Tozaltı kaynaklı spiral boru imalinde kaynak parametrelerinin boru kalitesine etkisi

Abstract

Tozaltı kaynaklı spiral boru imalinde kaynak parametrelerinin boru kalitesine etkisi isimli tez çalışmamda, tozaltı kaynaklı spiral boru imalatı dört ayrı bölümde incelenmiş olup, tezin her aşamasında teori ile pratik bilgiler kaynaştırılmaya çalışılmıştır. Tezin birinci bölümünde tozaltı kaynağı genel olarak ele alınmış olup; yöntemin temellerinden, uygulanan parametrelerden ve kullanılan malzeme ve ekipmanlar anlatılmıştır. İkinci bölümde tozaltı kaynaklı spiral boru imalatı, boru makinasından fotoğraflar verilerek anlatılmıştır. Burada tozaltı kaynağında kullanılan kaynak parametrelerinin kaynak kalitesine etkisi, spiral boru imalatında kullanılan malzemeler ve üretilen boruların kullanım alanları da incelenmiştir. Üçüncü bölümde ise spiral tozaltı kaynak dikişli borunun boru endüstrisindeki, kalite kontrolundan resimler ve fotoğraflar verilerek, anlatılmıştır. Ayrıca, kaynak dikişi tahribatsız kontrol yöntemleri hakkında bilgiler verilerek, kaynak dikişinde oluşan hataların çeşitlerinden, sebeplerinden radioscopik görüntülerle söz edilmiştir. Tezin dördüncü bölümünde, deneysel çalışmalara yer verilmiştir. Spiral boru imalatında kaynak ağız açısı, kaynak hızı, akım türü gibi parametrelerin değişimi ile elde edilen kaynak dikiş geometrisi incelenmiştir. Yapılan deneysel çalışmalar sonucunda optimum kaynak ağızının seçilmesi, en iyi kaynak kalitesini verecek hızın seçilmesi ve çok kafalı tozaltı kaynağına uygun akım türünün ve akım sırasının seçilmesi ile, hem maksimum üretimin yapılabileceği ve hemde kaliteli, standartlara uygun kaynak dikişinin elde edilebileceği görülmüştür.

In submerged arc welding, for obligation of good gualified welding joint, welding parameters should be properly applied and selected correctly. These parameters are welding current, flux type and its grain size, type of electrod, width and depth of flux layer, welding voltage, melding speed, electrod extension electrod size. Welding current is most influential variable of the submerged arc welding. Because, welding current effects and controls the deposition rate, depth of penetration and amount of melted base metal. If the welding current increases, depth of peneltration will increase but, in reaching excessive current level, material will be burn through by the filler metal. On the contrary, if the welding eurrent decreases, excessively distortions and contractions will happen and because of low current, in welding incomplete fusion and in adequate penetration will be resulted in. However, in submerged arc welding. AC or DC current type can be used. According to cuvrent type, joint form varies. For example, when we use DC- current, depth of penetration of the weld pool into the base metal and fusion rate of the base metal will increase. In using DC-current, it is available two different pobrity DC- positive and DC- negative while DC- positive polarity gives deeper penetration and higher welding speed in same parameters. DC-negative polarity gives melt-off rate about one- third greater than that of positive polarity. AC-current type, generally is recommend for trail arcs in, multiple applications and joints where have arc blowing problem. Welding voltage effects the shape of joint and external appearance of the joint for performing flatter and wider bead, resistance to porosity caused by rust welding voltage should be increased. Low welding voltage produces the arc which has resistance against arc blowing. Travel speed is, after welding current most important welding parameters in submerged are welding, for determining the depthof penetration of the joint and the bead size. Travel speed value dependon welding current value before starting welding operation travel speed is determined in dependent welding current. Excessive travel speed decreases wetting action and increases the tendency for undercut, arc blow, porosity, craking and uneven bead sgapes. For decreasing the tendency for porosity, trvel speed should be lowered. Excessively low travel speed results in hat shaped bead subjected to craking and electrod may born throvgh the base material. Electrod size is one of the parameters which affects the bead shape and depth of ponetrtion. For providing flexibility and comfortable movement in semiautomatic submerged arc welding, small diameter electrodes are used. And, this size of electrdes also used for multiple torches in submerged arc welding. Generally, large diameter electrodes are used in poor assembly of joint and joint which has lange root openings. Another effects of electrod size is in the deposition rate. At any given current, while a small diameter electrode will give higher deposition ratethan langer electrode. One ofthe parameters, which affects the bead appearence and soundness of joint is width and depth of the layer of granular flux. Deeper flux gives confined arc and a rough ropelik apperance of the bead. However, deeper flux increases the tendecy for becoming porosity because of the fact that during the welding, generated gases can not neadily escape. Generally, thickness range from tin material to thicker material can be welded by using submerged arc process. Various joint designs are svitable for applycation of submerged arc process. Joint edge can be prepared by using of oxy-acetilen or machining in the worrkshop. Thinner material should be welded in butt weld and thicker material, if possible, should be welded by preparing double edge joint design. Especially, submerged arc welding process is used in welding of thicker materiral in heavy industries. In submerged arc welding, using the multiple arc is possible. Especially, in industry, in terms of economical advantage submerged arc welding with multiple arc is preferred. In this kind of applications, current of type of first heelding head is DC-positive, because of the fact that DC-positive polarity gives good depthof penetratron. Other trail welding heads are AC. Becauce, generally trail arcs affect the bead size and appearance. It is important that in first head larger electroe size should be used, because of high current density. In submerged arc welding, selection of type of power supply is important for performing regular welding operation DC-constant voltage, DC-constant current and its combinalion or alternating current power sources can be used in submerged arc welding process. Used control equipments in submerged arc welding have too importance in determining and contrlling feeding electrod adjusting welding current and voltage, start-stop of welding supplying welding flux. They are produced in anolog or digital. Welding head includes electrode feed system, roller for electrode feed, welding torch, head fixtuer and positioner systems. Welding head travel in submerged arc welding is gererally provided by a tractor-type carriage, a side beam carriage, or a manupulator. When the travel units is not used, guided rollers are used, However, welding head control equipments, electrode feeding systems, flux supplying cone are mounted on the travel tractor. In submerged arc welding, flux recovery systems are used in. Because, in ternis of economy these systems provide advantage. In these systems, melted slag and un melted ganul flux are vacumed, arranged in good grin size, seperated inclusions and particules by using of magnetic units, dried and sent to flux cone again. Submerged arc weldind process is most popular process in welded tube manufac turing. These tubes may be longitudinally or spiral welded. Specilly, spiral welded tubes are preferred widely. Main principal of machine which produces spiral welded tube is that, rolled plate is pulled and is given spiral movement by using forming roller. Hydraulic power systems and planet type reducer is used in driving rolled plate. Angle between pulling plate and fonming spiral tube determines the tube diameter. This angle is determined by rolled plate (coil) width. When the coil which called rolled plate before, was purchased, all technologic tests are performed for determining the material quality. After coil number and heat number noticed to internol report coil is loaded to machine by using hydraulic equipments. Second coil is waited for loading to machine after finished first coil. XI After loading the coil to machine, for continuous produetion end of the first coil should be joined to end of the second coil. This kind of joint is called as skelpend weld in tube industry. In this joint, both of end is brought end by end, and by using grinding and oxy-acetilen, groove is prepared. After preparing the joint groore, by using of full automatic submerged arc welding equipment is welded. It is important that for skelp end weld, copper back plate should be used not to subjected to cracking and detrimental effect of the atmosphere. After performing skelp end weld, both side of steel plate is grooved by using machining equipments. Before grooving, both side of steel plate is cut by using hardened steel knives. Because, coil sides is not suitable for weldind operation. First stage of spiral submerged arc welding, in the area where the plate is formed spirally is internal submerged arc welding. In tube manufacturing, formed plate is welded in double side whichare called interial and external weld. Espesially, in internal meld, determined parameters mostly affects the tube quality. If the operator doesn't care seriously, inadequate penetration and tendency for porosity will increase. Second stage of spiral submerged arc welding is external weld. It is different from internal weld and approximately 2m. after, it is performed. In terms of penetration, operators give special care to external weld. Because of process, the area, where the external weld is performed, joint design is butt weld. Both of internal and external weld are used in performing multiple arc process. It varies dependent to plate thickness. Generally first welding head is powered by using DC-positive polarity because of the fact that first head gives the adequate penetration to bead. Other trail arcs are powered AC-current type. So, bead appearence and external shape of bead will bebetter. After finishing the tube production, last stage of production is that produced tubes iscut in required length. In tube industry for cutting tubes without stopping the production, special plasma-cutting equipments are used. In this equipment, cutting torch travel speed equalsto tube travel speed. So, without stopping the production tubes can be cut. For correct cutting plasma equipments should arrange properly and operator should give special care. In turkey, main producer of steel plates which are used in tube production is Ereğli Demir Çelik company. Generally, plates have 1500mm. length and 15-20 tonnes weight. Steel plates generally, are classified in dependent their mechanical properties. There are alot of standard foundations in around the world. Most popular of this standards is API (American Petroleum Instutie). According to API steel plates are classified by using its yield strength in Psi as Grade A, Grade B X42, X52, X60, X65, X70. For example, herein yiekl sterength X60 is 60 000 psi. Produced tubes are used in various fields in industry. For example, pile in foundation of harbours and buildings, severage and water line in, cities, natural gas and petroleum lines and etc. All tubes are designed in accordance with its using conditions. In accordance with standards, all tubes are examined by using destructive and non-destructive process. Tensile test, hardness test, bending test and notch-break Xll test are destructive tests. In tube industry, for examination non- destructive of tube, ultrasonic lamination, side lamination, ultrasonic weld control flouroscopic x-ray control, magnetic particul control are performed. Specially, with performing non-destructive technics, weld bead is examined %100. Ultrasonic examiration process, generally, means that, ultrasonic vawes are sent to material by using of proper probe and if there are any imperfections, inclusions or any kind of defects, these vawes comes back to probe and electronic circuit determines this difference. In structure of probe, there is a piezo crystal. This crystal converts the electrical vawes to ultrasonic vawes. Generally, these vawes if there is not any imperfection inthe steel plate, reflects from ather surface and come back probe again. If there is a differencebetween sent and came back vawes, electronic circuit determines this difference. Flouroscopic examiration is that X-rays is sent to material and passed X-rays form inside of the material shows the defects or imperfections in the screen. In spiral tube production using submerged arc welding, four wain defect are encountered in weld. These are porosity, crackins undercut and in adequate penetration. The most common cause of porosity is the presence of organic materials or ather gas producing contaminants in the joint. Other causes are contamination in wire, insufficient flux, contaminants influx, entrappet flux, segregation, inappropriate polarity. For preventing porosity; DC-positive polarity should be used, welding voltage can be increased, travel speed should be decreased before starting to weld, joints should be cleared and contaminants from flux should be seperated. Weld cracks mostly are seen in welding of thicker plates. Because, cooling time period is shorterhan thinner plates. Specially, in lower ambition temperature, tendency for cracking increases. However; opening root, wrong, polarity, using wrong electrode in first pass may cause in cracking. Preventing cracking, welding voltage and current should be adjusted correctly, negatif polarity should be selected, plate should be preheated and electrode size should be increased. Undercuts mostly are seen if high travel speed, high welding current or hoth are applied. Because of notch effect, undercuts is not appreciated by standarts. Preventing the undercut, negative polarity should be used in, electrode size should be increased, travel speed and welding current should be decreased. Inadequate penetration may happen in using lower welding current and higher travel speed. In addition to these, wrong polarity and joint design cause in undercut. To prevent the undercut, DC-positive polarity should be used or joint groove angle should be increased. xiu In this thesis below mentioned experimental workings and researchs was performed: -Changes of weld bead geometry, with joint angle. -Changes of weld bead geometry and microstructures, with travel speed. -Changes of weld bead geometry, with curret type. -Research of weld microstructure in skelp end weld. As mentioned before, joint angle affects joint geometry excessively. Especially, with good design of joint, required penetration can be obtained without increasing welding current and decreasing welding voltage. In this experimental working, five different joint angle (45°,60o,75o,90o,105°) were prepared in specimens by machining in the workshop. Afterwards, determined parameters were kept fix with wing of fullautomatic subwerged arc welding joint which was grooved the angle was welded. After welding operation specimens were taken from experiment plates for macro inspection and measuring the weld geometry. Finally, it was evaluated, when joint angle was increased, width of the bead shortened, depth of penetration increased. However, in 105° ansle, depth of penetration is high, it is unnecessary. Because, in macro appearence of 45° specimen, depth of penetration is adequate. As a result of this experiment, 45° joint angle is suitable for this kind of application. On the contrary, consumption of filler material and flux will increase. But, for polyethilen wrapped tubes for minimizing polyethilen consumption, joint angle can be selected 60° or 75°. Secondly performed experiment is reserch of the effect of the travel speed. During the production, determined parameters were applied and kept fix. Just travel speed was changed. Experiment was performed two different tube size (1016x9,52mm.-508x5,6mm.). After production, specimen was taken from tubes and machined in workshop for evanlating macro and micro structures and seeing bead geometry. In macro specimens, it was measured when the travel speed was increased, width of weld bead and depth of penetration was increased. But in highest valve of speed, depth of penetration and width of weld bead was apprecible. As a result of this evaluation, interms of production efficiency, highest speed which was given to machine was recorded as a normal production speed. In micro specimens, it was evaluted that given speeds didn't create any difference in welding zones (HAZ, internal weld and external weld). Once more, highest travel speed was recorded as a normal production speed. It was noticed that in HAZ and externa] weld widmanstatten ferrit and perlitic structures were seen. But in internal weld, equiaxis ferrit and perlitic structures were seen, because of the fact that internal weld performs about 2m. back from external weld. During external weld, heat input normalizes the microstructures of the internal weld. Another experiment includes in change of bead geometry, when the current type changes. In this experiment while all parameters was keeping fix, current type of XIV the external weld was converted from DC to AC. After, specimens was taken from tubes and macro specimens was prepared. In macro evalution, it is seen that depth of penetration and width of weld bead was not appreciable in AC weld bead. But, when DC in first head and AC in second head were used, best results was taken in depth of penetration and width of weld bead. Finally, for production of spiral welded tube, first head DC positive and trail arc's AC current type is correct application. Last experiment includes in research of micro structres of skelp end weld. Any tube which has skelp end weld was selected and skelp end weld was taken from tube. After, intersection zones of spiral weld and skelp end weld were prepared for microinspection. In micro inspection, a complex micro structure was seen. Micro structures include in coarse grain size ferrit and composition of widmanstatten ferrit and in semiantomatik submerged arc welding (external skelp end weld) was determined bainitic structures because of short cooling time conditions.