Abstract:
International Maritime Organization (IMO) have regulated the emissions of SOx and NOx in Marine applications by setting up the emission control area. In addition with the aim of reducing greenhouse gas emissions, IMO have toughen the emission levels & regulations in ship building and operation. In present era, LNG is quickly being adopted as fuel source in the maritime world as it is a technically proven and a commercially viable solution for shipping today. So there is a need of Vacuum insulated Cryogenic tanks and its Welding challenges are most successful concept for LNG fueled Tank in shipbuilding. In Marine Applications, Stainless Steel Materials (SA 240 TYP 201LN / 304) has a wide application due to its higher Tensile Strength and good Impact Properties at lower temperature services and high corrosion resistance Welding of LNG Fuel Tanks has various challenges of obtaining High Tensile Strength and required Toughness values at lower temperature along with least Distortion which requires advanced Welding Technology. To fulfill above mentioned requirements, Plasma GTAW, FORCE GTAW, GMAW, SAW and Manual GTAW with Pulse Technology and Hot Wire GTAW has been implemented depending upon type of material which can minimize Heat Input during fabrication and Maximizing the production level with less welding defects, and enhanced Client satisfaction.
Keywords: IMO, GTAW, Hot Wire TIG , Cryogenic, Welding, LNG Fuel Tanks
1.0 INTRODUCTION
The present trends in fabrication industries are to explore suitable welding process to obtain Good quality cost effective welds. The foremost objective of fabrication industries is to develop better quality weld with least or no repair. Welding is most critical operation of any fabrication type of company, and quality of welding has direct impact on quality of final product. Joining technology is an integral part of the manufacturing process and effort has been spent to develop and demonstrate the suitability of various processes for different applications. 201LN is a low nickel austenitic stainless steel. Nitrogen and manganese used for nickel substitution Increase the yield strength of the grade without impairing its ductility.
The main properties of 201LN are high strength and excellent ductility, good resistance to intergranular corrosion in the as - welded condition, excellent fabrication properties, low temperature toughness and superior mechanical properties allowing its use for cryogenic applications. 201LN is austenitic in the solution annealed condition (1000 - 1100°C) (1832 - 2012°F).
It contains a small amount of ferrite. 201LN is stable against martensitic transformations induced by deformation at low temperature. 201LN will usually provide weight savings in most structural or pressure vessels Applications for Ships and Maritime Industries.
Liquefied Natural Gas (LNG) is natural gas that has been cooled down to liquid form for ease of storage and transport. When Natural gas is cooled to minimum -1620 C, it becomes a clear, odorless, colorless liquid. It is neither corrosive nor toxic. LNG takes up about 1/600th volume of natural gas. It is primarily methane with low concentration of other hydrocarbons, water, carbon-dioxide, oxygen, nitrogen and some Sulphur components. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of the above components. The remaining natural gas is primarily methane with small amount of other hydrocarbons.
LNG weighs half the weight of water so it will float if spilled on water. LNG is better than any other fossil fuel for the environment. The combustion of natural gas releases significantly less COx, NOx and SOx and virtually no ash or particulates. And as it evaporates rapidly when exposed to the air, it leaves no residue on water or soil.
Welding Stainless Steel Materials of SA 240 TYP 201LN / 304 for Marine Application has various challenges of obtaining High Tensile Strength and required Toughness values at lower temperature along with least Distortion which requires advanced Welding Technology.
The present work was to deals with study and development of the Welding of SA240 TYP 201LN by advanced Welding Technology like GTAW Pulse Waveform Technology and Hot Wire TIG and for Welding of SA 240 TYP 304, Force GTAW, Plasma GTAW and SAW process has been selected. For this purpose, different test piece were welded and analyzed by means of Radiography Examination, Tensile test, Bend Test, Macro Examination as well as Micro Examination with Optimized Welding Parameter and appropriate Methodology for Welding Sequence on the Job with least Distortion and Warpages.
2.0 CRYOGENIC LNG FUEL TANK
2.1 TANK CATGORIZATION
The International code of safety for ships using Gases or other low-flashpoint Fuels (IGF) categorizes tanks into two main types: Integral tanks & Independent tanks. The integral tanks are mainly of membrane type and the independent tanks can be further classified into three subcategories, which are referred to as type A, type B and type C
Cryogenic LNG Fuel Tank are categorised as Independent tank Type C, designed based on pressure vessel criteria in accordance with recognized codes supplemented by IMO IGF code, statutory, class rules and regulations. Type C tanks store LNG at higher pressures with minimized Boil of Gas for long periods of time then other Types A & B are “atmospheric” tanks which are subject to “boil off”. Vacuum insulated tanks are most successful concept for gas-fueled vessels.
A vacuum insulated tank in principle, embodiment of two vessels into one, where one vessel is installed inside the outer vessel which is called jacket, and a vacuum shall be applied in the annular space. With very low boil-off rates, the tank pressure can be easily maintained below the opening pressure of the safety valves for storage of volumes up to approx. 1000 m3.
3.0 MATERIALS AND METHODOLOGY
3.1 Materials and Filler Wire
Stainless Steel material SA 240 TYP 201LN is the base material to be used for the Inner Tank fabrication purpose and SA 240 TYP 304 for Outer Tank Fabrication. The chemical composition of the SA 240 TYP 201LN and SA 240 TYP 304 in as received from the supplier is given in Table 1.
The base material of SA 240 TYP 201LN and SA 240 TYP 304 was cut into plates by using Grinding method. It is to be note that prior to the start of welding process, the plates were cleaned and degreased in order to remove different forms of contaminants. Welding Filler wire used for Welding of SA 240 TYP 201LN was ER 307 and for welding of SA240 TYP 304 was ER 308LSi.
The mechanical properties of Base Metal have been shown in Table 2.
3.2 Methodology
3.2.1 Edge Preparation
Edge preparation is one of the important aspects as whole quality of weld mainly depends on this feature. Acetone were used to clean the surface of the area around 25 mm where the welding to be carried out. In current case, test piece are preheated around 60 degree C by Propane torch or burners (Oxy Acetylene Gas). Porosity is a problem confined to the weld metal arises from gas dissolved in the molten weld metal becoming trapped as its solidifies, thus forming bubbles in the solidified weld. Hence Argon (95%) + Hydrogen (5%) mixture cylinders are preferable for Shielding in Hot Wire TIG and Force GTAW Process and Single Mixture Argon as Shielding Gas for GTAW Pulse Waveform and SAW process.
3.2.2 Welding Machine and Parameters
The arc welding process which is utilized for development is of GTAW Pulse Waveform process was performed at the using ESAB AC/DC welding machine. Hot Wire TIG process was performed using Fronius TransTIG5000 main power source having 80% duty cycle and a TransTIG2000 hot-wire power source. Force TIG process was performed on EWM 1000 Amps DC Power source and SAW was performed using WARPP Make 1000 Amps DC Power Source.
Welding was carried out in a defined work place which is clean, maintained and isolated from oil, grease, dirt particles etc.
Initially a bead on plate trials was carried out to study the weld bead profile. During Welding double check of all Hoses and Connections of Gas have been checked by Soap Solution to prevent any leakage and entrapment of Pores in the Weldment. Welding Parameters for different Welding Process utilized for Marine Applications are mentioned in Table 3 & Table 4 and after completing the face sided welding, back chipping were done on root side so as to remove unwanted weld metal which is excess and unfused.
3.3 Mechanical Test Results
Two Transverse Tensile test samples were prepared as per NR 216 through NR 529 .The test was carried out using Universal Testing Machine at NABL Approved laboratory. Each tensile specimen was tested and fracture location along with the Ultimate Tensile Strength noted for test piece. Two Root Bend and Two Face Bend or 04 nos. Side Bends (as applicable) were carried out as per NR 216 through NR 529 and found satisfactory. Impact properties at -196 Degree Centigrade has been carried out as per NR 216 through NR 529 for all Welding Process as mentioned. For Macro Examination, a cross section of welded material is polished and etched for examination. Porosity, lack of weld penetration, lack of side wall fusion, poor weld profile and other important defects were checked by this examination.
For Microstructural Characterization all samples were cut in the middle of the weld because it is believed that it takes few seconds for welding process to be stable. All weld samples were ground and polished. Samples formed at high and low heat input were selected and examined for microstructural changes using optical microscopy. Prior the microstructural examination, the polished samples were etched by Keller’s Etch. All Mechanical test results with different Welding Process are mentioned in Table 5 & Table 6.
4.0 FABRICATIONS AND TESTING: CHALLENGES
• Plate, Pipes and Valves Material was procured from class Approved Manufacturer duly witnessed by class surveyor. Test Certificate of Mechanical / Chemical properties were in accordance with ISO 10204 type 3.2 certificate.
• Marking of the tank was accomplished by the use of low stress stamps that do not cause local stress raisers on the tank.
• Dimensions were stringently control then specified for Welding. Fixtures were fabricated before welding to maintain uniform shell roundness. This is critical to maintain constant welding arc length during automatic welding.
• Square ness of shell was critical to maintain uniform root gap & groove angle to achieve defect free welding.
• Production weld test were performed for each 50 m of butt weld joints representing each welding position and WPS.Charpy V notch impact test and Transverse weld tensile test were performed on Production test weld
• All butt welds were require radiography and dye penetrant test over entire weld length.
• Tank were hydrostatically pressure tested to not less than1.5 times the design pressure. Test pressures were maintained for at least 2 hours.
• Perlite Quality, Density and its filling weight as per drawing are critical to have effective thermal insulation and were monitored precisely .
• Evacuation & Heating drying process were performed to remove moisture & maintaining desire vacuum level.
4.1 CRITICAL INSPECTION
• For vacuum insulated Type C- LNG tanks, the inner tank as well as the annular space (between the inner tank and the outer vacuum jacket) were typically not accessible for inspection after assembly, any leakage after assembly were checked by conducting a helium leak test (Inner vessel under pressure and annular space under vacuum by tracer probe method).
• Final vacuum and its retention were verified as per procedure and were accepted less than 130 mTorr for perlite insulated tank above 100 KL.
5.0 WELDING CHALLENGES INVOLVED:
• Surface area were cleaned by power brush and acetone before welding and it was ensured that area were free from any dust, grease and oil particle.
• Welding was carried out in isolated area free form any carbon steel particles etc.
• Chemistry of weld consumable were carefully selected. Low carbon welding consumable were used.
• Prior to start of welding of 201 LN materials, it was preheated to 600C.
• Skill of the welder were deployed to maintain constant parameter for controlling weld repair, heat input and distortion.
6.0 RESULTS AND DISCUSSION
Following are the Results and Discussion observed after Welding Test Samples:
1. Hot Wire TIG helps to increase the deposition rate due to which number of layers have been reduced ultimately enhancing good mechanical properties for SA240 TYP 201LN Base Metal.
2. GTAW with Pulse Waveform Technology helps to reduce Heat Input which helps to attain required mechanical properties and less distortion in SA240 TYP 201LN Base Metal.
3. Force GTAW with 3X Welding Speed helps to increase productivity along with Weld Quality and required Mechanical Properties of SA 240 TYP 304 Base Metal.
4. SAW and Plasma +TIG process helps to increase the productivity along with minimum weld repairs for SA240 TYP 304 Base Metal.
5. Distortion and Warpages have also been controlled by maintaining appropriate steps during Welding.
7.0 CONCLUSION
The effect of Welding Parameters on the mechanical properties of the SA 240 TYP 201LN and SA240 TYP 304 Weld joints have been successfully investigated in these developmental trials.
Following Conclusions were drawn from trails:
1. Force GTAW & Plasma +TIG ensure full penetration with 3X Welding Speed and minimum Weld Repairs for SA240 TYP 304 Base Metal.
2. Use of Preheat Maintenance and Inter pass Temperature during Welding has successfully contributed to avoid Distortion and Warpages.
3. Preheating needed for the removal of the moisture and slowing down the cooling rate prior to Welding of SA240 TYP 201LN.
4. Hot Wire TIG and GTAW with Pulse Technology helps to maintain Heat Input and due to which desired mechanical Properties have been achieved for SA240 TYP 201LN Base Metal.
5. Cleanliness of Edge Preparation as well as proper storage of Filler Wire is extremely important in achieving low levels of Porosity
REFERENCES
1. Barbara K. Henon; ‘Advances in Automatic Hot-Wire GTAW (TIG) Welding’
2. G. A. Hatt; ‘Improving TIG Welding Productivity Using the Hot Wire Technique’; The Welding Institute Research Bulletin, August 1985
3. BV Marine NR 529 and NR 216 Standards & IGF Code
4. Keyhole process in K-TIG welding on 4 mm thick 304 stainless steelShuangLin Cuia, ZuMing Liua,b,*, Yue Xiao Fanga, Zhen Luoa,Sunusi Marwana Manladana,c, Song Yi
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