This article discusses the fundamentals of welding, including welding safety, energy sources, electrode types, weld joint description, symbols, and properties of weld joints. Next, the types of welding processes are described, including shielded metal arc welding, gas tungsten arc welding, gas metal arc welding, and oxyacetylene welding. In the final portion, we review other methods including oxyacetylene torch cutting, oxyfuel cutting, air arc cutting with carbon, brazing, and soldering.
This section provides background information on the fundamentals of welding and includes a discussion of the following pertinent topics:
The dangers in welding come in three major categories: eye injuries, burns, and inhalation of fumes and gases. Any one of these can seriously injure a worker, and all three must be guarded against while on the job. Safety can never be taken for granted.
Welding gloves and a welding helmet are necessary for all arc welding tasks. Additional items of personal protective apparel are recommended as conditions and hazards become increasingly severe. Figure 1 shows personal protection for welding.
Figure 1: Personal Protection for Welding
Hot workpieces should always be handled with pliers or tongs; gloves help to prevent burns from accidental contact. Assume that all metal in the welding area is hot until proven otherwise.
Burns are a safety hazard that can be dangerous, and can cause a more serious injury. The burn can easily become infected because of the nature of the dirty hot metal. It can also cause a more serious injury when the person welding reacts instinctively to the pain of the hot metal scorching his or her skin. The red-hot welding rod in one hand and a welding torch in the other can become almost like weapons when the welder tries to remove hot slag from the skin surface. Sudden movements can place the welder and other nearby personnel in jeopardy.
Some ways to avoid getting burned while welding are as follows:
The major personal hazards in arc welding are the brilliant arc flash, harmful (infrared and ultraviolet) rays, and flying sparks. Never look directly at the welding arc without a dark eye shield. Looking at the arc even momentarily can be painful to the eyes and can cause temporary vision problems. Extended exposure with no protection can cause severe eye damage and permanent partial or total loss of sight. Always wear a welding helmet with a dark lens faceplate while arc welding for protection against the arc flash and weld spatter.
Some eye protection safety tips that should be used during welding are listed as follows:
Infrared rays from the welding arc have a penetrating heating effect. They cause more discomfort than harm when the exposure is of short duration. When welding for long periods, however, wear reflective apparel or use a reflective shield to minimize the effects of infrared rays.
The ultraviolet rays from an electric arc are the same rays that cause sunburn. However, because the rays are more concentrated in welding, the burn can be severe. For protection against "arc sunburn," simply do not expose skin to the rays from the arc. Ordinary clothing blocks the rays, so normal welding apparel should include a long-sleeved shirt buttoned at the sleeves and collar, full-length trousers, a welding helmet, and gloves. When stooping or crouching while welding, ensure that trouser legs do not "hitch up" to expose skin above the boot tops.
When welding, high-top boots or leggings that cover the instep ankle and the shin should be worn. Do not wear low-cut (oxford) shoes or footwear with exposed laces. Flying sparks can easily enter a low-cut shoe, and laces can trap and hold a spark or drop of molten metal. Wear a welding apron that extends below the tops of boots or leggings.
For overhead welding tasks and welds made at or above eye level, wear a cape. The cape protects the neck and upper body from the shower of sparks and hot metal falling from the weld area. A bib added to the cape extends protection to just below the waist. Also, wear a cap or hat to protect hair from falling sparks. Figure 2 shows a cape and a bib for overhead welding.
Figure 2: Cape and Bib for Overhead Welding
Always avoid breathing the fumes and smoke produced by arc welding. Certain metals and electrode coverings can produce especially hazardous smoke and fumes. They are particularly dangerous in a confined space, such as the inside of a tank or a boiler, or where ventilation or air circulation is poor.
Under most normal conditions, existing plant ventilation systems are adequate for maintenance or repair welding jobs of short duration. Permanent welding stations should have separate ventilation systems to adequately remove smoke and fumes. Under particularly hazardous conditions, such as in confined areas or where toxic gases may be present, it may be necessary to wear an air-fed respirator because of the lack of air that is safe to breathe. Determine what hazards might be encountered in a given welding work area, and use the necessary protective equipment.
Suggestions on how to avoid being overcome by toxic fumes are as follows:
Many of the safety precautions associated with welding also apply to cutting or brazing with an oxyfuel torch. Cutting is a much hotter operation than welding; therefore, the use of more protective clothing and additional safety practices will make the job much safer. Where severe smoke or fumes may be encountered, additional ventilation or a respirator may be necessary.
Dark goggles are generally worn for oxyfuel welding and cutting. The goggle lenses should meet National Bureau of Standards specifications, which have been assigned lens shade numbers for various types of welding and cutting operations. A minimum shade lens for light welding is a number four and for minimal flame cutting, a number five. For normal gas welding and cutting operations, a number six lens is recommended.
Before starting a cutting operation, remove all combustible material from the area. Use flame-retardant canvas or similar protective, nonflammable material to cover all objects that could be damaged or ignited by the spray of sparks produced during the cut. A spark from a cutting operation can travel 20 to 30 feet from where the work is being performed; readily combustible materials must be kept at least that far from the work area. To protect nearby workers, it is sometimes necessary to surround the area with welding screens. Before cutting through a metal wall or a large sheet, ensure that the opposite side has been cleared of personnel and materials that could be injured or damaged by the shower of sparks and hot metal. It may be necessary to post an observer on the other side of the wall to warn passersby of the danger.
The spray of sparks and hot metal from a cutting operation greatly increase the danger of fires. Always make sure an adequate fire extinguisher, such as a multipurpose ABC, is handy. To prevent explosions or the unexpected release of pressure, never make a cut in a tank, a pipe, or other sealed vessel until it is safe to do so. Refer to the safety regulations applicable when repairing closed or sealed vessels. Usually, such vessels must be drained, flushed, and vented before they are cut or welded.
Brazing and gas welding can be relatively safe operations if a few simple precautions are followed.
It is very important before beginning any task with oxyfuel equipment to ensure that the equipment is safe and in good working condition.
Do not use a cylinder without the proper pressure-reducing regulator. Never transport a cylinder without taking proper safeguards to protect against shifting or falling. Cylinders should not stand alone without being secured with a lashing or a chain to prevent them from toppling. Recap the cylinder whenever the gauges and regulators are removed. Do not use acetylene at a pressure exceeding 15 psi.
Perform an inspection to ensure that there are no open flames or other flammable sources in the immediate area and then crack the valve to blow out any dust or dirt before connecting the regulator. When setting up a rig, be sure to install a regulator in the line. Unscrew the regulator stem all the way out before pressurizing the regulator. Before removing it, close the cylinder valve and vent any residual gas from the regulator.
Because the threading is different for each, the oxygen gauge will fit only the oxygen cylinder and the fuel gas gauge fits only the fuel gas cylinder. The difference in the thread sizes prevents interchanging the connections. Observe each gauge frequently to note any trouble, such as unusual movement or jamming of the pointer. When any trouble is noted, close the cylinder valve and discontinue use until the trouble is located and corrected. Never lubricate gauges, regulators, valves, or connections; lubricant can react violently with oxygen. For the same reason, do not open valves while wearing greasy gloves.
If you hear a leak around the stem after opening the valve, try to stop the leak by tightening the connecting nut. If you are not immediately successful, close the valve and tag the cylinder, and then have the cylinder removed from service. Whenever you use a wrench to open or close a cylinder valve, be sure not to leave it on the stem where it could be accidentally hit and release gas. The wrench should be kept close by for use in an emergency.
Before beginning work, check torches for leaks in shutoff valves, hose couplings, or tip connections. Never use a defective torch. Inspect torch tips for clogging. Use a proper cleaning tool before beginning work to clear the blockage. Never light a torch from hot work or with a lighter or matched; use a friction lighter.
Inspect frequently for leaks. Do not depend on the valves to prevent gas leakage when a torch is left unattended at the worksite for extended periods of time. Always remove the torch and hose from enclosed spaces when work is interrupted or concluded. As an additional precaution, close shutoff valves on cylinders.
Hoses, the most fragile part of any burning or welding system, should be inspected frequently, especially before being used. Never use a system that does not distinguish (by color or other means) between the hose carrying oxygen and the hose carrying fuel. Couplings are designed not to be interchangeable; fuel gas couplings are left-hand threaded, while oxygen hose couplings are right-hand threaded. Connections that can be joined by being pushed or separated by a straight pull motion should not be used. Only couplings that require a rotary motion to be joined should be used.
Pressure test any hose that has had a flashback or, which on inspection, shows excessive wear. Fuel gas and oxygen hoses that have been disconnected from the torch or the gas-consuming device should be removed immediately from a confined space.
Heat treating is accomplished in three major stages: heating, soaking and cooling.
The primary objective in the heating stage is to maintain uniform temperatures. If uneven heating occurs, one section of a part can expand faster than another and result in distortion or cracking. Uniform temperatures are attained by slow heating.
The heating rate of a part depends on several factors. One important factor is the heat conductivity of the metal. A metal with a high-heat conductivity heats at a faster rate than one with a low conductivity. Also, the condition of the metal determines the rate at which it may be heated. The heating rate for hardened tools and parts should be slower than unstressed or untreated metals. Finally, size and cross section figure into the heating rate. Parts with a large cross section require slower heating rates to allow the interior temperature to remain close to the surface temperature that prevents warping or cracking. Parts with uneven cross sections experience uneven heating; however, such parts are less apt to be cracked or excessively warped when the heating rate is kept slow.
After the metal is heated to the proper temperature, it is held at that temperature until the desired internal structural changes take place. This process is called SOAKING. The length of time held at the proper temperature is called the SOAKING PERIOD. The is used for metals that require a rapid cooling rate, and soaking period depends on the chemical analysis of the oil mixtures are more suitable for metals that need a metal and the mass of the part. When steel parts are slower rate of cooling. Generally, carbon steels are uneven in cross section, the soaking period is deter- water-hardened and alloy steels are oil-hardened. Non-mined by the largest section. Ferrous metals are normally quenched in water.
During the soaking stage, the temperature of the metal is rarely brought from room temperature to the final temperature in one operation; instead, the steel is slowly heated to a temperature just below the point at which the change takes place and then it is held at that temperature until the heat is equalized throughout the metal. We call this process PRE-HEATING. Following preheat; the metal is quickly heated to the final required temperature.
When a part has an intricate design, it may have to be preheated at more than one temperature to prevent cracking and excessive warping.
NOTE: Nonferrous metals are seldom preheated, because they usually do not require it, and preheating can cause an increase in the grain size in these metals.
After a metal has been soaked, it must be returned to room temperature to complete the heat-treating process.
To cool the metal, you can place it in direct contact with a cooling medium composed of a gas, liquid, solid, or combination of these. The rate at which the metal is cooled depends on the metal and the properties desired. The rate of cooling depends on the medium; therefore, the choice of a cooling medium has an important influence on the properties desired.
Quenching is the procedure used for cooling metal rapidly in oil, water, brine, or some other medium. Because most metals are cooled rapidly during the hardening process, quenching is usually associated with hardening; however, quenching does not always result in an increase in hardness.
Some metals crack easily or warp during quenching, and others suffer no ill effects; therefore, the quenching medium must be chosen to fit the metal. Brine or water is used for metals that require a rapid cooling rate, and oil mixtures are more suitable for metals that need a slower rate of cooling.
All arc welding processes require a continuous welding energy source. This energy source, which is more commonly referred to as a welding machine or power source, must supply electrical current that is either alternating (AC) or direct (DC) to the welding electrode through a device that enables the precise control of the current. Welding machines are classified in accordance with the type of current (AC or DC) and the voltage output (variable or constant). A further classification designates the method by which energy is supplied to the welding machine such as directly from a power line or from a gasoline or diesel engine. The main function of any type of welding machine is to supply the type of current that is needed for welding.
Transformers, rectifiers, and generators are the three basic types of welding machines. The transformer welding machines are a voltage step-down transformer that changes high voltage, low amperage AC input current to low voltage, high amperage AC welding current. The transformer-rectifier welding machines are similar to the transformer machines. The difference between these machines is the addition of a rectifier that allows the transformer-rectifier welding machine to also produce DC welding current. Generators are either motor-driven or engine-driven. Motor-driven generators convert an AC input current into a DC welding current. Engine-driven (either gasoline or diesel) generators can produce both AC and DC welding current.
Figure 3 shows a basic welding circuit. This circuit represents a shielded metal-arc welding circuit in which the power supply is connected to the work piece through use of a ground cable. The current flows from the negative terminal of the power supply to the work piece (cathode). The electrode holder becomes the anode through connection of a welding cable between the electrode holder and the positive terminal of the power supply. When the power supply is energized and the electrode tip is touched to the work piece and then withdrawn and held close to the spot of contact, an arc is created across the gap. The arc produces a temperature of about 6,500°F at the tip of the electrode. This temperature is more than adequate for melting most metals.
Figure 3: Basic Welding Circuit
The circuit shown in Figure 3 is referred to as a reverse polarity welding circuit. In a straight polarity welding circuit, the current flows from the positive terminal of the power supply to the work piece (anode). The electrode holder is connected to the negative terminal of the power supply with a welding cable that becomes the cathode.
To understand the principle of a typical electric welding circuit, the nature of the current and the transport medium must be examined. During arc welding, the space between the electrode and the work piece is the point at which the arc is initiated and maintained. This point is referred to as the arc plasma. The welding arc is characterized as a high-current, low-voltage arc that requires a high concentration of electrons to carry the current. Negative electrons are emitted from the cathode (work piece) and flow along the negative ions of the plasma to the positive anode (electrode). Positive ions flow in the reverse direction.
The cathode, anode, and arc plasma are all areas of heat generation. Heat is mainly generated in the work piece when the positive ions strike the surface of the work piece. Heat at the electrode is mainly generated by the electrons. These electrons have been accelerated by the arc voltage as they pass through the plasma. The electrons then give up their energy as heat when they strike the electrode.
Several types of welding electrodes are available for different welding processes and materials. The original welding electrode was a piece of bare metal wire, and bare electrodes are still used today. Bare wire electrodes are manufactured in 36-inch straight lengths that range in sizes from 1/16 to 1/8-inch in diameter. Bare wire electrodes are also manufactured in continuous lengths that are wrapped on spools and that range in sizes from 0.035 to 0.045-inch in diameter. Covered electrodes are very common and are readily adaptable to field welding applications. These electrodes have a bare metal rod as a core and are covered with baked-on flux that provides such functions as shielding from the atmosphere, de-oxidation, and arc stabilization. Flux can also serve as a source of metallic additions to the weld. Flux cored electrodes are similar to coated electrodes. Each of these electrodes consists of a tubular wire filled with a flux material. These electrodes are generally manufactured in sizes from 0.045-inch up to 0.063-inch in diameter.
A consumable electrode is one that is consumed in the heat of the welding arc and adds metal to the weld. Consumable electrodes are considered to be filler metal. Figure 4 shows a consumable electrode that is used for shielded metal arc welding to make a weld on base metal. As the solid metal electrode is drawn near the base metal, the electrical circuit is completed and an arc is created. The solid metal electrode is heated in the arc, and, as a result, the electrode melts. As the electrode melts, small metal droplets are transferred from the solid metal electrode to the molten weld metal. As the molten weld metal cools under the slag, solidified weld metal that is fused to the base metal is formed. When the solid metal electrode is heated and burned in the arc, the covering on the solid metal electrode forms a protective gas over the molten weld metal. Some of the electrode covering material is metallic and becomes molten weld metal. The remainder of the electrode-covering material melts and forms over the solidified weld metal a ceramic cover called slag.
As the solid metal electrode melts and becomes shorter, the consumable electrode must be continuously lowered towards the base metal to maintain the proper arc length. Ultimately, when the usable portion of the consumable electrode has melted, the arc can no longer be maintained, and melting no longer occurs. The consumable electrode has then been fully consumed and another consumable electrode must be used to continue the welding process.
Figure 4: Consumable Electrode
A non-consumable electrode is one that is not consumed in the heat of the welding arc and that does not provide any metal to the weld. Figure 5 shows a non-consumable electrode that is used with the gas tungsten arc welding process. As the non-consumable electrode is drawn near the base metal, the electrical circuit is completed and an arc is created. Unlike the consumable electrode, non-consumable electrodes can effectively maintain the welding arc without melting. In this case, the non-consumable electrode is made from tungsten, which melts at about 6,200°F. Although the molten weld metal may reach and exceed this temperature, the actual temperature of the non-consumable electrode is well below the melting point of tungsten.
Because the non-consumable electrode does not melt, it is very easy to maintain a constant arc length with the base metal. Welding processes that use non-consumable electrodes do not have to be interrupted to replace the non-consumable electrode. In order to deposit filler metal when a welding process that uses non-consumable electrodes is being applied, a filler metal must be added to the welding arc.
Figure 5: Non-consumable Electrode
The electrodes used for SMAW are manufactured by having a thick flux covering surrounding a solid metal wire core. There are many variations on lengths, diameters, flux coverings, and wire core materials. The proper identification of a SMAW electrode is necessary for the welder to create a successful weld.
Classification of SMAW electrodes is based upon the metal wire core. When lengths or diameters are given for an electrode, these dimensions represent the wire core. The most common electrode lengths range from 11 to 18 inches. Common diameters come in a range from 1/16 to 3/8 inches and are available in 1/32-inch increments inside this range.
The flux covering is used to for several advantages.
These advantages include the following:
One problem that occurs with SMAW electrodes is that the flux covering can absorb moisture. This moisture will create excessive hydrogen during the welding process from the electrolysis of water. To prevent this occurrence, electrodes should not be exposed to an open-atmospheric condition. Storage of electrodes should be in either a drying oven or an airtight canister. The drying oven should be set for 250°F to maintain the welding electrode dry.
Proper electrode selection is crucial in the formation of a strong weld. Selection of the proper electrode is based upon several factors. These factors are as follows:
Some welding tasks will be very specific on the type of electrode to be used. If the weld task does not specify the exact electrode, the welder must rely on experience in the proper selection of an electrode.
The ability to strike an arc is one of the first aspects of SMAW to be mastered. The two most frequently used methods of initiating an arc are by using a glancing/scratching motion or a down-and-up motion. When striking the arc to the base metal, ensure that the location of the arc striking is performed where the weld bead needs to begin.
One common problem that occurs for a new welder is sticking the electrode to the base metal. This normally occurs from holding the electrode too close to the base metal. Another problem for the new welder is extinguishing the arc. This usually occurs because the welder withdraws the electrode too far away from the base metal.
At times during the welding process, the arc may continue to extinguish even if the electrode is held at an appropriate distance. This is because the current is set too low. Correction for this problem only requires a higher adjustment of the current setting. At other times, excessive spattering can occur while welding. This condition is almost always caused by having the current setting too high. Lowering the current setting will greatly reduce the amount of spattering that occurs. Care must be taken when adjusting the arc current to ensure that neither spattering nor extinguishing of the arc happens from the raising or lowering of the arc current.
To become a proficient welder, the welder must obtain the ability to identify the correct types of metals. This is important for the selection of the proper electrode, weld design, and welding techniques that will be used. All metals are broken down into two groups: ferrous metals and nonferrous metals.
Ferrous metals simply are some form of metal that contains iron. Carbon steel, cast iron, and stainless steel are some examples of ferrous metals. Nonferrous metals are metals that contain no or extremely small quantities of iron. Copper, brass, aluminum, and lead are all examples of nonferrous metals.
The best way to determine the type of metal being welded is to refer to any available manufacturers specifications. If none are available, there are many different tests for the identification of metals. One of the quickest methods of determining whether the metal is ferrous or non-ferrous with reasonable accuracy is by using a magnet. Magnets are attracted to iron and will stick to ferrous metals. The lack of iron in nonferrous metals causes the magnet not to attach to this type of metal. Other tests include spark testing, oxyacetylene torch testing, color testing, chip testing, sound testing, fracture testing, and density testing. The welder should use the appropriate test according to the materials available for the determination of the metal material.
Welds are made at the junction of at least two members. These weld junctions are called joints, which are defined as the location at which two or more members are to be joined. Parts that are joined by welding may be in the form of rolled plate, sheet, shapes, pipe, or the parts may be castings, forgings, or billets. The physical placement of the members that are to be joined defines the weld joint. Figure 6 shows the five basic types of weld joints (butt, corner, edge, lap, and tee) that are used to join members. In some instances, several weld joints may be used in combination to complete a weld.
Figure 6: Five Basic Weld Joint Types
Figure 7 shows a full penetration weld joint. The fusion zone shown in Figure 7 represents the area of base metal that was melted during the welding process. The boundaries of the fusion zone are between the weld metal and the fusion line. The actual fusion zone can only be determined through removal of a cross-section of the weld to examine the metallurgical structure of the base metal. The depth of the fusion zone depends on the amount of heat that was input to the weld joint during welding. As more heat is input to the weld joint during welding, the size of the fusion widens. As less heat is input to the weld joint, the size of the fusion zone narrows. The heat input to the weld joint is mostly controlled by the welding voltage and the electrode travel speed.
Figure 7: Full Penetration Weld Joint
The fusion line shown in Figure 7 represents the border of fusion during welding and the heat-affected zone. Beyond the fusion line, no melting of the base metal occurs.
The heat-affected zone shown in Figure 7 represents that portion of the base metal that has not been melted, but whose mechanical properties, or microstructure, has been altered by the heat of welding. The boundaries of the HAZ are between the base metal and the fusion line. When heat is input to a weldment from the electrode, the heat also transfers into the adjacent base metal. As the heat input during welding travels through the base metal, the heat dissipates as it gets farther from the weld. Even though the temperature may not be great enough to melt the base metal in the HAZ, the heat is sufficient to alter the microstructure and physical properties of the base metal in the HAZ next to the fusion zone.
The base metal shown in Figure 7 represents the material that is to be welded. In Figure 7, the base metal is plate material. The boundaries of the base metal include all of the material up to the HAZ. Although the area past the heat-affected zone has been heated during the welding process, changes to the microstructure and physical properties of the base metal have not occurred. However, this heat can cause warping of the base metal that could result in the improper alignment of welded components.
The American Welding Society (AWS) Standard A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination, is the standard for weld symbols. A weld symbol represents the weld joint on a drawing so that the details of the weld joint do not have to be shown. The specific weld symbols described in this section include fillet, butt, partial penetration, combination, and stud. To aid in the understanding of these weld symbols, each weld symbol includes an illustration of the desired weld detail that matches the weld symbol.
Figure 8 shows the basic components of a weld symbol including the reference line, arrow leader, tail, arrow side of the reference line, other side of the reference line, weld-all-around symbol, and field weld symbol. The reference line is the horizontal line from which all elements of a weld symbol are positioned. The arrow leader points to the joint to be welded. The tail is used only if additional reference information needs to be included. Weld symbols below the reference line are on the "arrow side," and the weld is made on the same side of the joint where the arrow leader points. Weld symbols above the reference line are on the "other side," and the weld is made on the opposite side of the joint from where the arrow leader points. Weld symbols that are placed both above and below the reference line are considered to be "both side," and the weld is made on both sides of the joint where the arrow leader points. The weld-all-around symbol means that a weld that extends around a series of connected joints must be completely welded around the entire series of connected joints. The weld-all-around symbol is not required for circumferential butt welds. The field weld symbol identifies those welds that must be made in the field.
Figure 8: Components of a Weld Symbol
Additional elements can be added to this basic weld symbol to provide more weld joint information. To establish uniformity among all possible weld symbols, the AWS has standardized the location of these elements on a weld symbol. Figure 9 shows the standard location of elements for any weld symbol that includes the finish symbol, contour symbol, root opening, groove weld size, depth of penetration, welding procedure specification (or other reference), basic weld symbol, number of spot, stud, or projection welds, pitch, length of weld, and groove angle.
Figure 9: Standard Location of Elements on a Weld Symbol
The dimensions of fillet welds are always shown on the same side of the reference line as the weld symbol, and they generally identify the size of the weld, the length of the weld, and the pitch of the weld. Figure 10 shows a weld symbol for a 5/16 inch fillet weld on the arrow side of the joint. Figure 10 also shows the desired weld.
Figure 10: Arrow Side Fillet Weld Symbol
Figure 11 shows a weld symbol for a 1/2-inch fillet weld on the arrow side of the joint and a 1/4-inch fillet weld on the other side of the joint. Figure 11 also shows the desired weld.
Figure 11: Both Side Fillet Weld Symbol
Figure 12 shows a weld symbol for a 1/4-inch (Member A) by 1/2-inch (Member B) fillet weld (unequal leg) on the arrow side of the joint. Figure 12 also shows the desired weld. Because the weld symbol convention does not provide sufficient detail, a note is required when it is necessary to accurately locate the 1/2-inch leg.
Figure 12: Unequal Leg Fillet Weld Symbol
Figure 13 shows a weld symbol for a 1/4-inch intermittent fillet weld on both sides of the joint that is 2-inch long with a pitch of 5 inches. Figure 13 also shows the desired weld.
Figure 13: Intermittent Fillet Weld Symbol
Figure 14 shows a weld symbol for a 3/8-inch staggered intermittent fillet weld that is staggered on both sides of the joint and that is 3 inches long with a pitch of 10 inches. Figure 14 also shows the desired weld.
Figure 14: Staggered Intermittent Fillet Weld Symbol
The dimensions of butt welds are also shown on the same side of the reference line as the weld symbol. Butt weld symbols generally identify the root opening, groove angle, contour symbol, and finish symbol. Figure 15 shows a weld symbol for a single-V groove weld with zero root opening, a 60° included bevel on the arrow side of the joint, and the desired weld.
Figure 15: Single-V-Groove Weld Symbol
Figure 16 shows a weld symbol for a double-V-groove weld with a 1/8-inch root opening, a 60° included angle, a 3/4-inch depth of preparation, and a ground convex face. Figure 16 also shows the desired weld.
Figure 16: Double-V-Groove Weld Symbol
Figure 17 shows a weld symbol for a single-U-groove with a 1/16-inch root opening, a 40° included angle, and a 7/8-inch depth of preparation on the other side of the joint. Figure 17 also shows the desired weld.
Figure 17: Single-U-Groove Weld Symbol
The dimensions of partial penetration welds generally identify the root opening, groove angle, depth of preparation, groove weld size, contour symbol, and finish symbol. Figure 18 shows a weld symbol for a single-V-groove weld with zero root opening, a 60° included angle on the arrow side of the joint, a depth of preparation of 1/4-inch, and a weld size of 3/8-inch. Figure 18 also shows the desired weld.
Figure 18: Partial Penetration Single-V-Groove Weld Symbol
Figure 19 shows a weld symbol for a double-V-groove weld with a zero root opening, a 60° included angle, a depth of preparation of 1/4_inch, and a weld size of 7/16-inch. Figure 19 also shows the desired weld.
Figure 19: Partial Penetration Double-V-Groove Weld Symbol
The dimensions of combination welds may include the root opening, groove angle, depth of preparation, groove weld size, contour symbol, finish symbol, fillet weld size, the length of the weld, and the pitch of the weld. Figure 20 shows a weld symbol for a square groove weld (both sides) with zero root opening, a weld size of 3/8-inch, and a 1/4-inch fillet weld (both sides). Figure 20 also shows the desired weld.
Figure 20: Combination Square Groove/Fillet Weld Symbol
Figure 21 shows a weld symbol for a single-V-groove weld (both sides) with a zero root opening, a 60° included angle, a depth of preparation of 1/4-inch, a weld size of 9/16-inch, and a 3/8-inch fillet weld (both sides). Figure 21 also shows the desired weld.
Figure 21: Combination Single-V-Groove/Fillet Weld Symbol
The symbol for a stud weld is a circle with a cross in the center. Figure 22 shows a stud weld symbol with a 0.25-inch stud diameter, a pitch of 1 inch, and five stud welds. The stud weld symbol does not indicate the welding of a joint in the ordinary sense; therefore, it has no arrow or other side significance. The stud weld symbol must be placed below the reference line, and an arrow must clearly point to the surface to which the stud is to be welded. As with other weld symbols, the dimensions must be placed on the same side of the reference line as the stud weld symbol. Because a stud weld symbol cannot locate the first and last stud weld, the drawing must also specify the exact location of the first and last stud welds that are in a single line. In Figure 22, the first and last studs are positioned 3/4 inch from the edges of the plate.
Figure 22: Stud Weld Symbol
The mechanical and physical properties of materials determine which materials are considered applicable in the design of a product. In the design of weldments, the properties of primary concern are those properties that indicate the behavior of metallic materials under various conditions of loading. These properties are determined in testing laboratories, where standardized procedures and equipment are used to gather data. The adequacy of a weld depends on whether the completed weld provides properties that are equal to or exceed those of the base metals being joined.
Properly executed welds generally have mechanical properties that are superior to the mechanical properties of the base metals that were joined. The following mechanical properties will be discussed in this section:
Tensile strength is the maximum strength developed in a metal tension test. The tension test is a method to determine the behavior of a metal under an axial stretch loading. To determine the tensile strength of a weldment, two base metals are welded together, sectioned, and machined to make a reduced-section tensile specimen. Under a tensile load, the tensile specimen will exhibit elastic elongation in proportion to the applied tensile load. At the yield point, the specimen will continue to exhibit plastic elongation without an increase in the load. Ultimately, the load is increased until the tensile specimen is pulled apart and fails. The ultimate load divided by the cross-sectional area of the tensile specimen determines the actual tensile strength of the welded assembly.
Figure 23 shows where to remove the reduced-section tensile specimens in weld test coupons. Figure 23 (A) shows the location of the two reduced-section tensile specimens in plate coupons, and Figure 23 (B) shows the location of the two reduced-section tensile specimens in pipe coupons. The reduced-section tensile specimens are located so as to provide representative tensile strength data for the entire weld test coupon.
Figure 23 also shows the location of the following specimens for purposes of reference:
Figure 23: Tensile Specimen Removal Locations
The ductility of a metal is the property that allows the metal to be stretched or otherwise changed in shape without breaking and then be able to retain the changed shape after the load has been removed. To determine the ductility of a weldment, two base metals are welded together, sectioned, and machined to make a guided-bend specimen. The guided-bend specimen is then bent in half to a specific radius that is based on the thickness of the specimen. Bending is accomplished with either a roller jig or a wrap-around jig as shown in Figure 24. The ductility of a weldment is very important because a higher ductility indicates a weld that would be less likely to crack in service.
Figure 24: Guided-Bend Test Jigs
The types of guided-bend specimens used to test weld ductility include face, root, and side. A root-bend specimen is a specimen in which the root of the weld becomes the convex surface of the bend specimen; a face bend specimen is a specimen in which the face of the weld becomes the convex surface of the bend specimen; a side bend specimen is a specimen in which one of the side surfaces of the weld becomes the convex surface of the bend specimen.
Figure 23 showed where to remove transverse bend specimens in weld test coupons. Figure 23 (A) illustrated the location of the four bend specimens (two face and two root) in plate coupons, and Figure 23 (B) illustrated the location of the four bend specimens (two face and two root) in pipe coupons. The root and face bend specimens are located to provide representative ductility data for the entire weld test coupon.
The hardness of a metal is defined as "the resistance of a metal to local indentation by a harder substance." Hardness testing is not a requirement of the fabrication codes, but it is often invoked by a job specification. Hardness data from a weldment provides an indication of the metallurgical effects of the welding process on both the weld metal and the heat-affected zone. Hardness data provides an indication of the approximate tensile strength of a metal, the ductility of the weldment, and the ability of the metal to withstand impact loads. The hardness of a weldment is important because very hard metallic surfaces indicate a weld that would be more likely to crack in service.
To determine the hardness of a weldment, a hardened steel ball or diamond is forced into the surface of the metal under a definite weight in a hardness testing machine. The amount of indentation is converted into a numerical value used to compare the relative hardness of a specific metallic surface. For welding procedure qualification coupons, the hardness testing must be performed on a cross-section of the weldment that has been etched to clearly show the base metal, weld metal, and HAZ, as shown in Figure 25. For production weldments, the hardness testing must be performed on the ground surface of the weld near the middle of the deposited weld bead. Additional hardness testing of the HAZ may be required by the applicable construction standard.
Figure 25: Hardness Testing of Weld Test Coupons
Impact strength is the ability of a metal to absorb the energy of a load that is rapidly applied to the member. A metal may have good tensile strength and good ductility under static loading, yet it may fracture if subjected to a high velocity impact. A material or weldment that does not have sufficient impact strength may be too brittle for the intended service. Because ductility allows the material or weldment to redistribute concentrated stresses and prevent material failures, adequate ductility is an important engineering consideration. Even if no stress concentrations are present in a brittle material, fracture will still occur suddenly because the yield stress and tensile strength are practically identical. Knowledge of the impact properties of materials and weldments is very important because a material or weldment that is ductile at room temperature can become brittle in the presence of stress concentrations, low temperature, high rates of loading, or embrittling agents such as hydrogen.
Impact strength testing is required by certain fabrication codes, such as ASME Section VIII, and it is most often determined by the Charpy V-notch test. To determine the impact strength of a weldment, two base metals are welded together, sectioned, and machined to make impact specimens. The impact strength of a material is determined by measuring the energy absorbed by the impact specimen while a weighted pendulum strikes and breaks the specimen. The absorbed energy is measured in foot-pounds. The temperature at which impact testing is performed depends on the application of the weldment. Impact testing at temperatures that are as low as -423°F (temperature of liquid hydrogen) is not uncommon.
Depending on the fabrication code, impact specimens that represent the weld metal, heat-affected zone, and base metal areas may have to be tested. A set of three impact specimens is generally required from each area to adequately characterize the impact strength of the welded assembly. Figure 26 shows where to remove impact specimens in weld test coupons used for pressure vessel construction. The impact specimens are removed so that the top of the specimens is 1/16" below the surface of the base metal. In relatively thick weld test coupons, additional specimens would be removed somewhere between the root of the weld and the middle of the weld.
Figure 26: Impact Specimen Removal Locations
There is no simple definition of metal; however, any chemical element having metallic properties is classed as a metal. Metallic properties are defined as luster, good thermal and electrical conductivity, and the capability of being permanently shaped or deformed at room temperature. Chemical elements lacking these properties are classed as nonmetals. A few elements, known as metalloids, sometimes behave like a metal and at other times like a nonmetal. Some examples of metalloids are as follows: carbon, phosphorus, silicon, and sulfur.
Although steelworkers seldom work with pure metals, we must be knowledgeable of their properties because the alloys we work with are combinations of pure metals. Some of the pure metals discussed in this article are the base metals in these alloys. This is true of iron, aluminum, and magnesium. Other metals discussed are the alloying elements present in small quantities but important in their effect. Among these are chromium, molybdenum, titanium, and manganese.
An alloy is defined as a substance having metallic properties that is composed of two or more elements. The elements used as alloying substances are usually metals or metalloids. The properties of an alloy differ from the properties of the pure metals or metalloids that make up the alloy, and this difference is what creates the usefulness of alloys. By combining metals and metalloids, manufacturers can develop alloys that have the particular properties required for a given use.
Very rarely do steelworkers work with elements in their pure state. We primarily work with alloys and have to understand their characteristics. The characteristics of elements and alloys are explained in terms of physical, chemical, electrical, and mechanical properties. Physical properties relate to color, density, weight, and heat conductivity. Chemical properties involve the behavior of the metal when placed in contact with the atmosphere, salt water, or other substances. Electrical properties encompass the electrical conductivity, resistance, and magnetic qualities of the metal. The mechanical properties relate to load-carrying ability, wear resistance, hardness, and elasticity. When selecting stock for a job, your main concern is the mechanical properties of the metal.
The various properties of metals and alloys were determined in the laboratories of manufacturers and by various societies interested in metallurgical development. Charts presenting the properties of a particular metal or alloy are available in many commercially published reference books. The charts provide information on the melting point, tensile strength, electrical conductivity, magnetic properties, and other properties of a particular metal or alloy. Simple tests can be conducted to determine some of the properties of a metal; however, we normally use a metal test only as an aid for identifying apiece of stock. Some of these methods of testing are discussed later in this lesson.
Welding is the only way to join two or more pieces of metal so that the pieces act as one piece. Welding is widely used to manufacture or repair all of the products made of metal. Almost everything made of metal is welded; some examples are the world's tallest building, rocket engines, nuclear reactors, pressure vessels, piping, home appliances, and automobiles.
The use of welding is still increasing because it is the most economical and efficient way to join metals. Welding has become complex and technical, and it requires considerable knowledge to select the proper welding process for critical work. The many advantages of welding include the following:
The process of welding produces localized coalescence of metals by heating the metals to a suitable temperature. Localized coalescence can occur with or without the application of pressure and filler metal. Coalescence is defined as a "growing together" or "growth into one body." Fusion is the complete blending of two base metals with the filler metal to form one weld. Filler metal is not used in all applications, and, when filler metal is absent, the welding process merely melts both base metals together.
Figure 27 shows a completed weld, which is a typical groove weld that is formed between two base metals that include the base metal, the weld metal, the heat-affected zone (HAZ), the fusion zone, and the fusion line. The weld metal is a combination of melted base metal and filler metal. The fusion zones are the junction between the base metal that has melted and the unmelted base metal. Typical welding arc temperatures range from 8,500F to 10,900F, and steel melts at approximately 2,800F.
Figure 27: Completed Weld
The purpose of welding is to join two pieces of metal together to provide a single piece that has mechanical properties that are equivalent to the weaker of the two base metals. These mechanical properties may include tensile and compressive strength, ductility and elasticity, shear strength, fatigue strength, impact strength, and hardness. More than forty different welding processes are available for the different applications. This section will focus on the following arc welding processes:
The shielded metal arc welding (SMAW) process, which is commonly called "stick" welding, is the most widely used arc welding process. SMAW is characterized by application versatility and flexibility, and relative simplicity of the equipment.
Shielded metal arc welding is a manual joining process in which coalescence of metals is produced by heat from an electric arc maintained between the tip of a covered electrode and the surface of the base metal in the weld joint. Figure 28 shows a simple schematic diagram of an SMAW welding circuit that includes the power supply, electrode holder, base metal, electric leads, and the electrode.
Figure 28: Schematic Diagram of SMAW Welding Circuit
Figure 29 details the SMAW process. The figure shows the base metal, solidified weld metal, molten weld metal, slag, arc, metal droplets, electrode wire, direction of travel, electrode covering, and protective gas from the electrode covering. As the electric arc melts the base metal and electrode wire, metal droplets are transferred to the weld and become solidified metal. The electrode covering also partially melts into the weld and burns to form a protective gas.
Figure 29: The SMAW Process
Either AC or DC current can be used for shielded metal arc welding. To select a power supply, the following factors should be considered:
A transformer-type of power supply would be used for AC welding, and a transformer-rectifier or a motor-driven generator power supply would be used for DC welding. The motor-driven generator would have to be used in remote field applications in which primary electrical power is not readily available.
Electric leads are used to connect the electrode holder and the ground clamp to the power supply. These electric leads are generally copper cables constructed for maximum flexibility to permit easy manipulation of the electrode holder and to prevent wear and abrasion resistance. The electric leads are jacketed with a synthetic rubber that has high toughness, high electrical resistance, and good heat resistance. Because of the rugged environment of most field welding operations, the durability requirements of the electric leads cannot be overemphasized. The size of the electric leads required for a particular application depends on the maximum amperage to be used during welding and the voltage drop between the electrode holder and the power supply. As the length of the electric leads increases, the associated voltage drop through the cable also increases. To compensate for this drop, larger diameter electric leads would be required.
The device that is used to hold and control the electrode in SMAW is known as an electrode holder. The electrode holder has metal jaws that firmly hold the electrode and that conduct the welding current from the electric lead to the electrode. These jaws are covered with insulation to keep the jaws from grounding to the base metal. An insulated handle on the electrode holder separates the welders hand from the welding current. Electrode holders come in several different sizes to accommodate specific ranges of welding current without overheating.
The filler metal form for SMAW is a covered electrode. The covered electrode has a bare metal rod as a core and is covered with baked-on flux. Typically, this flux consists of either iron powder-low hydrogen or cellulosic materials. The electrodes are readily available in sizes that range from 3/32" to 1/4" in diameter and are from 9" to 18" in length. The size of the covered electrode is based on the diameter of the bare wire core: It is not based on the overall diameter of the covered electrode.
The purpose of the electrode covering (flux) is to perform one or more of the following functions:
SMAW is one of the most versatile welding processes available for use in industry. SMAW is used to perform maintenance welding operations such as the repair of defective welds, the addition of corrosion-resistant material to pressure vessel internals, and the repair of failed mechanical equipment. SMAW is readily used on carbon steel, chrome-moly steel, stainless steel, and cast iron materials in the form of plates, shapes, pipe, castings, and forgings.
The SMAW process has the following advantages:
The SMAW process has the following disadvantages:
For any welder to form a strong joint that will hold under stress from any angle or to make a satisfactory weld, the primary talent that needs to be perfected is the formation of a bead. The bead formed is the result of drawing a weld pool after the electrode. This weld pool is formed from the welding arc that is struck between the electrode and the material being welded.
There are several conditions that the welder must control in order to form a satisfactory weld bead using the SMAW method. These factors are as follows:
When various electrode diameters are needed, the length of the welding arc must change accordingly. The normal arc length of covered electrodes will vary between 3/16" to 1/4". The welder needs to become proficient with welding using only one hand. There may be times when a welder must position a part with the opposite hand while tacking the piece with a weld bead.
Listening to the sound of the arc is a possible method for verifying that the arc length is correct. The welder should listen for a snapping or hissing noise. These noises are produced when there is a proper arc length. If the arc length is allowed to go below the nominal value of between 3/16" to 1/4", the arc may short out. If the arc length is allowed to go above the nominal value, excessive spattering will occur. Spattering is small solidified metal particles that are formed from the filler metal used for the weld. Even with maintaining the proper arc length, to form a good bead the welder must keep the electrode moving at a constant, forward speed. This constant forward movement is crucial to ensure a uniform height, width, and ripple spacing of the bead.
The welder has two forms of beads that can be placed on the material: the stringer or weaving bead.
The angle at which the electrode is held should be kept tipped in the direction of travel at a 20 angle. When forming a bead, the electrode should also be parallel to the weld line.
If necessary, clean the end of the electrode and position the electrode in the holder. Strike the initial arc approximately 3/8" ahead of the location where the bead should be formed. The electrode should be moved rapidly to the location where the weld pool should begin to form. This will allow the arc to stabilize prior to the initial formation of the weld pool. With a steady arc, begin moving the electrode in a straight line in the direction of the weld line. Ensure that a 20 angle is maintained at all times. The direction of drawing a weld bead is based on personal preference. Most right-handers find it most comfortable to form a bead from left to right, while left-handed welders find a right to left motion is the most practical. Whichever direction the bead is drawn, the important aspect is a motion that feels natural and allows an unobstructed view of the weld pool. The primary reason for maintaining the electrode at a 20 angle is the arc forces the molten metal to the back of the weld pool and will allow the desired bead ripples to form.
Forward motion begins when the bead width has reached the desired size. With the desired size reached, the welder will pull the electrode slightly forward and allow the pool to grow in size again. After the proper rate of advancing the electrode has been reached, the electrode should be moved forward in one continuous motion.
Two separate factors determine the speed of forward motion of the electrode. They are:
Correct height of the weld is shown by the formation of bullet-nose shaped ripples. The formation of these ripples can only be achieved by maintaining the proper forward speed. These ripples are the result of metal buildup above the surface of the metal. The welder must keep a close eye on the shape of the ripples being formed from the weld pool. If the bead takes on a pointed shape, then the speed of travel is too fast. If the bead is straight or does not have curvature, the speed of travel is too slow.
Another good measure of how fast to move the electrode can be determined after the weld is complete. To judge this, perform the following examinations:
A properly drawn stringer bead will be even in width, height, and ripples that are bullet-nose shaped. A good rule of thumb for bead height is one quarter of the bead width.
Even while maintaining the proper speed, angle, and position of the electrode, an unsatisfactory bead can still be formed if the proper current setting is not maintained. If the current has been set too low, the bead will not penetrate the metal and will pool too high. With the current too high, the electrode will be excessively hot and will produce enormous amounts of splatter. The bead will also have too much penetration and might even burn through the metal. The resulting weld will also be extremely porous with excessive gas pockets and impurities in the bead.
Poor penetration, overlapping of the ripples, and the bead too high are indications that the arc length is too short. If the bead height is too low and there is also poor penetration and undercut, then the arc length is too large. A bead that is wide and high indicates a bead that has been drawn too slowly. Low, thin beads indicate that the forward motion was too rapid.
If an SMAW bead is stopped before it is completely finished, a deep pit will be formed in the base metal. Complete care must be taken when restarting the arc and completing the bead. When the restarting is done carefully, it will be hard to see where the bead was stopped and then restarted.
Before the welder can re-strike the welding arc, the welder must first clean the previous bead. Re-strike the arc about 3/8" ahead of the forward edge of the pit. Then move the arc quickly backward until the new weld pool touches the very edge of the rear of the first pit. As soon as the two edges come in contact, the electrode is moved forward, and the welding continues. Again, if this is done carefully, the ripples of the old and new bead will match.
There are two ways to finish a bead or weld without leaving a crater. One method is to use a run-off tab. A piece of metal of the same type and thickness is tack welded to the end of the base metal being welded. The arc bead or weld is completed on the base metal and continued on the run-off tab. When the weld is stopped, the crater is on the run-off tab. The run-off tab is cut off, leaving a full-thickness bead at the end of the base metal.
A similar procedure is used to start a weld. A run-on tab is tacked to the base metal at the starting end. The welding arc is struck on the run-on tab. When the run-on tab is cut off, it leaves a well-defined, full-thickness bead at the beginning of the base metal.
By using a reverse electrode motion, the welder may also avoid a pit. As the end of the weld is reached, the electrode is moved to the trailing edge of the weld pool. The electrode is then lifted until the arc is broken when the weld pool is filled.
When shielded metal electrodes are used, a brittle slag coating is left, covering the weld bead. Prior to restarting a bead, this slag covering must be removed. It must also be removed after completing the bead, prior to welding over a bead, and before painting.
Many slag inclusions will result if the slag is not removed prior to restarting or welding over a bead. Slag inclusions are pieces of slag trapped or included in the weld. Slag is generally removed manually with a chipping hammer and a wire brush. The slag may also be removed mechanically by shot preening, wire brushing, or chipping.
The AC arc is quite stable once started. The DC arc, however, may have a tendency at times to wander from the weld line. This wandering, called arc blow, is usually caused by magnetic forces around the DC electrode. All electrical conductors are surrounded by a magnetic field when current is flowing. If the current travels continually in one direction, the magnetism can become very strong. Ac electrodes are not affected because of the constantly changing direction of the current. These reversals virtually cancel the magnetic blow effects in the AC circuit.
The magnetic fields or lines of flux travel easily in metal. They move through the air with more difficulty. When the arc is struck, a magnetic field is created around the DC electrode. The magnetic field prefers to travel in the base metal, not in the air. Therefore, the magnetic field forces the molten filler metal to blow inward from the end of the weld joint toward the center of the work. This is called forward arc blow. In the center area of the weld joint, the arc and molten filler metal act normally.
The magnetic flux intensifies ahead of the electrode as the welder nears the end of the joint. This happens as the magnetic flux tries to stay in the metal rather than travel out into the air. The arc and molten metal are now blown back toward the beginning of the weld. This action is known as backward arc blow. Very seldom does arc blow occur across the weld axis (sideways).
Certain preventive or corrective measures can be taken if the arc blow is extremely strong. One or more of the following may be used to correct magnetic arc blow:
The backstep method makes use of a number of short welds. The welds bead is divided into several sections. The first segment is started away from the beginning of the joint. The weld is made toward the beginning of the joint. Each section is welded back toward the previous section.
There is a wide variety of different flaws that can exist in finished welds. There are two classifications of weld flaws: imperfections and defects. Any weld flaw is referred to as an imperfection. If this flaw is visible to the naked eye, it is referred to as a defect. It is possible to see many flaws with the naked eye; however, some will require specialized destructive or nondestructive testing to detect.
Visual inspection of a weld will show the welder many serious flaws that are indicative of a poor weld. These flaws are:
Welds that have the proper proportions should have a width of about three times the dimension of the electrode. Also, welds should have a ripple-shaped appearance and a slight contour or crown. Undercutting is caused by excessive penetration and will result in grooves cut along the sides (toe) of the weld bead. This is normally caused by too high of a current setting and is accompanied with high amounts of spattering. Lack of penetration is usually the result of too low of a current setting. In this weld, the weld bead overlaps the toe of the weld, and there is also little penetration. Surface flaws and defects can be any sort of spatter, cracks, holes, or pitting that can occur in the weld bead or on the base metal.
Using the SMAW process, the welder has several styles of welding techniques to use. The application of the weld style used is dependent on the joint to be used. No matter which joint is used, the proper alignment of the pieces is crucial. Maintaining proper alignment can be accomplished by the use of jigs, clamps, or by tack welding. Tack welding is the process of placing a small weld on the pieces to hold them into place. Occasionally during the application of a weld, the tack weld can become brittle and break. This can cause the pieces to shift out of alignment. To prevent this, it is recommended to place a tack weld every 3 inches along the weld path.
The different types of joints that are used in SMAW are as follows:
The edge joint is a weld that is made between the edges of two or more parallel or nearly parallel parts. Edge preparation is normally only required for materials greater than 1/4 inch. During the application of an edge joint, the welder should wait to move the electrode until the weld pool comes in contact with the outside of the pieces. For thicker pieces, the welder should wait until the weld pool fills the groove prepared for the weld.
Lap joints are performed on base metals that are parallel to one another, but the edges of the pieces are not aligned. A fillet weld is usually performed to weld this joint in place. The ideal weld bead should be convex in shape. The width of the bead should reach up to the top of the edge on the one piece and extend on the face of the second piece the same amount as the weld travels up the edge. For example, if the edge piece is 1/4 inch thick, the weld should extend on the face of the second piece 1/4 inch. Several passes may be required for thicker sections of metals.
Corner or T-joints are performed when the edge of one piece rests on the face of another piece. The weld may be performed on the inside or outside of the corner. Placement of the weld will depend on application of the piece and direction of suspected stresses. Fillet welds are normally used; however, for large and wide joints, a weaving bead should be used.
Butt joints are used when the pieces are in contact edge to edge. It is recommended to prepare the edges of the metal with a V-shaped groove at a 45 angle down to within 1/16 inch of the bottom. The two pieces should be welded together with a 1/16 inch gap between to two pieces. More than one bead may be required. If additional weld beads are required, the welder needs to clean the weld prior to starting the next bead to ensure that no slag is trapped in the weld.
Because of the high quality welds that are produced, the gas tungsten arc welding (GTAW) process, which is often called "TIG" (tungsten inert gas) welding, has become an indispensable welding process for many industries.
Gas tungsten arc welding can be a manual or an automatic joining process in which coalescence of metals is produced by heat from an electric arc maintained between the tip of a tungsten electrode (non-consumable) and the surface of the base metal in the weld joint. Figure 30 shows a simple schematic diagram of a GTAW welding circuit that includes the power supply, torch, base metal, electric leads, filler metal, inert gas supply, cooling water supply, and water return.
Figure 30: Schematic Diagram of GTAW Welding Circuit
Figure 31 details the GTAW process. As the electric arc melts the base metal, the filler metal is introduced to the arc area where the filler metal also melts. After the filler metal cools, it becomes solidified weld metal. GTAW requires the simultaneous use of both hands; one hand to control the torch and one hand to control the addition of filler metal. Because of this technique, gas tungsten arc welding requires more skill of the welder. Care must be taken to ensure that the non-consumable tungsten electrode does not come into contact with the molten weld puddle. Such contact would cause the electrode tip to melt and distort.
Figure 31: The GTAW Process
Either AC or DC current may be used for gas tungsten arc welding. The following factors should be considered when selecting a power supply:
Typically, a transformer-rectifier or an engine-driven generator power supply would be used for gas tungsten arc welding. For more specific applications, such as thin sheet metal, the power supplies are equipped with a pulsed DC welding current, which results in a lower overall heat input to reduce distortion and warping. A high frequency feature is used on some power supplies to stabilize or "stiffen" the welding arc during precision applications, at very low currents, and in outdoor areas.
Electric leads are used to connect the torch and the ground clamp to the power supply. The ground lead and the torch lead are identical to the leads used for shielded metal arc welding; however, the short torch lead also has an internal tube to convey shielding gas to the torch.
The torch that is used in GTAW holds the non-consumable tungsten electrode that conducts welding current to the arc and provides a means to convey the shielding gas to the arc zone. Torches are rated in accordance with the maximum welding current that can be used without overheating the torch. For high current (300 to 500 amps) welding applications, torches are available with a continuous flow of water through internal passageways to cool the torch. Torches are available in several different head configurations to facilitate unique welding positions and welder comfort. GTAW torches often have auxiliary switches and valves to control current and gas flow.
The filler metal form for GTAW is a bare wire. The bare wires are readily available in 36" length and in diameters that range from 1/16" to 3/16". For automatic welding applications, the filler metal is a continuous wire, as small as 0.020", wound on a spool. Extra care must be exercised to keep the filler metal clean and free of all contaminants, such as oil and moisture. Clean, uncontaminated filler metal helps to ensure high quality welds.
The purpose of the shielding gas is to provide an inert atmosphere that prevents excessive oxygen contamination of the molten filler metal during solidification. Shielding gas is usually supplied to the torch from a local high pressure gas storage cylinder. Typical inert gases that are used with GTAW are argon and helium; however, depending on the application, argon-hydrogen and argon-helium blends are sometimes used. Argon is heavier than air, and it tends to cover the weld. Argon generally provides a smooth welding arc with adequate penetration at a low cost. Helium is lighter than air, and it does not provide adequate shielding unless the flow is significantly increased. Helium provides greater penetration of the welding arc, and it is usually preferred on thick materials.
When root passes are made on certain materials such as stainless steel and nickel-based alloys, the air on the back side of the weld joint can actually corrode the weld. To avoid this problem, the air must be purged from the root pass to prevent the formation of oxide and scale that would result in loss of passivity in stainless steel and nickel based alloys. Figure 32 shows, for a piping weld, a purge scheme that identifies the purge gas inlet, the purge gas outlet, the weld joint, and the pipe. Generally, this purge gas is the same as the shielding gas that is used in the torch, and the gas must be conveyed to the back side of the weld through a separate hose that can be regulated independently of the torch shielding gas. By flowing the purge gas into the pipe, one can force the air out through the weld joint and the purge gas outlet.
Figure 32: Purge Scheme for a Piping Weld
Because GTAW produces high quality welds, it is commonly used to weld the root pass of pipe butt welds for high alloy materials such as stainless steel and nickel base alloys. Open butt GTAW root pass welds are used on pipes to provide excellent radiographic quality welds, a smooth shallow bead contour that does not affect flow conditions, a weld that can be made from only one side, and a weld that can be made in all positions around the pipe. On thin wall materials, GTAW may be used to weld the complete joint without a significant loss of productivity. When radiographic quality welds are required on carbon steel weld joints, GTAW may also be used to ensure complete fusion of the root pass. GTAW is the process of choice for small diameter (less than 2") piping welds that include butt, fillet, and socket welds. Maintenance welding operations are performed with GTAW on small intricate parts and on materials that are very thin because of the excellent low current control of this process.
The GTAW process has the following advantages:
The GTAW process has the following disadvantages:
Inert shielding gases surrounding the weld area allow the GTAW to produce high-quality welds. The most common shielding gas used for AC and DC GTAW is argon (Ar). Compared to helium, argon provides a smoother, quieter arc. Argon requires a lower arc voltage and provides a better cleaning action than helium. Argon is used in AC welding because of the lower required arc voltage. Argon is ten times heavier than helium, which makes argon heavier than air. The weight of argon allows it to provide better shielding than helium, so less gas is required.
Helium (He) provides a higher available heat at the workpiece than does argon. GTAW performed with helium gas produces deeper penetration than GTAW performed with argon. This deeper penetration makes helium a better choice as a shielding gas on thicker sections of metal.
When a situation arises where deep penetration and good cleaning action are required, argon and helium gas mixtures are used. These mixtures contain up to 75 percent helium. This allows the higher available heat for the deeper welds and the better cleaning (shielding) action of argon.
The use of hydrogen (H2) to mix with argon when welding stainless steel, nickel-copper, or nickel-based alloys permits increased welding speeds. Hydrogen is not recommended for use on other metals, because it produces hydrogen cracks in the welds. Table 1 lists the shielding gases and current types for various metals. For manual gas tungsten arc welding, argon is the most common shielding gas. Argon-helium mixtures are sometimes used. When doing automatic GTAW, argon, helium, argon-helium, and argon-hydrogen mixtures are used. Gas mixtures can be obtained in cylinders from local welding gas distributors or can be mixed using a gas mixer.
1. Ar (argon) 2. He (helium) 3. AC-HF (alternating current, high frequency) 4. DCEN (direct current electrode negative, also DCSP) 5. H2 (hydrogen)
The flow rate is measured in cubic feet per hour (ft3/hr or cfm) or liters per minute (L/min). This flow rate varies with the base metal being welded, the thickness of the base metal, and the position of the welded joint. Higher gas flow rates are needed when welding overhead. This is necessary because argon, which is heavier than air, tends to fall away from the overhead joint.
Once the correct gas mixture and required flow rate have been determined, the flow rate needs to be correctly adjusted on the flowmeter. The most common flowmeter is the vertical tube gas flowmeter.
The supply valve for the gas cylinder must be opened before setting the flowmeter. The procedure for opening the shielding gas cylinder is as follows:
Performing a successful gas tungsten arc weld requires the selection of the correct type and diameter of tungsten electrode. The types of tungsten electrodes, with their color codes shown in parentheses, are as follows:
Pure tungsten or zirconiated electrodes are favored when AC welding. These electrodes will develop a ball or hemisphere on the tip of the electrode when they are heated. The ball shape is desirable because it reduces current rectification and allows the AC to flow more easily. Ceriated electrodes will also do fairly well when used for AC welding.
To form a ball or hemisphere, set the current to DCEP. Use a clean piece of copper to strike an arc because copper does not melt easily and does not quickly contaminate the electrode. Raise the current until the tip of the electrode forms a ball about the same diameter as the electrode.
When using either a pure or zirconiated tungsten electrode, the electrode is not sharpened to a point to achieve full current capacity. Instead, a ball the same size as the electrode diameter will be produced after the arc is struck. The ball formed may be up to 1 1/2 times the width of the electrode diameter. A ball larger than 1 1/2 times the electrode diameter may melt off and fall into the weld. A ball this size also indicates that the current might be set too high. Electrodes used with AC can be sharpened to a taper, so that a smaller ball will form on the tip, which will reduce the current capacity of the electrode.
DC welding is normally done with thoriated tungsten electrodes. Thoria raises the electron emissions from a tungsten electrode. The addition of 1 to 2 percent thoria also prevents the electrode tip from forming and maintaining a ball. Thoriated electrodes and other electrodes used for DC welding are ground to a point, and the point is then blunted.
If a thoriated electrode is used with AC, it will initially form a ball on the end. While welding, the ball breaks down quickly and forms a number of small projections at the tip. These projections will cause an uncontrollable motion of the arc. Because of this, thoriated electrodes should only be used for direct current GTAW.
Electrodes that can be used for both AC and DC welding are those with ceria and lanthana added. These electrodes do not form a ball as readily as pure or zirconiated tungsten.
Tungsten electrodes of any type need to be provided protection from contaminants. The electrode should not come in contact with the base metal, the weld metal, or the filler metal while it is hot. Such contamination prevents the electrode from emitting or receiving electrons effectively, and this contamination must be removed. One way to remove the contamination is to break off the tip of the electrode and reshape it.
Oxygen, nitrogen, and dirt in the air may contaminate the hot tungsten electrode and metal in the weld area. A shielding gas is used to prevent this contamination. After the arc is broken, the shielding gas is allowed to flow over the electrode and the weld area. The setting of the timing of the shielding gas postflow is on the arc welding machine panel.
Tungsten electrodes are manufactured in sizes from 0.010" to 1/4". The most common lengths are 6" and 7". Each diameter and type of electrode has a maximum amperage associated with it. This amperage limit varies with the current, polarity, and shielding gas used.
Grinding wheels that are used for pointing tungsten electrodes should be used only for this purpose. This segregation will help to keep the electrodes free from contamination. The most recommended grinding wheels are silicon carbide or alumina oxide. Alumina oxide wheels cut slower but last longer than silicon carbide wheel. An 80-grit grinding wheel should be used for the rough grinding of the electrode tip. A 120-grit grinding wheel should perform the finish grinding. A grinding wheel having an open structure is preferred because the grinding wheel will run cooler and collect less contamination.
Grinding electrodes should be performed in a lengthwise direction. This method of grinding provides the optimum current-carrying characteristics. Always wear safety goggles when grinding.
Another method of accurately pointing a tungsten electrode is by using a flame. A very repeatable tip is created using this method.
Determine what type and thickness of metal is to be welded. After selecting the electrode diameter, obtain a collet and collet body for that electrode diameter. Thread the collet body into the GTAW torch. Place the collet into the top of the torch, and then thread the end cap onto the torch. Place the electrode into the torch collet, and tighten the end cap to secure the electrode in place.
An electrode may be installed so that it is even with the end of the nozzle. It may extend 1/2" or more. As a rule, the electrode should extend a distance equal to one to two electrode diameters but not more than the nozzle diameter. Some joints, such as corner or T-joints, require a longer electrode extension. As the electrode extension increases, higher gas flows must be used or the use of a gas lens is required.
Nozzles used on gas tungsten arc welding torches vary in size and method of attachment. The end of the nozzle varies in design to allow for attachment to different manufacturers torches.
Most nozzles used for GTAW are manufactured from ceramic materials. The exit diameter is manufactured in an assortment of sizes. GTAW nozzles are also made in various lengths from short to extended nozzle.
Manufacturers have their own distinct part number for each nozzle. However, the exit diameter for any nozzle is specified with a number that represents the diameter in 1/16" increments.
The basis for the size of the nozzle is to allow coverage of the entire weld area with the shielding gas. If the exit diameter is too large or too small, poor shielding gas coverage will result.
The choice of a nozzle size is often a compromise that is necessary to meet the specific job requirements. Small-diameter nozzles are often used to permit a constant arc length. In the situation of welding a root pass, a small-diameter nozzle is used. The nozzle is constantly touching the sides of the groove as it is rocked back and forth across the root opening. It is also kept in contact with the groove opening as the weld moves forward slowly. The choice of a small nozzle diameter allows the welder to reach the bottom of the groove. It also allows the welder to keep a constant arc length.
Filler metal used for GTAW is normally bare wire. The filler metal can be purchased in coils or in precut lengths. Coiled wire is cut to the preferred length by the welder. If a wire feeder is used for semiautomatic or automatic welding, the wire is fed from the coil. Precut wire is usually purchased in lengths of 24" or 36".
Filler metal alloy must be selected for the specific job. Steel welding rods are not copper-coated, as with oxyfuel gas welding because this would cause spatter and could contaminate the tungsten electrode.
Common diameters of filler wires range from 1/16" to 1/4". Smaller-diameter wire is readily available in coils to 0.015". Precut filler rods are readily available up to 1/4".
The GATW method can achieve high quality welds. Just this method alone is not enough to form a quality weld. Cleaning the surface of the base metal is required. Any oil, grease, dirt, rust, or any other material that can be trapped in a weld bead as an impurity must be removed. With increased impurities in a weld bead, fragile and imperfect welds will be formed.
There is no difference between the joint designs for GTAW and SMAW. The edge on the base metal is usually mitered or shaped to the preferred groove shape when gas tungsten arc welding metal thicker than 3/16".
Thermal or mechanical processes can be used to miter or shape the edges of the base metal. To produce clean, even, and precise cuts, plasma arc cutting is preferred. Machining, sawing, shearing, or grinding are the mechanical means normally used.
The parts of a weldment need to be secure in place prior to welding. This can be accomplished by the use of jigs or fixtures, or tack welding the pieces into place.
A backing strip or ring should be used to control the depth of penetration. The backing ring or strip may also be used when welding metals that may exhibit weakness when they reach their melting temperature. This usually occurs on such metals as aluminum, magnesium, and copper. As metal reaches its melting temperature, pieces of the metal may fall away. Large holes then form in the base metal. A backing strip or ring will help to prevent holes from forming on the metal.
On metals such as magnesium, titanium, and zirconium, a shielding gas may be applied to the backside of the metal. Application of this gas can be performed through a metal channel fastened to the backside of the weld. Filling the inside of the weldment is commonly performed on pipes or closed containers. When a closed container is welded, a vent needs to be provided. This is to prevent the overpressurization of the enclosed gases that would prevent adequate penetration.
Welding the parts in a chamber is another technique used to prevent contamination from entering the weld. Preventing the spread of contamination is accomplished by maintaining the chamber at a vacuum or filling with a shielding gas. This technique will ensure that all parts of the weld and base metal are protected. A chamber is often used to weld titanium.
The following methods can be used to start a gas tungsten arc:
To start the arc, the remote finger- or foot-operated contactor switch must be depressed. This switch also causes the shielding gas to flow prior to starting the arc.
The electrode is touched to the base metal when touch starting is used, and then it is withdrawn about 1/8". The arc will stabilize after a few seconds. The arc length may then be reduced to an arc length of 1/32" to 3/32". Touching the tungsten electrode to the base metal may contaminate the weld area because of some base metal or tungsten transfer.
Touch the nozzle to the metal to initiate the arc using superimposed high frequency. The contactor switch is turned on to start the high-frequency current. A foot- or finger-operated remote switch is used as the machine contactor switch. Another technique is to hold the electrode horizontally about 1" on top of the metal when using high-frequency start. Rotate the electrode toward a vertical position. The high frequency will jump the gap to initiate the arc as the electrode comes near the base metal. The high frequency will shut off automatically when the arc is stabilized if using direct current. When using alternating current, the high frequency should remain on constantly.
A high-voltage surge is used for high-voltage starting. As in high-frequency starting, the electrode is brought close to the base metal. A high-voltage surge causes the arc to jump the gap and start when the contactor switch is pressed. The voltage surge stops automatically after the arc is stabilized.
The ability to make a weld with a small heat-affected zone around it is one advantage of GTAW. Oxyfuel gas and SMAW heat a large area as the metals temperature is increased to the melting temperature. This creates a large heat-affected zone and possibly a weaker metal area around the weld.
Another advantage of GTAW is the absence of metal transfer through the arc. During GTAW, there is no spattering of metal globules from the welding arc or weld pool. The completed weld is of high quality, and the arc action is very quiet.
Using the lowest current necessary to melt the metal is the proper method to accomplish GTAW. The welding speed should be the highest possible that will ensure a sound weld.
The arc is directed to the area to be melted once it is struck. A molten weld pool is created under the arc, and the filler rod is added to fill the pool. When making stringer beads, the width of the pool should be about 2 to 3 times the diameter of the electrode used. A weaving bead is used if the bead must be wider. Several stringer beads may also be used to fill a wide groove joint. Sufficient shielding gas must flow to protect the molten metal in the weld area from becoming contaminated.
The filler rod must not be withdrawn from the area protected by the shielding gas. The filler rod will become contaminated if it is withdrawn while it is molten. The weld will become contaminated if the filler rod is then melted into the weld.
Heat a spot until a molten pool forms after the arc is struck. The electrode should be held at about a 60 to 75 angle from the workpiece. Hold the filler rod at about a 15 to 20 angle to the workpiece. Add the filler rod to the pool when the molten pool reaches the desired size. The electrode should be moved to the back of the weld pool when the filler rod is to be added. The filler rod is then added to the forward part of the molten pool. This technique of adding the filler metal to the weld pool may be used for all weld joints in all positions.
Shielding gas continues to flow for a few seconds each time the arc is broken. This protects the weld metal, electrode, and filler metal from contamination by the surrounding atmosphere. The gas also continues to flow after the torch or foot-operated contactor switch is turned off.
The GTAW torch should be hung on an insulated hook if welding is stopped for a short time. The station should be shut down when welding is to be stopped for a long period of time. Hang up the torch after the gas postflow period, and then shut off the shielding gas cylinder. Turn on the torch or foot-operated contactor switch to start the gas flow. This is done to drain the complete shielding gas system of gas. Turn out the regulating screw on the regulator to turn it off. Screw in the flowmeter to shut it off. The float ball will hit the top of the flowmeter very hard when the regulator is opened again if the flowmeter is not turned off. Turn off the arc welding power source switch.
Welding joint designs for GTAW are the same as those used for oxyfuel gas and shielded metal arc welding. The flat or down-hand position is generally the easiest position in which to weld.
The metal should be bent upwards. This bent-up edge is then melted to form a weld, so no filler rod will be required. The electrode should be held at about 60 to 75 from the base metal. The GTAW arc is applied to the flanged metal to form a weld pool on both edges. The weld pool will touch the outer edges of the flange as it increases in size. The electrode must then be moved ahead to the front edge of the weld pool. The electrode is again moved ahead when the pool again touches the outer edges of the flange. This motion will become smooth and will create a weld bead with uniformly spaced ripples. This is repeated until the weld is completed.