Doğal Gaz Boru Hatları İçin Yüksek Gerilimli Kaynak Ana Malzemesi Teknolojisinin Geliştirilmesi (borkay)

Abstract

Birincil enerji kaynağı olan doğal gaz ve petrole duyulan ihtiyaç her geçen gün artmaktadır. Artan bu ihtiyacı güvenli, hızlı ve ekonomik bir şekilde karşılamak için yüksek gerilimli çeliklerden yapılan (örneğin X80), büyük çaplı (örneğin 1422 mm, 56 inç), kalın duvarlı (örneğin 22 mm) ve kaynak edilmesi yüksek teknoloji ve sarf malzemesi gerektiren boruların kullanılacağı yeni doğal gaz boru hatlarının yapımı planlanmaktadır. Bu tür boru hatlarının dünyanın birçok bölgesinde giderek artan örnekleri görüldüğü gibi, ülkemizde de NABUCCO doğal gaz boru hattı bunlardan birisidir. Yeni doğal gaz boru hattı projelerinde kullanılan çeliklerin kalitesi ve mekanik dayanım değerleri gittikçe artmaktadır. Artan kalite ve dayanım değerleri parça/maliyetinde küçük bir miktar artışa neden olsa da azalan boru kalınlığı ve buna bağlı olarak daha az kaynak sarf malzemesi, kaynak süresi ve boru taşıma maliyetlerinden dolayı toplam proje maliyetlerini azalmaktadır. Bunlara ek olarak daha yüksek basınçlarda çalışma imkânı sağladıklarından gaz taşıma verimliliğini de arttırmaktadırlar. Bu nedenlerle günümüzde X52 ve X60 gibi akma değerleri düşük olan çeliklerin yerine, yüksek gerilimli X70 ya da X80 çelik teknolojileri kullanılmaktadır. Dünyanın bazı bölgelerinde doğal gaz boru hatları proje çalışmalarında X100 ve X120 tipi çeliklerinde kullanılabileceği araştırılmıştır. Bunların başında ExxonMobil’ in Alaska Pipeline projesi için X120 tipi çelik boru hatları için kapsamlı bir çalışma yapmıştır. Bu gelişmelerin yanı sıra, ülkemizden geçmesi planlanan 3300 km‘lik NABUCCO doğal gaz boru hattı projesinde 1422 mm çapında ve yaklaşık 20-22 mm et kalınlığında, X80 kalitede çeliklerin kullanılması da ön görülmektedir. Bu tür yüksek gerilime sahip ve yüksek iç basınç altında çalışması gerekecek çelik borularının üretimi (tandem ya da çift telli toz altı kaynak teknolojisi) ve daha sonra birbirleri ile birleştirme (elektrot, gaz altı, orbital otomasyon) kaynaklarında kullanılan kaynak sarf malzemelerinin (elektrodlar, masif teller ve özlü teller) kaynak yapılan çeliklerden daha yüksek akma ve kopma gerilimlerine sahip olması gerekmektedir. Bu mekanik özelliklerinin yanı sıra, bazı proje ve uygulamalarda kaynak bölgesi -60°C derecelere varan düşük sıcaklıklarda yüksek kırılma tokluğuna sahip olması gerekmektedir. Bu tür yüksek mekanik özelliklere sahip olan kaynak ürünlerinin gerektirdiği teknoloji ve metalürjik bilgilerin acil olarak ülkemizde geliştirilmesi ülke sanayisi ve ekonomisi için öncelikli konu teşkil etmektedir. Günümüzde kullanılan kaynak elektrot ve tellerinin kimyasal bileşimlerini yeniden düzenlemek, var olanları geliştirmek (örneğin Ni, Ti, B vs miktarlarında hassas dengeleri kurarak) gerekmektedir. Kısa adı Borkay olan bu projede, şu anda Türkiye’de üretilemeyen yüksek gerilimli kaynak elektrotlarının, masif tellerinin ve özlü tellerinin gerektirdiği teknolojik ve bilimsel bilgiye ulaşmak istenmektedir. Avrupa standardı EN 10208/2 ve Amerikan standardı API 1104 kaynak bölgesinde kullanılan çelikten daha yüksek akma gerilimi istemektedir. Bunun yanı sıra düşük sıcaklıklarda yüksek kırılma tokluk değerlerinde kaynak gerekmektedir. Yapılan araştırmada düşük karbonlu (<0.1%) ve Si, Mn, Ni, Mo ve çok az miktarda Ti ihtiva eden kaynak elektrot, masif tel ve özlü tellerinin karmaşık kimyasal birleşiminin geliştirilmesi amaçlanmıştır. Yapılan deneyler metalurjik mikroyapı incelemeleri, tahribatsız muayene (Radoyragfik Muayene) ve çeşitli sıcaklıklarda yapılan mekanik deneylerle (Çekme deneyi, eğme deneyi ve Charpy-V) kontrol edilerek standartlara ve proje hedeflerine uygun olup olmadığı belirlenmiştir. Yapılacak deneylerde elektrik ark kaynağı, tozaltı kaynağı ve gazaltı kaynağı olmak üzere üç ana kaynak türü kullanılması hedeflenmiştir. Projenin bu tez çalışma aşamasında sadece elektrik ark ve gazaltı kaynakları yapılmıştır. Seçilen kaynak sarfları API 1104 standartlarında tavsiye edilen elektrotlardan, masif tellerden ve özlü tellerden oluşmaktadır. Seçilen elektrotlar AWS A5.5 standardına, masif teller AWS A5.17 ile ASW A5.23 standartlarına, özlü teller ise AWS A5.20 ile AWS A5.29 standartlarına uygundur. Yapılan kaynak deneyleri ile çeşitli miktarda element içeren elektrot, masif tel ve özlü tellerin kaynaklanabilirlikleri, mekanik ve mikroyapısal özellikleri birbirleri ile karşılaştırılmıştır. Böylece her bir alaşım elementin kaynak bölgesi üzerindeki etkileri saptanmıştır. Daha sonra bu veriler değerlendirilerek yeni özlü tel tasarımları yapılmıştır. Yapılan üç özlü tel tasarımından bir tanesi istenilen mekanik değerlere ulaşmışarak başarılı olmuştur.

Demand to the natural gas and petroleum are increasing gradually. To fulfill this energy requirements with the greatest possible safety and efficiency, natural gas line pipes, which are highly stressed (e.g. X80), large diameter (e.g. 1422mm, 56”), thick wall (e.g.22 mm) and requiring high – tech and consumable for welding are planned to use. Number of examples of such pipelines can be seen al around the world and NABUCCO pipeline, which is going through the Turkey, will be one example to these. Transporting oil and gas through long distance by using pipeline with high pressure and large diameter is an economical way. In this case, pipeline steels require high performance, high strength and toughness, thicker and larger to guarantee a safe and reliable service at such high pressure, as well as to reduce the cost of construction and operation for a pipeline project. For this reason, it is possible to use high strength materials like X80, having a further reduced carbon content and thereby excellent field weldability. The use of X80 steel pipes for pipeline construction started on a trial basis as early as 1985 and is increasingly using in large diameter pipeline construction worldwide and it may be expected it should soon be recognized as an engineering standard material for pipeline construction in the same manner as X70 was 10 years ago. Mechanical strength and grade of the steels used in natural gas line pipes are increasing progressively. In spite of the fact that the higher quality and strength values cause a small increase in price/ton, reduction in pipe wall thickness, and thus, lower consumable consumption, shorter weld time and lower pipe transportation cost lead to reduction in overall cost of the construction. In addition to that, high strength steels make possible to operate pipelines with higher pressure resulting an increase in gas transportation efficiency. Therefore, X80 and X100 grade high stressed steels are preferred in natural gas and crude oil pipelines instead of conventional grades such as X52 and X65. It has been studied that, X100 and X120 grade steels can be used in pipeline projects in some districts around the world. Furthermore, for ExxonMobil’s Alaska Pipeline project, an extensive research were conducted for X120 grade pipelines. In addition to these, 1422mm diameter, 20 – 22mm thick X80 grade steels are anticipated to use in 3300 km long NABUCCO gas pipeline project which will run via Turkey. Welding consumables (electrodes and wires) for the production (tandem or dual torch sub merged arc welding technology) and at the substantial girth welding operations of these kind of highly stressed steels, which are going to work under high internal pressure, should have higher tensile and yield strength than the base material. Additionally, in some projects and applications, welding zone should have high fracture toughness at low temperatures such as -60ºC. The immediate development of required welding technologies and metallurgical knowledge in our country for such products constitute great importance for national industry and economy. It is needed that to arrange chemical composition of present welding electrodes and wires (e.g. by balancing the Ni, Ti, B addings). Especially within the following decade, consideration of the number of natural gas and crude oil pipelines which is planned to pass through Turkey, it is a must for Turkish industry to develop required technology and supply the product to enter such projects and market. This study focused on girth welding of X80 pipes. Various electrodes have been studied to have perfect weldability. Mechanical and microstructural analyses have been perfomed on parts welded. Notch impact toughness of welded parts have been studied and the relation among microstructure and mechanical properties were investigated. The project, which acronym is Borkay, is aimed at reaching to the required technology and knowledge for high stressed welding electrodes and wires that are not being produced in Turkey at the present time. European norm EN 10208/2 and American Standard API 1104 requires higher yield strength value from the pipe welding zone. Additionally, high fracture toughness at low temperature is needed from the weld metal. With this research complex chemical composition of low carbon (<0.1%) and Si, Mn, Ni, Mo, and too few amount of Ti containing welding electrode and flux wire developments are investigated. Projected values of weld zone conformation according to standats and project goals by metallurgical microstructure inspection, NDT (Radyography Inspection) and mechanical tests at various temperatures (Tensile Test, Bending Test, Charpy-V) are investigated. In this study, selected welding consumables which are not produced in Turkey and used to weld for X80 pipe steels have API 1104 and AWS standards. These welding consumables’s weldability, mechanical and chemical properties are examined. To do that, development of welding consumables is aimed. In this experimental study, pipes made of X80 steel which have 22mm thickness were used. Test pieces were cut from welded plates of 600 mm (width) ×450 mm (length) ×22mm (thickness). This base metal with a 60º V-shaped groove was used and multipass (8 passes) welding procedure was carried out in flat position with the interpass temperature of about 150ºC. To constitute an experimental design, welding consumables which have API 1104 and AWS standards are selected. Welding experiments are applied according to experimental design. As three main welding methods, which are electrical arc welding, submerged metal arc welding, and gas metal arc welding was aimed to apply. At this part of the project electrical arc welding and gas metal arc welding are applied. With different diameter electrodes, welding wires, and flux-cored wires were used to deposited the X80’s weld beads. Electrodes, welding wires, and flux cored wires, which mechanical properties are suitable for X80 pipes have chosen according to the AWS A5.5, AWS A5.17, AWS A.23, AWS A5.20 and AWS A5.29 requested in API 1104. Electrodes, welding wires and flux-cored wires with different amount of alloying elements are used in experiments. Their weldability and mechanical properties are tested and welding metal microstructures are examined. In this way, each alloying element’s effects of welding structure are researched. Then these informations are used to develope new flux-cored wire for high strength pipeline steels. Three flux-cored wires are designed for the project. One of them has enough mechanical properties to weld X80 pipeline steels. Radiography test is applied to welded specimens after welding experiments. Radiography test results are evaluated according to API 1104 standard. Weld seams radiography test result are agreeable to API 1104. After radiography tests, nick break test which is a destructive test is applied to welded specimens. Nick break test specimens are prepared according to API 1104 standard and nick break test is applied and appreciated according to API 1104 standard. Most of nick break test results of specimens are not suitable for API 1104 whereas, used welding consumables in this study are produced according to AWS standard and used at the present. This case is a proof of human factor has an important effect to welding defect. Human factor causes welding defects in this study. Tensile and fracture toughness tests are applied to welded specimens for define mechanical properties of weld seam. Tensile test specimens are prepared according to API 1104 standard and tensile test is applied and appreciated according to API 1104 standard. Rupture point of welded specimen is requested from base metal according to API 1104. Some experimental group’s welded specimens rupture from base metal, and some experiment group’s welded specimens rupture from weld metal. Fracture toughness test specimens are prepared according to API 1104 standard and fracture toughness test is applied and appreciated according to API 1104 standard. The aim is achieving 47 J toughness at -60oC. A flux cored wire named as E81 T1-Ni1 H4 which has AWS A5.20 and AWS A5.29 standards, achieved 58 J toughness at -60ºC. Another flux cored wire named as E70 T-5 M J which has AWS A5.20 and AWS A5.29 standards, achieved 55 J toughness at -60ºC. Theese toughness values are satisfying project’s goal. These flux cored wire’s chemical compounds are considered while designing new flux cored wire. Chemical compounds of weld metal are determined by optical emission spectrometer. Influences of elements in weld metal to mechanical and microstructural properties are examined. It has seen that Ni can be used to improve the impact toughness of weld metals especially at low temperatures. Ni also improves tensile strength but it causes decrease yield strength. If Mn content in the weld metal is high, it causes low toughness values, after all Mn is improving tensile strength. To achieve high toughness at low temperature, Carbon content must also be decrease. But, Mn and C increase yield strength, so it is important to make a good contact between toughness and tensile strength. Besides, decreasing the grain size increases the toughness of steel, i.e. decreases the ductile-brittle transition temperature (DBTT). Mo decreases the grain size which cause increases the toughness of weld metal. Mo also improves yield and tensile strength. Microstructural examination was carried out on cross section of the weldments. The specimens were mounted later flatted and then grounded using grinding discs with grit ranges from 220 to 1200. Then the sample were then polished using 6 &#956;m, 3 &#956;m, and 1 &#956;m diamond suspansion. Samples were then washed, cleaned by alcohol and then dried, followed by etching with 3 % Nithal solution. SEM examination samples were performed using a JEOL JEOL JSM-7000F. In this study, microstructure of weld metal is examined for existence of martensite microstructure whether or not. Microstructures of weldmetal and heat affected zone are examined by using scanning electron microscope. Martensite microstructers are unwanted microstructures to reach high toughness at low temperatures. Ferritic microstructures are observed for all weld specimens. Ferritic microstructures are best microstructures to reach high toughness at low temperatures. Heat affected zones of weld specimens are also have ferritic microstructure. To improve flux cored wire’s mechanical properties, experimental results are appreciated. First design has (chemical compound of flux cored wire %; C:0,05 / Si:0,55 / Mn:1.35 / Mo:0,2 / Ni:0,07) 540 MPa tensile strength. That value don’t provide X80 steel’s mechanical properties. Because of this result, Mn, Ni, and Mo elements are added to second design of flux cored wire. Second design of flux cored wire (chemical compound of flux cored wire %; C:0,05 / Si:0,55 / Mn:1.6 / Mo:0,35 / Ni:0,7) 711 MPa tensile strength and 620 MPa yield strength. Second design has also has 52 J at -60oC which is bigger than project’s goal. Second design has successfully provides required mechanical properties for X80 steel. C, Mn, and Ni elements are added for third design to more improve tensile strength of flux cored wire. Tensile strength is improved but toughness is decreased. Third design (chemical compound of third flux cored wire %; C:0,08 / Si:0,56 / Mn:1,7 / Mo:0,32 / Ni:0,9) is failed to reached mechanical requirements of X80 steel.