EW-492 TECHNICAL GUIDE Flux Cored Arc Welding HHOOBBAARRTT IINNSSTTIITTUUTTEE OOFF WWEELLDDIINNGG TTEECCHHNNOOLLOOGGYY®®,, 440000 TTRRAADDEE SSQQUUAARREE EEAASSTT,, TTRROOYY,, OOHHIIOO 4455337733 UU..SS..AA.. Table of Contents Chapter Page 1. Introduction to the Process . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Equipment for Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Shielding Gases and Electrodes . . . . . . . . . . . . . . . . . . . . . 14 5. Welding Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6. Cost of Flux Cored Arc Welding . . . . . . . . . . . . . . . . . . . . . 32 7. Welding Metallurgy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 8. Weld and Joint Design . . . . . . . . . . . . . . . . . . . . . . . . . . 43 9. Welding Procedure Variables . . . . . . . . . . . . . . . . . . . . . . . 59 10. Welding Procedure Schedules . . . . . . . . . . . . . . . . . . . . . . 69 11. Preweld Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 12. Welding Discontinuities and Defects . . . . . . . . . . . . . . . . . . . 79 13. Postweld Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 14. Welder Training and Qualification . . . . . . . . . . . . . . . . . . . . . 87 15. Welding Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Appendix: Sources for Standards . . . . . . . . . . . . . . . . . . . . 104 © 2012, 2002, 1994 Hobart Institute of Welding Technology ISBN 978-1-936058-18-1 This publication includes information available at the time of production. The Hobart Institute of Welding Technology presents this information as a guideline. Relevant standards may have been updated and should be reviewed together with this book for accuracy. Federal or other laws and standards may govern different operations and facilities. Hobart Institute of Welding Technology disclaims liability for any injury to persons or to property, or other damages of any na- ture whatsoever, whether special, indirect, consequential or compensatory, directly or indirectly resulting from the publication, use of or reliance on this book. Hobart Institute of Welding Technology makes no guarantee or warranty as to the accuracy or completeness of any information published herein. CHAPTER 1 INTRODUCTION TO THE PROCESS Flux cored arc welding (FCAW) is an arc weld- Nozzle ing process in which the heat for welding is pro- Gas (not required for self-shielding wires) duced by an arc between a continuously fed tu- Molten Metal bular electrode wire and the work. Shielding is Solidified Flux Cored obtained by a flux contained within the tubular Weld Metal Electrode electrode wire or by the flux and an externally Molten supplied shielding gas. Some trade names for Slag Slag Arc this process are FabCO®, Fabshield®, Inner- shield®, and Dual Shield®. A diagram of the process is shown in Illustration 1-1. Base Metal Flux cored arc welding is similar to gas metal arc welding in many ways. The flux cored wires Direction of Travel used for this process give it different character- istics. Flux cored arc welding is widely used for Illustration 1-1 – Flux Cored Arc Welding welding ferrous metals and is particularly good for applications where high deposition rates are desir- the disintegration of electrode coatings. Results of this able. At high welding currents, the arc is smoother and analysis showed that the predominate gas given off by more manageable when compared to using large diame- electrode coatings was carbon dioxide. This discovery ter gas metal arc welding electrodes with carbon dioxide. led quickly to the use of carbon dioxide gas for shielding The arc and weld pool are clearly visible to the welder. when welding carbon steel with the gas metal arc weld- A slag coating is left on the surface of the weld bead, ing process. Early experiments with carbon dioxide gas which must be removed. Since the filler metal transfers were not successful. However, continued research and across the arc, some spatter is created and some smoke experimentation with electrode wire composition and is produced. equipment did develop a practical gas metal arc process for the welding of carbon steels. The gas-shielded metal arc welding process had its be- ginning in the early 1920’s. Experiments conducted at that CO shielded gas metal arc welding became very popular 2 time proved that weld metal properties were improved during the mid-1950’s. Its most successful applications when the welding arc and welding puddle were protected were for fully automatic welding installations in the au- from atmospheric gases by an inert gas shield. However, tomotive industry. High metal deposition rates and fast the development of coated electrodes in the late 1920’s travel speeds characterized the process. Because of the reduced the interest in gas-shielded processes. extremely fluid weld puddle, the process was limited to flat position and horizontal fillet welds. Also, the process The gas tungsten-arc welding (GTAW) process became was so fast that it was difficult to do semiautomatic weld- available commercially in the early 1940’s and was the ing with manual travel. Weld spatter was a problem. forerunner of the modern gas-shielded metal arc welding processes which includes flux cored and metal cored arc Flux cored or “inside out” coated electrode wires were welding. In the GTAW process, a non-consumable elec- experimented with in the early 1920’s but, as with the trode is used and the arc area is shielded by an inert gas. gas-shielded systems, the development of covered elec- trodes reduced interest in the inadequately developed This development was followed in the late 1940’s by the flux cored electrode wires. gas metal arc welding (GMAW) process when a consum- able electrode wire replaced the tungsten electrode. The The joining of the CO gas shielding system to the flux 2 gas metal arc or MIG (Metal Inert Gas) process was used cored consumable wire development provided a process for nonferrous metals and employed an inert gas, usually that overcame many deficiencies. Operating character- helium or argon. As gas metal arc welding developed, istics were improved by the addition of the core mate- engineers learned that mild carbon and low alloy steels rials and weld quality was improved by eliminating at- could be welded also. However, the high cost of inert mospheric contamination. The process was introduced shielding gas prevented this process from competing publicly at the American Welding Society Exposition held economically with the less costly manual shielded metal at Buffalo, New York, in May 1954. The electrodes and arc welding. equipment were refined and introduced in essentially the present form in 1957. Since the introduction, improve- Research work conducted on manual coated electrode ments have been made on the equipment and consum- welds dealt with an analysis of the gas produced in ables for the process. – 1 – A major development in flux cored arc welding in the ADVANTAGES AND LIMITATIONS early 1960’s was the introduction of flux cored wires, not requiring external gas shielding. These electrode wires Flux cored arc welding has many advantages for a wide became popular for many field construction applications variety of applications. Flux cored arc welding often because the need for a gas shielding system was elimi- competes with shielded metal arc welding, gas metal arc nated, which made the equipment more portable. welding, and submerged arc welding for many applica- tions. Some of the advantages of this process are: Continuing work has been done on improving the me- chanical properties, especially the impact toughness, of 1) It has a high deposition rate and faster travel speeds welds made by flux cored arc welding. The electrodes are often used available today produce much better results than the wires originally introduced in the late 1950’s and early 2) Using small diameter electrode wires, welding can be 1960’s. More flux cored wires for welding higher alloy done in all positions steels have been developed. During the 1960’s metal cored wires were introduced. An important difference 3) Some flux cored wires do not need an external supply between a flux cored wire and a metal cored wire is the of shielding gas which simplifies the equipment absence of heavy slag on the weld. 4) The electrode wire is fed continuously so there is very Other major improvements in flux cored wires include im- little time spent on changing electrodes proving the welder appeal. This has been done by reduc- ing the smoke and spatter levels produced and improving 5) A higher percentage of the filler metal is deposited the weld bead appearance characteristics. Development when compared to shielded metal arc welding on these and other aspects of flux cored arc welding is continuing. 6) Better penetration is obtained than from shielded metal arc welding METHODS OF APPLICATION Although flux cored arc welding may be applied semi- automatically, by machine, or automatically, the process is usually applied semiautomatically. In semiautomatic welding, the wire feeder feeds the electrode wire and the power source maintains the arc length. The welder ma- nipulates the welding gun and adjusts the welding pa- rameters. Flux cored arc welding is also used in machine welding where, in addition to feeding the wire and main- taining the arc length, the machinery also provides the joint travel. The welding operator continuously monitors the welding and makes adjustments in the welding pa- rameters. Automatic welding is used in high production applications. In automatic welding, the welding operator only starts the operation. – 2 – CHAPTER 2 PRINCIPLES OF OPERATION The flux cored arc welding process uses the heat of an atmosphere. Most self-shielding wires also operate using electric arc between a consumable, tubular electrode and longer electrode extensions than gas-shielded wires. the part to be welded. Electric current passing through an ionized gas produces an electric arc. The gas atoms and ARC SYSTEMS molecules are broken up and ionized by losing electrons and leaving a positive charge. The positive gas ions then The flux cored arc welding process may be operated on flow from the positive pole to the negative pole and the both constant voltage and constant current power sourc- electrons flow from the negative pole to the positive pole. es. Any welding power source can be classified by its About 95% of the heat is carried by the electrons and the volt-ampere characteristics as either a constant voltage rest is carried by the positive ions. The heat of the arc (also called constant potential) or constant current (also melts the electrode and the surface of the base metal. called variable voltage) type, although there are some ma- The molten weld metal, heated weld zone, and electrode chines that can produce both characteristics. Constant are shielded by one of two methods. One is by the de- voltage power sources are preferred for a majority of flux composition of the flux core of the electrode. The sec- cored arc welding applications. ond method is by a combination of an externally supplied shielding gas and the decomposition of the flux core of In the constant voltage arc system, the voltage deliv- the electrode wire. The flux core has essentially the same ered to the arc is maintained at a relatively constant level purpose as the coating on an electrode for shielded metal which gives a flat or nearly flat volt-ampere curve, as arc welding. The molten electrode filler metal transfers shown in Illustration 2-1. This type of power source is across the arc and into the molten weld puddle. A slag widely used for the processes that require a continuously forms on top of the weld bead which can be removed fed wire electrode. In this system, the arc length is con- after welding. trolled by setting the voltage level on the power source and the welding current is controlled by setting the wire The arc is struck by starting the wire feed which causes feed speed. As shown in Illustration 2-1, a slight change the electrode wire to touch the workpiece and initiate the in the arc length will produce a large change in the weld- arc. Arc travel is usually not started until a weld puddle is ing current. formed. The welding gun then moves along the weld joint manually or mechanically so that the edges of the weld Most power sources have a fixed slope that is built in for joint are joined. The weld metal solidifies behind the arc a certain type of flux cored arc welding. Some constant which completes the welding process. A large amount of voltage welding machines are equipped with a slope con- flux is contained in the core of a self-shielding wire as trol which is used to change the slope of the volt-ampere compared to a gas-shielded wire. This is needed to pro- curve. Illustration 2-2 shows different slopes obtained vide adequate shielding and because of this, a thicker from one power source. The slope has the effect of limit- slag coating is formed. In these wires, deoxidizing and ing the amount of short-circuiting current that the power denitrifying elements are needed in the filler metal and supply can deliver. This is the current available from the flux core because some nitrogen is introduced from the power source on the short circuit between the electrode 36 40 35 30 S S 30 HIGH S T T T L L L O 25 O O V V MEDIUM V C. C. 25 D. D. LOW 20 20 0 0 0 200 300 400 500 600 0 200 300 400 500 600 WELD CURRENT (AMPERES) LOAD CURRENT (D.C. AMPERES) LOAD CURRENT (D.C. AMPERES) Illustration 2-1 – Constant Voltage Volt-Ampere Curve Illustration 2-2 – Different Slope Obtained from a Constant Voltage Power Source – 3 – wire and the work. This is not as important in flux cored The constant current arc system provides a nearly con- arc welding as it was in gas metal arc welding because stant welding current to the arc which gives a drooping short-circuiting metal transfer is not encountered except volt-ampere characteristic, as shown in Illustration 2-3. with alloy cored, low flux content wires. This arc system is used with the shielded metal arc weld- ing and gas tungsten arc welding processes. The welding A slope control is not required, but may be desirable, current is set by a dial on the machine and the welding when welding with small diameter, alloy cored, low flux voltage is controlled by the arc length held by the welder. content electrodes at low current levels. The short-circuit current determines the amount of pinch force available This system is necessary for manual welding because on the electrode. The pinch force causes the molten the welder cannot hold a constant arc length which electrode droplet to separate from the solid electrode. causes only small variations in the welding current. The flatter the slope of the volt ampere curve, the higher When flux cored arc welding is done with a constant the short-circuit and the pinch force. The steeper the current system, a special voltage sensing wire feeder is slope, the lower the short-circuit and pinch force. The used to maintain a constant arc length. pinch force is important with these electrodes because it affects the way the droplet detaches from the tip of the For any power source, the voltage drop across the weld- electrode wire. When a high short-circuit and pinch force ing arc is directly dependent on the arc length. An in- are caused by a flat slope, excessive spatter is created. crease in the arc length results in a corresponding in- When a very low short-circuit current and pinch force crease in the arc voltage and a decrease in the arc length are caused by a steep slope, the electrode wire tends to results in a corresponding decrease in the arc voltage. freeze in the weld puddle or pile up on the work piece. Another important relationship exists between the weld- When the proper amount of short-circuit current is used, ing current and the melt off rate of the electrode. With very little spatter is created. low current, the electrode melts off slower and the metal is deposited slower. This relationship between welding The inductance of the power supply also has an effect current and wire feed speed is definite, based on the on the arc stability. When the load on the power supply wire size, shielding gas type and type of electrode. A changes, the current takes time to find its new level. The faster wire feed speed will give a higher welding current. rate of current change is determined by the inductance of the power supply. Increasing the inductance will reduce In the constant voltage system, instead of regulating the the rate of current rise. The rate of the welding current wire to maintain a constant arc length, the wire is fed rise increases with the current which is also affected by into the arc at a fixed speed and the power source is de- the inductance in the circuit. Increased arc time or induc- signed to melt off the wire at the same speed. The self- tance produces a flatter and smoother weld bead as well regulating characteristic of a constant voltage power as a more fluid weld puddle. Too much inductance will source comes about by the ability of this type of power cause more difficult arc starting. source to adjust its welding current in order to maintain a fixed voltage across the arc. 36 Open circuit voltage 30 Constant current S power SS ST TT TL source LL LO 25Arc OO OV VV VC. length: Cvoolntasgtaen t D. Long power 20Normal source Short 0 0 200 300 400 500 600 WWEELLDD CCUURRRREENNTT ( A(AMMPPEERREESS) ) WELD CURRENT (AMPERES) LOAD CURRENT (D.C. AMPERES) Illustration 2-3 – Constant Current Illustration 2-4 – Volt-Ampere Curves Volt-Ampere Curve – 4 – With the constant current arc system, the welder would of transfer. These are spray transfer, globular transfer, change the wire feed speed as the gun is moved toward and short-circuiting transfer. The metal transfer of most or away from the weld puddle. Since the welding cur- flux cored electrodes resembles a fine globular transfer, rent remains the same, the burn-off rate of the wire is sometimes referred to as small droplet transfer. Some unable to compensate for the variations in the wire feed flux cored electrodes have a spray transfer. Only the al- speed, which allows stubbing or burning back of the wire loy cored, low flux content wires can produce a short- into the contact tip to occur. To lessen this problem, a circuiting metal transfer similar to gas metal arc welding. special voltage sensing wire feeder is used, which regu- lates the wire feed speed to maintain a constant voltage On flux cored electrodes, the molten droplets build up across the arc. The constant voltage system is preferred around the periphery or outer metal sheath of the elec- for most applications, particularly for small diameter wire. trode. By contrast, the droplets on solid wires tend to With smaller diameter electrodes, the voltage sensing form across the entire cross section at the end of the system is often not able to react fast enough to feed at wire. A droplet forms on the cored wire, is transferred, the required burn-off rate, resulting in a higher instance of and then a droplet is formed at another location on the burn-back into the contact tip of the gun. metal sheath. The core material appears to transfer inde- pendently to the surface of the weld puddle. Illustration Illustration 2-4 shows a comparison of the volt ampere 2-5 shows the metal transfer in flux cored arc welding. curves for the two arc systems. This shows that for these particular curves, when a normal arc length is used, the At low currents, the droplets tend to be larger than at current and voltage level is the same for both the con- higher current levels. If the welding current using a 3/32 stant current and constant voltage systems. For a long in. (2.4 mm) electrode wire is increased from 350 to 550 arc length, there is a slight drop in the welding current for amps, the metal transfer characteristics will change. the constant current machine and large drop in the cur- Transfer is much more frequent and the droplets become rent for a constant voltage machine. For constant volt- smaller as the current is increased. At 550 amperes some age power sources, the volt-ampere curve shows that of the metal may transfer by the spray mode, although when the arc length shortens slightly, a large increase in the globular mode prevails. There is no indication that welding current occurs. This results in an increased burn- higher currents cause a transition to a spray mode of off rate, which brings the arc length back to the desired transfer, unless an argon-oxygen shielding gas mixture level. Under this system, changes in the wire feed speed, is used. caused by the welder, are compensated for electrically by the power source. The larger droplets at the lower currents cause a certain amount of “splashing action” when they enter the weld METAL TRANSFER puddle. This action decreases with the smaller droplet size. This explains why there is less visible spatter, the arc Metal transfer, from consumable electrodes across appears smoother to the operator, and the deposition ef- an arc, has been classified into three general modes ficiency is higher, when a wire is used with a high current density rather than at the low end of its current range. Illustration 2-5 –Metal Transfer in Flux Cored Arc Welding – 5 – CHAPTER 3 EQUIPMENT FOR WELDING The equipment used for flux cored arc welding is very In general, these lower duty cycle machines are the similar to that used for gas metal arc welding. The basic constant current type, which are used in plants where arc welding equipment consists of a power source, con- the same machines are also used for shielded metal trols, wire feeder, welding gun and welding cables. A ma- arc welding and gas tungsten arc welding. Some of the jor difference between the gas-shielded electrodes and smaller constant voltage welding machines have a 60% self-shielded electrodes is that the gas-shielded wires duty cycle. also require a gas shielding system. This may also have an effect on the type of welding gun used. Fume extrac- Types of Current tors are often used with this process. For machine and automatic welding, several items, such as seam followers Flux cored arc welding uses direct current. Direct cur- and motion devices, are added to the basic equipment. rent can be connected in one of two ways, which are Illustration 3-1 shows a diagram of the equipment used electrode positive (reverse polarity) or electrode negative for semiautomatic flux cored arc welding. (straight polarity). The electrically charged particles flow between the tip of the electrode and the work as shown in POWER SOURCES Illustration 3-2. Flux cored electrode wires are designed to operate on either DCEP or DCEN. The wires designed The power source or welding machine provides the elec- for use with an external gas shielding system are gener- tric power of the proper voltage and amperage to main- ally designed for use with DCEP. Some self-shielding flux tain a welding arc. Most power sources operate on 230 or cored wires are used with DCEP while others are devel- 460 volt input power, but machines that operate on 200 oped for use with DCEN. Electrode positive current gives or 575 volt input are available as options. Power sources better penetration into the weld joint. Electrode negative may operate on either single phase or three-phase input current gives lighter penetration and is used for welding with a frequency of 50 to 60 Hz. thinner metal or where there is poor fit-up. The weld cre- ated by DCEN is wider and shallower than the weld pro- duced by DCEP. Power Source Duty Cycle The duty cycle of a power source is defined as the ratio Types of Power Sources of arc time to total time. Most power sources used for flux cored arc welding have a duty cycle of 100%, which The power sources generally recommended for flux cored indicates that they can be used to weld continuously. arc welding are direct current constant voltage types. Some machines used for this process have duty cycles Both rotating (generator) and static (single or three-phase of 60%, which means that they can be used to weld six transformer-rectifiers) are employed. Any of these types of every ten minutes. of machines are available to produce constant current or Control System Shielding Wire Feed Control Gas Source (Optional) Feeder Contactor Control Gas Gun Out Control Without Gas Wire Feed Electrode Drive Motor Voltage With Gas Control Power Electrode Lead Source Workpiece Lead Base Metal Illustration 3-1 – Equipment for Flux Cored Arc Welding – 6 – constant voltage output, or both. The same power sourc- es used with gas metal arc welding are employed with flux cored arc welding. Flux cored arc welding generally uses higher welding currents than gas metal arc weld- ing, which sometimes requires a larger power source. It is important to use a power source that is capable of producing the maximum current level required for an ap- plication. Generator Welding Machines The generator welding machines used for this process can be powered by an electric motor, for shop use, or an internal combustion engine, for field applications. The gasoline or diesel engine driven welding machines have either liquid or air-cooled engines and many of them pro- vide auxiliary power for emergency lighting, power tools, etc. Many of the engine driven generators used for flux cored arc welding in the field are combination constant current-constant voltage types. These are popular for ap- plications where both shielded metal arc welding and flux cored arc welding can be accomplished using the same power source. Illustration 3-3 shows an engine driven Illustration 3-3 – Gas Powered Welder/Generator Photo courtesy of Miller Electric Manufacturing Co. generator machine used for flux cored arc welding. The motor driven generator welding machines are gradu- transformer circuit. A rectifier is an electrical device which ally being replaced by transformer-rectifier welding ma- changes alternating current into direct current. These chines. Motor-driven generators produce a very stable machines are more efficient electrically than motor-gen- arc, but they are noisier, more expensive, consume more erator welding machines and they provide quieter opera- power and require more maintenance than transformer- tion. There are two basic types of transformer-rectifier rectifier machines. welding machines; those that operate on single phase input power and those that operate on three-phase input Transformer-Rectifier Welding Machines power. The most widely used welding machines for flux cored The single phase transformer-rectifier machines provide arc welding are the transformer-rectifiers. A method of DC current to the arc and a constant current volt-am- supplying direct current to the arc, other than the use pere characteristic. These machines are not as popular of a rotating generator, is by adding a rectifier to a basic as three-phase transformer-rectifier welding machines for flux cored arc welding. When using a constant cur- + - rent power source, a special variable speed or voltage sensing wire feeder must be used to maintain a uniform current level. A limitation of the single phase system is that the power required by the single phase input power may create an unbalance of the power supply lines which is objectionable to most power companies. These ma- chines normally have a duty cycle of 60%. + - - + The most widely used type of power source for this pro- cess is the three-phase transformer-rectifier. These ma- + - - + chines produce DC current for the arc and, for flux cored arc welding, most have a constant voltage volt ampere + - - + characteristic. When using these constant voltage ma- - + chines, a constant speed wire feeder is employed. This type of wire feeder maintains a constant wire feed speed with slight changes in welding current. The three-phase input power gives these machines a more stable arc DCEP DCEN than single phase input power and avoids the line unbal- ance that occurs with the single phase machines. Many Illustration 3-2 – Particle Flow for DCEP and DCEN of these machines also use solid state controls for the – 7 – welding. A 650 amp solid state controlled power source is shown in Illustration 3-4. This machine will produce the flattest volt-ampere curve of the different constant voltage power sources. Most three-phase transformer- rectifier power sources are rated at a 100% duty cycle. CONTROLS The controls for this process are located on the front of the welding machine, on the welding gun, and on the wire feeder or a control box. The welding machine controls for a constant voltage ma- chine include an on-off switch, a voltage control, and of- ten a switch to select the polarity of direct current. The voltage control can be a single knob, or it can have a tap switch for setting the voltage range and a fine voltage control knob. Other controls are sometimes present, such as a switch for selecting CC or CV output on combination machines, Illustration 3-4 – Three phase Power Source or a switch for a remote control. On the constant cur- with Multi-Process Capability rent welding machines there is an on-off switch, a current Photo courtesy of Miller Electric Manufacturing Co. level control knob, and sometimes a knob or switch for selecting the polarity of direct current. welding, a separate control box is often used to control The trigger or switch on the welding gun is a remote con- the wire feed speed. On the wire feeder control box, there trol that is used by the welder in semiautomatic weld- may also be switches to turn the control on and off and ing to stop and start the welding current, wire feed, and gradually feed the wire up and down. shielding gas flow. Other controls for this process are used for special appli- For semiautomatic welding, a wire feed speed control is cations, especially when a programmable power source normally part of, or close by, the wire feeder assembly. is used. An example is a timer for spot welding. Controls The wire feed speed sets the welding current level on that produce a digital read-out are popular because more a constant voltage machine. For machine or automatic concise control is easier. 4-Roll Gear Box Photo courtesy of Miller Electric Manufacturing Co. 2-Roll Gear Box Illustration 3-7 – Wire Feeder Assembly – 8 –