Welding Inspection Technology PDF

WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS tions can occur if proper precautionary measures are not followed. Most welding and cutting operations employ some type of electrical equipment. For example, automatic oxyfuel gas cutting machines use electric motor driv...

WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS tions can occur if proper precautionary measures are not followed. Most welding and cutting operations employ some type of electrical equipment. For example, automatic oxyfuel gas cutting machines use electric motor drives, controls and systems. Lightning-caused electrical accidents may not be avoidable. However, all others are avoidable, including those caused by lack of proper training. gas valve should not be opened beyond the point necessary to provide adequate flow. Opened in this way, it can be shut off quickly in an emergency. Usually, this is less than one turn of the handle. If the immediate valve controlling the burning gas is inaccessible, another upstream valve may cut off the flow of gas. Most fuel gases in cylinders are in liquid form or dissolved in liquids. Therefore, the cylinders should always be used in the upright position to prevent liquid surges into the system. Electric shock occurs when an electric current of sufficient amount to create an adverse effect passes through the body. The severity of the shock depends mainly on the amount of current, the duration of flow, the path of flow, and the state of health of the person. The current is caused to flow by the applied voltage. The amount of current depends upon the applied voltage and the resistance of the body path. The frequency of the current may also be a factor when alternating current is involved. A fuel gas cylinder can develop a leak and sometimes result in a fire. In case of fire, the fire alarm should be sounded, and trained fire personnel should be summoned immediately. A small fire near a cylinder valve or a safety device should be extinguished. When possible, extinguish the fire by closing the valve, using water, wet cloths, or fire extinguishers. If the leak cannot be stopped, after the fire is extinguished, the cylinder should be removed by trained fire personnel to a safe outdoor location, and the supplier notified. A warning sign should be posted, and no smoking or other ignition sources should be allowed in the area. Shock currents greater than about 6 milliamperes (mA) are considered primary because they can cause direct physiological harm. Steady state currents between 0.5 mA and 6 mA are considered secondary shock currents. Secondary shock currents can cause involuntary muscular reactions without normally causing direct physiological harm. The 0.5 mA level is called the perception threshold because it is the point at which most people just begin to feel the tingle from the current. The level of current sensation varies with the weight of the individual and to some extent between men and women. With a large fire at a fuel gas cylinder, the fire alarm should be actuated, and all personnel should be evacuated from the area. The cylinder should be kept wet by fire personnel with a heavy stream of water to keep it cool. It is usually better to allow the fire to continue to burn and consume the issuing gas rather than attempt to extinguish the flame. If the fire is extinguished, there is danger that the escaping gas may ignite with explosive violence. Most electrical equipment, if improperly installed, used, or maintained, can be a shock hazard. Shock can occur from lightning-induced voltage surges in power distribution systems. Even earth grounds can attain high potential relative to true ground during severe transient phenomenon. Such circumstances, however, are rare. Shielding Gases Argon, helium, carbon dioxide (CO2), and nitrogen are used for shielding with some welding processes. All, except carbon dioxide, are used as brazing atmospheres. They are odorless and colorless and can displace air needed for breathing. In welding and cutting work, most electrical equipment is powered from AC sources of between 115 V and 575 V, or by engine-driven generators. Most welding is done with less than 100 arc volts. (Fatalities have resulted with equipment operating at less than 80 V.) Some arc cutting methods operate at over 400 V and electron beam welding machines at up to about 150 kV. Most electric shocks in the welding industry occur as the result of accidental contact with bare or poorly insulated conductors operating at such voltages. Therefore, welders must take precautions against contacting bare elements in the welding circuit, and also those in the primary circuits. Confined spaces containing these gases must be well ventilated before personnel enter them. If there is any question about the space, it should be checked first for adequate oxygen concentration with an oxygen analyzer. If an analyzer is not available, an air-supplied respirator should be worn by anyone entering the space. Containers of these gases should not be placed in confined spaces, as discussed previously. Electrical resistance is usually reduced in the presence of water or moisture. Electrical hazards are often more severe under such circumstances. When arc welding or cutting is to be done under damp or wet conditions including heavy perspiration, the inspector must wear Electric Shock Electric shock can cause sudden death. Injuries and fatalities from electric shock in welding and cutting opera- 2-17 CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS WELDING INSPECTION TECHNOLOGY Terminals for welding leads and power cables must be shielded from accidental contact by personnel or by metal objects, such as vehicles and cranes. Connections between welding leads and power supplies may be guarded using (1) dead front construction and receptacles for plug connections, (2) terminals located in a recessed opening or under a nonremovable hinged cover, (3) insulating sleeves, or (4) other equivalent mechanical means. dry gloves and clothing in good condition to prevent electric shock. The welding inspector should be protected from electrically conductive surfaces, including the earth. Protection can be afforded by rubber-soled shoes as a minimum, and preferably by an insulating layer such as a rubber mat or a dry wooden board. Similar precautions against accidental contact with bare conducting surfaces must be taken when the welding inspector is required to work in a cramped kneeling, sitting, or lying position. Rings and jewelry should be removed before welding to decrease the possibility of electric shock. The workpiece being welded and the frame or chassis of all electrically powered machines must be connected to a good electrical ground. Grounding can be done by locating the workpiece or machine on a grounded metal floor or platen. The ground can also be connected to a properly grounded building frame or other satisfactory ground. Chains, wire ropes, cranes, hoists, and elevators must not be used as grounding connectors or to carry welding current. The technology of heart pacemakers and the extent to which they are influenced by other electrical devices is constantly changing. It is impossible to make general statements concerning the possible effects of welding operations on such devices. Wearers of pacemakers or other electronic equipment vital to life should check with the device manufacturer or their doctor to find out whether any hazard exists. The work lead is not the grounding lead. The work lead connects the work terminal on the power source to the workpiece. A separate lead is required to ground the workpiece or power source to earth ground. Electric shock hazards are reduced by proper equipment installation and maintenance, good operator practice, proper clothing and body protection, and equipment designed for the job and situation. Equipment should meet applicable NEMA or ANSI standards, such as ANSI/UL 551, Safety Standard for Transformer Type Arc Welding Machines. Care should be taken when connecting the grounding circuit. Otherwise, the welding current may flow through a connection intended only for grounding, and may be of a higher amount than the grounding conductor can safely carry. Special radio-frequency grounding may be necessary for arc welding machines equipped with high-frequency arc initiating devices. If significant amounts of welding and cutting are to be done under electrically hazardous conditions, automatic machine controls that safely reduce open circuit voltage are recommended. When special welding and cutting processes require open circuit voltages higher than those specified in NEMA publication EW-1, Arc Welding Power Sources, insulation and operating procedures that are adequate to protect the welder from these higher voltages must be provided. Connections for portable control devices, such as push buttons carried by the operator, must not be connected to circuits with operating voltages above 120 V. Exposed metal parts of portable control devices operating on circuits above 50 V must be grounded by a grounding conductor in the control cable. Controls using intrinsically safe voltages below 30 V are recommended. Electrical connections must be tight and be checked periodically for tightness. Magnetic work clamps must be free of adherent metal particles and spatter on contact surfaces. Coiled welding leads should be spread out before use to avoid overheating and damage to the insulation. Jobs alternately requiring long and short leads should be equipped with insulated cable connectors so that idle lengths can be disconnected when not needed. A good safety training program is essential. Employees must be fully instructed in electrical safety by a competent person before being allowed to commence operations. As a minimum, this training should include the points covered in ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes (published by the American Welding Society). Persons should not be allowed to operate electrical equipment until they have been properly trained. Equipment, cables, fuses, plugs, and receptacles must be used within their current-carrying and duty cycle capacities. Operation of apparatus above the current rating or the duty-cycle results in overheating and rapid deterioration of insulation and other parts. Actual welding current may be higher than that shown by indicators on the welding machine if welding is done with short leads or low Equipment should be installed in a clean, dry area. When this is not possible, it should be adequately guarded from dirt and moisture. Installation must be done to the requirements of NFPA 70, National Electrical Code, and local codes. This includes disconnects, fusing, and types of incoming power lines. 2-18 WELDING INSPECTION TECHNOLOGY CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS Engine driven machines must be turned off before refueling, and any fuel spills should be wiped up and fumes allowed to dissipate before the engine is restarted. Otherwise, the ignition system, electrical controls, spark producing components, or engine heat may start a fire. voltage, or both. High currents are likely with general purpose welding machines when they are used with processes that use low arc voltage, such as gas tungsten arc welding. Welding leads should be the flexible type of cable designed especially for the rigors of welding service. Insulation on cables used with high voltages or high-frequency oscillators must provide adequate protection. The recommendations and precautions of the cable manufacturer should always be followed. Cable insulation must be kept in good condition, and cables repaired or replaced promptly when necessary. Key Terms and Definitions ACGIH—American Conference of Governmental and Industrial Hygienists. This group is concerned with the proper, safe levels of exposure to hazardous materials. Welders should not allow the metal parts of electrodes, electrode holders, or torches to touch their bare skin or any wet covering of the body. Dry gloves in good condition must always be worn. The insulation on electrode holders must be kept in good repair. Electrode holders should not be cooled by immersion in water. If watercooled welding guns or holders are used, they should be free of water leaks and condensation that would adversely affect the welder’s safety. Welders should not drape or coil the welding leads around their bodies. adiabiatic recompression—the term given to the temperature rise that can occur when some gases at high pressures are released suddenly. (Normal pressure gas releases usually result in a cooling of the gas by the decompression.) ANSI—American National Standards Institute. An organization promoting technical and safety standards. ANSI Z49.1—Safety in Welding, Cutting, and Allied Processes, a document outlining safe practices for welding and cutting operations. A welding circuit must be de-energized to avoid electric shock while the electrode, torch, or gun is being changed or adjusted. One exception concerns covered electrodes with shielded metal arc welding. When the circuit is energized, covered electrodes must be changed with dry welding gloves, not with bare hands. De-energization of the circuit is desirable for optimum safety even with covered electrodes. ANSI Z87.1—Practice for Occupational and Educational Eye and Face Protection. asphyxiation—loss of consciousness as a result of too little oxygen or too much carbon dioxide in the blood. When a welder has completed the work or has occasion to leave the work station for an appreciable time, the welding machine should be turned off. Similarly, when the machine is to be moved, the input power supply should be electrically disconnected at the source. When equipment is not in use, exposed electrodes should be removed from the holder to eliminate the danger of accidental electrical contact with persons or conducting objects. Also, welding guns of semiautomatic welding equipment should be placed so that the gun switch cannot be operated accidentally. AWS—American Welding Society. AWS is the technical leader in welding and related issues. combustibles—any material that can easily catch fire. cryogenic—very cold service, usually well below 0°F. DOT—Department of Transportation. A federal or state agency covering the transport of materials. filter lens—in welding, a shaded lens, usually glass, that protects the eyes from radiation from the welding arc and other heat sources. Welding lenses are numbered, with the higher numbers offering the greatest protection. See Table 2.1, Lens Shade Selector, for appropriate lens selection. Fires resulting from electric welding equipment are generally caused by overheating of electrical components. Flying sparks or spatter from the welding or cutting operation, and mishandling fuel in engine driven equipment are among other causes. Most precautions against electrical shock are also applicable to the prevention of fires caused by overheating of equipment. Avoidance of fire from sparks and spatter was covered previously. fire watch—a person whose primary responsibility is to observe the work operation for the possibility of fires, and to alert the workers if a fire occurs. flammable—anything that will burn easily or quickly. (Inflammable has the same meaning.) The fuel systems of engine driven equipment must be in good condition. Leaks must be repaired promptly. 2-19 CHAPTER 2—SAFE PRACTICES FOR WELDING INSPECTORS WELDING INSPECTION TECHNOLOGY fume plume—in welding, a smoke-like cloud containing minute solid particles arising directly from molten metal. OSHA—Occupational Safety and Health Act. This federal law outlines the requirements for safety in the workplace. fuse plug—a plug filled with a material, usually a metal that has a very low melting point. Often used as a heat and/or pressure relief device. pascal (Pa)—in the metric system, the unit for pressure, or tensile strength. The U.S. customary equivalent is psi, pounds per square inch. One psi equals 6895 Pa. fume release—a general term given to the unexpected and undesired release of materials. pinch points—any equipment geometry that can lead to pinching parts of the body, especially the hands or feet, while working on the equipment. galvanized material—any material having a zinc coating on its surface. Common galvanized items are sheet metal and fasteners. safety glasses—spectacles with hardened and minimum thickness lenses that protect the eyes from flying objects. Improved eye protection occurs when side shields are attached to the safety glasses. hot work permit—a form designed to ensure that all safety precautions have been considered prior to any operation having open flames or high heat. standby—in welding, a person trained and designated to stand by and watch for safety hazards, and to call for help if needed. Most often used for vessel entry safety. lock, tag, and try—the phrase noting the physical locking-out of equipment, tagging it for identification, and trying the equipment to make sure it is not operable prior to beginning any repair work. TLV—Threshold Limit Value. The permissible level of exposure limits for hazardous materials. MSDS—Material Safety Data Sheet. A document that identifies materials present in products that have hazardous or toxic properties. toxic—poisonous. vapors—the gaseous form of a substance. NEMA—National Electrical Manufacturers Association. 2-20 CHAPTER 3 Metal Joining and Cutting Processes Contents Introduction ..................................................................................................................................................................3-20 Welding Processes ........................................................................................................................................................3-40 Brazing and Soldering Processes ................................................................................................................................3-36 Cutting Processes..........................................................................................................................................................3-38 Summary .......................................................................................................................................................................3-45 Key Terms and Definitions ..........................................................................................................................................3-45 3-1 CHAPTER 3—METAL JOINING AND CUTTING PROCESSES WELDING INSPECTION TECHNOLOGY Chapter 3—Metal Joining and Cutting Processes Introduction ing this knowledge will help the inspector gain the cooperation of the welders and others involved with the fabrication operation. Since the welding inspector is primarily concerned with welding, knowledge of the various joining and cutting processes can be very helpful. While it is not mandatory that the inspector be a qualified welder, any hands-on welding experience is beneficial. In fact, many welding inspectors are selected for that position after working as a welder for some time. History has shown that former welders often make good inspectors. The processes discussed here can be divided into three basic groups: welding, brazing/soldering and cutting. Welding and brazing/soldering describe methods for joining metals, while cutting results in the removal or separation of material. As each of the joining and cutting processes are discussed, there will be an attempt to describe their important features, including process advantages, process limitations, equipment requirements, electrodes/filler metals, techniques, applications, and possible process problems. There are certain aspects of the various joining and cutting processes which the successful welding inspector must understand in order to perform most effectively. First, the inspector should realize the important advantages and limitations of each process. The inspector should also be aware of those discontinuities which may result when a particular process is used. Many discontinuities occur regardless of the process used; however, there are others which can occur during the application of a particular process. There are numerous joining and cutting processes available for use in the fabrication of metal products. These are shown by the American Welding Society’s Master Chart of Welding and Allied Processes, shown in Figure 3.1. This chart separates the welding and joining methods into seven groups, Arc Welding, Solid-State Welding, Resistance Welding, Oxyfuel Gas Welding, Soldering, Brazing, and Other Welding. Allied processes include Thermal Spraying, Adhesive Bonding, and Thermal Cutting (Oxygen, Arc and Other Cutting). The welding inspector should also have some knowledge of the equipment requirements for each process, because often discontinuities occur which are the result of equipment deficiencies. The inspector should be somewhat familiar with the various machine controls and what effect their adjustment will have on the resulting weld quality. With so many different processes available, it would be difficult to describe each one within the scope of this course. Therefore, the processes selected for discussion include only those which are applicable for the AWS Certified Welding Inspector examination. On that basis, the following processes will be described: When the welding inspector has some understanding of these process fundamentals, he or she is better prepared to perform visual welding inspection. This knowledge will aid in the discovery of problems when they occur rather than later when the cost of correction is greater. The inspector who is capable of spotting problems inprocess will be a definite asset to both production and quality control. Welding Processes • Shielded Metal Arc Welding • Gas Metal Arc Welding Another benefit of having experience with these welding methods is that the production welders will have greater respect for the inspector and resulting decisions. Also, a welder is more likely to bring some problem to the inspector’s attention if he or she knows that the inspector understands the practical aspects of the process. Possess- • Flux Cored Arc Welding • Gas Tungsten Arc Welding • Submerged Arc Welding • Plasma Arc Welding 3-2 WELDING INSPECTION TECHNOLOGY CHAPTER 3—METAL JOINING AND CUTTING PROCESSES Figure 3.1—Master Chart of Welding and Allied Processes 3-3 CHAPTER 3—METAL JOINING AND CUTTING PROCESSES WELDING INSPECTION TECHNOLOGY lescence of metals or nonmetals produced either by heating the materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone and with or without the use of filler metal.” Coalescence means “joining together.” Therefore welding refers to the operations used to accomplish this joining operation. This section will present important features of some of the more common welding processes, all of which employ the use of heat without pressure. • Electroslag Welding • Oxyacetylene Welding • Stud Welding • Laser Beam Welding • Electron Beam Welding • Resistance Welding Brazing Processes As each of these welding processes is presented, it is important to note that they all have certain features in common. That is, there are certain elements which must be provided by the welding process in order for it to be capable of producing satisfactory welds. These features include a source of energy to provide heating, a means of shielding the molten metal from the atmosphere, and a filler metal (optional with some processes and joint configurations). The processes differ from one another because they provide these same features in various ways. So, as each process is introduced, be aware of how it satisfies these requirements. • Torch Brazing • Furnace Brazing • Induction Brazing • Resistance Brazing • Dip Brazing • Infrared Brazing Cutting Processes • Oxyfuel Cutting • Air Carbon Arc Cutting Shielded Metal Arc Welding (SMAW) • Plasma Arc Cutting The first process to be discussed is shielded metal arc welding. Even though this is the correct name for the process, we more often hear it referred to as “stick welding.” This process operates by heating the metal with an electric arc between a covered metal electrode and the metals to be joined. Figure 3.2 shows the “business end” and the various elements of the shielded metal arc welding process. • Mechanical Cutting Welding Processes Before our discussion of the various welding processes, it is appropriate to define what is meant by the term welding. According to AWS, a weld is, “a localized coa- ELECTRODE COVERING CORE WIRE SHIELDING ATMOSPHERE WELD POOL METAL AND SLAG DROPLETS SOLIDIFIED SLAG WELD METAL PENETRATION DEPTH BASE METAL DIRECTION OF WELDING Figure 3.2—Shielded Metal Arc Welding, Including Schematic of Details 3-4