Research On The Weldability Of Ultralow-Carbon Microalloyed X100Pipeline Steel

Actually, Oil and gas pipeline project is a large-scale and long welding welding molding installation. Thus, research on the Weldability of pipeline in their application is very important. Although China’s high-level pipeline has been born, live production has been mature, but there is still a big gap with the developed countries. Currently, there are not enough researches on the weldability of X100pipeline steel, study was not a lot of the welding processesneed further study. In this thesis, welding thermal simulation and actual welding technology are used to study the weldability of ultralow-carbon microalloyed X100pipeline steel. The main research contents are carried out in this thesis as follows:(1) By the experiment of simulated HAZ continuous cooling transformation, the SH-CCT curve of tested steel is measured. The influence of cooling rate on microstructure and hardness of CGHAZ and FGHAZ was obtained to reveal the phase transition of HAZ at different cooling rates. The results indicate that whether it is CGHAZ or FGHAZ, it can obtain the granular bainite structure in low cooling speed, especially when ts/s reaches600s, there will be the characterized by degenerate pearlite group. When the cooling rate raised to about10～20℃/s (t8/5to15-30s), the group mainly transform into bainite ferrite. In addition, with the decrease of the cooling rate, in the low cooling speed range, the microhardness of CGHAZ and FGHAZ values were both increased with the increase of cooling rate, but when reaching a certain cooling rate (t8/5cooling time is about30s), its hardness value almost kept at about325HV. However, compared to FGHAZ, CGHAZ improves the transformation start temperature and expands the phase transition temperature range at the same time.(2) To simulate different welding heat input on microstructure and toughness of CGHAZ impact experiment found that the hardness and impact toughness of CGHAZ of welding decreased with the increase of welding heat input. When t8/5is less than30s, the organization mainly for bainitic ferrite (BF) and lath martensite (LM), its hardness and toughness increased more than the parent material. When t8/5to85s, appears a large number of M-A components in the organization, the hardness and the toughness decrease a lot, showing the welding local softening and the embrittlement phenomenon. So, chosing the most appropriate heat input is important to ensure the welding joint toughness.(3) Simulation two thermal cycles on microstructure and toughness of CGHAZ impact experiment found that the hardness value and toughness of heat affected zone after the second thermal cycle are reduced compared to the first thermal cycle coarse grain zone (CGHAZ). The secondary peak temperature for600,900,1000,1200℃, the ductile damage degree is smaller; The secondary peak temperature of critical temperature700℃or800℃, the organization is given priority to granular bainite, though partial recrystallization happens during this time, but the microstructure is not refined,"chain" structure of the mesh tissue in group M-A element appear at the grain boundaries, resulting in X100HAZ toughness is poor, performance for critical coarse grained region (ICCGHAZ) local brittle.(4) To simulate preheat temperature on microstructure and toughness of CGHAZ experiment found that the welding CGHAZ toughness almost don’t reduce when welding line energy Q is1.5KJ/mm thermal input and the preheat temperature is not greater than150℃; When Q is3.0KJ/mm, preheating temperature exceeds150℃, X100pipeline steel toughness is significantly reduced. Therefore, from the view of prevention of welding cold crack, selecting100～150℃to preheat in X100pipeline steel welding process.(5) The welding cold crack sensitivity of X100is studied and forecasted by the theoretical method and the cold cracking experiment. Theoretical calculation of available experimental steel carbon equivalent CEN is0.2836%, the welding cold crack sensitivity index Pcm is0.21%, It shows that X100belongs to welding steel and has a low welding crack sensitivity, but there is still cold cracking tendency, It should be considered the appropriate preheating before welding; Inclined Y-shaped groove welding crack experiments show that X100steel has good crack resistance, Under low preheat temperature75℃can completely avoid the welding cold cracks; Welding HAZ maximum hardness experiments show that with the increase of energy and temperature, the highest hardness decreases. When the welding line energy Q is1.12KJ/mm and preheat temperature is20℃, welding HAZ maximum hardness is353HV>350HV, there also is a tendency to smaller cold cracking. Therefore, in order to avoid the occurrence of cold cracking, proper preheat and heat treatment after welding should be given.(6) By using the submerged arc welding and CO2arc welding, the welding procedure of X100pipeline steel has been researched systematically on the system. By tensile, impact, bending, hardness testing, experimental fracture morphology and microstructure observation of the welded joints, systematic analysis of the welding heat input, welding heat treatment on microstructure and properties of welded joints were studied to reveal the microscopic organizational strength and toughness of the joint and the experimental steel welding process adaptability. The experimental results show that when the line energy were1.65KJ/mm for submerged arc welding and it is1.17KJ/mm for CO2arc welding, after120℃preheat and post weld heat treatment, welded joint can obtain best tolerating match, the comprehensive mechanical properties and characteristics are in the optimal state.