Welding
Welding
Welding is a group of processes used to join non-metallic and metallic materials, by applying heat, pressure, or a combination of both. Most welding procedures require heat, although some procedures require only extreme pressure (cold welding). The welding process chosen to join materials together depends upon the mechanical, physical, and chemical properties of the materials to be joined, and the use for which the product is intended. The welding processes most commonly used today are resistance welding, gas welding, and arc welding. Special welding processes used include electrogas, electroslag, plasma arc, submerged arc welding, underwater, electron beam, laser beam, ultrasonic, friction welding, thermit, brazing, and soldering.
In industry, welding usually refers to joining metals, although materials such as plastics or ceramics are welded. Thermoplastics, such as polyvinylchloride, polyethylene, polypropylene, and acrylics can be welded. Like metal, plastics are welded with localized heat. New welding processes have been developed as new metals, alloys, plastics, and ceramics have been created. Welding is a means of construction, and a method for maintenance and repair. Various welding processes are used in numerous industries, such as aircraft, automotive, mining, nuclear, railroad, shipping, building construction, tool-making, and farm equipment. Welding by robots is one of the more common—and spectacular—applications of robots in industry.
When welding, wearing protective clothing is necessary to avoid injury from sparks, metal fragments, flames, and ultraviolet and infrared rays. Different welding processes require specific clothing. Clothing should be flame resistant, hair and skin should be covered, and special goggles must be worn. Sometimes leather clothing and helmets are recommended, as are steel toed boots. The work area should be properly ventilated; some welding procedures are required to be done in specially vented areas or booths to avoid toxic fumes.
Tests have been devised to inspect welds for flaws and defects. There are two types of testing, nondestructive and destructive. Often, a visual inspection is all that is needed, but to test for internal or extremely small defects, other methods are necessary. Some nondestructive methods include air pressure leak tests, and ultrasonic, x ray, magnetic particle, and liquid penetrant inspections. Nondestructive tests do not damage the weld. Destructive tests are used to test the physical properties of the weld. Usually a test piece is removed from the weld, or a sample weld is made and then tested, completely destroying the weld. Some examples of destructive tests are tensile, hardness, bend, impact, pressure, and fillet testing.
Evolution of welding
The oldest type of welding is forge welding, a process that dates to 2000 BC. Forge welding is a pressure-heat procedure used by blacksmiths and artisans to form metal into specific shapes, and to join metals such as copper and bronze together. Toward the beginning of the twentieth century, several new welding techniques were developed. The discovery of acetylene gas in 1836 by Edmund Davy led to oxyacetylene welding. Resistance welding was invented in 1877 by the British-born American electrical engineer Elihu Thomson (1853-1937). The electric arc, discovered by Sir Humphry Davy, was first used for welding by Auguste de Meritens in 1881. In the United States, C. L. Coffin received a patent for a bare metal electrode arc welding process in 1892. As the arc metal welding process was developed and improved, welding replaced riveted joints as a method of joining pieces of metal. In 1918, the first all-welded ship was launched, and in 1920 the first all-welded building was constructed. Electric arc processes were used extensively during the post-World War I (1914-1918) period. During World War II (1939-1945) inert gas welding was developed, and the gas shielded welding process was developed in 1948. Today, there are around 40 welding processes in use. Some newer welding processes include electron beam welding, laser beam welding, and solid state procedures such as friction and ultrasonic welding.
Welding methods
A weld is defined as a blend or coalescence of two metals (or nonmetals) by heating them until they reach a critical temperature and flow together. Upon cooling, the metal becomes hard. The piece of metal to be welded is called the base metal, workpiece, or work. The edges of the base metal are often specially prepared for welding by, for example, machining, shearing, or gouging. There are five basic weld joints: butt, lap, corner, “T,” and edge. The American Welding Society has developed a system of symbols that are added to mechanical drawings, to convey precisely how a welding site should be prepared, what type of weld should be made, and any other considerations.
Fusion welding, a heat process that sometimes requires the use of a filler metal, uses either electricity (arc welding) or gas (gas welding) as its source of heat energy. Solid state processes, such as friction welding and ultrasonic welding, weld metals at a temperature below their melting points, without the addition of a filler metal. Pressure is always used to achieve a weld with this method. When most metals are heated, a reaction takes place between the base metal and the surrounding atmosphere. For example, some metals oxidize when melted, which can interfere with the quality of the weld. Other common atmospheric contaminants are nitrogen and hydrogen. To control this
KEY TERMS
Acetylene —Colorless fuel gas. When burned with oxygen, acetylene produces one of the highest flame temperatures.
Arc —A stream of bright light or sparks formed as a strong electric current jumps from one conductor to another. In welding, an arc is formed when an electrode connected to a power supply touches the base metal.
Base metal —Metal to be welded or joined. It is also called workpiece or work.
Brazing —Process of welding in which two base metals are joined with a filler metal heated to above 800°F (427°C), but below the melting point of the base metals. The piecework is grooved, the brazing rod is melted, and the molten filler metal flows into the grooves via “capillary action.”
Chemical properties —The way in which a material reacts in a given environment. Some examples are oxidation resistance and corrosion resistance.
Electrode —Terminal point to which electricity is brought to produce the arc for welding. Some electrodes are melted and become part of the weld.
Electron beam welding —Process in which a focused beam of electrons heats and fuses the material being welded.
Filler metal —Metal or alloy added to the base metal to make welded, brazed, or soldered joints.
Flux —A material to facilitate melting and the removal of unwanted contaminants.
Laser beam welding —Welding process that uses the energy of a laser beam to fuse materials.
Mechanical properties —The way a material reacts under loads or forces, such as hardness, brittleness, ductility, and toughness.
Physical properties —The characteristics used to describe or identify a metal, such as color, melting temperature, or density.
Resistance —The property of a material to oppose the passage of an electric current. In welding, metal “resists” the electrical current and heats up. Shielding gas —A gas that is used to guard the weld from surrounding air contamination.
Soldering —A group of welding processes that join materials by heating a filler metal or solder to around 800°F (427°C), which is below the melting point of the base metal.
problem, fluxes and inert gases are used to rid the welding area of impurities, and to protect the area from the atmospheric gases by displacing the surrounding air from the weld site. Welding is done mechanically or manually with welding guns or torches, and can also be performed by robots.
Arc welding
The electric arc used in welding processes is created between a covered or bare metal electrode and the base metal or workpiece. With shielded metal arc welding (SMAC), an electric circuit is set up between the welding machine (AC or DC continuous power source), the workpiece, the electrical cables, the electrode holder, electrode, and ground wire. To strike an arc, the electrode must be touching the base metal; this is usually done by scratching or pecking the base metal with the electrode. As the electricity begins flowing, the electrode is held away from base metal, creating a gap. The electrical current flows across the gap, resulting in an arc. The intense heat from the arc melts the workpiece and the electrode, which contains metal powder that, when melted, becomes the filler metal. The covering or coating on the metal electrode is a flux material that melts, which removes impurities from the weld and sometimes creates a gas that shields the area from atmospheric contamination. Essentially, the electrode and its shielding control the mechanical, chemical, and electrical characteristics of the weld. The heat—6,000–9,000°F (3,351–4,982°C)—of the electric arc brings the base metal and the consumable electrode to molten state, within a matter of seconds.
Gas metal arc welding (GMAW or MIG) is similar to SMAW. A direct current (DC) is always used with this process and there is a gas supply apparatus. A consumable electrode is housed within a nozzle that supplies an inert shielding gas such as helium or argon. GMAW has several advantages over SMAW. With GMAW, welding speed is faster, no slag is produced, there is deeper penetration, and the electrode wires are continuously fed so that longer welds can be made. A type of arc welding that does not use a consumable electrode is gas tungsten arc welding (GTAW or TIG). An arc is produced between the base metal and a tungsten electrode, a shielding gas is used, and there must be a water supply to cool off the torch. Instead of a consumable electrode, a metal rod or welding rod is used to provide filler metal, if required. This type of welding is also called heliarc welding.
Gas welding
Gas welding, also called oxyfuel gas welding (OFW), refers to a group of welding processes that use gas as the source of heat energy. Oxyacetylene and oxyhydrogen are two types of fuel gases used. Acetylene is commonly used for welding because, when combined with oxygen, the flame temperature can reach 5,600°F (3,093°C), the highest temperature produced by any fuel gas-oxygen combination. A filler metal rod may or may not be used with this type of welding process. The fuel gas and oxygen are contained in separate pressurized tanks or cylinders. Specially designed hoses run from the gas cylinders and connect to the welding torch. The welding torch has valves that control the amount of incoming gases, and a mixing chamber where the gases are mixed. The blended gases flow to the tip of the torch where the flame is ignited by a torch lighter, usually a flint and steel sparklighter. After the flame is lit, it must be adjusted until the correct balance of gases is achieved (a neutral flame). Oxyfuel gas welding was the primary welding process during the first part of the twentieth century. As newer methods of welding and new materials were developed, other, more suitable, welding processes replaced the oxyfuel gas process. Currently, the oxyacetylene process is used for braze welding, brazing, and soldering.
Resistance welding
Resistance, or spot, welding (RW) is a process where two or more layers or pieces of metal, stacked together, are welded together by a combination of pressure and heat. An electrical current, along with an appropriate amount of pressure, is applied to the area or spot of the desired weld. When the electricity flows through the metal, it heats up, due to the metal’s resistance to the flow of electricity. When two or more metals are touching, the heat flows through from one piece to the next. The greatest amount of heat is generated at the spot where the two metals are touching. As the temperature reaches the critical point where the metals melt, a weld is created. Pressure is applied through air pressure, hydraulic pressure, or mechanical leverage. Electrodes used for spot welding are not consumable, and can be manufactured into specific shapes. Some electrodes are shaped like wheels for seam welding.
See also Metallurgy; Solder and soldering iron.
Resources
BOOKS
Althouse, Andrew D., et al. Modern Welding. 10th ed. South Holland, IL: Goodheart-Willcox, 2003.
Christine Miner Minderovic
Welding
Welding
Welding is a group of processes used to join non-metallic and metallic materials, by applying heat , pressure , or a combination of both. Most welding procedures require heat, although some procedures require only extreme pressure (cold welding). The welding process chosen to join materials together depends upon the mechanical, physical, and chemical properties of the materials to be joined, and the use for which the product is intended. The welding processes most commonly used today are resistance welding, gas welding, and arc welding. Special welding processes used include electrogas, electroslag, plasma arc, submerged arc welding, underwater, electron beam, laser beam, ultrasonic, friction welding, thermit, brazing, and soldering.
In industry, welding usually refers to joining metals, although materials such as plastics or ceramics are welded. Thermoplastics, such as polyvinylchloride, polyethylene, polypropylene, and acrylics can be welded. Like metal , plastics are welded with localized heat. New welding processes have been developed as new metals, alloys, plastics, and ceramics have been created. Welding is a means of construction, and a method for maintenance and repair. Various welding processes are used in numerous industries, such as aircraft , automotive, mining , nuclear, railroad, shipping, building construction, tool-making, and farm equipment. Welding by robots is one of the more common—and spectacular—applications of robots in industry.
When welding, wearing protective clothing is necessary to avoid injury from sparks, metal fragments, flames, and ultraviolet and infrared rays. Different welding processes require specific clothing. Clothing should be flame resistant, hair and skin should be covered, and special goggles must be worn. Sometimes leather clothing and helmets are recommended, as are steel toed boots. The work area should be properly ventilated; some welding procedures are required to be done in specially vented areas or booths to avoid toxic fumes.
Tests have been devised to inspect welds for flaws and defects. There are two types of testing, nondestructive and destructive. Often, a visual inspection is all that is needed but to test for internal or extremely small defects, other methods are necessary. Some nondestructive methods include air pressure leak tests, and ultrasonic, x ray, magnetic particle, and liquid penetrant inspections. Nondestructive tests do not damage the weld. Destructive tests are used to test the physical properties of the weld. Usually a test piece is removed from the weld, or a sample weld is made and then tested, completely destroying the weld. Some examples of destructive tests are tensile, hardness, bend, impact, pressure, and fillet testing.
Evolution of welding
The oldest type of welding is forge welding, a process that dates to 2,000 b.c. Forge welding is a pressure-heat procedure used by blacksmiths and artisans to form metal into specific shapes, and to join metals such as copper and bronze together. Toward the beginning of the twentieth century, several new welding techniques were developed. The discovery of acetylene gas in 1836 by Edmund Davy led to oxyacetylene welding. Resistance welding was invented in 1877 by the British-born American electrical engineer Elihu Thomson (1853-1937). The electric arc , discovered by Sir Humphry Davy, was first used for welding by Auguste de Meritens in 1881. In the United States, C. L. Coffin received a patent for a bare metal electrode arc welding process in 1892. As the arc metal welding process was developed and improved, welding replaced riveted joints as a method of joining pieces of metal. In 1918, the first allwelded ship was launched, and in 1920 the first all welded building was constructed. Electric arc processes were used extensively during the post-World War I period. During World War II, inert gas welding was developed, and the gas shielded welding process was developed in 1948. Today, there are around 40 welding processes in use. Some newer welding processes include electron beam welding, laser beam welding, and solid state procedures such as friction and ultrasonic welding.
Welding methods
A weld is defined as a blend or coalescence of two or metals (or nonmetals) by heating them until they reach a critical temperature and flow together. Upon cooling, the metal becomes hard. The piece of metal to be welded is called the base metal, workpiece, or work. The edges of the base metal are often specially prepared for welding by, for example, machining, shearing, or gouging. There are five basic weld joints: butt, lap, corner, "T," and edge. The American Welding Society has developed a system of symbols that are added to mechanical drawings, to convey precisely how a welding site should be prepared, what type of weld should be made, and any other considerations.
Fusion welding, a heat process that sometimes requires the use of a filler metal, uses either electricity (arc welding) or gas (gas welding) as its source of heat energy . Solid state processes, such as friction welding and ultrasonic welding, weld metals at a temperature below their melting points, without the addition of a filler metal. Pressure is always used to achieve a weld with this method. When most metals are heated, a reaction takes place between the base metal and the surrounding atmosphere. For example, some metals oxidize when melted, which can interfere with the quality of the weld. Other common atmospheric contaminants are nitrogen and hydrogen . To control this problem, fluxes and inert gases are used to rid the welding area of impurities, and to protect the area from the atmospheric gases by displacing the surrounding air from the weld site. Welding is done mechanically or manually with welding guns or torches, and can also be performed by robots.
Arc welding
The electric arc used in welding processes is created between a covered or bare metal electrode and the base metal or workpiece. With shielded metal arc welding (SMAC), an electric circuit is set up between the welding machine (AC or DC continuous power source), the workpiece, the electrical cables, the electrode holder, electrode, and ground wire. To strike an arc, the electrode must be touching the base metal; this is usually done by scratching or pecking the base metal with the electrode. As the electricity begins flowing, the electrode is held away from base metal, creating a gap. The electrical current flows across the gap, resulting in an arc. The intense heat from the arc melts the workpiece and the electrode, which contains metal powder that, when melted, becomes the filler metal. The covering or coating on the metal electrode is a flux material that melts, which removes impurities from the weld and sometimes creates a gas that shields the area from atmosphericcontamination . Essentially, the electrode and its shielding control the mechanical, chemical, and electrical characteristics of the weld. The heat—6,000–9,000°F (3,351–4,982°C)—of the electric arc brings the base metal and the consumable electrode to molten state, within a matter of seconds.
Gas metal arc welding (GMAW or MIG) is similar to SMAW. A direct current (DC) is always used with this process and there is a gas supply apparatus. A consumable electrode is housed within a nozzle that supplies an inert shielding gas such as helium or argon. GMAW has several advantages over SMAW. With GMAW, welding speed is faster, no slag is produced, there is deeper penetration, and the electrode wires are continuously fed so that longer welds can be made. A type of arc welding that does not use a consumable electrode is gas tungsten arc welding (GTAW or TIG). An arc is produced between the base metal and a tungsten electrode, a shielding gas is used, and there must be a water supply to cool off the torch. Instead of a consumable electrode, a metal rod or welding rod is used to provide filler metal, if required. This type of welding is also called heliarc welding.
Gas welding
Gas welding, also called oxyfuel gas welding (OFW), refers to a group of welding processes that use gas as the source of heat energy. Oxyacetylene and oxyhydrogen are two types of fuel gases used. Acetylene is commonly used for welding because, when combined with oxygen , the flame temperature can reach 5,600°F (3,093°C), the highest temperature produced by any fuel gas-oxygen combination. A filler metal rod may or may not be used with this type of welding process. The fuel gas and oxygen are contained in separate pressurized tanks or cylinders. Specially designed hoses run from the gas cylinders and connect to the welding torch. The welding torch has valves that control the amount of incoming gases, and a mixing chamber where the gases are mixed. The blended gases flow to the tip of the torch where the flame is ignited by a torch lighter, usually a flint and steel sparklighter. After the flame is lit, it must be adjusted until the correct balance of gases is achieved (a neutral flame). Oxyfuel gas welding was the primary welding process during the first part of the twentieth century. As newer methods of welding and new materials were developed, other, more suitable, welding processes replaced the oxyfuel gas process. Currently, the oxyacetylene process is used for braze welding, brazing, and soldering.
Resistance welding
Resistance, or spot, welding (RW) is a process where two or more layers or pieces of metal, stacked together, are welded together by a combination of pressure and heat. An electrical current, along with an appropriate amount of pressure, is applied to the area or spot of the desired weld. When the electricity flows through the metal, it heats up, due to the metal's resistance to the flow of electricity. When two or more metals are touching, the heat flows through from one piece to the next. The greatest amount of heat is generated at the spot where the two metals are touching. As the temperature reaches the critical point where the metals melt, a weld is created. Pressure is applied through air pressure, hydraulic pressure, or mechanical leverage. Electrodes used for spot welding are not consumable, and can be manufactured into specific shapes. Some electrodes are shaped like wheels for seam welding.
See also Metallurgy; Solder and soldering iron.
Resources
books
Althouse, Andrew D., et al. Modern Welding. South Holland, IL: Goodheart-Willcox, 1988.
Bowditch, W., and K. Bowditch. Welding Technology Fundamentals. South Holland, IL: Goodheart-Willcox, 1992.
Jeffus, L., and H. Johnson. Welding, Principles and Applications. 2nd ed. Albany, NY: Delmar, 1988.
Christine Miner Minderovic
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- Acetylene
—Colorless fuel gas. When burned with oxygen, acetylene produces one of the highest flame temperatures.
- Arc
—A stream of bright light or sparks formed as a strong electric current jumps from one conductor to another. In welding, an arc is formed when an electrode connected to a power supply touches the base metal.
- Base metal
—Metal to be welded or joined. It is also called workpiece or work.
- Brazing
—Process of welding in which two base metals are joined with a filler metal heated to above 800°F (427°C), but below the melting point of the base metals. The piecework is grooved, the brazing rod is melted, and the molten filler metal flows into the grooves via "capillary action."
- Chemical properties
—The way in which a material reacts in a given environment. Some examples are oxidation resistance and corrosion resistance.
- Electrode
—Terminal point to which electricity is brought to produce the arc for welding. Some electrodes are melted and become part of the weld.
- Electron beam welding
—Process in which a focused beam of electrons heats and fuses the material being welded.
- Filler metal
—Metal or alloy added to the base metal to make welded, brazed, or soldered joints.
- Flux
—A material to facilitate melting and the removal of unwanted contaminants.
- Laser beam welding
—Welding process that uses the energy of a laser beam to fuse materials.
- Mechanical properties
—The way a material reacts under loads or forces, such as hardness, brittleness, ductility, and toughness.
- Physical properties
—The characteristics used to describe or identify a metal, such as color, melting temperature, or density.
- Resistance
—The property of a material to oppose the passage of an electric current. In welding, metal "resists" the electrical current and heats up.
- Shielding gas
—A gas that is used to guard the weld from surrounding air contamination.
- Soldering
—A group of welding processes that join materials by heating a filler metal or solder to around 800°F (427°C), which is below the melting point of the base metal.