RED hose to the fuel-gas regulator and the GREEN hose to the oxygen regulator. Notice the left-hand threads on the fuel-gas connection.
- To blow out the oxygen hose, turn the regulator screw in (clockwise) and adjust the pressure between 2 and 5 psig. After the hose has been purged, turn the screw back out again (counterclockwise) to shutoff the oxygen. Do the same for the fuel-gas hose, but do it ONLY in a well-ventilated place that is free from sparks, flames, or other possible sources of ignition.
- Connect the hoses to the torch. The RED (fuel-gas) hose is connected to the connection gland with the needle valve marked “FUEL.” The GREEN (oxygen) hose is connected to the connection gland with the needle valve marked “OXY.” l With the torch valves closed, turn both regulator screws clockwise to test the hose connections for leaks. If none are found, turn the regulator screws counterclockwise and drain the hose by opening the torch valves.
- Select the correct cutting tip and install it in the cutting torch head. Tighten the assembly by hand, and then tighten with your gang wrench.
- Adjust the working pressures. The fuel-gas pressure is adjusted by opening the torch needle valve and turning the fuel-gas regulator screw clockwise.
- Adjust the regulator to the working pressure needed for the particular tip size, and then close the torch needle valve. To adjust MAPP gas, you should set the gauge pressure with the torch valves closed. To adjust the oxygen working pressure, you should open the oxygen torch needle valve and proceed in the same manner as in adjusting the fuel-gas pressure.
In lighting the torch and adjusting the flame, always follow the manufacturer’s directions for the particular model of torch being used. This is necessary because the procedure varies somewhat with different types of torches and, in some cases, even with different models made by the same manufacturer.
In general, the procedure used for lighting a torch is to first open the torch oxygen needle valve a small amount and the torch fuel-gas needle valve slightly more, depending upon the type of torch. The mixture of oxygen and fuel gas coming from the torch tip is then lighted by means of a spark igniter or stationary pilot flame.
NEVER use matches to light the torch; their length requires bringing the hand too close to the tip. Accumulated gas may envelop the hand and, upon igniting, result in a severe burn. Also, never light the torch from hot metal.
After checking the fuel-gas adjustment, you can adjust the oxygas flame to obtain the desired charac-teristics for the work at hand, by further manipulating the oxygen and fuel-gas needle valves according to the torch manufacturer’s direction.
There are three types of gas flames commonly used for all oxygas processes. They are carburizing, neutral, and oxidizing. To ensure proper flame adjustment, you should know the characteristics of each of these three types of flame. Figure 4-17 shows how the three differ-ent flames look when using MAPP gas as the fuel.
Figure 4-17.—MAPP-gas flames.
A pure fuel-gas flame is long and bushy and has a yellowish color. It takes the oxygen it needs for combus-tion from the surrounding air. The oxygen available is not sufficient enough to burn the fuel gas completely; therefore, the flame is smokey and consists of soot. This flame is not suitable for use. You need to increase the amount of oxygen by opening the oxygen needle valve until the flame takes on a bluish white color, with a bright inner cone surrounded by a flame envelope of a darker hue. It is the inner cone that develops the required operating temperature.
CARBURIZING FLAME.— The carburizing flame always shows distinct colors; the inner cone is bluish white, the intermediate cone is white, the outer envelope flame is light blue, and the feather at the tip of the inner cone is greenish. The length of the feather can be used as a basis for judging the degree of carburization. The highly carburizing flame is longer with yellow or white feathers on the inner cone, while the slightly carburizing flame has a shorter feather on the inner cone and becomes more white. The temperature of carburizing flames is about 5400°F.
Strongly carburizing flames are not used in cutting low-carbon steels because the additional carbon they add causes embrittlement and hardness. These flames are ideal for cutting cast iron because the additional carbon poses no problems and the flame adds more heat to the metal because of its size.
Slightly carburizing flames are ideal for cutting steels and other ferrous metals that produce a large amount of slag. Although a neutral flame is best for most cutting, a slightly carburizing flame is ideal for producing a lot of heat down inside the kerf. It makes fairly smooth cuts and reduces the amount of slag clinging to the bottom of the cut.
NEUTRAL FLAME.— The most common preheat flame for oxygas cutting is the neutral flame. When you increase the oxygen, the carburizing flame becomes neutral. The feather will disappear from the inner flame cone and all that will be left is the dark blue inner flame and the lighter blue outer cone. The temperature is about 5600°F.
The neutral flame will not oxidize or add carbon to the metal you are cutting. In actuality, a neutral flame acts like the inert gases that are used in TIG and MIG welding to protect the weld from the atmosphere. When you hold a neutral preheat flame on one spot on the metal until it melts, the molten puddle that forms looks clear and lies very quietly under the flame.
OXIDIZING FLAME.— When you add a little more oxygen to the preheat flame, it will quickly be-come shorter. The flame will start to neck down at the base, next to the flame ports. The inner flame cone changes from dark blue to light blue. Oxidizing flames are much easier to look at because they are less radiant than neutral flames. The temperature is about 6000°F.
The oxidizing flame is rarely used for conventional cutting because it produces excessive slag and does not leave square-cut edges. Oxidizing flames are used in conjunction with cutting machines that have a high-low oxygen valve. The machine starts the cut with a oxidizing flame then automatically reverts to a neutral flame. The oxidizing flame gives you fast starts when using high-speed cutting machines and is ideal for piercing holes in plate. Highly oxidizing flames are only used in cutting metal underwater where the only source of oxy-gen for the torch is supplied from the surface.
CUTTING MILD-CARBON STEEL
To cut mild-carbon steel with the oxygas cutting torch, you should adjust the preheating flames to neutral. Hold the torch perpendicular to the work, with the inner cones of the preheating flames about 1/16 inch above the end of the line to be cut (fig. 4-18). Hold the torch in this position until the spot you are heating is a bright red. Open the cutting oxygen valve slowly but steadily by pressing down on the cutting valve lever.
Figure 4-18.—Position of torch tip for starting a cut.
When the cut is started correctly, a shower of sparks will fall from the opposite side of the work, indicating that the flame has pierced the metal. Move the cutting torch forward along the line just fast enough for the flame to continue to penetrate the work completely. If you have made the cut properly, you will get a clean, narrow cut that looks almost like it was made by a saw. When cutting round bars or heavy sections, you can save preheating time by raising a small burr with a chisel where the cut is to begin. This small raised portion will heat quickly, allowing you to start cutting immediately.
Once you start the cut, you should move the torch Slowly along the cutting mark or guide. As you move the torch along, watch the cut so you can tell how it is progressing. Adjust the torch as necessary. You must move the torch at the correct speed, not too fast and not too slow. If you go too slowly, the preheating flame melts the top edges along the cut and could weld them back together again. If you go too rapidly, the flame will not penetrate completely, as shown in figure 4-19. When this happens, sparks and slag will blow back towards you. If you have to restart the cut, make sure there is no slag on the opposite side.
Figure 4-19.—The effect of moving a cutting torch too rapidly across the work.
Cutting Thin Steel
When cutting steel 1/8 inch or less in thickness, use the smallest cutting tip available. In addition, point the tip in the direction the torch is traveling. By tilting the tip, you give the preheating flames a chance to heat the metal ahead of the oxygen jet, as shown in figure 4-20.
Figure 4-20.—Recommended procedure for cutting thin steel.
If you hold the tip perpendicular to the surface, you decrease the amount of preheated metal and the adjacent metal could cool the cut enough to prevent smooth cutting action. Many welders actually rest the edge of the tip on the metal during this process. If you use this method, be careful to keep the end of the preheating flame inner cone just above the metal.
Cutting Thick Steel
Steel, that is greater than 1/8 inch thick, can be cut by holding the torch so the tip is almost vertical to the surface of the metal. If you are right-handed, one method to cut steel is to start at the edge of the plate and move from right to left. Left-handed people tend to cut left to right. Either direction is correct and you may cut in the direction that is most comfortable for you. Figure 4-21 shows the progress of a cut in thick steel.
Figure 4-21.—Progress of a cut in thick steel. A. Preheat flames are 1/16 to 1/8 inch from the metal surface. Hold the torch in this spot until the metal becomes cherry red. B. Move the torch slowly to maintain the rapid oxidation, even though the cut is only partially through the metal. C. The cut is made through the entire thickness; the bottom of the kerf lags behind the top edge slightly.
After heating the edge of the steel to a dull cherry red, open the oxygen jet all the way by pressing on the cutting lever. As soon as the cutting action starts, move the torch tip at a even rate. Avoid unsteady movement of the torch to prevent irregular cuts and premature stopping of the cutting action.
To start a cut quicker in thick plate, you should start at the edge of the metal with the torch angled in the opposite direction of travel. When the edge starts to cut, bring the torch to a vertical position to complete the cut through the total thickness of the metal. As soon as the cut is through the metal, start moving the torch in the direction of travel.
Two other methods for starting cuts are used. In the first method, you nick the edge of the metal with a cold chisel at the point where the cut is to start. The sharp edges of the metal upset by the chisel will preheat and oxidize rapidly under the cutting torch, allowing you to start the cut without preheating the entire edge of the plate. In the second method, you place an iron filler rod at the edge of a thick plate. As you apply the preheat flames to the edge of the plate, the filler rod rapidly reaches the cherry red temperature. At this point, turn the cutting oxygen on and the rod will oxidize and cause the thicker plate to start oxidizing.
CUTTING CAST IRON
It is more difficult to cut cast iron than steel because the iron oxides in cast iron melt at a higher temperature than the cast iron itself. Before you cut cast iron, it is best to preheat the whole casting to prevent stress fractures. Do not heat the casting to a temperature that is too high, as this will oxidize the surface and make cutting more difficult. A preheat temperature of about 500°F is normally satisfactory.
When cutting cast iron, adjust the preheating flame of the torch to a carburizing flame. This prevents the formation of oxides on the surface and provides better preheat. The cast-iron kerf is always wider than a steel kerf due to the presence of oxides and the torch movement.
The torch movement is similar to scribing semi-circles along the cutting line (fig. 4-22). As the metal becomes molten, trigger the cutting oxygen and use its force to jet the molten metal out of the kerf. Repeat this action until the cut is complete.
Figure 4-22.—Torch movements for cutting cast iron.
Because of the difficulty in cutting cast iron with the usual oxygas cutting torch, other methods of cut-ting were developed. These include the oxygen lance, carbon-arc powder, inert-gas cutting, and plasma-arc methods.
GOUGING MILD STEEL
Cutting curved grooves on the edge or surface of a plate and removing faulty welds for rewelding are additional uses for the cutting torch. The gist of groove cutting or gouging is based on the use of a large orifice, low-velocity jet of oxygen instead of a high-velocity jet. The low-velocity jet oxidizes the surface metal only and gives better control for more accurate gouging. By varying the travel speed, oxygen pressure, and the angle between the tip and plate, you can make a variety of gouge contours.
A gouging tip usually has five or six preheat orifices that provide a more even preheat distribution. Automatic machines can cut grooves to exact depths, remove bad spots, and rapidly prepare metal edges for welding.
Figure 4-23 shows a typical gouging operation. If the gouging cut is not started properly, it is possible to cut accidently through the entire thickness of the plate. If you cut too shallow, you can cause the operation to stop. The travel speed of the torch along the gouge line is important. Moving too fast creates a narrow, shallow gouge and moving too slow creates the opposite; a deep, wide gouge.
Figure 4-23.—Typical gouging operation using a low-velocity
BEVELING MILD STEEL
Frequently, you must cut bevels on plate or pipe to form joints for welding. The flame must actually cut through 2.8 inches of metal to make a bevel cut of 45 degrees on a 2-inch steel plate. You must take this into consideration when selecting the tip and adjusting the pressures. You use more pressure and less speed for a bevel cut than for a straight cut.
When bevel cutting, you adjust the tip so the pre-heating orifices straddle the cut. Apiece of l-inch angle iron, with the angle up, makes an excellent guide for beveling straight edges. To keep the angle iron in place while cutting, you should use a heavy piece of scrap, or tack-weld the angle to the plate being cut. Move the torch along this guide, as shown in figure 4-24.
Figure 4-24.—Using angle iron to cut bevels on steel plate
ELECTRIC DRIVE CUTTING TORCH CARRIAGE
An improvement over mechanical guides is an electric motor-driven cutting torch carriage. The speed of the motor can be varied allowing the welder to cut to dimensions and to cut at a specific speed. A typical motor driven carriage has four wheels: one driven by a reduction gear, two on swivels (castor style), and one freewheeling. The torch is mounted on the side of the carriage and is adjusted up and down by a gear and rack. The rack is a part of the special torch. The torch also can be tilted for bevel cuts. This machine comes with a straight two-groove track and has a radial bar for use in cutting circles and arcs. A motor-driven cutting torch cutting a circle is shown in figure 4-25. The carriage is equipped with an off-and-on switch, a reversing switch, a clutch, and a speed-adjusting dial that is calibrated in feet per minute.
Figure 4-25.—Electric motor-driven carriage being used to cut a circle in steel plate.
Figure 4-26 shows an electric drive carriage on a straight track being used for plate beveling. The operator must ensure that the electric cord and gas hoses do not become entangled on anything during the cutting operation. The best way to check for hose, electric cord, and torch clearance is to freewheel the carriage the full length of the track by hand.
Figure 4-26.—Electric motor-driven carriage being used on straight track to cut a beveled edge on steel plate.
You will find that the torch carriage is a valuable asset during deployment. This is especially true if your shop is called upon to produce a number of identical parts in quantity. Such an assignment might involve the fabrication of a large supply of handhole covers for runway fixtures, or another assignment might be the production of a large quantity of thick base plates for vertical columns. When using the torch carriage, you should lay the track in a straight line along a line parallel to the edge of the plate you are going to cut. Next, you light the torch and adjust the flame for the metal you are cutting. Move the carriage so the torch flame preheats the edge of the plate and then open the cutting oxygen valve and turn on the carriage motor. The machine begins moving along the track and continues to cut automatically until the end of the cut is reached. When the cut is complete, you should do the following: promptly turn off the cutting oxygen, turn off the current, and extinguish the flame--in that order. The cutting speed depends upon the thickness of the steel being cut
CUTTING AND BEVELING PIPE
Pipe cutting with a cutting torch requires a steady hand to obtain a good bevel cut that is smooth and true. Do not attempt to cut and bevel a heavy pipe in one operation until you have developed considerable skill. First, you should cut the pipe off square, and ensure all the slag is removed from the inside of the pipe. Next, you should bevel the pipe. This procedure produces a cleaner and better job; it is ideal for use by an inexperienced welder.
When cutting a piece of pipe, you should keep the torch pointed toward the center line of the pipe. Start the cut at the top and cut down one side. Then begin at the top again and cut down the other side, finishing at the bottom of the pipe. This procedure is shown in figure 4-27.
Figure 4-27.—Cutting pipe with an oxygas cutting torch.
Figure 4-28.—Fabricating a T.
When you make T and Y fittings from pipe, the cutting torch is a valuable tool. The usual procedure for fabricating pipe fittings is to develop a pattern like the one shown in figure 4-28, view A-1.
After you develop the pattern, wrap it around the pipe, as shown in figure 4-28, view A-2. Be sure to leave enough material so the ends overlap. Trace around the pattern with soapstone or a scribe. It is a good idea to mark the outline with a prick punch at 1/4-inch intervals. During the cutting procedure, as the metal is heated, the punch marks stand out and make it easier to follow the line of cut. Place the punch marks so the cutting action will remove them. If punch marks are left on the pipe, they could provide notches from which cracking may start.
An experienced welder can cut and bevel pipe at a 45-degree angle in a single operation. A person with little cutting experience should do the job in two steps. In that case, the first step involves cutting the pipe at a 90-degree angle. In the second step, you bevel the edge of the cut to a 45-degree angle. With the two-step procedure, you must mark an additional line on the pipe. This second line follows the contour of the line traced around the pattern, but it is drawn away from the original pattern line at a distance equal to the thickness of the pipe wall. The first (90-degree) cut in the two-step procedure is made along the second line. The second (45-degree) cut is made along the original pattern line. The primary disadvantage of the two-step procedure is it is time consuming and uneconomical in oxygen and gas consumption.
The one-step method of cutting and beveling pipe is not difficult, but it does require a steady hand and a great deal of experience to turn out a first-class job. An example of this method for fabricating a T is shown in figure 4-28. View A of figure 4-28 outlines the step-by-step procedures for fabricating the branch; view B shows the steps for preparing the main section of the T; and view C shows the assembled T, tack-welded and ready for final welding.
Step 3 of view A of figure 4-28 shows the procedure for cutting the miter on the branch. You should begin the cut at the end of the pipe and work around until one half of one side is cut. The torch is at a 45-degree angle to the surface of the pipe along the line of cut. While the tip is at a 45-degree angle, you should move the torch steadily forward, and at the same time, swing the butt of the torch upward through an arc. This torch manipulation is nec-essary to keep the cut progressing in the proper direction with a bevel of 45 degrees at all points on the miter. Cut the second portion of the miter in the same reamer as the first.
The torch manipulation necessary for cutting the run of the T is shown in Steps 3 and 4 of view B in figure 4-28. Step 3 shows the torch angle for the starting cut and Step 4 shows the cut at the lowest point on the pipe. Here you change the angle to get around the sharp curve and start the cut in an upward direction. The completed cut for the run is shown in Step 5 (figure 4-28, view B).
Before final assembly and tack welding of any of the parts of a fabricated fitting, you must clean the slag from the inner pipe wall and check the fit of the joint. The bevels must be smooth and have complete fusion when you weld the joint.
The cutting torch is a valuable tool for piercing holes in steel plate. Figure 4-29 shows the steps you should use to pierce holes in steel plate. First, lay the plate out on firebricks or other suitable material so the flame does not damage anything when it burns through the plate. Next, hold the torch over the hole location with the tips of the inner cone of the preheating flames about 1/4 inch above the surface of the plate. Continue to hold the torch in this position until a small spot has been heated to a bright red. Then open the cutting oxygen valve gradually, and at the same time, raise the nozzle slightly away from the plate. As you start raising the torch and opening the oxygen valve, rotate the torch with a slow spiral motion. This causes the molten slag to be blown out of the hole. The hot slag may fly around, so BE SURE that your goggles are tightly fitted to your face, and avoid placing your head directly above the cut.
Figure 4-29.—Piercing a hole with an oxygas cutting torch.
If you need a larger hole, outline the edge of the hole with a piece of soapstone, and follow the procedure indicated above. Begin the cut from the hole you pierced by moving the preheating flames to the normal distance from the plate and follow the line drawn on the plate. Round holes are made easily by using a cutting torch with a radius bar attachment.
The cutting torch is an excellent tool for removing rivets from structures to be disassembled. Rivet cutting procedures are shown in figure 4-30. The basic method is to heat the head of the rivet to cutting temperature by using the preheating flames of the cutting torch. When the rivet head is at the proper temperature, turn on the oxygen and wash it off. The remaining portion of the rivet can then be punched out with light hammer blows.
Figure 4-30.—Using a cutting torch to remove a rivet head,
The step-by-step procedure is as follows:
- Use the size of tip and the oxygen pressure required for the size and type of rivet you are going to cut.
- Heat a spot on the rivet head until it is bright red.
- Move the tip to a position parallel with the surface of the plate and turn on the cutting oxygen slowly.
- Cut a slot in the rivet head like the screwdriver slot in a roundhead screw. When the cut nears the plate, draw the nozzle back at least 1 1/2 inches from the rivet so you do not cut through the plate.
- When cutting the slot through to the plate, you should swing the tip through a small arc. This slices half of the rivet head off.
- Swing the tip in an arc in the other direction to slice the other half of the rivet head off.
By the time the slot has been cut, the rest of the rivet rope strands from unlaying during cutting, seize the wire head is at cutting temperature. Just before you get through the slot, draw the torch tip back 1 1/2 inches to allow the cutting oxygen to scatter slightly. This keeps the torch from breaking through the layer of scale that is always present between the rivet head and the plate. It allows you to cut the head of the rivet off without damaging the surface of the plate. If you do not draw the tip away, you could cut through the scale and into the plate.
A low-velocity cutting tip is best for cutting button-head rivets and for removing countersunk rivets. A low-velocity cutting tip has a cutting oxygen orifice with a large diameter. Above this orifice are three preheating orifices. Always place a low-velocity cutting tip in the torch so the heating orifices are above the cutting orifice when the torch is held in the rivet cutting position.
CUTTING WIRE ROPE
You can use a cutting torch to cut wire rope. Wire rope consists of many strands, and since these strands do not form one solid piece of metal, you could experience difficulty in making the cut. To prevent the wire rope on each side of the place where you intend to cut.
Adjust the torch to a neutral flame and make the cut between the seizings. If the wire rope is going to go through sheaves, then you should fuse the strand wires together and point the end. This makes reeving the block much easier, particularly when you are working with a large-diameter wire rope and when reeving blocks that are close together. To fuse and point wire rope, adjust the torch to a neutral flame; then close the oxygen valve until you get a carburizing flame. With proper torch manipulation, fuse the wires together and point the wire rope at the same time.
Wire rope is lubricated during fabrication and is lubricated routinely during its service life. Ensure that all excess lubricant is wiped off the wire rope before you begin to cut it with the oxygas torch.
CUTTING ON CONTAINERS
Never perform cutting or welding on containers that have held a flammable substance until they have been cleaned thoroughly and safeguarded. Cutting, welding, or other work involving heat or sparks on used barrels, drums, tanks, or other containers is extremely dangerous and could lead to property damage or loss of life.
Whenever available, use steam to remove materials that are easily volatile. Washing the containers with a strong solution of caustic soda or a similar chemical will remove heavier oils.
Even after thorough cleansing, the container should be further safeguarded by falling it with water before any cutting, welding, or other hot work is done. In almost every situation, it is possible to position the container so it can be kept filled with water while cutting or other hot work is being done. Always ensure there is a vent or opening in the container for the release of the heated vapor inside the container. This can be done by opening the bung, handhole, or other fitting that is above water level.
When it is practical to fill the container with water, you also should use carbon dioxide or nitrogen in the vessel for added protection. From time to time, examine the gas content of the container to ensure the concentration of carbon dioxide or nitrogen is high enough to prevent a flammable or explosive mixture. The air-gas mixture inside any container can be tested with a suit-able gas detector.
The carbon dioxide concentration should beat least 50 percent of the air space inside the container, and 80 percent or more when the presence of hydrogen or carbon monoxide is detected. When using nitrogen, you must ensure the concentration is at least 10 percent higher than that specified for carbon dioxide.
Carbon dioxide or nitrogen is used in apparently clean containers because there may still be traces of oil or grease under the seams, even though the vessel was cleaned and flushed with a caustic soda solution. The heat from the cutting or welding operation could cause the trapped oil or grease to release flammable vapors that form an explosive mixture inside the container.
A metal part that is suspiciously light maybe hollow inside; therefore, you should vent the part by drilling a hole in it before heating. Remember: air or any other gas that is confined inside a hollow part will expand when heated. The internal pressure created may be enough to cause the part to burst. Before you do any hot work, take every possible precaution to vent the air confined in jacketed vessels, tanks, or containers.
Judging Cutting Quality
To know how good of a cutting job you are doing, you must understand know what constitutes a good oxygas cut. In general, the quality of an oxygas cut is judged by four characteristics:
- The shape and length of the draglines
- The smoothness of the sides
- The sharpness of the top edges
- The amount of slag adhering to the metal
Drag lines are line markings that show on the surface of the cut. Good drag lines are almost straight up and down, as shown in figure 4-31, view A. Poor drag lines, as shown in figure 4-31, view B, are long and irregular or curved excessively. Drag lines of this type indicate a poor cutting procedure that could result in the loss of the cut (figure 4-31, views B and C). Draglines are the best single indication of the quality of the cut made with an oxygas torch. When the draglines are short and almost vertical, the sides smooth, and the top edges sharp, you can be assured that the slag conditions are satisfactory.
Figure 4-31.—Effects of correct and incorrect cutting procedures.
A satisfactory oxygas cut shows smooth sides. A grooved, fluted, or ragged cut surface is a sign of poor quaility.
TOP EDGE SHARPNESS The top edges resulting from an oxygas cut should be sharp and square (figure 4-31, view D). Rounded top edges, such as those shown in view E of figure 4-31, are not satisfactory. The melting of the top edges may result from incorrect preheating procedures or from moving the torch too slowly.
An oxygas cut is not satisfactory when slag adheres so tightly to the metal that it is difficult to remove.
In all cutting operations, you must ensure that hot slag does not come in contact with combustible material. Globules of hot slag can roll along the deck for long distances. Do not cut within 30 to 40 feet of unprotected combustible materials. If you cannot remove the combustible materials, cover them with sheet metal or other flameproof guards. Keep the fuel gas and oxygen cylinders far enough away from the work so hot slag does not fall on the cylinders or hoses.
Many of the safety precautions discussed in other lessons in this course apply to cutting as well as to welding. Be sure you are completely familiar with all the appropriate safety precautions before attempting oxygas cutting operations.
BACKFIRE AND FLASHBACK
Improper operation of the oxygas torch can cause the flame to go out with a loud snap or pop. This is called a “backfire.” Close the torch valves, check the connections, and review your operational techniques before relighting the torch. You may have caused the backfire by touching the tip against the work, by overheating the tip, or by operating the torch with incorrect gas pressures. A backfire also may be caused by a loose tip or head or by dirt on the seat.
A flashback occurs when the flame burns back inside the torch, usually with a shrill hissing or squealing noise. You should close the torch oxygen valve that controls the flame to stop the flashback at once. Then you should close the gas valve and the oxygen and gas regulators. Be sure you allow the torch to cool before relighting it. Also, blow oxygen through the cutting tip for a few seconds to clear out soot that may have accumulated in the passages. Flashbacks may extend back into the hose or regulators. Flashbacks indicate that something is wrong, either with the torch or with the way it is being operated. Every flashback should be investigated to determine its cause before the torch is relighted. A clogged orifice or incorrect oxygen and gas pressures are often responsible. Avoid using gas pressures higher than those recommended by the manufacturer.
Gas cylinders are made of high-quality steel. High-pressure gases, such as oxygen, hydrogen, nitrogen, and compressed air, are stored in cylinders of seamless construction. Only non-shatterable high-pressure gas cylinders may be used by ships or activities operating outside the continental United States. Cylinders for low-pressure gases, such as acetylene, may be welded or brazed. Cylinders are carefully tested, either by the factory or by a designated processing station, at pressures above the maximum permissible charging pressure.
Identification of Cylinders
Color warnings provide an effective means for marking physical hazards and for indicating the location of safety equipment. Uniform colors are used for marking compressed-gas cylinders, pipelines carrying hazardous materials, and fire protection equipment.
Five classes of material have been selected to represent the general hazards for dangerous materials, while a sixth class has been reserved for fire protection equipment. A standard color has been chosen to represent each of these classes and is shown in table 4-2.
Table 4-2.—Standard Colors
Since you work with fuel gas and oxygen, you must become familiar with the colors of the cylinders in which these gases are contained. The fuel-gas cylinder is yellow, and the oxygen cylinder is green.
In addition to color coding, the exact identification of the material contained in a compressed-gas cylinder must be indicated by a written title that appears in two locations-diametrically opposite and parallel to the longitudinal axis of the cylinder. Cylinders, having a background color of yellow, orange, or buff have the title painted black Cylinders, having a background color of red, brown, black, blue, gray, or green, have the title painted white.
COLOR WARNINGS.— The appearance on the body, top, or as a band(s) on compressed-gas cylinders of the six colors specified should provide a warning of danger from the hazard involved.
CYLINDER COLOR BANDS.— Cylinder color bands appear upon the cylinder body and serve as color warnings when they are yellow, brown, blue, green, or gray. The bands also provide color combinations to separate and distinguish cylinders for convenience in handling, storage, and shipping.
DECALS.— Two decals may be applied on the shoulder of each cylinder. They should be diametrically opposite and at right angles to the titles. They should indicate the name of the gas, precautions for handling, and use. A background color corresponding to the primary warning color of the contents should be used.
SHATTERPROOF CYLINDERS.— A shatter-proof cylinder should be stenciled with the phrase “NON-SHAT’’ longitudinally 90 degrees from the titles. Letters must be black or white and approximately 1 inch in size.
SERVICE OWNERSHIP.— On cylinders owned by or procured for the Department of Defense, the bottom and the lower portion of the cylinder body opposite the valve end may be used for service ownership titles.
The six colors identified in Table 4-2 are used on the body and top of, or as a band on, a compressed-gas cylinder to serve as a warning of the hazard involved in handling the type of material contained in the cylinder.
Figure 4-32.—Titles and color codes for compressed-gas cylinders.
Figure 4-32 shows titles and color codes for compressed-gas cylinders most often found on construction sites or in a public works department where welders are working. Figure 4-33 shows how cylinders are identified by the overall painted color code and by the stenciled name of the gas. It should be noted that the color code of cylinders shown in figure 4-32 is military only; the commercial industry does not necessarily comply with these color codes. Commercial U.S. and ISO color-code standards have yet to be established.
Figure 4-33.—Identifying color patterns for gas cylinders.
Handling and Storing Gas Cylinders
Each compressed-gas cylinder carries markings indicating compliance with Interstate Commerce Commission (ICC) requirements. When the cylinders are at your work site, they become your responsibility. There are several things you should not do when handling and storing compressed-gas cylinders.
- Never fill your own cylinders. It requires special training and special equipment.
- Never alter or fix the safety devices on a cylinder. It is illegal and also stupid. The only personnel permitted to work on cylinder safety devices are the cylinder owners and suppliers.
- Never store cylinders near a heat source or in direct sunlight. Heat causes the gas inside a cylinder to expand. This could result in cylinder failure or fire.
- Never store cylinders in a closed or unventilated space. If one of the cylinders were to leak, it could cause an explosion or asphyxiate someone entering the space.
- Store cylinders in protected, well-ventilated, and dry spaces. Protect the cylinder valves and safety devices from ice and snow. A safety device may not work if it is frozen.
- Never store fuel cylinders and oxidizers within the same space. Oxidizers must be stored at least 50 feet from fuel cylinders. Use fire-resistant partitions between cylinder storage areas.
- Never mix empty cylinders with full cylinders.
- Do not mix cylinders that contain different gases.
- Always replace the cylinder cap and mark the cylinder “Empty” or “MT.” Store the cylinders in a cool, dry place ready for pickup by the supplier. Even in storage, chain the cylinders when they are stored in the upright position.
- Never drag a cylinder to move it. When available, use a cylinder truck. If at all possible, leave the cylinders on the hand truck and operate them from there; otherwise, tilt the cylinder slightly and roll it on the bottom edge. Always install the cylinder cap before moving the cylinder. Never use slings or magnets to carry cylinders. If you lift a cylinder upright by the cap, make sure that it is screwed on tightly. If the cylinder cap comes off, the cylinder could fall and either crush your foot or snap the valve off. If a cylinder is dropped and the valve breaks, it could launch itself like a rocket.
When cylinders have been stored outside in freezing weather, they sometimes become frozen to the ground or to each other. To free the cylinders, you can pour warm water (not boiling) over the frozen or icy areas. As a last resort, you can pry them loose with a prybar. If you use a prybar, never pry or lift under the valve cap or valve.