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ОглавлениеIn Chapter 1, I mentioned oxyacetylene welding as an ideal process to learn and practice in order to gain the fundamental knowledge of fusion welding. Melting of the base material occurs slowly and is easy to observe. Filler metal is added separately and the two-hand technique used is similar to what is employed for TIG welding. Since it is a slower process than TIG, the manual skills are easier to practice. Maintaining a fixed distance from the welding tip to the work is not as critical as with TIG, which also helps develop manual skills.
In addition, the equipment needed is relatively inexpensive and can be used for welding, brazing, cutting, and heating. With the proper attachments, it is ideal for cutting steel of any thickness or heating metal for bending.
Therefore, although it may not be widely used in automotive work for general welding, it is good to review the basics before discussing TIG welding (see Chapter 4). It may be the proper tool to weld a very lightweight chassis made from small-diameter 4130 chrome-moly tubing (see Chapter 7). In addition, there are some unique joining applications, such as repairing a crack in cast iron, when braze welding with oxyacetylene is often preferred. The simplicity and relative low cost of oxyacetylene welding is another reason to consider its use for a number of applications.
Fig. 3.1. Oxyacetylene welding is an ideal process to learn and practice to gain the fundamental knowledge of fusion welding. The equipment is inexpensive and flexible to use. With the proper attachment, it is ideal for cutting steel of any thickness or heating metal for bending.
Fig. 3.2. Two cylinders, one containing oxygen and the other acetylene, supply these gases through long hoses to the torch. The gases are mixed before exiting through a small hole in the torch tip and ignited. For welding, acetylene is the only practical gas to use. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
An oxyacetylene welding rig consists of two cylinders with regulators and hoses attached to a torch.
One cylinder contains oxygen and the other acetylene; they supply these gases to the torch through long hoses. Valves in the torch control the flow of gases. The gases mix before exiting through a small hole in the tip and are ignited. Different-size tips are available for welding various thicknesses of materials. For welding, acetylene is the only practical gas to use. It has the hottest inner cone temperature of any fuel gas (5,720 degrees F). Other fuel gases are acceptable for cutting because the hottest flame is not necessary.
Oxygen Regulator—Avoiding Explosions
Oxygen is usually supplied in a high-pressure cylinder with a pressure of 2,000 to 2,600 psi (pounds per square inch). This high pressure must be reduced to less than 15 psi for welding the typical material thicknesses used in automotive applications.
Connecting an oxygen regulator and adjusting the pressure must be done following the manufacturer’s instructions. Be sure to read and understand the operating and safety precautions. When installing the regulator on a cylinder and opening the cylinder valve, very high oxygen pressure enters the regulator passages. This sudden high-pressure surge can cause a high temperature similar to what occurs in a Diesel engine combustion chamber when air is compressed. Most materials, including stainless steel and copper, burn and melt in the presence of pure oxygen. If not handled properly, the regulator could explode causing serious injury.
The internal design of an oxygen regulator must consider the possibilities of an oxygen fire or explosion. For these regulators to function efficiently, a valvespring is located on the high-pressure side of a diaphragm to regulate the pressure. This valvespring attaches to a valvestem and valvestem guide, which smoothes out the movement of the valvestem, so the regulator does not chatter due to rapid opening and closing of the valve.
Fig. 3.3. An oxygen regulator reduces the approximate 2,600 psi in a high-pressure cylinder to 5 to 15 psi, so it can be safely used for welding. Proper precautions must be followed when installing an oxygen regulator to avoid the possibility of a fire or explosion. Always follow the manufacturer’s instructions.
Here are the steps to avoid an explosion:
1. Before installing a regulator on a cylinder, visually check the cylinder outlet to make sure there is no debris.
2. Crack open the cylinder contents valve to clear any potential contaminants from the opening.
3. A slight opening is all that is needed. Quickly close the valve.
4. The oxygen regulator must have an inlet filter, so carefully check to see that it’s in place. If it is missing or not working, do not use the regulator. Take it to your gas supplier and have one installed or have the existing one repaired.
5. With an inlet filter in place, make sure it is clean, and free of all oil, grease, or contamination. This is particularly important in a location where grease or oil may be in the area where the regulator was placed when changing cylinders. Locate a clean location to place the regulator while swapping cylinders. The workbench needs to be free of grease or oil so you do not contaminate the regulator, and never put the regulator on the floor. Remember that even a small amount of oil or grease located on the regulator inlet can cause a regulator explosion.
6. After the regulator is connected, tighten the nut with no more than a 12-inch-long wrench. The sealing seats on the regulator inlet and cylinder valve are carefully machined metal-to-metal surfaces. Never use thread sealer on any regulator threads because regulator threads are not sealing threads, rather they are only for pulling the seats together. If the seats leak, the threads do not block the flow of gas. With oxygen service, not using thread sealant is even more important because contamination of the inlet occurs.
7. If the seats are clean, they do not need high torque to properly seat. If they do need high torque, either the cylinder or regulator may have a defective seat.
8. If a leak is found after testing, remove the regulator, clean the seat with a clean cloth, and reinstall. If the leak is still present, replace the cylinder or the regulator.
The way a cylinder valve is opened is very important. A valve on any high-pressure cylinder should always be opened very slowly. One reason is the pressure gauge has a small, sealed, curved tube that bends when subjected to pressure. When the tube bends, it actuates levers that move the gauge pointer, and rapidly opening the cylinder stresses the tube.
An explosion is possible if the valve on the oxygen sensors is opened too quickly (I have witnessed many tests of oxygen regulator explosions in a laboratory environment, so this next step is reinforced in my mind). Before opening the oxygen cylinder valve, be sure the regulator pressure adjusting screw is turned out fully. Leaving the pressure adjusting screw turned in to maintain a setting is a very dangerous practice. All regulator manufacturers warn about the need to fully turn the adjusting screw out By leaving the pressure adjusting screw in, even slightly, the high-pressure oxygen gas passes through the open valve seat, exposing the regulator diaphragm and other internal parts.
With the pressure-adjusting screw fully open, very slowly open the cylinder contents valve. One suggestion is to not look at the contents gauges while opening the valve. In fact, take an extra precaution and stand to the side of the regulator. In lab tests, when an explosion occurs, most (but not all) of the flame comes out of the front and sometimes the rear of the regulator. One reason for opening slowly is to avoid a shock to the pressure gauge, but another reason is that a regulator explosion might be caused by high pressure rapidly entering the small gauge tube that bends when pressure is applied.
If even a small amount of hydrocarbon contaminant enters the tube, and the pressure suddenly increases from 14.7 to 2,600 psi of pure oxygen, spontaneous combustion can occur. It is like a Diesel engine combustion chamber. No spark is needed, just fuel and high pressure.
Fig. 3.4. Acetylene regulators only allow pressures up to 15 psi because, beyond that, pressure acetylene gas is unstable. At 29 psi the gas can spontaneously explode. When acetylene is contained in a cylinder, the gas is dissolved in acetone.
A Diesel engine with a high compression ratio of 25:1 has a maximum cylinder pressure of 368 psi of air (25 × 14.7). That is sufficient to ignite Diesel fuel in air. With 2,600 psi of oxygen, it takes little fuel to cause ignition! Once the fire starts, all materials burn in an oxygen environment—the brass regulator body and even a stainless steel diaphragm.
An oxygen regulator burnout is not an everyday occurrence. There are hundreds of thousands of oxygen regulators on cylinders in the United States and just a few burns occur each year. Evidence collected over the years shows that when one does occur the inlet filter was often missing. While filters are necessary for all uses and in all environments, clean filters are especially important in auto body shops, garages, or any place where hydrocarbon products are on benches or floors.
When a full cylinder is being exchanged for an empty cylinder, the oxygen regulator might be placed on a dirty bench or floor. The inlet nipple could pick up some grease or oil and enter the opening. Burnouts have occurred in coal mines where safety precautions are always emphasized and taught to all workers. Could it be the regulators were exposed to coal dust when swapping cylinders?
Most burnouts occur when a new full cylinder is being installed. If the inlet filter is always checked before installing an oxygen regulator, the pressure adjusting screw is backed all the way out, and the cylinder valve is opened very slowly, an oxygen regulator burnout or explosion should never be experienced.
There are also oxygen regulators designed to contain an explosion should one occur. Ask your welding supplier about them.
Acetylene Regulator
Acetylene is potentially unstable at pressures over 15 psi. To increase a cylinder’s capacity, while providing a safe environment, the acetylene is dissolved in acetone and held in a porous media contained in the cylinder. Cylinder pressures of 250 psi can then be used. The acetylene regulator lowers the pressure to the 5 to 8 psi required for welding. Consult the manufacturer’s operating and safety instructions for a particular regulator and welding tip to define the pressure setting recommended. Never set pressures above 15 psi.
Torches
Most oxyacetylene welding systems are purchased as kits that include a torch handle, a cutting attachment, and several welding tips and cutting nozzles for various thicknesses of material. Some kits include a multi-flame heating head. This is similar to a large welding tip but instead of one hole with a single flame, it has multiple holes for multiple flames. This allows heating a wider area without a single hot flame melting the material.
Fig. 3.5. High-pressure gas cylinders, such as those used for oxygen, are made of high-strength steel and have a significant safety factor in their design. They are usually filled to about 2,500 psi, 170 times atmospheric pressure. Acetylene cylinders only contain 250 psi when full. They are filled with diatomaceous earth or a ceramic material and store the acetylene in acetone.
The torch handle has two valves used to adjust the flow rate of oxygen and acetylene. These require fine adjustment to obtain the proper flame properties. However, since the oxygen valve is also used for cutting where higher flow rates are required, it must also pass a high rate of gas flow when needed. The flow rates of gas may vary from 4 to 200 cfh (cubic feet per hour), a very wide range.
The welding tip often contains a mixer where the oxygen and acetylene are combined so they can burn. Two types of mixers are in use today. The first is called a medium-pressure type, where the gases are supplied at about equal pressures. The pressures, when using medium-pressure mixers, are typically similar for oxygen and acetylene, at about 2 to 7 psi depending on the tip size. The second is an injector type, where the oxygen is supplied at a high pressure (55 psi or higher) and the acetylene is supplied at a low 1 psi.
In the injector type, the oxygen passes through a very small orifice in the injector, and the expansion of the oxygen as it leaves the orifice pulls the acetylene into the mixing chamber. The mixer is located in the inlet of the welding tip because there is a relationship between the mixer and the tip size. A single mixer cannot satisfy all of the requirements.
Fig. 3.6. Most oxyacetylene welding systems are purchased as kits that include a torch handle, a cutting attachment, several welding tips, and cutting nozzles for various thicknesses of material. Some include a multi-flame heating head that allows heating a wider area with flames that do not melt the material.
Fig. 3.7. The torch handle has two valves that are used to adjust the flow rate of oxygen and acetylene. These require fine adjustment to obtain the proper flame properties and high-flow capacity for cutting. Flow rates of gas may vary from 4 to 200 cfh.
In addition, all the passages in the welding head must be designed so that if the flame is forced back into the head, it is extinguished without damage to the head or torch. This could occur by momentary contact of the torch tip against the work. This can lead to a flashback.
One advantage of an injector mixer design is it can use the last amount of acetylene left in the cylinder. One thing to watch for when the gas level is very low is it not only pulls out the acetylene gas but the acetone as well!
Cylinders
High-pressure gas cylinders, like those used for oxygen, are made of high-strength steel and have a significant safety factor in their design. They are usually filled to about 2,500 psi, which is 170 times atmospheric pressure. Gas cylinders are periodically pressure tested and examined for damage before they are allowed to be refilled.
A common, large oxygen cylinder is about 5½ feet high and 9 inches in diameter. When full it contains about 2,640 psi and 325 cubic feet of oxygen. The physical internal volume of the cylinder is about 1.8 cubic feet, yet it holds about 325 cubic feet of oxygen. That is the volume of the gas when it exits the cylinder and is at room temperature and atmospheric pressure of 14.7 psi, which is what you pay for.
That volume of gas is proportional to the absolute pressure, which is gauge pressure plus 14.7 psi. Therefore, the gas stored in a high-pressure oxygen cylinder (2,640 gauge reading + 14.7 psi ÷ 14.7 psi) is 180 times denser than if at atmospheric pressure. Then 1.8 cubic feet of physical cylinder volume times 180 higher density = 325 cubic feet of gas at what is called STP, standard temperature and pressure.
An empty cylinder weighs about 140 pounds. It is deep-drawn from a single piece of high-strength steel or forged from billet steel. The final shape is heat treated after forming. The cylinder is pressure tested at 3,360 psi before put in service. It must also be tested at least once every 10 years while it is in service. The U.S. Department of Transportation establishes the regulations that cover construction, testing, marking, filling, and maintenance issues.
On the top shoulder of the cylinder body is the date it was put in service and the dates when it was retested. The brass oxygen cylinder valve has a threaded outlet that is machined to Compressed Gas Association (CGA) standards, and the American National Standards Institute (ANSI) accepts these standards. All oxygen regulators sold in the United States and Canada have a mating outlet fitting that conforms to these standards. The connection is designated CGA 540 and is recognized for oxygen service only.
Never use an adapter to connect a regulator to any high-pressure cylinder. Oxygen regulators are specially made for that service. Every cylinder valve is also equipped with a bursting disk, which ruptures and allows the contents to vent in the event the cylinder reaches a pressure near the test pressure, as might occur in a fire.
Acetylene cylinders are constructed differently than oxygen cylinders. First, they are not under high pressure and only contain 250 psi when full. As mentioned, acetylene at any pressure above 15 psi is unstable and should never be used. In fact, acetylene at 29 psi becomes self-explosive, and it does not need oxygen for this explosion occur. It decomposes into carbon black and hydrogen.
How is it possible to have acetylene in a cylinder at 250 psi when the gas cannot be used above 15 psi? The gas in the cylinder is dissolved in acetone, and therefore, it does not exist as a gas within the cylinder.
The inside of the cylinder has a unique construction. It is completely filled with porous materials. Newer cylinders are filled with diatomaceous earth or a ceramic material. Older cylinders were filled with materials such as balsa wood, charcoal, and shredded asbestos. These fillers decrease the size of the open spaces in the cylinder.
Acetone, a colorless, flammable liquid, is added until about 40 percent of the porous material is filled. The filler acts as a large sponge to absorb the acetone, which absorbs the acetylene. In this process, the volume of the acetone increases as it absorbs the acetylene, while acetylene decreases in volume.
In the cylinder, there’s a safety plug with a small hole through the center that is filled with a metal alloy that melts at approximately 212 degrees F or releases the contents at 500 psi. When a cylinder is overheated, the plug melts and permits the acetylene to escape before a dangerous pressure can build up. The safety plug hole is too small to permit a flame to burn back into the cylinder if the escaping acetylene should become ignited.
Hose Types
Hose for oxyacetylene welding must be the correct type. The CGA defines these grades as follows:
• Grade R and Grade RM for acetylene use only
• Grade T for all fuel gases
Why are Grades R and RM for acetylene only? For many years, the predominant fuel gas in the industry was acetylene. Acetylene has little or no adverse effects on rubber and was only used at a maximum of 15-psi pressure. Therefore, no requirements for fuel gas compatibility were initially specified. Different types of fuel gases (propane, natural gas, methyl-acetylene-propadiene, propylene, hydrogen, etc.) became popular over time, particularly for cutting. Many of these fuel gases are detrimental to certain types of rubber. With the use of these fuel gases, combining the wrong hose and fuel could lead to premature hose failure.
Grades R, RM, and T are compatible with acetylene. If a fuel gas other than acetylene is used, Grade T hose must be used.
Fig. 3.8. Hoses for oxyacetylene welding must be of the correct type. The Compressed Gas Association defines three grades: Grade R, Grade RM, and Grade T. For fuel gases other than acetylene, only Grade T should be used.
Flame Types
In oxyacetylene welding, the torch tip never touches the material being welded; only the flame touches. The type of flame produced depends on the ratio of oxygen to acetylene.
A neutral flame is produced when there is a 1:1 ratio of oxygen to acetylene. This type of flame has no chemical effect on the weld metal so it does not oxidize the weld metal nor cause an increase in carbon. The excess acetylene flame is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. This is often called a carburizing flame.
An excess-acetylene flame causes an increase in the weld carbon content when welding steel.
An oxidizing flame is created when the proportion of acetylene in the mixture is lower than that required to produce a neutral flame. It ozonizes or “burns” some of the weld metal.
Chemistry of the Flame
When acetylene burns in the air, carbon dioxide and water vapor are the byproducts. It takes 2 cubic feet of acetylene and 5 cubic feet of oxygen or 2½ times as much oxygen as acetylene. Yet a neutral flame burns at 1:1 oxygen/acetylene ratio and a neutral flame does not have an excess of either gas. This is not a contradiction because the combustion process is more complex than simply the volume of oxygen and acetylene gas supplied to the torch. The actual combustion takes place in two stages. In the first stage, the mixture leaving the torch tip supplies the oxygen, and in the second stage, the air surrounding the flame supplies the oxygen.
In the first stage of combustion, the acetylene breaks down into carbon and hydrogen. The carbon reacts with the oxygen to form carbon monoxide, which requires one molecule of oxygen for each molecule of acetylene.
In the second stage of combustion, the carbon monoxide reacts with the oxygen from the air to form carbon dioxide. The hydrogen reacts with the oxygen from air to form water.
The two-stage combustion process produces the well-defined inner cone in an oxyacetylene flame. The first stage takes place at the boundary between the inner cone and the blue outer flame. The second stage takes place in the outer flame. If the proportion of acetylene supplied to the tip is increased, a white “feather” appears around the inner cone. This feather contains white-hot particles of carbon that cannot be oxidized to carbon monoxide in the inner cone boundary due to the lack of oxygen in the original mixture. On the other hand, if the proportion of oxygen fed to the tip is increased, the inner cone shortens noticeably and the noise of the flame increases.
Fig. 3.9. The neutral flame has a 1:1 ratio of oxygen to acetylene. Excess acetylene causes an increase in carbon content or carburizing when welding steel. An oxidizing flame is created when the proportion of acetylene in the mixture is lower than that required to produce a neutral flame, so it ozonizes or “burns” some weld metal.
Flame Adjustment
For most welding, a neutral flame is desired. Even a skilled oxyacetylene weldor has difficultly telling the difference between a true neutral flame and a slightly oxidizing flame. However, it is relatively easy to tell the difference between a neutral flame and a slight-excess acetylene flame. Therefore, it is always best to adjust the flame to neutral from a slight-excess acetylene flame.
Start with an excess-acetylene flame. Increase the flow of oxygen until the excess-acetylene feather is almost gone. This feather is visible in Figure 3.9, and it extends beyond the concentrated white flame cone at the torch tip.
Filler Rods
AWS classifies welding filler rods for oxyacetylene welding according to their chemical composition. AWS specification A5.2 designation of R45 is a common alloy. The rod contains low carbon and manganese alloy additions. It produces an all-weld tensile strength of about 45 ksi (1 ksi equals 1,000 psi. The use of ksi eliminates all of the zeros.)
For added strength, the R60 rod designation contains higher carbon, manganese, and some silicon.
In addition to meeting minimum chemical requirements, a weld must be made as defined in AWS A5.2 specification and produce a minimum of 60-ksi tensile strength. An even stronger alloy is available that contains other alloying elements and meets a minimum tensile strength of 65 ksi.
A note of interest is that R45 has very little alloy, which is probably similar to that of a coat hanger. For oxyacetylene welding, where only a low strength is needed a coat hanger could work. However, its chemistry may not be consistent and if you try it, be sure all of the paint or clear coating is removed.
Oxyacetylene is often more useful than other welding processes for braze welding. Unlike fusion welding, braze welding does not melt the base metal, so the melting point of the filler rod is below the melting point of the material being welded. When joining cast iron, for example, the joint does not have to deal with a mixture of the very high carbon-based material.
Fig. 3.10. One area in which oxyacetylene can be very useful over other welding processes is when braze welding. The melting point of 60-percent copper and 40-percent zinc filler rod is about 1,630 degrees F, well below that of cast iron, for example. This allows a quality joint to be made without having to melt the very-high-carbon cast iron.
A common brazing alloy with 60-percent copper 40-percent zinc has a melting point of about 1,630 degrees F, which is well below that of cast iron. Weld strength typically exceeds 45 ksi. The rod melts and wets the surface but does not melt the cast iron. A flux ensures a very clean surface and assists in the wetting process.
The other advantage of braze welding cast iron is that the high heat input and inherent slow cooling reduces the shrinkage stresses in the cast iron and avoids cracks. Braze welding is a preferred method of repairing cracks in cast-iron parts, particularly some types that are very difficult to fusion weld.
Welding equipment can be purchased from many sources, including the Internet. However, welding equipment operates in a difficult environment and sometimes requires repair, so make sure businesses are available to repair the product if it fails.
Oxyfuel equipment is unique; it is essential to match the torches, regulators, and hose with the fuel gas. With the increasing cost of acetylene, alternative fuel gases may be best for you, and you should discuss the available options with your gas supplier. Some equipment is sold with two cylinders, but determine if your gas supplier is willing to fill them or swap them with filled cylinders instead. If you are braze welding, heating, and cutting rather than truly welding, a gas distributor may recommend a system that uses propane, propalyene, or another alternative to acetylene.
Safety
These general safety guidelines should be followed, but the following short overview of general safety issues is not meant to replace the instructions supplied with the oxyfuel welding and cutting outfit or other equipment. Read and understand the manufacturer’s instructions for safe use of the product.
General Precautions
• Do not use oil. Oil, grease, coal dust, and other organic ingredients are easily ignited and burn violently in the presence of oxygen. Never allow such materials to come in contact with oxygen or oxygen-fuel gas equipment. Oxygen-fuel gas equipment does not require lubrication.
• A serious accident can occur if oxygen is used as a substitute for compressed air. Oxygen must never be used to power pneumatic tools, to blow out pipelines, to dust clothing, or for pressure testing.
• Never use acetylene at pressures above 15 psi.
• Never use torches, regulators, or other equipment in need of repair. If a regulator creeps up in pressure, it has a seat leak and should be repaired or replaced.
• Do not connect an oxygen regulator to a cylinder unless it has a filter on the inlet. If it is on the input nipple, check to see it is installed every time the regulator is put on a cylinder.
• Always use the equipment manufacturer’s recommended operating pressures. Using pressures higher than recommended not only makes flame adjustment difficult, but it can cause a flashback fire inside the torch.
• Always used fully enclosed goggles or a full-face helmet when working with a lighted torch. Goggles with a number-4 shade are generally satisfactory for oxyacetylene welding and oxyfuel cutting.
• Do not use matches to light a torch. Always use a friction lighter to avoid having your hands near the flame when lighting.
• Wear suitable clothing: fire resistant gauntlet gloves and long-sleeved shirts. Wool is more fire resistant than cotton or synthetic fabric.
• Before starting to weld or cut, check the area to make sure sparks, flames, hot metal, or slag will not start a fire.
• Never weld or cut without adequate ventilation.
• Use particular caution when welding and cutting in dusty or gassy locations. These atmospheres necessitate extra precautions to avoid explosions or fires from sparks, matches, or open flames of any type.
• Never weld or cut on containers that have held flammable or toxic substances until the container has been thoroughly cleaned and flammable gases have been neutralized.
Precautions for Containers with Flammable Substances
• Assume the container may contain residue.
• Wash with a strong solution of caustic soda to remove heavy oil.
• If possible, fill the container with water to within a few inches of the working area before welding. When impractical to fill with water, an inert gas such as nitrogen or carbon dioxide can be used to purge the container of oxygen and flammable vapors. Maintain the gas purge during welding.
Other Precautions
• Make sure that jacketed or hollow parts are sufficiently vented before heating, welding, or cutting. Air, gas, or liquid confined inside a hollow container expands when heated. The pressure created can cause a violent rupture.
Fig. 3.11. The American Welding Society classifies oxyacetylene welding filler rods based on their chemical composition. A common alloy has an AWS specification A5.2 designation of R45. The rod contains little alloy having low carbon and manganese additions. The rod produces an all weld tensile strength of about 45 ksi.
• Remove or securely fasten in place any bushings in a casting before heating the casting. Bronze bushings expand more than cast iron when heated to the same temperature. If a bushing is left in place, the casting may be damaged or expansion may cause the bushing to fly out. If it cannot be removed, bolting large washers or plates over the ends may be possible.
• Protect cylinders, hoses, and your legs and feet when cutting. Do not cut material in such as position that allows sparks, hot metal, or a cut part to fall against a gas cylinder, the hoses, or your legs and feet.
• Take special care to make certain that a flame, sparks, hot slag, or hot metal do not reach combustible material and start a fire. This is particularly important in cutting operations. Have someone stand by to watch the sparks and give warning if sparks are going into an area that could cause fire problems.
• An appropriate fire extinguisher, a pail of water, a water hose, or a sand bucket should be located and readily available near the area. A person stationed on fire watch should have firefighting equipment and a fire extinguisher immediately at hand.
Have someone remain in the area for at least a half hour after the welding or cutting is finished to watch for smoke from a smoldering fire.
This book is not intended to teach detailed manual welding techniques. Oxyacetylene welding, however is, a useful process to learn and may not be familiar to those who have some skill at MIG and TIG welding.
Project: Welding Plate Practice
EXERCISE 1: WELDING BEADS
This exercise starts with weld beads and butt welds made downhand in 1/8-inch-thick steel, and then progresses to welds made in tubing. Select a number-2 or -3 welding tip size or one recommend by the manufacturer for use with 1/8-inch-thick steel. Manufacturers also supply recommended oxygen and acetylene pressures for various tip sizes and gas mixers used with their torches. Run a weld bead on a plate. Make the first weld passes without filler metal. Hold the inner cone about 1/8 inch from the plate and hold the torch stationary until a small molten puddle forms. Move the torch tip in a small semicircle and direct it slowly along the plate.
Fig. 3.12. Some simple oxyacetylene welding practical exercises help develop skills. A good exercise is to run a weld bead on plate using a 1/6-inch-diameter filler rod and hold as shown. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 2: PRACTICE FORMING A WELD PUDDLE
For the next practice welds, hold a 1/16-inch rod as shown in Figure 3.12. Insert the rod into the leading edge of the weld puddle until a few drops flow into the deposit. Then pull the rod back slightly and advance the torch a small amount, allow a puddle to form, and repeat the process of inserting the rod.
Fig. 3.13. To fill gaps you need two pieces of steel 9 inches long and 1/8 inch thick. Place them 1/16 inch apart at one end and 3/16 inch apart at the other end. Make a tack weld at the narrower end first and then tack weld the larger gap. Advance the torch tip as the weld progresses. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 3: ADD FILLER WELD BETWEEN TWO PLATES
Filling gaps is the next thing to learn. Place two pieces of steel approximately 9 inches long, 3/32 to 1/8 inch thick, 1/16 inch apart at one end and 3/16 inch at the other. Make a tack weld at the tighter end and tack weld the end with the 3/16-inch gap. The next tack weld is somewhat more difficult because of the greater gap. Move the flame slowly from one edge to the other. As the edges melt, place a small amount of filler metal on the edge and allow it to cool slightly. This is usually best performed when the flame is on the opposite side of where the metal is added. Increase the size of the tack by adding more filler rod.
After tack welding the piece, start at the tighter end and add filler as you did for the tack; move the flame in a small arc from side to side. Advance the torch tip as the weld progresses.
Figure 3.14 is a view of the middle of the joint. The technique becomes obvious as the weld progresses. More time is spent at the edges as the torch is moved side to side. Too much time spent in the middle of the joint causes the metal to burn through. The end objective is to have the weld bead drop through slightly at the bottom while being reinforced about 1/16 inch on top.
Test the weld quality by cutting a section about 3 inches long and placing one plate in a vice and striking the other with a hammer. You should be able to bend it at a 90-degree angle without breaking it.
Fig. 3.14. This is the perspective in the middle of the joint. The technique becomes obvious as the weld progresses. More time is spent at the edges as the torch is moved side to side. Too much time spent in the middle of the joint causes the metal to burn through. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
Fig. 3.15. To weld heavier sections use two pieces of 1/4-inch-thick steel. Bevele each plate at a 45-degree angle with a 1/16-inch nose. Concentrate the flame on melting the square-edge nose and forming the underbead. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 4: ESTABLISH ROOT WELD
Heavier sections can be welded using a similar technique (but with the appropriate tip size and filler rod). Use two pieces of steel 1/4-inch thick. Bevel each plate at a 45-degree angle. Use a grinder to make a flat nose at the bottom about 1/16 inch in width. This leaves a 90-degree bevel 3/16 inch deep. Assuming the test plates are about 9 inches long, gap them 1/8 inch at one end and 3/16 inch on the other.
Make tack welds on both edges as was done for the thinner sheet metal. Once the tack welds have been made, make a weld as before, but it should only fill up about half the thickness. This is the first of two weld passes. Focus on making a good underbead. The top edges of the plate should not have melted. The flame is concentrated on melting the square-edge nose and forming the underbead.
As the root pass approaches completion, the top corners of both pieces should still be sharp and unmelted. This first pass achieves complete root penetration and little else. The appearance of the top of the weld is not very important because it will be melted and covered by the finish pass. The puddle can be kept relatively small, which is also a help if the gap varies.
Make sure the bottom-squared nose is brought up to melting temperature before the weld puddle advances across them. Be sure not to add too much filler metal at one time.
Move the filler rod in and out of the weld puddle to control the rate of filler addition.
Move the flame across the joint, spending more time on the sides of the V, being sure they are melted up to about halfway up to the top edge of the plate.
Fig. 3.16. The objective of the first pass is to complete root penetration (here, the root pass is approaching completion). Keep in mind the appearance of the top weld is not very important. Move the flame across the joint, spending more time on the sides of the V so they are melted up to about halfway. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
Fig. 3.17. With the second pass, keep the end of the rod in the puddle at all times. Rub the end against the solid metal below the puddle, maintaining a constant back-and-forth motion across the joint. When the flame is melting one side of the joint, the rod is pushing on the puddle on the opposite side. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 5: COMPLETE FINISH WELD
Make a second pass over the first one. A larger 1/8-inch-diameter rod can be used as the weld progresses slowly and more filler rod is needed. Start at the beginning and form a weld puddle and proceed to fill the joint completely.
Much more filler is required in this pass. Keep the end of the rod in the puddle at all times, actually rubbing the end against the solid metal below the puddle and keeping it in a constant back-and-forth motion across the joint. The flame should move in longer arcs than was needed for the root pass. It should dwell at the sides of the V so they are melted before the puddle moves forward. Use a sideways motion of the filler rod to move the puddle back.
When making this pass, heat from the weld puddle, not the flame, melts the filler rod. The flame and rod motion must be controlled because the flame is melting one side of the joint and the rod is pushing on the puddle on the opposite side. The width of the flame movement need not be as large as the rod movement. If the rod is not moved a sufficient amount, a weld undercut occurs. There may also be places where the weld bead did not reach the top surface of the plate, creating an underfill defect.
In Figure 3.18, a backhand technique (also called drag) is used for the second pass. The flame is angled toward the completed weld, and the rod is angled toward the finishing end. Although most oxyacetylene welding is done with a forehand or push technique (as shown in all previous illustrations) in some situations, a backhand technique may be desired. The rod and flame move in an oval pattern along the weld line. The rod moves backward as the flame moves forward. The finished weld has heavier ripples than a good forehand weld deposit.
Fig. 3.18. Make the second pass with a drag, or backhand, technique. Angle the flame toward the completed weld and the rod toward the finishing end. Use a forehand technique for most oxyacetylene welding, but in some situations, a backhand technique is useful. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
Project: Welding Pipe Practice
Although this series of welds shows welding larger-diameter pipe, it can be used for welding small-diameter tubes. The oxyacetylene welding process is still used to weld 4130 tubing for aircraft. My old friend Butch Sosnin taught sprinkler pipe weldors that oxyacetylene welding was ideal for small-diameter pipe welding, so it certainly should be considered.
Fig. 3.19. Welding a pipe is another useful exercise. The oxyacetylene welding process is still used to weld 4130 tubing for aircraft. An application of welding 1/2-inch-diameter 4130 tubing is discussed in Chapter 7. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 1: MAKE BOTTOM-TO-TOP WELD
Place tacks in the joint, as shown in Figure 3.20. With material thickness less than 1/8 inch, you can make the weld in one pass. Point the torch flame approximately toward the centerline on the pipe. Hold the filler rod tangentially to the joint. Make the weld from bottom to top, adding filler to the leading edge of the puddle. Completing the weld requires welding in the flat, vertical, and overhead positions. Stop the weld, repositioning your hand and start a puddle for each position.
Fig. 3.20. With pipe material thickness less than 1/8 inch, make the weld in one pass. Point the torch flame toward the centerline on the pipe. Hold the filler rod tangentially to the joint. The weld is made from bottom to top, adding filler to the leading edge of the puddle. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 2: MAKE TOP-TO-BOTTOM WELD
In Figure 3.21, the weldor is using a backhand technique, and the weld is made from top to bottom. There are certain situations in which welding out of position is necessary. In some welding circumstances, the flame is used to hold up the weld puddle. The aim is to keep the puddle from running onto metal that has not melted. As noted when backhand welding flat plate, the relative movements of the rod and flame are different from forehand welding. The rod and flame move in an oval path with one forward when the other is backward and vice versa.
Fig. 3.21. Use a backhand technique as the weld is made from top to bottom. There are times when welding out of position is necessary. Use the flame to hold up the weld puddle. Move the rod and flame in an oval path with one going forward and the other going backward. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
EXERCISE 3: PERFORM OVERHEAD WELD
Welding overhead is not much different from welding vertically. Surface tension holds the molten metal up, as does the force of the flame. If forehand and backhand techniques are an option, a backhand technique can be used to start the weld at the top and move downhill. When the bottom is reached, a forehand technique is used to weld to the top. If you weld with only a forehand technique, start at the bottom and work up one side, change position, and work up the other side.
Fig. 3.22. Welding overhead is similar to welding vertically. Surface tension and the force of the flame hold up the molten metal. If forehand and backhand techniques are used, the weld can be started at the top and welded downhill with a backhand technique. When the bottom is reached, use a forehand technique to weld to the top. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
Application: Oxyacetylene Welding Cast Iron
Oxyacetylene welding is very effective for repairing cracks in cast-iron parts. Some types of cast iron can be fusion welded, although with difficulty and the potential for cracking, while others cannot. However, they all can be braze welded. With this approach, the filler material melts and wets the cast iron, but that occurs well below the melting point of the cast iron.
Fig. 3.23. Oxyacetylene welding is very effective for repairing cracks in cast-iron parts. Some types of cast iron can be fusion welded, although with difficulty and the potential for cracking, while others cannot. However, they can all be braze welded.
Fig. 3.24. The finished brazed joint is sound and well wet into the cast iron. The brazing material is also very ductile. When the cast iron expands and cools as the part is heated and cooled, the brazing alloy can expand and contact as needed and does not excessively stress the cast iron.
Repairing an exhaust manifold is a typical example of this.
To simulate a crack repair job, a groove representing a crack was placed in the manifold. The weldor ground through the full thickness of the manifold and a V-joint was placed in it. In addition to the advantage of braze welding not melting the cast iron, it also requires heating the material to a high temperature. This essentially preheats the assembly and avoids high cooling rates. This high heating reduces the stress on the braze weld area when completed, so the brittle cast iron is less likely to crack.
AWS RBCuZn-C (58-percent copper and 41-percent zinc), a common brazing alloy, is used to braze weld this simulated crack. The white outside coating on the rod is a brazing flux, which provides good wetting of the cast iron. To further promote wetting of the cast iron, brazing alloy can butter the surfaces to be joined and this bridges the joint. In addition, it allows the wetting to be seen more clearly. The flux is applied to remove oxides from the cast-iron surface and promote wetting. There is sufficient coating on the rod so there’s no need for adding more. The brazing flux is available in a metal can. It’s applied to the joint or by dipping the bare rod into the flux as the braze welding progresses.
Fig. 3.25. Braze welding does not melt the cast iron, and also requires heating the material to a high temperature. This essentially preheats the assembly and avoids high cooling rates. Stress on the joined area is reduced as a result, so the brittle cast iron is less likely to crack.
The finished brazed joint is sound and wet into the cast iron. The brazing material is also very ductile. When the cast iron expands and cools as the part is heated and cooled, the brazing alloy can expand and contract as needed and does not excessively stress the cast iron. To be safe, the part is slow cooled after brazing.
Fig. 3.26. AWS RBCuZn-C, a common flux-coated brazing alloy, was used to braze weld this simulated crack. A flux is needed to ensure good wetting of the cast iron. It removes oxides from the cast-iron surface and promotes wetting. There is sufficient coating on the rod to avoid the need for adding more.
