| X90 pipeline steel has broad application prospects in oil and gas pipeline construction with high toughness, low brittle ductile transition temperature and good corrosion resistance. Welding technology of pipeline steel is the key to pipeline manufacturing and laying in the construction field. During the welding procedure, thermal cycle have influence on the microstructure and chemical composition of the weld seam, which cause the properties of weld seam changed, affecting the service life of pipeline. Studying in welding technology of X90 pipeline steel is particularly necessary. Microstructure, mechanical properties, welding properties and ductile brittle transition temperature of X90 pipeline steel are studied in this paper, and then the influence of three kinds of welding process on microstructure, mechanical properties and corrosion resistance of welded joints are discussed. A thorough comparative analysis is carried out, and the reasonable welding process is put forward in most applications.X90 pipeline steel belongs to ultra-fine grain steel and the grain size is about 25 ?m, whose structure consists of polygonal ferrite, lath bainite, a small amount of granular bainite distributed in the ferrite matrix and M/A islands. The tensile strength of base metal can reach 760MPa, and its yield strength exceeds 680MPa. Impact absorbed energy is 296J at room temperature, and decreases with the decrease of temperature. When the temperature dropped to -40?, the impact absorbed energy reduced to 210J, micro porous aggregated in the fracture, there was a large number of tough nest and a small amount of NbC precipitated. Mechanical properties of X90 pipeline steel exceeded requirement of the API 5L for pipeline steel.The carbon equivalent of X90 pipeline steel is 0.51, and the welding property is general. When the preheat temperature is 130?, the surface crack rate and the section crack rate of the welded joint decreased from 27.95% and 58.28% to 0 respectively. Series impact test showed that the ductile brittle transition temperature of X90 pipeline steel was -80??90?, and the impact absorbed energy decreased with the decrease of temperature. The fracture presented tough rupture before -40? and brittle fracture tendency appeared at -60?. Impact absorbed energy reduced by 62J when the temperature is -60?, and the brittle fracture rate was more than 50%, dimples decreasing and the characteristics of the river fan pattern appeared.Fusion of Joints welded by three kinds of welding process was good, and nondestructive testing detected no defects. The grain size in the weld area is small. The microstructure of SMAW weld seam is mainly composed of massive ferrite and granular bainite. The microstructure of GMAW weld seam is composed of lath bainite, whose width is around 0.6?m, and M/A islands distributed in the matrix. The M/A islands is small, whose size is about 0.1 ?m. The microstructure of SAW weld seam mainly composed of acicular ferrite and a small amount of granular bainite. The grain size of CGHAZ coarsens seriously, and mainly consists of the lath bainite, granular bainite and M/A. The width of the laths increased to 1 ?m. Because of the fine M/A, granular bainite distributed in the soft ferrite matrix phase and lath bainite, the weld seam of X90 pipeline steel presented good strength and toughness. Carbon content of the fusion zone of joints is higher. The content of element Fe, Mn and Si is close to that of base metal, and the content of Ni, Cr and Mo are increased. Al oxide appeared in the SMAW fusion zone with rarely Nb content. Alloy element distribution of SMAW is uneven, and the weld quality is lower than GMAW and SAW.Mechanical tests showed that the tensile strength of welded joints of SMAW, GMAW and SAW are 790MPa,771MPa and 721MPa respectively, whose plasticity decreased from 25% to 20%, and the yield ratio is about 0.9. Impact absorbed energy of SAW and GMAW samples are about 40J higher than SMAW samples, decreasing with the decrease of temperature. When the temperature is -40?, the minimum impact absorbed energy is 141J. All fractures are ductile fracture with micro porous aggregated. Dimples and metal carbide particles appeared in the fracture of the section. When the weld seam was bending to 120 degrees, joint specimen of SMAW appeared crack, while the GMAW and SAW specimens were still found no defects under the bending of 150 degrees. The hardness distribution trend of root welding layer, filling layer and surface layer of three welded joints is consistent, and the hardness of surface layer is the highest. Hardness of weld zone is slightly lower than that of base metal, and the hardness of fusion zone appear trough value. Near the base metal side, hardness increased significantly after the fusion line.The hardness of SAW joints here is about 284HV and the hardness decreases when getting into the softening zone, and then gradually close to 255HV of base metal. Mechanical performance test shows that, in the same case, welding quality of SAW is slightly higher than GMAW, and SMAW is poor.Immersion corrosion test results of welded joint in different concentration or pH of HC1, NaOH and NaCl solution showed that the corrosion rate of the three kinds of welded joints increased with the increase of the concentration of the solution. The corrosion resistance of the welded joints of the three kinds of welding processes is as followed: GMAW>SAW> SMAW. The welding joints have better corrosion resistance to alkali environment, and the acid and electrolyte salts have obvious corrosion effect on the weld joint. Corrosion is close to the mild level at lower concentration or pH, while corrosion of SMAW welded joints in pH=1HCl solution and 5% NaCl solution can reach high level. Electrochemical corrosion tests show that the SMAW sample has the highest self-corrosion current and the lowest impedance, and the corrosion of three kinds of welding joints are high level, so it is recommended to take protective measures in strong electrolyte solution. |
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