- 14.3 ARC SHIELDING NATURE OF THE ARC / 14.4. - Since the joining is by inter- mixture of the substance of one part with the substance of the other part, with or without an intermediate of like substance, the final weldment has the potential for exhibiting at the joint the same strength properties as the metal of the parts. - This is in sharp contrast to nonfusion processes of joining—such as soldering, brazing, or adhesive bonding—in which the mechanical and physical properties of the base materials cannot be duplicated at the joint.. - Most welding in the manufacture of steel products where filler metal is required, however, is accom- plished with the second type of electrode—the type that supplies filler metal as well as providing the conductor for carrying electric current.. - The arc produces a temperature of about 650O 0 F at the tip of the electrode, a temperature more than adequate for melting most metals. - The heat produced melts the base metal in the vicinity of the arc and any filler metal supplied by the electrode or by a separately introduced rod or wire. - The result is a fusion bond and the metallurgical unification of the work- pieces.. - FIGURE 14.1 The basic arc-welding circuit. - Using the heat of an electric arc to join metals, however, requires more than the mov- ing of the electrode with respect to the weld joint. - Should the metal in the molten pool come in contact with air, oxides and nitrides would be formed, which upon solidification of the molten pool would destroy the strength properties of the weld joint. - This is referred to as arc shielding, and such shielding may be accomplished by various techniques, such as the use of a vapor-generating covering on filler-metal-type electrodes, the covering of the arc and molten pool with a sepa- rately applied inert gas or a granular flux, or the use of materials within the cores of tubular electrodes that generate shielding vapors.. - Whatever the shielding method, the intent is to provide a blanket of gas, vapor, or slag that prevents or minimizes contact of the molten metal with air. - The shielding method also affects the stability and other characteristics of the arc. - Thus the core materials in a flux-core electrode may perform a deox- idizing function as well as a shielding function, and in submerged-arc welding, the granular flux applied to the joint ahead of the arc may add alloying elements to the molten pool as well as shielding it and the arc.. - Figure 14.2 illustrates the shielding of the welding arc and molten pool with a covered "stick". - electrode—the type of electrode used in most manual arc welding.. - The extruded covering on the filler metal rod, under the heat of the arc, generates a gaseous shield that prevents air from coming in contact with the molten metal. - It also supplies ingredients that react with deleterious substances on the metals, such as oxides and salts, and ties these substances up chemically in a slag that, being lighter than the weld metal, rises to the top of the pool and crusts over the newly solidified metal. - This slag, even after soldification, has a protective function: It minimizes con- tact of the very hot solidified metal with air until the temperature lowers to a point where reaction of the metal with air is lessened.. - FIGURE 14.2 How the arc and molten pool are shielded by a gaseous blanket developed by the vapor- ization and chemical breakdown of the extruded cov- ering on the electrode in stick-electrode welding.. - While the main function of the arc is to supply heat, it has other functions that are important to the success of arc-welding processes. - Negative electrons are emitted from the cathode and flow—along with the negative ions of the plasma—to the pos- itive anode, as shown in Fig. - Heat is generated in the cathode area mostly by the positive ions striking the sur- face of the cathode. - In the central column of the plasma, electrons, atoms, and P o s i t i v e j o n s are j n accelerated motion a n d a r e con-. - FIGURE 14.3 Characteristics of the 1 ^ distribution of heat or voltage drop in arc. - (The Lincoln Electric Company.) the three heat zones can be changed. - In welding, not only does the arc provide the heat needed to melt the electrode and the base metal, but under certain conditions it must also supply the means to transport the molten metal from the tip of the electrode to the work. - If the electrode is consumable, the tip melts under the heat of the arc, and molten droplets are detached and transported to the work through the arc column. - Any arc- welding system in which the electrode is melted off to become part of the weld is described as metal arc. - More of the heat developed by the arc ends up in the weld pool with consumable electrodes than with nonconsumable electrodes, with the result that higher thermal efficiencies and narrower heat-affected zones are obtained. - Since there must be an ionized path to conduct electricity across a gap, the mere switching on of the welding current with a cold electrode poised over the work will not start the arc. - The mechanics of stick-electrode welding are such that electric contact with the elec- trode cannot be made immediately above the arc—a technique that would circum- vent much of the resistance heating.. - The length between the tip of the electrode and the point of electric contact is then inadequate for enough resistance heating to take place to overheat the electrode in advance of the arc, even with currents two or three times those usable with stick-electrode welding.. - This solving of the point-of-contact problem and circumventing of the effects of resistance heating in the electrode constituted a breakthrough that substantially lowered welding costs and increased the use of arc welding in industrial metals joining. - The shielded metal-arc process—commonly called stick-electrode welding or manual welding—is the most widely used of the various arc-welding processes. - With this process, an electric arc is struck between the electrically grounded work and a 9- to 18-in length of covered metal rod—the electrode. - The welder grips the insulated handle of the electrode holder and maneuvers the tip of the electrode with respect to the weld joint. - When the welder touches the tip of the electrode against the work and then withdraws it to establish the arc, the welding circuit is completed. - The heat of the arc melts base metal in the immedi- ate area, the electrode's metal core, and any metal particles that may be in the electrode's covering. - This periodic changing of electrodes is one of the major disadvantages of the process in production welding. - It decreases the operating factor, or the percent of the welder's time spent in the actual laying of weld beads.. - High amperages, such as those used with semiauto- matic guns or automatic welding heads, are impractical because of the long (and varying) length of electrode between the arc and the point of electric contact in the jaws of the electrode holder. - The welding current is limited by the resistance heating of the electrode. - The electrode temperature must not exceed the break- down temperature of the covering. - The versatility of the process—plus the simplicity of equipment—is viewed by many users whose work would permit some degree of mechanized welding as over- riding its inherent disadvantages. - The result of these efforts was the development of the semiautomatic and full-automatic pro- cesses for welding with continuous flux-cored tubular electrode "wires.". - FIGURE 14.4 Principles of the self-shielded flux-cored arc-welding process.. - The electrode may be viewed as an inside-out construction of the stick elec- trode used in shielded metal-arc welding. - The only other major difference is that the weld metal of the electrode surrounds the shield- ing and fluxing chemicals rather than being surrounded by them.. - Full-automatic welding with self-shielded flux-cored electrodes goes one step further in mechanization—the removal of direct manual manipulation in the utiliza- tion of the open-arc process.. - One of the advantages of the self-shielded flux-cored arc-welding process is the high deposition rates that are made possible with the hand-held semiautomatic gun. - Another advantage of the process is its tolerance of poor fitup, which in shops often reduces rework and repair without affecting final product quality. - The toler- ance of the semiautomatic process for poor fitup has expanded the use of tubular steel members in structures by making possible sound connections where perfect fitup would be too difficult or costly to achieve.. - The shielding gas—helium, argon, carbon dioxide, or mixtures thereof—protects the molten metal from reacting with constituents of the atmosphere. - It is a preferred process for the welding of aluminum, magnesium, cop- per, and many of the alloys of these reactive metals. - Most of the irons and steels can be satisfactorily joined by MIG welding, including the carbon-free irons, the low- carbon and low-alloy steels, the high-strength quenched and tempered steels, the chromium irons and steels, the high-nickel steels, and some of the so-called super- alloy steels. - FIGURE 14.5 Principle of the gas metal-arc process. - 14.6.4 The Gas-Shielded Flux-Cored Process. - FIGURE 14.6 Principles of the gas-shielded flux-cored process. - Upon solidification of the molten area, unification occurs. - Both the tip of the nonconsumable tungsten electrode and the tip of the filler rod are kept under the protective gas shield as welding progresses.. - It is one of the few processes that is satisfactory for welding such tiny and thin-walled objects as transistor cases, instru- ment diaphragms, and delicate expansion bellows.. - FIGURE 14.7 Principles of the gas tungsten-arc process. - The nature of the flux is such that very little smoke or visible fumes are developed.. - Flux may also be applied in advance of the welding operation or ahead of the arc from a hopper run along the joint. - During welding, the heat of the arc melts some of the flux along with the tip of the electrode, as illustrated in Fig. - The tip of the electrode and the welding zone are always surrounded and shielded by molten flux, surmounted by a layer of unfused flux. - FIGURE 14.8 The mechanics of the submerged-arc process. - Currents as high as 600 A can be carried on electrodes as small as 5 /64 in, giving a density of the order of 100 000 A/in 2 —6 to 10 times that carried on stick electrodes.. - Because of the high current density, the melt-off rate is much higher for a given electrode diameter than with stick-electrode welding. - Mechanical properties at least equal to those of the base metal are consistently obtained. - Thus in such welds the base metal may greatly influence the chemical and mechanical prop- erties of the weld. - For this reason, it is sometimes unnecessary to use electrodes of the same composition as the base metal for welding many of the low-alloy steels.. - The high quality of submerged-arc welds, the high deposition rates, the deep pen- etration, the adaptability of the process to full mechanization, and the comfort char- acteristics (no glare, sparks, spatter, smoke, or excessive heat radiation) make it a preferred process in steel fabrication. - Automatic submerged-arc installations are also key features of the welding areas of plants turning out mass- produced assemblies joined with repetitive short welds.. - Various adaptations of the arc-welding processes described have been made to meet specialized joining needs. - Electroslag welding is an adaptation of the submerged-arc process for joining thick materials in a vertical position. - Figure 14.9 is a diagrammatic sketch of the elec- troslag process. - It will be noted that whereas some of the principles of submerged- arc welding apply, in other respects the process resembles a casting operation.. - A starting pad at the bottom of the joint prevents. - the fall-out of the initially deposited weld metal and, since it is penetrated, ensures a full weld at this point. - Water-cooled dams, which may be looked on as molds, are placed on each side of the joint. - The electrode or electrodes may be oscillated across the joint if the width of the joint makes this desirable.. - At the start of the operation, a layer of flux is placed in the bottom of the joint and an arc is struck between the electrode (or electrodes) and the work. - Heat for melting the electrode and the base metal subsequently results from the electrical resistance heating of the electrode section extending from the contact tube and from the resistance heating within the molten slag layer. - In conventional practice, the weld deposit usually contains about one-third melted base metal and two-thirds electrode metal—which means that the base metal substantially contributes to the chemical composition of the weld metal. - Weld quality with the electroslag process is generally excellent, because of the protective action of the heavy slag layer. - A disadvantage of the process is that it requires an external source of shielding gas. - Stud arc welding is a variation of the shielded metal-arc process that is widely used for attaching studs, screws, pins, and similar fasteners to a large workpiece. - At a press of the trigger, current flows through the stud, which is lifted slightly, creating an arc. - The fundamentals of the process are illustrated in Fig. - The flux may include any of the agents found in a regular electrode covering. - Plasma-arc (or plasma-torch) welding is one of the newer welding processes which is used industrially, frequently as a substitute for the gas tungsten-arc process.. - The workpiece may or may not be part of the electric circuit. - In the transferred- arc system, the workpiece is a part of the circuit, as in other arc-welding processes.. - FIGURE 14.10 Principles of stud welding, using a ceramic ferrule to shield the pool, (a) The stud with ceramic ferrule is grasped by the chuck of the gun and positioned for welding.. - As a result of the recombining, the heat of dissociation absorbed in the arc is liberated, supplying the heat needed for fusing the base metal and any filler metal that may be introduced.. - The arc supplies the heat through the intermediate of the molecular-dissociation, atom- recombination mechanism. - These are almost insoluble in molten metal and either bubble out of the weld pool vigorously or become entrapped in the solidifying metal, resulting in porosity.. - With the exception of the gases, all the commonly used consumables are covered by AWS specifications.
Xem thử không khả dụng, vui lòng xem tại trang nguồn hoặc xem
Tóm tắt