Gemi inşaatında kaynak işleminin mekanizasyonu

Abstract

Gemi ve Deniz yapılarında kullanılan çelik malzemeler de özellikle kaynak teknolojisinin gelişimi ile kaynak çat lamalarının önlenmesi, ve soğuk ortamlarda sünek davranış gösterecek şekilde geliştirilmeleri önem kazanmıştır. Bil hassa daha az malzeme kullanılarak (Mukavemet/ Ağırlık oranı artırılarak) Mukavemetleri artırılacak yönde fakat çok düşük sıcaklıklarda belirli bir sünekliğe sahip tehlikesizce kaynak edilebilir çeliklerin gelişimi önem kazanmıştır. önceleri metaller kaynak edilebilir veya edilemez diye sınıflandırılırken, günümüzde kaynak kabiliyeti üzerinde durulmaktadır. Bu nedenle mevcut kaynak yöntemlerinin geliştirilmesi ve yenilerinin bulunması yolunda yapılan araştırmalar ve çabalar büyük bir hız kazanmıştır. Günümüzde en büyük gelişme olanağı bulan kaynak yön temleri MIG-MAG ve tozaltı kaynak yöntemleridir. Tozaltı kaynak yöntemi tam mekanize edilmiş kaynak için tipik bir örnektir. Burada ilave metal kaynak yerine yalnız otoma-r tik olarak gelmemekte, aynı zamanda uygun donanımlarla kaynak kafası ile iş parçası arasında izafi bir hareket de sağlanmaktadır. Tozaltı kaynağında birden fazla tel beslemeli sistemlerin uygulanmasıyla kaynak hızı önemli ölçüde artırılmaktadır. Hemen hemen bütün eritme kaynak yöntemlerinin uygulama alanı bulduğu gemi inşaatında MIG-MAG kaynak yöntemi günümüzde bir yere sahiptir. örtülü elektroda nazaran daha yük sek bir erime gücüne, ve diğer tel sürme tertibatlı yöntemlere nazaran da her pozisyonda uygulanabilme özelliğine sahiptir. Orta büyüklükteki bir gemi de dahi kilometrelerce kaynak dikişinin varlığı bu iki özelliğin bu endüstri dalın da ne derece önemli olduğunu ortaya koymaktadır. Gemi inşaatında ilk adımlardan biri malzemelerin kesilerek imalata hazırlanmasıdır. Günümüzde malzemelerin kesilerek imalata hazırlanmasında otomatik kesme makinaları kullanılmaktadır. Optik ve nümerik kontrollü oksijenle kesme makinaları kullanıma girerek başarı ile uygulanabilmektedir. Böylece kesim işleminde sürat ve kalite bakımın dan önemli gelişmeler sağlanmıştır. Yüksek yatırım maliyeti ve pahalı gazların kullanılma sı itibariyle "Plazma Ark" ile kesme oksijenle termik kesme ve bunun değişik uygulamalarının sonuç vermediği durumlarda başarı ile uygulanabilmektedir. Plazma ark ile kesme yöntemi tüm elektrik iletkenlerine, yani metallerin kesilme işleminde kullanılabilmektedir.

The steels used in shipbuilding are defined mainly by requirements on chemical composition, mechanical properties. and in some cases also on heat treatment. Up to a few years ago only mild steel was used in shipbuilding. During recent years, steels with higher tensile strength are used to an increasing extent in shipbuilding. In selecting a process for production welding, ability primary consideration is the ability of the process to give the required quality at the lowest cost and to complete the wark in a certain limited time. The most used welding method today in shipbuilding is manual-arc welding with stick electrodes. It is charac terized by application versality and flexibilityand relative simplicity in the equipment. The second most important welding method in shipbuilding is submerged arc welding. The main feature of this method is of course the high productivity of the welding process for this reason it is used to an increasing extent. Submerged-arc welding is surtable for both semi-automatic and full-mechanized welding. MIG welding is today used only to a small extent in shipbuilding. Welding is either semi-automatic, using a hand-held gun to which electrode is fed automatically, or full-automatic. It is quite possible that this method will be used more in future, as it can combine the flexiblity of manual-arc welding with the productivity of submerged-afc Welding Plasma-arc welding is one of the newer welding and cutting processes. It offers greater welding speeds, better weld quality, and less sensivi-ty to process variables than the conventional processes it replaces. IX Shielded Metal-arc Welding The shielded metal-afc process-commoriy called "stick electrode" or manual arc welding is the most widely used of are welding processes in shipbuilding. In is characterised by application versality and flexibility and relative simp licity in the equipment. For manual arc welding a very large number of electrodes are available for different kind of steels and for different welding positions. Manual arc welding therefore very suitable for the very different applications that exist in shipbuilding. Shielded metal-arc welding requires relatively low currents (10 to 500 amp) and voltages (17 to 45) depending on the type and size electrode used. The current may be either AC or DC or a combination of AC/DC welder. The fact that the shielded metal-arc process can be used with so many electrode types and sizes in all posi tions, on a great variety of materials and with flexiblity in operator control makes it the most versatile <3f all wel ding processes. Gravity Welding For fillet welding the gravity welding process is widely used throught the shipbuilding industry as being the most economical method of producing long fillet welds. The origin of the gravity welding comes from a «Swedish patent back in 1940-42. However, the popularity of the method was almost none ultil the improvement of iron powder electrodes and suitable machines in early sixties made it possible for one operator to use 4 to 6 gravity welders. The fematic gravity welder consists of a bar and a welder base. Gravity feed is provided by the guide bar on which slides the electrode carriage with the electrode holder. A bend at the lower end of the guide bar gives the carriage the movement which swithes off the arc. It is possible to adjust the inclination and thus the length of from each electrode. The support arm is also equipped with a disconnector being switched on to start the arc and off when the arc automatically extinguished. In this manner, the electrode can be changed without the operator needing to touch live parts, The productivity of fematic welding has been very thoroughly" studied to determine the factors which have biggest influence. The length of the electrode, fusion x time of the electrode, number of feeders per welder and the type of workpiece are among the factors investigated. For shipyard use where large panels with long uninterrupted stiffeners have welded 700 mm seems to give the optimum length. A welder should be able to look after at least four feeders and his productivety should be about five times that of a welder depositing 450 mm long electrodes manually. Gas Matal-arc Welding Gas metal-afc welding, popularly known as MIG welding uses a continues electrode for filler metal and externally supplied gas or gas mixture for shielding. The shielding gas-helium, argon, carbon dioxide, or mixtures thereof protects the molten metal from reacting with constifuents of the atmosphere. Alth ugh the gas shield is effective in shielding the molten metal from the air, deoxidizers are usually as alloys in the electrode. Welding is either semiautomatic, using a hand-held gun to which electrode is fed automatically, or full-auto matic equipment is used. The guns and welding heads for semiautomatic and full-automatic welding with the gas- shielded process are more complex. Fugure l shows a schematic for a full-automatic welding facility with either self shielded or gas shielded flux-cored electrode; the datted line indicates the addition requirred with the gas- shielded version when using CO2 as the shielding gas. Electrode Shielding gas. Gas cup Work "© Fig:l. Schematic For a Full-automatic Welding With Either Self-Shielded or Gas Shielded Flux-Cored Electrode Metal transfer with the MIG process is by one of two methods: "Spray-arc" or short circuiting, With spray-arc, drops of molten metal detach from the electrode and move through the arc column to the work. The short-circuiting technique, metal is transferred to the work the molten tip of the electrode contacts the molten puddle. Spray-arc MIG welding process produces an intensively hot, higher voltage arc, and, thus, gives a higher deposi tion rate short-arc welding, The spray arc tecpnique is recommended for thicker sections, requiring heavy single or multipass welds or any filler-pass application where high deposition rate is advantageous. To use short-arc welding efficiently, special power sources with adjustable slape, voltage, and inductance characteristics are requirred. These power sources produce the predictable and controllable current surges needed for successful use of the short-arc technique. The gas-shielded flux-cored process is used for welding mild and low alloy steels. It gives high deposition efficiencies, and high operating factors. Radiographic quality welds are easily produced, and the weld metal with mild and low allxy steels has good ductility and toughness. MIG welding is today used only to a small extent in shipbuilding, it is, however, quite poss ble that this method will be used more in future because it can, ofter some modifications of the welding equipment, conbine the flexibility of manual welding with the productivity of submerged arc welding. TIG welding is an arc welding process wherein coalessence is produced by heating with an arc between a tungten electrode and the work. A filler metal may or may not bu used. Shielding is obtained with a gas or gas mixia ture. The gases employed in the TIG process are argon and helium and mixtures of the two. Materials weldable by the TIG process are most grades of carbon, also, and staintess steels; aluminum and most of its alloys; magnesium and most of its alloys; copper and various brasses and brunzes. Submerged Arc Welding Xll With proper selection of equipment, submerged arc is widely applicable to the welding requirements of shipbuil ding industry. It can be used with all types of joints, and permits welding a full range of carbon and low alloy steels. It is also applicable to some high-alloy, heat- treated, and stainless steels, and is fovored process for rebuilding and hardsurf acing. Any degree of mecpanization can be used from the hand-held semi-automatic gun to boom or track-carried and fixture-held multiple welding heads. The high quality of submerged-arc welds, the high deposition rates, the deep penetration, the adaptability of the process to full mechanization, and the comfort characteristics (no glare, sparks, spatter, smoke, or - excessive heat radiation) make it a preferred process in shipbuilding industry. It is used extensively in fabrica ting ship plate, structural beams, girders, webs, colummns where long welds are reguired in ship, barge and off-shore buildings. The high deposition rates attained with submerged-arc chiefly responsible for the economies achieved with the r process. The cost reduction when changing from the shielded metal-arc process to submerged-arc frequently dramatic. Thus, a hand-held submerged-arc fun with mechanized travel may reduce welding costs more than 50%; with fully automatic multiarc equipment, it is not unusual for the costs to be but 10 % of those attained with stick-electrode welding. When submerged -arc equipment is operated properly, the weld beads are smooth and uniform, so that grinding or machining are rarely required. Since the rapid heat input of the process minimizes distortion, the costs for straig- tening finished assemblies are reduced, especially if a carefully planned welding sequence has been followed. Where the deep-penetration characteristics of the process permit the elimination or reduction of joint preparation, expense is lessened. After the weld has been run, cleanning costs are minimized, because of the elimination of spatter by the protective flux. Submerged-arc weyding differs from other arc welding processes in that a blanket of fusible, granular material (flux) is used for shielding the arc and the molten metal. The arc is strack between the workpiece and a bare wire electrode, the tip of which submerged in the flux. Since the arc is completely covered by the flux, it is not visible and the weld is run without the flash, spatter and sparks that characterize the open-arc processes. xiii The process is either semiautomatic or full-automatic with electrode fed mechanically to the welding gun, heads In Serai-automatic welding, the weldor moves the gun usually equipped with a flux-feeding device, along the joint. Semiautomatic welding equipment appears quite simple compared to full-automatic welding equipment, but possibly represents even a higher stage in mecnanization ingenuity because of its simplicity. Semi-automatic welding attampts to put a degree of mechanization in the great bulk of wel ding work that is not subject to full mechanization. This work requires extreme flexibility, which is impossible with any full mechanized equipment. With fully mechanized welding, a great variety of equipment is used. In a fully mechanized installation, the manual skill of the weldor is replaced entirely. Machines supply the skill and the operators of such machines provide the instructionfe and watch over the machines to make sure that instructions are followed. The welding head replaces the welding gun at the fully mechanized stage of arc-welding development. Head may be stationary with the work moving under it, or it may move across the work. The head may feed one wire or two wires (Twinare), and two or more heads may be used together (tandem-arc and multiarc). Fugure shows sketches of various wire-feeded arrangements for full-automatic submer* ged-arc welding. Full automatic welding is not limited to fixed loca tions. By use of self-propelled trackless tractor, the welding equipment may be taken to the work, rather than the work brought to the welding equipment. Self-propelled units such as DC-AC tandem submerged-arc Welder, are widely used in ship and barge building; beam, girder and column fabrica tion, either in the shop or in the field. xiv Fig: 2. Schematic of Various Wire-Feeder Arrangements For Full-Automatic Submerged Arc Welding. 3- O'clock Welding Automatic welding in the horizontal-vectical position is only made to a limited extent. In the building dock ' there is also limited space, which does not enable bigger machines to run between the side hull and the dock wall, but where it is possible, automatic welding is performed using submerged arc welding with twin wire. In modern shipbuilding where one section is added to another with planned intervals the welding has to be perf or med in a certain limitid time. Not only the machine but everything around has to be set before welding can start. Sometimes the joints has to be adjusted, maybe the gap is too big and a root run has to be done by hand welding. All this takes time and it happens there is simply no time for a machine set up before next section has tobe erected. To overcome these drawbacks, efforts have been made to design machines with a very short set up time, practically negligable. To use automatic 3-o' clock welding in xv shipbuilding overcomes these problems Electros lag welding is an adaptation of the submerged- arc welding process for joining thick materials in the ver tical pasition. In figure 3. a square butt joint in heavy plates in illustrated, but the electroslag process is also applicable to T joints, corner joints, girth seams in heavy wall cylinders, and other joints, with modifications in equipment and techniques. 1- Electrode guide tube 2- Electrode 3- Woter-cooled copper shoes 4- Finished weld 5- Base metal 6- Molten slag 7- Molten weld metal 8- Solidified weld metal Figure: 3. Schematic Sketch Of Electroslag Welding. Weld quality withelectroslag process is generally excellent, due to the protective action of the heavy slag layer. Sometimes, hewever, the copper dams are provided with" orifices jüst above the slag layer, through which a protective gas argon or carbon dioxide is introduced to flush out the air above the weld and, thus, give additional assurance against oxidation. Such provisions are sometimeo considered worthwhile when welding highly alloyed steels or that contain easily oxidizing elements. The electroslag process has various advantages including no need for special edge preparations, a desirable seguence of cooling that places the outside surfaces of weld under rat her than tensile stresses, and relative relative freedom from porosity problems. xvi Electrogas welding is similar to electroslag welding in that the equipment is similar and the joint is in the vertical position. The shielding is by carbon dioxide or an inert gas. A thin layer of slag, supplied by the flux- cored electrode, covers the molten metal, and the heat is supplied by an arc rather than by resistance heating as in the electroslag process. A disadvantage of the process is thcXt it requires on external source of shielding gas. However, one arvantage is that if the welding is stopped the electrogas prociss can be started again with less difficult than the electros lag process. Plasma-arc Welding Plasma-arc welding is one of the newer welding process In some applications, it offers greater welding speeds, better weld quality, and less sensivity to process variable than the conventional process it replaces. With the plasma arc, temperature as high as 60000° P are developed and, theoretically, temperatires as high as 200000° F are passible The heat in plasma-arc welding originates in an arc, but this arc is not diffused as in ordinary welding arc. Intead, it is constricted by being forced through a rela tively small orifice,. The "orifice" or plasma gas may be supplemented by an auxilury source of shielding gas. "Of if ice" gas refers to the that is directed is directed into torch to surround the electrode. It becomes ionized in the arc to from the plasma and emerges from the orifice in the toch nozzle as a plasma jet. If a shielding gas in used, it is directed onto the warkpiece from an outer shielding ring. The workpiece may or may not be part of the electrical circuit. In the "tarnsferred-arc" system, the workpiece is a part of the circuit, as in other arc-welding processes. In the "nontrrinsferred" system, the constricting nozzle surrounding the electrode acts as an electrical terminal, and are is struck between it and the electrode tip; the plasma gas then carries the heat to the workpiece. The arvantages gained by using a constricted-arc process over the gas tungsten-arc procebb include greater energy concentration, improved arc stability, higher welding speeds and lower width-to-depth ratio for a given penetration. "Keyhole" welding or penetrating completely through the workpiece is possible.