Welding is a general term for various processes used to join metal parts by producing a coalescence, called a weld, at a joint. This is usually done by applying heat and energy when bringing the pieces of metal together.
Welding has many applications. Some welded products include ships, aircraft, automobiles, electric and electronic parts, and in building and construction work. Although over 50 welding processes are used today, the most common ones are gas welding and arc welding.
Welding is a hazardous process that joins materials together by melting a metal work piece along with a filler metal to form a strong joint.
What is Welding?
Coalescence occurs when two metals seem to pull together, or grow into one body, of the base metal parts when there is the slightest contact. There are two basic requirements for coalescence: heat and intimacy of contact.
Heat: Welding processes differ depending on the source of heat, the manner in which heat is applied or generated, and the intensity of the heat. The fuel used as a heat force may be:
- acetylene or hydrogen in air or in oxygen;
- an electric arc;
- an electric, gas, or oil furnace;
- the resistance of metal to the flow of electric current; or
- a chemical reaction between a metal oxide and finely divided aluminum.
The intensity of heat applied or generated at the joint varies according to the metals being joined and to the welding process being used. All welding processes, except brazing, use temperatures high enough to melt the base metals. However, all welding, cutting, and brazing processes generate enough heat to seriously injure workers.
Intimacy of Contact: The second basic requirement for coalescence, intimacy of contact, is accomplished in two ways: pressure processes and non-pressure processes. In pressure processes, there is no space between the surfaces being joined. Welders apply pressure while the contact surfaces are at a high enough temperature to allow plastic flow of the metal. In non-pressure processes, the space between the joined surfaces is filled with molten metal.
Oxyacetylene welding or cutting is also called torch or gas welding or cutting. Two metals are joined by melting or fusing their adjoining surfaces in the process. This is done by directing a flame from burning gas (usually acetylene) to melt metal at a joint to be welded, and is a common method for welding iron, steel, cast iron, and copper.
Oxyacetylene Cylinders: Oxyacetylene equipment consists of a cylinder of acetylene, a cylinder of oxygen, two regulators, two lengths of hose with fittings, a welding torch with tips, and either a cutting attachment or a separate cutting torch.
Accessories include a friction igniter to light the torch, an apparatus wrench to fit the various connections on the regulators, the cylinders, and the torches; goggles with filter lenses for eye protection; and gloves for protection of the hands. Flame-resistant clothing is worn when necessary.
Regulators: Reduce the gas pressure in a cylinder to a suitable working pressure before it can be used. This is done by a regulator or reducing valve. Regulators are either the single-stage or the double-stage type:
- Single-stage regulators reduce the pressure of the gas in one step;
- two-stage regulators do the same job in two steps or stages. Less adjustment is generally necessary when two-stage regulators are used.
Acetylene regulators and oxygen regulators are of the same general type, although those designed for acetylene are not made to withstand such high pressures as are those designed for use with oxygen cylinders.
Welding Torches: The oxyacetylene welding torch is used to mix oxygen and acetylene gas in the proper proportions and to control the volume of these gases burned at the welding tip.
- Torches have two needle valves, one for adjusting the flow of oxygen and the other for adjusting the flow of acetylene.
- They have a handle (body), two tubes (one for oxygen and one for acetylene), a mixing head, and a tip.
- Welding tips are made from a special copper alloy, which dissipates heat (less than 60 percent copper), and are available in different sizes to handle a wide range of plate thicknesses.
Hoses: Hoses used to make the connection between a torch and a regulator are strong, nonporous, and flexible and light enough to make torch movements easy. It is made to withstand high internal pressures, and the rubber used in its manufacture is specially treated to remove sulfur to avoid the danger of spontaneous combustion.
The hoses used for acetylene and oxygen are the same in grade, but they differ in color and have different types of threads on the hose fittings. The color codes are as follows:
- The oxygen hose is GREEN.
- The acetylene hose is RED.
For added protection against mixing of the hoses during connection:
- The oxygen hose has right-hand threads and the acetylene hose has left-hand threads.
- The acetylene fittings have a notch that goes around the circumference of the fittings for an additional identification factor.
Electric Arc Welding and Cutting
Arc welding is the process in which fusion is produced by heating with an electric arc that is generated between an electrode and the surface of the base metal.
Arc cutting is the process in which the cutting or removal of metals is done by melting with the heat of an arc between an electrode and base metal.
In electric welding, electrodes form a part of the electrical circuit. In gas tungsten arc welding, electrodes melt off and are a source of the filler metal supply.
- Solid Electrodes- These electrodes are consumable (composed of steel, copper, aluminum, various alloys, and other metals) or non-consumable (primarily tungsten). They produce less fumes, compared with flux-cored wire or coated electrodes.
- Covered and Coated Electrodes- These are the largest group of electrodes used in welding. The covering provides the flux from the weld. Major metals from the coatings include fluoride, nickel, iron, chromium, manganese, copper, and molybdenum.
Overexposure to these substances can cause injury and illness over the long-term.
For more information on arc welding, read Safe Arc Welding by Lincoln Electric.
Brazing is a welding process using nonferrous filler alloys that do not contain iron or steel and have a melting point above 840°F but below that of the base metal. Brazing is also called ‘hard soldering’ or ‘silver soldering.
Brazing is the only welding process in which the melting of the base metal is not necessary for coalescence. Coalescence occurs when two metals seem to pull together, or grow into one body, of the base metal parts when there is the slightest contact. Click here for more information on brazing alloys.
Soldering is a joining process using non-ferrous filler alloys. Soft soldering uses alloys that melt between 190°F to 840°F and is used in electronics, plumbing, and joining sheet metal parts. Soldering is not considered a welding process. Lead and tin are common alloys used in soldering, but there is also less common lead-free solder to decrease environmental impacts.
Defects in welds can cause unforeseen injuries and accidents. Common weld defects to be familiar with to ensure your safety include:
- Incomplete fusion: Incomplete fusion occurs when the weld fails to fuse one side of the joint in the root. The most common type of incomplete fusion is overlap.
- Inadequate joint and root penetration: Inadequate joint and root penetration is cause for rejection of a weld even if it is sound in all other respects. The strength required in a weldment is achieved only when the specified joint and root penetration is achieved.
- Spatter: Spatter is the term used to describe metal particles or globules expelled during welding and that do not form part of the weld. When spatter occurs, small balls of metal are stuck to the surface of the base metal along the line of weld.
- Overlap: Overlap is a protrusion of the weld metal beyond the bond at the toe of the weld. This is the most common type of incomplete fusion.
- Undercut: An undercut is a groove melted into the base metal adjacent to the toe and not filled with weld metal.
- Root cracks: Root cracks are similar to toe cracks except that they occur at the root of the weld. Root cracks may be in the weld metal or in the base metal.
- Toe cracks: Toe cracks occur in the base metal, at the toe of the weld.
- Crater cracks: One common kind of crack is the crater crack. This occurs in the crater or depression at the termination of a weld bead in gas or arc welding.
- Underbead cracks: Underbead cracks occur in the heat-affected zone underneath a bead and do not extend to the surface of the metal.
- Voids: Voids, also called gas pockets or blow holes, occur as the result of gas being absorbed during the welding and then trapped as the metal solidifies.
- Inclusions: Slag inclusion is the term used to describe the weld defect in which non-metallic solid material is trapped in the weld metal or at the bond between the weld metal and the base metal.
Protecting yourself when performing welding operations depends on your understanding of the hazards involved and the proper way to control them. Controlling welding hazards includes avoiding eye injury, respiratory protection, ventilation of the work area, protective clothing, and having safe equipment to use.
Gas Welding Hazards
Since gas welding is slower and easier to control than electric arc welding, it is commonly used in general maintenance work, brazing, and soldering.
Equipment: Oxyacetylene equipment consists of a cylinder of acetylene, a cylinder of oxygen, two regulators, two lengths of hose with fittings, a welding torch with tips, and either a cutting attachment or a separate cutting torch. Accessories include a friction igniter to light the torch, an apparatus wrench to fit the various connections on the regulators, the cylinders, and the torches; goggles with filter lenses for eye protection; and gloves for protection of the hands. Flame-resistant clothing is worn when necessary.
Acetylene (chemical formula C2H2): a fuel gas made up of carbon and hydrogen. When burned with oxygen, acetylene produces a very hot flame, having a temperature between 5700°F and 6300°F. Acetylene gas is colorless, but has a distinct, easily recognized odor.
MAPP (methylacetylene-propadiene) gas: an all-purpose industrial fuel that has the high flame temperature of acetylene and the handling characteristics of propane.
- MAPP is not sensitive to shock and nonflammable in the absence of oxygen. There is no chance of an explosion if a cylinder is bumped, jarred, or dropped. The cylinders may be stored or transported in any position with no danger of an explosive air pocket being formed.
- MAPP toxicity is rated “very slight,” but high concentrations (5,000 ppm) may have an anesthetic effect. Local eye or skin contact with MAPP gas vapor causes no adverse effect. However, the liquid fuel will cause dangerous frostlike burns due to the temperature at which MAPP gas should be stored.
Oxygen: a colorless, tasteless, odorless gas that is slightly heavier than air. Oxygen will not burn by itself, but it will support combustion when combined with other gases.
- Take extreme care to ensure compressed oxygen does not become contaminated with hydrogen or hydrocarbon gases or liquids.
- A highly explosive mixture will be formed if uncontrolled compressed oxygen becomes contaminated. Oxygen should NEVER come in contact with oil or grease.
- Oxygen cylinders are supplied in several sizes. The size most commonly used 9 1/8 inches in diameter, weighs about 145 pounds, and has a capacity of 200 cubic feet. At 70°F, the gas is under a pressure of 1800 psi.
Arc Welding Hazards and Precautions
Safe procedures and practices must always be used when working around or with arc welding equipment to avoid being injured.
Avoid the following hazards in arc welding:
- radiation from the arc, in the form of ultraviolet and infrared rays
- flying sparks and globules of molten metal
- electric shock
- metal fumes
Radiation: Radiation from the arc is hazardous to the eyes. Eyes should be protected from radiation from the arc by use of an arc welding helmet or face shield with approved lenses.
- Cover the face, hands, arms, and other skin surfaces to prevent exposure to the radiation.
- Gloves should be worn and other parts of the body covered by clothing of sufficient weight to shut out the rays of the arc.
- Without proper clothing, burns comparable to sunburn will result.
Arc Flash: When possible, shield arc-welding operations so no one may accidentally look directly at the arc or have it shine or reflect into his or her eyes.
- An arc “flash” may cause a person to be temporarily blinded.
- The severity of an arc flash and the time it will take to recover varies with the length of time a person was exposed to the arc.
- Long exposure has been known to cause permanent damage to the retina of the eye.
- If someone is severely “flashed,” medical personnel should provide special treatment at once.
Electric Shock: Avoid the possibility of dangerous electric shock by using insulated electrode holders and wearing dry leathers and gloves.
- When possible, avoid using arc-welding equipment in wet or damp areas.
- Perform arc-welding only in an area that is well-ventilated.
Flying sparks: usually accompany arc welding. These present a hazard if they strike unprotected skin, lodge on flammable clothing, or hit any other flammable material.
- When arc welding, wear suitable weight clothing and cuffless trousers.
- Cover pockets so they will not collect sparks, and remove any flammable materials, such as matches, plastic combs, or gas lighters.
- Wear the proper foot protection. Wear high top boots with steel toes.
Hot Metal and Burns: Hot metal and flying sparks have the potential to cause serious burns. Never handle it with bare hands until it has cooled naturally or has been quenched in the quenching tank.
Be sure to use leather gloves with tight fitting cuffs that fit over the sleeves of the jacket. Many welders wear a full set of leathers that consists of the following:
- jacket or set of sleeves
- gauntlet gloves
- welders hat liner
In gas welding, the high temperatures of the welding flame and the sparks will burn skin. Gas welding can also cause radiation burns due to infrared rays emitted by the red-hot material. Wear flame-resistant or flame-retardant clothing and hair protection at all times.
Gases and Fumes: Fluxes used in certain welding and brazing processes produce vapors that are irritating to the eyes, nose, throat, and lungs. Welding fumes and gases generally come from the following sources:
- base material being welded or the filler material used
- coatings and paints on the metal or electrode coatings;
- shielding gases supplied from cylinders;
- chemical reactions due to ultraviolet light of the arc and heat; and
- contaminants in the air from cleaners and degreasers.
Perform welding in a well-ventilated area and always wear approved safety goggles. Here are a few options:
- The darkest shade of the goggles that still show a clear outline of the work without producing eyestrain is recommended.
- Sunglasses are not adequate.