Читать книгу Welding Essentials - William Galvery - Страница 9
ОглавлениеThe chapter of knowledge is a very short one, but the chapter ofaccidents is a very long one.
Philip Dormer Stanhope
Introduction
Oxyacetylene welding was first used industrially in the early years of the twentieth century. Although this process makes excellent welds in steel, it is little used for welding today except for a few specialties (light aircraft, race car frames and American Petroleum Institute natural gas distribution), since there are other more efficient welding processes available. However, oxyacetylene has many other important uses: cutting, hardening, tempering, bending, forming, preheating, postheating, brazing, and braze welding. Because of the precise control the welder has over heat input and its high-temperature flame, together with its low equipment cost, portability, and versatility, it remains an essential tool. No industrial shop is complete without an oxyacetylene outfit. As with all effective tools, using oxyacetylene carries risk. We will cover the theory and use of oxyacetylene equipment so you can use them with confidence and safety. It will also prepare you for the next chapter on oxyfuel cutting, because many components and issues are common to both processes.
Process Name
What is the name covering all welding processes using oxygen and a fuel gas?
Oxygen fuel (oxyfuel) welding.
What is the American Welding Society (AWS) abbreviation for oxyfuel welding?
The abbreviation for all oxyfuel welding processes; those using oxygen and any fuel gas is OFW.
A particularly important member of the OFW process family is oxyacetylene welding. What is the AWS abbreviation for this process?
The abbreviation for oxyacetylene welding is OAW. Note that OAW is just one member of the OFW family.
Equipment
Figure 1-1 shows and labels the components of the basic oxyacetylene welding equipment (outfit) showing how they are connected.
Figure 1–1A drawing of oxygen acetylene welding equipment
Process
How does oxyacetylene equipment perform the welding process?
Oxygen and acetylene gases when combined, in the proper proportions, in the torch handle’s mixing chamber, produce an approximately 5600°F (3100°C) flame at the torch tip. This flame melts the edges of the base metals to be joined into a common pool. Sometimes additional filler metal is added to the molten pool from a welding rod. When this common pool cools and the metal freezes solid, the joined metals are fused together and the weld complete.
What are some advantages of the OAW process?
•Low cost
•Readily portable
•Excellent control of heat input and puddle viscosity
•No external power required
•Good control of bead size and shape
•Fuel mixture is hot enough to melt steel
Setup (and related safety)
List the steps for installation of oxygen and acetylene pressure regulators on full cylinders. Be sure to include all safety precautions.
•Put on your welding safety equipment: tinted safety goggles (or tinted face shield), cotton or wool shirt and pants, high-top shoes, and welding gloves at a minimum.
•Make sure the valves on previously used or empty cylinders are fully closed and their valve protection covers are securely screwed in place. Then remove the empty cylinders from the work area and secure them against tipping during the wait for a refill shipment. Secure the newly replaced or full cylinders to a welding cart, building column, or other solid anchor to prevent the cylinders from tipping over during storage or use.
•Momentarily open each cylinder valve to the atmosphere and reclose the valve quickly purging the valve; this is known as cracking a valve. Cracking serves to blow out dust and grit from the valve port and to prevent debris from entering the regulators and torch.
•With a clean, oil-free cloth, wipe off the cylinder valve-to-regulator fittings on both cylinders to remove dirt and grit from the fittings’ connection faces and from the fittings’ threads. Do the same to both regulators’ threads and faces. Remember, never use oil on high-pressure gas fittings. Oxygen at high pressures can accelerate combustion of oil into an explosion.
•Make sure reverse-flow check valves are installed on the torch or the regulators.
•Check to see that both the oxygen and acetylene regulator pressure adjustment screws are unscrewed, followed by threading each regulator to its respective cylinder. Snug up the connections with a wrench. Caution: Oxygen cylinder-to-regulator threads are right-handed; so are oxygen hose-to-torch screw fittings. Acetylene cylinder-to-regulator fittings and acetylene hose-to-torch fittings threads are left-handed. This arrangement prevents putting the wrong gas into a regulator or torch connection.
•Stand so the cylinders are between you and the regulators, S-L-O-W-L-Y open the oxygen cylinder valves. Open the oxygen cylinder valve until it hits the upper valve stop and will turn no further. Also standing so the cylinders are between you and the regulator, open the acetylene cylinder valve gradually and not more than 1 turns. If there is an old-style removable wrench on the acetylene cylinder, keep it on the valve in case you must close it in an emergency.
•Look at the high-pressure—cylinder side—pressure gauges to indicate about 225 psi (15.5 bar) in the acetylene cylinder and 2250 psi (155 bar) in the oxygen cylinder. Note: 1 bar = 1 atmosphere = 14.5 psi = 0.1 MPa. Cylinder pressures vary with ambient temperature. The pressures given above are for full cylinders at 70°F (21°C).
•Purge each torch hose of air separately: Open the oxygen valve on the torch about three-quarters of a turn, then screw in the pressure control screw on the oxygen regulator to your initial pressure setting—about 6 psi (0.4 bar). After several seconds, close the torch valve. Do the same for the acetylene hose. Comment: We do this for two reasons, (1) to make sure we are lighting the torch on just oxygen and acetylene, not air, and (2) to set the regulators for the correct pressure while the gas is flowing through them.
•Caution: never adjust the acetylene regulator pressure above 15 psi (1 bar) as an explosive disassociation of the acetylene could occur.
•Recheck the low-pressure gauge pressures to make sure the working pressures are not rising. If the working pressure rises, it means the regulator is leaking. Immediately shut down the cylinders at the cylinder valves as continued leaking could lead to a regulator diaphragm rupture and a serious accident. Replace and repair the defective regulator.
•Test the system for leaks at the cylinder-to-regulator fittings and all hose fittings with special leak detection solutions; bubbles indicate leaks.
If you are using a small to medium torch tip on a job for the first time, what regulator pressures should be set as a starting point?
Set both the acetylene and the oxygen regulator pressures to 6psi (0.4 bar).
What are the steps for adjusting the torch to a neutral flame?
•Open the acetylene valve no more than turn and use a spark lighter to ignite the gas coming out of the tip. A smoky orange flame will be the result, Figure 1–2 (A).
•Continue to open the acetylene valve until the flame stops smoking (releasing soot). Another way to judge the proper amount of acetylene is to open the acetylene valve until the flame jumps away from the torch tip, leaving about inch gap (1.6 mm), Figure 1–2 (B). Then close the valve until flame touches the torch tip.
•Open the oxygen valve slowly. As the oxygen is increased, the orange acetylene flame turns purple and a smaller, white inner cone will begin to form. With the further addition of oxygen, the inner cone goes from having ragged edges, Figure 1–2 (C), to sharp, clearly defined ones. The flame is now neutral and adding oxygen will make an oxidizing flame, Figure 1–2 (D).
•If a larger flame is needed while keeping the same tip size, the acetylene may be increased and the oxygen further increased to keep the inner cone’s edges sharp. This process of increasing the acetylene, then the oxygen is usually done in several cycles before the maximum flame available from a given tip is achieved. Adjusting the flame below the minimum flow rate for the tip orifice permits the flame to ignite inside the nozzle. This is flashback and makes a popping sound. If you need a smaller flame, use a smaller torch tip. See the section on flashback.
Figure 1–2Shows flame adjustments from carburizing to a neutral flame
What it the hottest part of a neutral flame?
The tip of the inner cone is the hottest part of the flame. The inner cone is where the optimum mixture of oxygen and acetylene burn. The outer envelope where any unburned acetylene burns with oxygen from the atmosphere. A neutral flame is when enough oxygen is present in the flame to be burning all of the acetylene gas and is used for most welding processes. See Figure 1–3.
Figure 1–3Graph of an oxyacetylene flame temperature profile
What effect do oxidizing and carburizing flames have on molten metal in the weld pool?
An oxidizing flame contains more oxygen than the flame can burn and this oxygen combines (or burns) with carbon in the steel to carbon dioxide gas. The result is the weld metal has a change in carbon content and in its properties. Strength is always degraded and brittleness increased.
A carburizing flame contains more acetylene than the flame can burn and the carbon in the acetylene adds to the carbon in the weld pool causing gas bubbles in the weld. When the weld freezes these gas bubbles create porosity holes.
What are the proper steps to shut down an oxyacetylene torch and its cylinders?
•First turn off the oxygen and then the acetylene with the torch handle valves. Turning off the acetylene first can cause a flashback.
•Turn off the oxygen and acetylene cylinder valves at the upstream side of the regulators.
•Separately, open and reclose the oxygen and acetylene valves on the torch handle to bleed the remaining gas in the hoses and regulator into the atmosphere. Verify that both the high-pressure and low-pressure gauges on both regulators indicate zero.
•Unscrew the regulator pressure adjustment screws on both cylinders in preparation for the next use of the equipment. The regulator screws should be loose but not about to fall from their threads.
Gases
What is acetylene gas?
It is a clear gas having a specific gravity slightly lighter than air at 0.906 (air = 1.000). Acetylene’s chemical formula, C2H2, indicates that each molecule of this hydrocarbon compound contains two carbon atoms and two hydrogen atoms.
What is the odor of acetylene gas?
It has a distinctive garlic odor. Because the liquid acetone in the acetylene cylinder also has an odor, this acetone odor is frequently mistaken for that of acetylene, when in fact it is the odor of the mixture of both acetylene and acetone.
How is acetylene made?
Acetylene results from dissolving calcium carbide in water and capturing the resulting gas. One pound of calcium carbide generates about 10 cubic feet of acetylene (1 kg calcium carbide generates about 618 liters of acetylene).
Where does calcium carbide come from?
Calcium carbide results from an industrial process where lime and coke are smelted in an electric furnace. A gray, hard solid, it is supplied in a variety of forms: bricks, powders, pellets, or granules.
What is the chemical equation of combustion of acetylene in a neutral flame?
C2H2 + 2.5O2 → 2CO2 + H2O
What does this equation tell us?
One part acetylene and two and a half parts oxygen combine to produce a neutral flame (a neutral flame has just the right amounts of fuel and oxygen so there is neither an excess of oxygen or fuel after combustion). What the equation does not tell us is that equal volumes of acetylene and oxygen from the compressed gas cylinders combine with another one and a half parts of oxygen from the atmosphere to make the flame.
How is oxygen made for welding?
Atmospheric air is repeatedly cooled and compressed until it becomes a very cold liquid. This liquid is gradually warmed, and as each component gas of the liquid air reaches its vaporization temperature, it comes out of the liquid air, and separates itself. This is the fractional distillation of liquid air. Other gases important in welding—nitrogen, carbon dioxide, and argon—are also made using this process. Oxygen can also be made by electrolysis of water, but this is not a cost-effective process to make industrial quantities.
What are the two main ways of supplying welding shops with oxygen?
Compressed gas cylinders are used in smaller shops; liquid oxygen cylinders in larger shops. The liquid oxygen flows from its cylinder into a radiator that warms the liquid oxygen, and converts it into gaseous oxygen.
How are large welding shops supplied with acetylene?
Multiple cylinders are manifolded together and their output piped around the plant to each welding or cutting station.
What is peculiar about the filling and draining of acetylene cylinders?
Because the acetylene is dissolved in acetone, not just pumped into the pressure vessel, the filling process takes seven hours as the absorption process occurs. Similarly, an acetylene cylinder can only deliver one-seventh of its capacity per hour as the acetylene will not come out of solution in the acetone faster. More acetylene capacity will require cylinders manifolded together. This can become an important issue when using large multi-flame heating tips (in the industry called a rosebud tip) which consume many times more gas than a welding tip.
Why is acetylene potentially so dangerous?
Acetylene will form explosive mixtures with air at all concentrations between 2.5 and 80%. This is the widest range of any common gas and almost insures an explosion if leaking gas is ignited.
Can other fuel gases be used in place of acetylene?
Certainly, but their maximum heat potential is below that required for welding steel. Acetylene is the best gas for welding because it:
•Has the highest temperature of all fuel gases.
•Acetylene delivers a higher concentration of heat than other fuel gases.
•Has the lowest chemical interaction with the weld pool’s molten metal than all other gases.
However, other gases such as natural gas, methylacetylene-propradene stabilized (also called MPS or MAPP® gas), propane, hydrogen, and proprietary gases based on mixtures of these are frequently used for other non-welding processes for cost reasons. They work well for soldering, brazing, preheating, and oxygen cutting, and are seldom used for welding. Small changes, like different torch tips, may be necessary to accommodate alternate fuel gases. Table 1–1 shows the maximum temperature achievable with different fuel gases. Where even lower temperatures are needed (sweating copper tubing and many small soldering tasks) a single cylinder of fuel gas using only atmospheric oxygen is effective and economical.
Table 1–1 Combustion properties of fuel gases
Compressed Gas Cylinders
What is the difference between acetylene cylinder and oxygen cylinder construction?
Oxygen cylinders are seamless vessels of special high-strength alloy steel. They are made from a single billet by a draw-forming process and they contain no welds. Acetylene cylinders are fabricated and contain welds.
What materials other than acetylene are found inside acetylene cylinders?
Under certain conditions above 15 psi (1 bar), acetylene may spontaneously disassociate into its components of carbon and hydrogen. Acetylene cylinders are packed with an inert porous monolithic filler to prevent this dangerous disassociation. Acetylene cylinders are also contains acetone that can dissolve 25 times its own volume of acetylene per atmosphere of pressure. This greatly increases the cylinder’s acetylene capacity.
Figure 1–4Oxygen and acetylene cylinder cross sections
What is the purpose of safety valves and plugs in oxygen and acetylene cylinders?
Their purpose is to prevent the cylinder bursting from overpressure when it is heated. Oxygen cylinders have a small metal diaphragm in a section of the valve which ruptures, releasing cylinder pressure to the atmosphere and preventing a cylinder burst. Disk rupture occurs above 3360psi (232 bar), the cylinder test pressure.
Acetylene cylinders contain one to four fusible safety plugs depending on their capacity. These fusible plugs, made of a special metal alloy, melt at 212°F (100°C).
They also release the cylinder contents to atmosphere to prevent rupturing (and then exploding) when the cylinder is exposed to excessive temperatures, usually from a fire. Acetylene cylinders may have the plugs on the top, or top and bottom.
Figure 1–5Detail of pressure safety relief on oxygen valve
Why should the welder open the oxygen cylinder valve all the way, but open the acetylene cylinder valve just one turn?
Because the oxygen cylinder is filled to such a high pressure (2250 psi or 155 bar) to prevent leakage around the valve stem, oxygen and all other high-pressure cylinders have a second valve seat to make a solid seal around the valve stem when the valve is open. See Figure 1–6. Because the acetylene cylinder valve sees a relatively low pressure (225 psi or 15.5 bar), leakage around the valve stem in use is small and a single seat is used. Since the acetylene valve can deliver adequate volume with one turn open, opening the valve more just increases the closing time in an emergency. For similar reasons the welder must never remove the removable wrench from the valve of old-style acetylene cylinders while the cylinder is in use.
Figure 1–6Details a cross section of oxygen valve
Why should the welder position the cylinders between himself and the regulators when opening the cylinder valves?
If a regulator fails internally, releasing high-pressure gas from a cylinder into the regulator’s low-pressure side, the regulator housing and gauges may explode. Fatalities have resulted from such malfunctions.
If an acetylene cylinder has been incorrectly transported on its side, why should the welder avoid immediate use?
The acetylene gas and the acetone in which it is dissolved may become mixed in the area just below the valve, resulting in both gaseous acetylene and liquid acetone at the top of the cylinder. This is where acetylene exits the cylinder and goes through the valve to enter the regulator. Both acetylene gas and liquid acetone will be drawn into the regulator possibly ruining the rubber components of the regulator and torch and creating a safety hazard. The weld metallurgy may also be contaminated.
What should the welder do knowing that a newly delivered acetylene cylinder has been incorrectly transported on its side?
Upright the cylinder and wait at least one-half hour before connecting and using the cylinder to allow the liquid phase of the acetone to separate from the acetylene gas in the upper portion of the cylinder. That way no acetone will be drawn into the regulator possibly damaging its seals. Also, acetone in the weld flame will contaminate the weld pool and spoil the weld.
How can one readily distinguish between the oxygen fitting swivel nut and one for acetylene?
Acetylene, like most other fuel gas handling equipment, has a notch or groove cut in the middle of the edges of the hexagonal faces of the swivel nut. This is a flag for a left-handed thread. See Figure 1–7.
Figure 1–7Compares connector nuts used on oxygen and acetylene equipment
Are all oxygen cylinders painted green?
Frequently, oxygen cylinders are painted green or have a green band, but the only sure way to determine the contents of a compressed gas cylinder is to read the adhesive label on it. This label is required by law and should not be removed. Do not go by its color as there is no color code. Unlike civilian industry, the US armed forces do color code their cylinders.
What pressures should full oxygen cylinder and full acetylene cylinder gauges show at 70°F (21°C)?
The acetylene should show 225 psi (15.5 bar) and the oxygen 2250 psi (155 bar). Note that these pressures will fluctuate with ambient temperature.
What do the letters and numbers stamped on the neck of high-pressure cylinders indicate?
The stampings indicate which US Department of Transportation specifications the cylinder meets, what type steel was used, who fabricated it, and when.
•Steel stamp markings such as “DOT-3A-2400” indicate the cylinder was made to US Government Department of Transportation (DOT) specifications, the “3A” denotes chrome manganese steel (or “AA” for molybdenum steel), and the “2400” the maximum filling pressure in psi.
•The oldest date indicates the month and year of manufacture. Subsequent dates, usually at five year intervals, indicate when mandatory hydrostatic pressure testing was performed and by whom. See Figures 1–8 and 1–9.
Figure 1–8High-pressure cylinder markings
Figure 1–9Acetylene cylinder markings
What are common oxygen cylinder sizes?
Figure 1–10 shows high-pressure cylinder sizes. Many gases in addition to oxygen, like nitrogen, carbon dioxide, and argon come in high-pressure cylinders.
Figure 1–10Oxygen cylinder sizes
What factors govern the choice of oxygen and acetylene cylinder size?
Smaller cylinders are suitable for refrigeration repairman who must climb ladders with OAW equipment, but they are impractical for most work. For example, a 55 ft3 (1557 liter) oxygen cylinder would last under two hours cutting inch (3 mm) steel plate. For the larger cylinders, their size and weight can be major drawbacks where forklifts and loading docks are not available. In general the mid-sized cylinders offer the best compromise of economy and convenience.
What are the common acetylene cylinder sizes?
See Figure 1–11.
Figure 1–11Acetylene cylinder sizes
Why are pressure gauges on acetylene cylinders poor indicators of remaining gas quantity?
Gas pressure remains nearly constant at a given temperature as acetylene gas is withdrawn from solution in acetone until very little gas remains. The best way to determine a cylinder’s remaining contents is to compare its current weight with its empty weight. Note that the weight of the empty cylinder is stamped on its top. There are 14.6 ft3 of acetylene for every pound of cylinder weight over its empty weight, or one liter of acetylene for every 1.1 grams of cylinder weight over the empty weight. Drawing acetylene from cylinders at pressures below 25 psi (1.7 bar) can cause acetone to be withdrawn from the cylinder.
Under what ownership arrangement may compressed gas cylinders be offered to users by welding equipment suppliers?
•Outright sale of the cylinder with the right to exchange it for a filled one of equal size by paying for refill is most economical in the long run. Usually one gas supplier will accept the cylinders you obtained from another at no additional charge. There will be a problem swapping cylinders if an embossed owner’s name appears on the neck ring.
•Cylinders may be rented by the month or year. Excellent when you don’t have a long-term need for them, just an immediate one.
•Some distributors lease cylinders for a year or more; some for 99 years.
•There is a very silly practice that is worth mentioning. Some gas distributors try to sell you a brand new cylinder for more money than one with previous use—an “old” one, but since most cylinders cannot be refilled immediately, chances are that you will exchange your “new” one for an “old” one when you get your first refill. The “old” ones usually cost less. And since cylinders can last well over 30 years and the cylinders due for pressure hydro testing are tested at the gas supplier’s expense, there is little point in paying extra to get your own “new” cylinder.
Regulators
What is the purpose of pressure regulators?
Regulators reduce the pressures of welding gases from the very high cylinder pressures to the low pressures needed by the torch to function properly. Also, as the cylinder pressure falls with gas consumption, the regulator maintains the constant pressure needed by the torch, even though the cylinder supply pressure drops greatly. For example, an oxygen cylinder may contain oxygen at 2250 psi (155 bar) and the torch requires about 6 psi (0.4 bar) to operate. Similarly, a full acetylene tank may contain gas at 225 psi (15.5 bar) and the torch needs fuel gas at 6 psi (0.4 bar).
How does a single-stage pressure regulator work?
There are two designs for single-stage regulators, the stem-type and the nozzle-type. In the stem-type, the balance of forces on each side of the diaphragm and attached stem perform pressure regulation. There are four forces acting on the diaphragm and stem. In Figure 1–12 the combined forces of the large upper spring and atmospheric pressure act to open the regulator valve and admit gas into the regulator and hoses; in the opposite direction the combined forces of the high-pressure gas on the lower side of the diaphragm and the small stem spring act to close the regulator. When the adjusting screw is unscrewed (or in the up position in the diagram), there is little pressure exerted downward on the diaphragm by the large spring and the regulator stays closed. When the adjusting screw is tightened downward to increase regulator pressure, the increased pressure on the attached spring exerts more pressure on the diaphragm and opens the valve, admitting high-pressure gas to the lower chamber and hose. As gas continues to enter this chamber, chamber pressure rises. When it rises above the pressure called for, the high-pressure gas in the lower chamber partially or fully closes the valve to maintain the desired pressure.
Figure 1–12Single-stage stem-type regulator
The nozzle-type regulator is very similar to the stem-type regulator, but instead of the valve being closed by inlet or cylinder pressure as in the stem-type, the inlet pressure works to open the valve. The result is the same: a balance of pressure across the diaphragm accurately controls pressure to the torch.
How does a two-stage regulator work?
A two-stage regulator is basically two single-stage regulators connected in series inside the same housing with the total pressure drop being split across the two regulator stages. The first stage pressure is factory-set; the second stage pressure is user-set. See Figure 1–14.
Figure 1–13Single-stage nozzle-type regulator
What are the advantages and disadvantages of a two-stage pressure regulator over a single-stage one?
The two-stage regulator’s advantage is that a higher volume of gas may be withdrawn from the cylinder with less pressure fluctuation than produced by a single-stage regulator. The combination of two regulators working together in series maintains a very constant torch pressure over wide cylinder pressure changes. Its disadvantage is cost. They are only needed when large gas volumes are needed as with multiple stations or rosebud tips.
Figure 1–14Two-stage regulator
Torches, Tips, and Hoses
What are the major parts of an oxyacetylene torch?
Shown below is the most common oxyacetylene torch design. Other designs are available. Some have very small flames for jewelry and instrument work, while others take no accessories and are much lighter in weight than standard torch designs to reduce operator fatigue.
Figure 1–15Oxyacetylene torch and tip
Besides a selection of tip sizes for different sized jobs, what other devices can be put on the torch handle and what are they used for?
•Cutting heads also called cutting attachments (see Chapter 2).
•Multi-flames for heating metals prior to bending, brazing, or heat-treating.
Figure 1–16Oxyacetylene torch attachments: cutting head (left), welding tips (center) and multi-flame tip (right)
Why are there different size torch tips?
Matching the size of the flame and the resulting volume of gas to the thickness of the metal in the weld is important. Too much flame and the base metal around the weld may be damaged, too little and there is inadequate heat to melt metal for full penetration.
How are torch tip sizes designated?
There is no industry standard; each torch manufacturer has its own numbering system. Cross-reference tables compare each manufacturer’s tip sizes with numbered drill sizes.
The American Welding Society (AWS) has been urging tip manufacturers to stamp tips with the material thickness size to eliminate the confusion of tip size numbers. The AWS C4.5M Uniform Designation System for Oxy-Fuel Nozzles calls for tips to be stamped with the name of the manufacturer, a symbol to identify the fuel gas, the maximum material thickness, and a code or part number to reference the manufacturer’s operating data; many manufacturers are not in compliance. Most companies making welding tips do provide information booklets available to cross reference their tip sizes to tip drill sizes. See Table 1-2 tip drill size to material thickness.
Table 1-2Matching welding tip size to weld material thickness
How can the drill size of a tip be determined?
Using a tip cleaner find the round file which fits into the tip easily but snuggly then check the drill size of that file listed on the body of the tip cleaner cover.
When should the torch tip be cleaned and how is it performed?
When sparks from the weld puddle deposit carbon inside the nozzle and on the tip face. These act as spark plugs and cause premature ignition of the gas mixture. Torch tips should be cleaned at the start of each day’s welding and whenever flashback occurs, the flame splits, or when the sharp inner cone no longer exists. To clean, select the largest torch tip cleaning wire file that fits easily into the nozzle and use the serrated portion to remove any foreign material. Be careful not to bend the tip cleaner file into the tip which can cause the cleaning file to break inside the tip; if the file breaks inside the tip it is nearly impossible to remove. Also be sure not to enlarge the existing hole. Then touch up the face of the tip with a file or emery cloth to remove any adhering dirt. Use compressed air or oxygen to blow out the tip. Never use a twist drill to clean the tip; it will cause bell-mouthing.
Why is it important to purge each gas hose separately and not simultaneously?
All possibility of permitting gas to enter the wrong hose and regulator must be prevented as it can lead to a deadly explosion.
How are gas hoses color coded?
Hoses for oxygen and acetylene welding and cutting are coded red for acetylene and green or black for oxygen.
What is flashback and what hazards does it present?
Flashback occurs when a mixture of fuel and oxygen burns inside the mixing chamber in the torch handle and reaches the hoses to the regulators or cylinders. Such burning in the hoses, regulators, or cylinders is likely to cause an accident with burns, a major fire, explosion, shrapnel injuries, and fatalities. If either through operator horseplay (like turning on both the acetylene and the oxygen with the torch tip blocked), or through regulator failure, an explosive mixture of acetylene and oxygen is forced back toward the cylinders. This explosive mixture may enter:
•One hose, or
•One hose and one regulator, or
•One hose, one regulator, and one cylinder.
The stage has been set for a catastrophic explosion. When the torch is lit, this explosive mixture will go off. See Figure 1–17.
Figure 1–17How flashback can occur
How can flashback be prevented?
Flashback is easily prevented by installation of flashback arrestors consisting of both a check valve and a flame arrestor. One flashback arrestor fits between each hose and the torch handle hose fitting. The check valves prevent the gas from one hose from entering the torch handle and then crossing to the other gas hose inside the back of the mixing chamber. Without the mixing of gases into an explosive mixture in the hoses, there can be no explosion in the hoses, regulator, or cylinder. The flame arrestor consists of a compressed stainless steel or sintered metal cylinder. The flame arrestor cylinder tends to stop fire from passing through it by both lowering the temperature of the flame front by absorbing its heat and by forcing the flame through small passages.
These devices are about the diameter of the gas hoses and about 1 inches long. Some newer torch designs incorporate check valves and flashback arrestors into the torch handle itself. Some arrestors fit between the regulator and the hose. See Figure 1–18. The best arrestors include a thermally-activated, spring-loaded shut-off valve which closes on sensing a fire.
Figure 1–18Reverse-flow check valve flashback arrestor cross section
What is backfire and what hazards does it present?
A backfire is a small explosion of the flame at the torch tip. The biggest hazard is that the detonation from the tip may blow molten weld metal five to ten feet from the weld and injure someone. Also, a series of repeated, sustained backfires, which can sound like a machine gun, may overheat the tip or torch, permanently damaging them.
How can backfire be prevented?
The most frequent cause of backfire is pre-ignition of the mixed acetylene and oxygen. Here are the most common causes of pre-ignition and their solutions:
•The mixed welding gases are flowing out through the tip more slowly than the flame front burns and the flame front ignites the gas in the tip and/or mixing chamber causing a pop. Solution: Slightly increase both the oxygen and acetylene pressures and if this results in too large a flame for the job, reduce the torch tip size.
•The tip may be overheated from being held too close to the weld or from working in a confined area like a corner. Solution: Let the tip cool off and try again holding the tip farther from the weld pool.
•Carbon deposits or metal particles inside the tip act like spark plugs prematurely igniting the mixed gases. Solution: Let the tip cool, then clean it thoroughly with your tip cleaning kit.
You are using a multi-flame (rosebud) tip that has a large flame for heating metal prior to welding, bending, or brazing. Soon after the torch is lit, it starts to pop (either once, or in a series of pops), or begins to squeal. What is the most likely problem and how is it best corrected?
This is flashback. Most likely low acetylene gas pressure is not pushing the oxygen/fuel mixture out of the tip faster than the flame can burn back on itself inside the tip. This allows the flame to burn inside the torch either in a single pop, a series of pops, or in a rapid series of pops that sounds like a squeal. Not only can one ruin a tip by allowing this to continue unchecked, but if the flame burning inside the tip reaches back into the hoses, these can explode and/or burn off and leave the welder holding a burning rubber hose, a very serious condition. To avoid this hazard: Immediately turn off the torch, oxygen first, then the acetylene. Allow the torch to cool down for several minutes, increase the acetylene regulator pressure setting to 15 psi (1 bar), reignite the torch and open the acetylene valve to obtain full flow, followed by adjusting the oxygen.
What is the proper way to set-up, light, adjust and use a multi-flame?
When using a multi-flame tip you first set the acetylene pressure at or just below 15psi (1 bar) and the oxygen pressure at 30psi (2 bar); open the acetylene torch valve far enough to light the acetylene flame once the flame is ignited open the acetylene valve until you have full flow of gas; now you can open the oxygen torch valve and adjust the flame to slightly carburizing. You may now use the multi-flame (rosebud) to heat materials but keep the sharp inner cone flame away from the material and only touch the carburizing flame to the material being heated. A heat sensing device such as a pyrometer or temperature sensing stick can be applied to the material to indicate the temperature of the material being heated.
Flames
What are the three types of flames that different ratios of oxygen and acetylene can produce and what are the characteristics of these flames?
•Oxidizing flames result when there is an excess of oxygen over acetylene. This flame will change the metallurgy of the weld pool metal by lowering the carbon content as it is converted to carbon dioxide.
•Neutral flames result when there is just enough oxygen to burn all the acetylene present. This flame has the least effect on weld pool metal as only carbon monoxide and hydrogen combustion products result and is most frequently used in welding common materials.
•Carburizing flames result when there is an excess of acetylene gas over the amount that can be burned by the oxygen present. The opposite of an oxidizing flame, it adds carbon to the weld pool and can change its metallurgy, usually adversely.
Which of the three types of flames, oxidizing, neutral, and carburizing produces the hottest flame?
An oxidizing flame is significantly hotter than the other two flames, but is less useful as it will introduce more contaminants into the weld pool.
For what applications is an oxidizing flame used?
An oxidizing flame is often used in braze welding or in fusion welding of heavy, thick parts with brass or bronze rod. In these applications, we are not concerned with weld pool contamination by carbon. An oxidizing flame is required for oxygen-fuel cutting.
Applications
For what type jobs is OAW best suited?
•Repair and maintenance where one type of equipment can perform many different repairs
•Welding of thin sheet, tubing, and small diameter pipe
•In field operations for natural gas distribution systems up to four inch diameter schedule forty pipes
For what type jobs are OAW definitely not a good choice?
OAW welds of thick sections are not economical when compared with shielded metal arc welding, flux-cored arc welding, or gas metal arc welding.
What disadvantages does OAW have over other welding processes?
In general most other processes are faster: they can apply more weld filler metal in a given time.
What metals can the OAW process readily weld?
•Copper
•Bronze
•Lead
•Low alloy steels
•Wrought Iron
•Cast steel
What materials can be welded by the OAW process if additional steps are taken?
Aluminum and stainless steel may be welded, provided one or more of the following steps are taken—preheat, postheat, use of fluxes, or special welding techniques.
What are three major problems associated with welding aluminum?
•Does not change color prior to melting, so it requires extra welder skill to control heat input.
•Has hot shortness—lacks strength at high temperatures.
•Exposed aluminum has a very thin oxide layer that requires the use of flux and also the oxide surface does not let the welder see a wet-looking molten weld pool.
Besides welding, what other processes can an oxyacetylene welding assembly perform?
With minor additional equipment, it may perform:
•Brazing and soldering
•Case hardening
•Descaling
•Post-heating
•Pre-heating
•Stress relieving
•Oxyacetylene cutting
•Flame hardening
•Flame straightening
•Shrink-to-fit parts assembly
•Surface treatment
•Forging
•Heating for bending and forming
•Tempering and annealing
Why is the carbon content of steel important to the welder?
Carbon content determines its weldability and controls the steel’s tendency to harden upon rapid cooling. The greater the carbon content, the harder it may become.
Why should clothes hangers not be used as welding rod?
Both safety and quality suggest clothes hangars should not be used for welding. Hangers are usually painted and may release toxic fumes, as they burn; they may also be plated, also possibly toxic. From a quality standpoint, their metal content is unknown, variable and unlikely to provide a good weld.
What factors are important in the selection of filler metal (welding rod)?
Usually the filler metal is a close match to the base metal. Sometimes the filler metal will have deoxidizers added which will improve the weld more than just a base metal match. Rod diameters vary from to inch diameter. The prefix R in the description of the oxy-acetylene welding wire means rod which is followed by two or three numbers designating the ultimate tensile strength of the as welded filler material in thousands of pounds per square inch (psi). See Table 1–3.
Table 1–3 Oxyacetylene steel welding rods
What procedures should be followed in welding common metals and what welding rods would make a good starting point?
For all metals, begin by removing all surface dirt, scale, oxide, grease, and paint. Refer to Table 1–4 for technique, flux, flame, and suggested method.
Table 1–4 Information for welding various metals
Weld Preparation
What joint preparation is used for OAW butt welds?
See Figure 1–19.
Figure 1–19Preparation for OAW butt welds
Figure 1–20Correct and defective butt weld profiles
Weld Profiles
On a butt weld, what do the following weld profiles look like: correct weld, poor penetration, excessive reinforcement, undercutting, and excessive root reinforcement?
See Figure 1–20.
Safety
What essential pieces of safety equipment are needed to begin OAW?
See Figure 1–21.
•Non-synthetic fabric (cotton or wool) long-sleeved shirt buttoned to the top to prevent sparks from entering.
•Tinted welding goggles with minimum of number 5 shade lenses.
•Leather gloves.
•Spark igniter.
•Pliers for moving hot metal.
Figure 1–21Oxyacetylene safety equipment
What are the main hazards of OFW and what safety equipment can prevent these injuries?
•External eye injuries from welding or grinding sparks are prevented using welding goggles, safety glasses, or safety shields.
•Internal (retinal) eye damage from viewing hot metal and the radiation being emitted during welding and while cooling (until the metal is no longer red), prevented by using a number 5 tinted lens.
•Burns from weld sparks and hot metal prevented by leather or heavy cotton welding gloves, fire retardant clothing, leathers or specially treated welding jacket or cape-sleeves and bibs when working overhead, cuffless pants, high-top leather shoes.
•Fume hazards from the vapors of metals and flux, must be avoided by proper ventilation, fume filters, and welder air supplies to the welding hood.
•Fires from the welding process prevented by moving flammables away from the weld zone and having water or fire extinguishers close at hand.
What are 15 important welding safety practices whose violations can lead to serious accidents?
•Never use oxygen in place of compressed air.
•Never use oxygen for starting engines or cleaning clothing.
•Store and use acetylene and propane cylinders valve end up.
•Secure cylinders to prevent them from being knocked over in use.
•Use valve protection caps on cylinders while moving them.
•Never leave a lighted torch unattended.
•When a cylinder is empty, close the valve and mark it EMPTY (MT).
•Do not attempt repair of cylinder valves or regulators; send them to a qualified repair shop.
•Never use compressed gas cylinders as rollers.
•Never attempt welding on a compressed gas cylinder.
•Keep power and welding cables away from compressed gas cylinders.
•Prevent sparks from falling on other persons, combustible materials, or falling through cracks in the floor.
•On old-style acetylene cylinders with a removable valve wrench, always leave the wrench in place when using the equipment, so it can be shut off quickly in an emergency.
•When transporting compressed gas cylinders by vehicle have the cylinder caps in place and secure the cylinders so they will not move around as the vehicle starts and stops. Never transport cylinders with the regulators in place.
•Never carry compressed gas cylinders inside a car or car trunk.
What is the best way to weld on a sealed cylinder, tank, vessel or container?
Never weld on a sealed container regardless of its size. Even if the vessel is clean and empty, penetration of the shell could release hot gases from the interior. They could also drive the torch flame back towards the welder. If the cylinder is empty and contains no residual vapors, vent it to atmosphere by opening a valve, hatch, bung, or by drilling a hole. An even more dangerous situation results when the cylinder contains residual flammable vapors whether it is vented to atmosphere or not. This will almost certainly result in an explosion. Clean or purge the cylinder with an inert gas, then have it checked for lack of explosive vapors by a qualified person. Vent it to the atmosphere and begin welding. In some cases filling the vessel with water, or other liquid and welding below the liquid is acceptable, but this is an area for experienced, knowledgeable welders.
Why use a striker to light an oxyfuel torch and not a match?
The striker keeps your fingers away from the flame that can ignite into a large flame. The use of a butane cigarette lighter for torch ignition can cause a large fire or explosion with the potential power of a half-stick of dynamite.
Why should the welding area be well ventilated to draw the weld fumes away from the welder?
Many fumes from the welding process are poisonous and must be avoided. Welding fumes from cadmium plating, galvanized sheet metal, lead, brass (which contains zinc), and many fluxes (especially those containing fluorine) are poisonous. They can have both immediate and long-term adverse health effects. Welding supply companies, welding equipment manufacturers, and materials suppliers will provide MSDSs (Material Safety Data Sheets) on request. Often they are available for downloading via the Internet from the manufacturer. They detail the hazards of materials and equipment and show how to deal with them safely. They are particularly helpful in understanding the fume hazards of fluxes, solders, and brazing materials.
