Gemi inşaatında kaynaktan dolayı meydana gelen deformasyonları minimuma indirecek kaynaklı dizayn
Distortions and distortion fairing with a gas burning torch for correction distortion in a welded ship
- Tez No: 39605
- Danışmanlar: DOÇ.DR. DEMİR SİNDEL
- Tez Türü: Yüksek Lisans
- Konular: Gemi Mühendisliği, Marine Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 230
Özet
ÖZET Bir metalik malzeme önce ısıtılıp, sonra da soğutulursa düzgün ve serbest bir uzama ve kendini çekme oluşur. Bu fiziksel kural, yalnız hacim bakımından serbest hareket edebilen ve düzenli olarak ısıtılıp soğuyan parçalar için geçerlidir. Diğer taraftan bu ideal durum kaynak sırasında söz konusu olmadığından, kendini çekme ve büzülme yani distorsiyon problemleri ortaya çıkar. Kaynak işlemi sırasında oluşan kaynak gerilmelerinin ve çarpılmaların meydana gelmesine sebep olan üç ana faktör vardır. Bunlar tavlama gerilmeleri, konstrüksiyonun rijitliği ve malzemenin metalurjik durumudur. Bir kaynak işleminde, işlemin özelliklerine yeteri derecede dikkat edilmezse, bunun neticesi olarak büyük çapta kendini çekmeler, çarpılmalar ve gerilmeler oluşur. Bu distorsiyonların da kaynaktan sonra ortadan kaldırılması için büyük masraflara neden olan doğrultma ve düzeltme işlemine gerek duyulur. Doğrultma işlemleri zaman kaybına sebebiyet verdiği gibi, parçada bir çok yeni gerilmelerin ortaya çıkmasına sebep olur. Burada, bir tersanede kaynak yapılan bir gemiye ait perdenin kaynak ve düzeltme işlemleri için sarfedil'en kamanı örnek olarak alırsak, alın bağlantılarının ve takviyelerin kaynağının 22 saat ve bunlara ait distorsiyon düzeltme işi 21.5 saat sürdüğünü görürüz. Bu da bize doğrultma ve düzeltme için sarf edilen zamanın, kaynak için harcanan zamana hemen hemen eşit olduğunu gösterir. Kaynaktan sonra meydana gelen distorsiyonun minimuma indirilmesi için, uygun bir kaynak planının hazırlanması ve düzeltme içinde gerekli sürenin kaynak zamanının AlO ila %20 arasında tutulması gerekir. Uygulamada bu sınırın üzerine çıkılmaması istenir. Gemi konstrüksiyonlarında genellikle özel sistemlere şekil veren kapalı sistemlerde görülen ısıtılan şeritler veya kaynaklar birbirine göre farklı olarak bulunurlar. Bu nedenle yönleri ve büyüklükleri önceden bilinmeyen karmaşık gerilmeler meydana gelir. Kaynak sırası hataları, kaynak ağzı hazırlanmasında teknoloji hatası, hatalı konstrüksiyon ve sistem elemanlarının dikkatsizce birleştirilmeleri gibi hatalı kaynak işlemleri nedeniyle bu gerilmeler üst üste toplanabilir ve ciddi blok çarpılmalarına, konstrüksiyon bozulmalarına veya kaynak çatlamalarına sebep olur. Kaynak çatlakları meydana gelmese bile bu gerilmeler konstrüksiyonun rij itliğini yenip sistemi deforme eder.Bu çalışmada, özellikle atelye bazında çalışma yapan Gemi İnşaatı Mühendislerinin teknenin inşaası sırasında sık sık karşılaştıkları blok çarpılmaları ve konstrüksiyon bozulmalarına karşı, gemi konstrüksiyonunun kaynağı tamamlandıktan sonra tav şaluması vasıtasıyla termik metodlarla deformasyon doğrultmalarının yolu ve şartları incelenmiştir. Gemi klaslama müesseselerinin termik olarak doğrultulmalarına müsade ettiği ¥.25 karbonlu çeliklerden yapılacak konstrüksiyonlara uygulanacak doğrultma metodları ve işleminin önemi bu yüzden çok fazladır, çarpılmanın meydana gelişinin konstrüksiyon detaylarının doğru uygulanmasıyla önüne geçilebilineceği bir gerçektir. Bu çalışmada ayrıca tersanelerde uyulması gereken gemi konstrüksiyon kaynak planı ve gerilmeleri, çarpılmaları minimum düzeye indirgeyecek kaynaklı konstrüksiyon detayları ve pratik çözümleri önerilmiştir. XI
Özet (Çeviri)
DISTORTIONS AND DISTORTION FAIRING WITH A GAS BURNINIG TORCH FOR CORRECTION DISTORTION IN A WELDED SHIP SUMMARY This study explains the precuations which should be taken, from design to the completion of the fabrication in welding of the ship construction. The ship building engineers know that wrong welding influences may occur in welding as in any proses. These fault may arise from bad workmanship, incorrect procedures, wrong materials used, wrong direction of weld, wrong order of sequence etc... However distortion is probably the most powerful of the influences in ship building construction. There are fundamentally, two methods which may be used to ensure that the finished construction is free from distortion. 1) The fabrication is so designed and so welded that its shape and dimensions will be within the specialied tolerances without further treatment, 2) The fabrication is allowed to distort and after welding, the distortion is suitable corrected. DISTORTION When metal is heated it will expand, and when it is cooled it will contract. This expansion and contraction may set up severe internal stresses and may deform the part being welded. If a weld is deposited lengthwise along a stripe of steel, the strip will bow upward at both ends as the weld cools. This is due XIIto the contraction of the weld reinforcement above the plate surface. Weld beads that are small, or that have deep penetration and are flat, will not cause as much deformation as those that are convex. By depositing more weld beads on the opposite side the strip can be brought back to its original form. If two plates are butt welded together and they are free to move, they will be drawn together at the opposite end from that being welded. This is known as transverse contraction. Transverse contraction can be controlled. If the seam to be welded is not very long it may be tack welded at the opposite end. If the seam is long it may have to be tacked in several places. Long seams may also be controlled by the use of clamping devices and wedges. The wedge is advanced along the seam with the welding operation. Clamping devices help to keep the plates on the same plane. Pre-spacing is another means of controlling expansion and contraction. The amount of spacing depends upon a number of variables such as length of seam, thickness of plate, speed of welding etc. In welding beveled edges of thick plates such as single V-butt joints and U groove joints it will be found that the plates will be pulled out of line with each other. This is so because the openning at the top of the groove is greater than at the bottom, resulting in more weld metal hence more contraction. The greater the number of passes the greater will be this wrapping, which can be counteracted by setting up plates before welding so that they bow in the opposite direction. This is not always possible, however, and the use of various clamping devices may be used. XIIIDESIGN AND PROCEDURES FOR REDUCING DISTORTION In order to develop effective means for reducing distortion, it is essential to understand how distortion is formed and how they are affected by design and welding procedure. Consequently, some of the discussions given in the preceding sections should be useful in determining appropriate designs and procedures. The preceding questions and answer section is useful for reducing distortion. CONTROL OF DISTORTION DESIGN The most economic design for a welded fabrication is the one which requires the fewest number of parts and a minimum of voiding; such a design also assists in reducing distortion, and is to bo recommended on that account. The type of joint preperation is important, particularly for unrestrained butt welds, because the joint preperation dan influence the amount of angular distortion which will result from welding. Distortion cannot be controlled solely by good design and well-chosen joint details, but they serve to reduce the magnitude of the problem and make it easier during manifacture to apply practical measures to achieve the desired results. ASSEMBLY PROCEDURE Once it is accepted that distortion in some form of another is inevitable, it becomes necessary to consider how it can be controlled. Experience has shown that success may be achieved by adopting one or the other of two assembly methods: XIV1) By estimating the amount of distortion likely to occur during welding and then assembling the job with the members press to compensate for the distortion and, 2) By assembling the job so that it is nominally correct before welding and by employing some form of restraint to minimize the welding distortion after welding. The first method is attractive because the parts have almost complete freedom to move during welding and there will be less residual stress than with the second method. However it is a difficult method to apply, except on relatively simple fabrication. For this reason, it is usual to break down the work into sub assemblies that can be welded without restraint, when dealing with fabrications comprising a large number of parts. The subassemblies are then assembled together and welded to complete the job, but often this final welding has to be carried out under conditions of restraint. The second or restrained assembly method is the one more generally preffered because of its comparative simplicity. The restraint may be applied by clamps, the use of fixtures, or simply by adequate tack welding. While this method minimizes distortion, it can result in high residual stresses. ' These high residual stresses and the risk of cracking can often be reduced by the use of a suitable welding sequence and, with heavy sections, by preheating. Where service requirements demand the removal of resudial stresses, a stress relieving heat treatment must be applied after welding. DISTORTION PREVENTION Good design should ensure as few welded joints as possible in a structure, particularly when it is made up of thin section plate. XVWhere they exist, welded joints should be accessible, preferably for downhand welding. The edge preperation of joints can be arranged to reduce distortion. For example a single V preperation joint with four runs of welding will distort a double V preperation joint welded with for runs will only exhibit slight shrinkage of the joined plates. Restraint is the usual method of distortion prevention in shipbuilding. Where units are faired ready for welding they are tack welded to hold them in place during welding. The parts will then remain dimensionally correct and rijidity of the structure will usually restrain any distortion. Strongbacks or clamping arrengements are also used on butt and fillet welds. All welds“shrink”, so the use of the correct procedure in welding can do much to reduce distortion. The fewer runs involved in a welding joint the less will be the distortion. Symetrical welding either side of a joint with a double V preperation willproduce a distortion - free weld. Simultaneous welding by two operators are therefore a useful technique which should be practised whenever possible. Welding should always take place towards the free or unrestrained end of a joint. For long welding runs several techniques are used to minimize distortion one of them is the backstep method. Here the operator welds the joint in sections in the numerical order and direction shown. A variation of this is“skip”welding, and likewise progresses in the numerical order and direction shown. Distortion may then be controlled by balancing the welding as much as possible and allowing the weld shrinkage to occur freely. Welding sequencestaking this into account should be well thought our before welding commences. XVICORRECTION OF DISTORTION, It is not always possible to control distortion within acceptable limits, especially when dealing for the first time with a new design or fabrication. There are bound to be instances where, despite the most careful planning, the distortion becomes excessive. In such circumstances, it is usually possible to rectify the trouble by one of the following methods. Distortion can be removed by producing adequate plastic deformation in the distorted member of section. The required amount of plastic deformation can be obtained by the thermal or mechanical methods discussed in the following paragraphs: 1) Thermal Straightening: Thermal or flame straightening has been used most successfully in the shipbuilding industry to remove distortion. The area to be straightened is heated to between 595 - 65 0 °C and then quenched with a water spray; repeated applications of heat in specific areas in a selected sequence or pattern are normally needed to straighten a distorted member or structure. The patterns are usually spot or linear heating tecniques. 2) Pressing: Distorted members can be straightened in a press if the members can be moved and if the pressis large enought handle them; heat may or may not be required for straightening. 3) Other Methods: Jacking is closely related to pressing in that distortion is removed by the application of pressure with or without added heat. Although its use is frowned upon, hammering of locally heated areas is sometimes used for straightening also. Flame straightening, which is relatively simple method and requires a minimum of equipment, has been widely used especially for structures in low carbon steel. However, there has been a concern XVIIover the possible material degradation when flame straightening is used for structures in high strength steels. XVIII
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