Kaynaklı bir gemide oluşan distorsiyonlar ve giderilme esasları

Abstract

Bir metalik malzeme ısıtıldığında genleşir, soğuduğunda ise kendini çeker. Eğer malzemenin durumu elastik sınırın ötesinde ise veya malzemeyi sınırlayan etkenler varsa distorsiyon oluşur. Distorsiyon oluşumun sebepleri ısı etkisi ve dış kuvvet etkisidir. Gemi inşaatında meydana gelen distorsiyonların hemen hemen hepsi kaynağın ısı etkisinden meydana gelir. Gemi konstrüksiyonunda karşılaşılan distorsiyonların ortadan kaldırılması için büyük masraflara neden olan doğrultma ve düzeltme işlemlerine gerek duyulur. Doğrultma işlemleri zaman kaybına neden olduğu gibi yeni yeni gerilmelerde doğurur. Doğrultma ve düzeltme işinde genellikle ısıl işlem uygulanır. Çalışmada, gemi inşaatı sırasında karşılaşılan distorsiyonların oluşum nedenleri, düzeltilmesi esasları incelenmiştir. Tersanelerde uygulanması gereken gemi konstrüksiyon kaynak planı ve gerilmeleri, çarpılmaları minumum düzeye indirgeyecek kaynaklı konstrüksiyon detaylarıda incelenmiş ve pratik çözümler önerilmiştir.

In selecting a process for production welding, ability consideration is the ability of the process to give the required quality at the lowest cost and to complete the work in a certain limited time. The steels used in shipbuilding are defined mainly by requirements on chemical composition, mechanical properties and is 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. Stell is expanded by heating and shrunken by cooling, also deformed by adding of force. If the state of material is beyond elastic limit or the restraint of material is not relieved, so called distortion occurs in material. The causes of distortion are roughly divided into the fallowing two classes. 1- Distortion by heating 2- Distortion by external force DISTORTION BY HEATING The distortion is grown by heat effect generally, Heating sources of the effect are gas cutting, electric welding, arc air gouging. etc. Particularly on present arc welding, the high.temperature of 2.000°c and above is generated and it becomes the cause of distortion in material. VI Almost of distortion in shipbuilding are caused by the heating effect af uielding. Especially the effect to thin plate of less rigidity is serious. Besides, the steel plate is locally heated by the above mentioned heating sources and the temperature differences appear in each direction of plate thickness, breadth and length (heating progress direction). Consequently a lack of uniformity occurs in the plate elongation and results in the distortion by plasticization. Sometimes no distortion appears at a glance but internal stresses remain in the steel members, in this case distortion occurs by a little heat input. If it appears on the surface of steel member as the deformation it is an abject of the fairing work, but if it is hidden inside as internal stresses, we have no means to take measures. Then we have to take proper measures whenever it appears as the distortion on surface. This kind of distortion is apt to appear on the thin plate structure of accommodation quarters of the ship. DISTORTION BY EXTERNAL FORCE The distortion is grown during assembly, erection and transportation processes of the steel members and hull blocks. The distortion described on distortion by heating is caused by the shrinkage (by cooling after elongation), but the distortion by external force is all caused by the elongation of steel members, and grown on the following working stages. i)- Turnover of hull block ii)- Storage of hull block (Mis-piling, Mis-arrangement of cribbing) iii)- Transportation of hull block iv)- Erection of hull block v)- Concaved damage by falling of steel member of tool vi)- Concaved damage of ships1 bottom due to mis- arrangement of cribbing vii)- Impact during docking and undocking Vll viii)- Concaved damage by water hammering etc. The causes of distortion have many varieties. The form of distortion is very irregular, then you have to take adequate measures according to the form of distortion on the fairing work. FORM DF DISTORTION The distortion caused by the heat is classified into the following groups according to the appearence a)- Sharp pointed deformation The distortion caused by fillet welding appears mainly on the costruction having face plate, that is built-up longitudinals transverse webs, etc. b)- Curved deformation The distortion appears as a whole of steel member and caused by the wrong sequence of welding and the difference of shrinkage between welding parts. c)- Panel deformation (skinny horse back) The distortion occurs by fillet welding, and appears throughout the ship, that is, on shell plating, skin plating of accommodation structure, etc. even though on the thicker plate. d)- Waved deformation The distortion is caused by the shrinkage difference of steel plate by heating and is apt to appear on the complicated hull block. It has always a special feature, that is curved with hump and hollow having inflection paints on frame lines. vm e)- Twisted deflection The distortion is caused by the imperfect restraint defective jigs, improper welding sequence, etc. at assembly stage. Also at the fairing work on hull block, if the work is carried out on the twisted block condition caused by the use of defective setting jigs, the twisted deflection is plasticized occasionally. f)- Knuckled deformation necessary to correct the distorted part by cutting off. Sufficient reinforcement such as strong back is important consequently we can prevent the growth Df knuckled deformation. DISTORTION REMOVAL The methods of the distortion removal are classified as follows: a)- Use of heat effect Use of heat effect is the most popular. It is a general saying that steel is shrunken after heating. It is true so far as we see the phenomena come to the surface, but we have to understand the process of that If not you will fail in the distortion removal by the misdirected heating, so it is necessary to pay attention, In case of the heat effect utilization, it is accompanied with the quality change of steel material because of an excessive heating on high temperature and water cooling. Then, it is necessary to consider the reduction of heat input aa for as possible in the distor tion removal. Especially in the distortion removed on IX high tensile steel and normalized steel, a severe heat control is necessary. The heating methods are classified as follows: 1- Line Heating Method The line heating method is the most papular method and a 9D % or more of the fairing work is carried out by this method. A knuckled deformation occurs on steel plate by line heating. 2- Spot Heating Method This is one of the heating method and the whole thickness of steel plate will be heated in the method. 3- Triangle Heating Method This method can be applied if you understand the line heating method, and is applied for the fairing and bending of angle bar and flat bar. k- Red-Hot Heating Method This method applied for an excessive deformation and different from the above mentioned line heating method. The deformations are unable to correct easily by line heating, such as an excessive, concave dent caused by impact, an concave dent caused by supporting fittings during hull block storage, a sharp concave dent appeared during turnover and erection of hull block etc. b)- Use of enternal force Correction force by proper physicgl tools is used together with line heating for fairing. Deformation sccurs on the steel plate convex to the heating surface side.- This is an inverse deformation against on expected knuckle. Accordingly when external force is applied to restrain the deformation, the transformation will occur. Namely the whole difference of elongation caused by temperature difference is absorbed by the inflation of steel plate and the amount of inflation is larger than the case without use of external force. In other words, the same amount of inflation is got by the use of enternal force with lesser heat input. Consequently the distortion induced in adjacent parts is lessen and the deterioration of steel material is prevented. 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. Where they exist, welded joints should be accessible, preferably for downhand welding. The edge preparation of joirats can be arranged to reduce distortion. For example A single ~\1 preparation joint with four runs of welding will distort A double-V perparation joint welded with four runs will only exhibit slight shrinkage of the joined plates. Restraint is the usual method of distortion preven tion 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 the rigidity of the structure will usually restrain any distortion. Strongbacks or clamping arrangements are also used on butt and fillet welds. Despite the most stringent methods to eliminate it, distortion can still occur. Where the distortion in a joint is considered unacceptable the joint must be gouged, grooved or completely split, and then rewelded. Strongbacks man be placed across the joint to restrain distortion during rewelding. Straightforward mechanical means may be used, such as hydraulic jacks or hammering on localised areas of distortion or buckling. Where such methods involve straining the welds, they should be examined for cracks after correction. Every effort should be made to avoid mechanically straightening structures far this reason. xi The application of concentrated heat fromaa gas- burning torch may be used for corrocting distortion in steels other than the higher tensile, quenched and tempered types. A small area is heated on the side where the contraction mould bring about an improvement. The steel is heated to a "red heat" and the torch slowly moved along a previously drawn line, at such a speed that the "redVheat" does not pass right through the material. The recrystallisation absorbs the expansion and, on cooling, conctraction occurs which brings about a favourable distortion, thus correcting the original distorted structure.