Examination of welding distortion via applying thermal tensioning technique on austenitic stainless steels

Abstract

Under the construction of German Mine Hunter Consortium (Abeking&Rasmussen and Lürssen), Istanbul Shipyard/Pendik is building Aydın Class Mine Countermeasures (MCM) Vessels for Turkish Navy. Aydın Class Mine Countermeasures (MCM) Vessels distinct from the other world navies because of their hull made of austenitic stainless steel. Nowadays, German, Italian and Canadian navies use this type of steel for ship hulls. Austenitic stainless steel has many advantages, like resistant to the corrosion of sea water, high yield point and non-magnetic structure. But these metals are having also disadvantages, like poor heat conductivity and high thermal expansivity. These disadvantages create more distortions than normal steels in welding conditions. Fittings of welded structure are very important section in shipbuilding, so geometry of welded structure need to be accurate. For straightening of distortions, flame straightening method is widely using in shipbuilding industry. Nowadays for quality products it is necessary to reduce price. Straightening of distortions via flame straightening method is more time, manpower and material consuming. So improving of straightening of distortions will be beneficial for budget of government. The aim of this thesis is to show benefits of usage of Thermal Tensioning Technique as a method of straightening of distortions for Aydın Class Mine Countermeasures (MCM) Vessels which are built at the Istanbul Shipyard/Pendik. Straightening of distortions via Thermal Tensioning Technique is examined for ship building industry as a preventing method with many academic researches. The stiffened panel made of austenitic stainless steel is used in the experiment of the Strathclyde University/ Scotland. In specific condition under gas protection weld (MIG), thermal camera is used for distribution of heat, thermo couples are used for temperature measurement and laser appliance is used for distortion measurements. This paper focuses on the transient temperature fields created by the large heat inputs from arc welding which is the driving force for distortion in the plates. The use of a thermal imaging camera allows the thermal fields to be viewed on a whole-field basis. The use of a welding rig in this kind of investigation is essential as it allows parameters to be kept constant throughout. The moveable track is controlled by stepper motors such that the travel speed of the weld can be adjusted to a tenth of a millimeter per second. The torch is held static while the plate is pulled along at a constant speed by a series of pulleys and small stepper motors. The welding machine was connected to the computer and so all parameters could be entered via the computer so that all the variables could be controlled. The camera was held in place with scaffolding and the tripod for the camera fixed to a bracket that allowed movement in all directions. The CCD thermal imaging camera used here was the ThermaCam SC500 from FLIR systems. The camera is able to provide thermo graphic images in the infrared spectrum from 3 to 12μm. Following initial tests with austenitic stainless steel plates the amount of background radiation reflected from the plate to the camera was seen to be high. This led to the painting of the plates with a high emissivity, two-part zinc silicate primer made up in two parts. To verify thermal images thermocouples were also present at distances of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100mm. The laser appliance is used for distortion measurements. All of austenitic stainless steel plates are scanned before welding and again after welding. The data was used to compare the distortion effects under different restraint conditions. The distortion analysis was carried out using an automated laser scanner which was programmed to cover the whole plate recording data at intervals throughout the scan. The surface deformation data obtained from Labview program was evaluated on the graphics. Austenitic stainless steel plates are tested under four conditions. These are free welding, restrained welding, restrained and additional heat input welding, free and additional heat input welding. As a result, the experiments in Strathclyde University proved that Thermal Tensioning Technique prevent buckling distortions and improves angular distortions on stiffened welded plates. Usage of Thermal Tensioning Technique for Aydın Class Mine Countermeasures (MCM) Vessels and also other type of ship building industry is additional benefit and assistance for economy.

Özet

Türk Deniz Kuvvetlerinin modernizasyonu kapsamında İstanbul Tersanesi Komutanlığı/Pendik’te, Alman Abeking&Rasmussen ve Lürssen firmalarının konsorsiyumu ile Aydın Sınıfı mayın avlama gemileri inşa edilmektedir. Aydın Sınıfı mayın avlama gemilerinin, genel olarak dünya bahriyesindeki mayın avlama gemilerinden farkı, bu sınıf gemi inşasında kullanılan ostenitik paslanmaz çeliklerdir. Hali hazırda bu çeliği, Alman, İtalyan ve Kanada donanmaları kullanmaktadırlar. Anılan çeliğin, mayın avlama gemilerindeki kullanım amacı, manyetik olmamaları, deniz suyu korozyonuna dayanımlı olmaları ve yüksek mukavemete sahip olmalarıdır. Ancak manyetik olmayan ostenitik paslanmaz çeliklerin, ısı iletimlerinin kötü, buna mukabil ısıl genleşmelerinin yüksek olması, bu çeliklerin kaynak sonrası distorsiyonlarının, diğer çeliklere oranla, daha fazla olmasına neden olmaktadır. Bu durum, kaynaklı parçaların montajları esnasında, birleştirme problemlerine yol açtığından, her kaynak işleminden sonra kaynaklı parçalar, alevle düzeltme işlemine tabi tutulmaktadır. Bu çalışmanın amacı, İstanbul Tersanesi Komutanlığı/Pendik’te imali devam eden Aydın Sınıfı mayın avlama gemilerinin kaynak işlemi sonrasında meydana gelen distorsiyonlarını en aza indirmek amaçlı, kaynak esnasında uygulanan ilave ısı girdisinin bir yöntem olarak uygulanabilirliğinin ispat edilmesidir. İskoçya Strathclyde Üniversitesi’nde yapılan deneyler sonucunda, kaynak esnasında parçaya ilave ısı girdisi verilerek, Isıl Gerilme Tekniği uygulanmasının, T kaynaklı parçalarda, ondüle şeklindeki (buckling) distorsiyonlarını önlediği ve açısal çarpılmaları azalttığı ispatlanmıştır.