| With the growing demand for energy in industry,the development direction of oil and gas transportation pipelines is developing towards high-grade steel,large-diameter,high-pressure,long-distance transportation and other technologies.The East China-Russia Natural Gas Pipeline is a long-distance natural gas pipeline with the largest pipe diameter and wall thickness,the largest unit throughput,and the highest pressure that has been built in China.For the first time,X80 grade welded pipe with an outer diameter of Φ1422mm was used on its trunk line.The extremely cold service environment on the Sino-Russian East Line caused a decrease in the toughness of the welds of the pipeline,which seriously affected the normal operation of the entire oil and gas pipeline.Therefore,how to control and improve the low temperature toughness of the weld has become a hot issue that needs to be solved urgently.This article starts with the technical performance requirements of X80 pipeline steel straight seam welded pipe and Based on the influence mechanism of alloy elements on weld performance and welding metallurgical characteristics,the submerged arc welding wire for X80 grade pipeline steel welded pipe suitable for extremely cold service environment was designed by controlling the alloy composition variable method.The Mn and Ni elements in submerged arc welding wire are designed respectively,and the microstructure,low temperature toughness,microhardness,tensile strength and nanoindentation characteristics of 5 groups of submerged arc welds are compared and analyzed.The interaction mechanism between the content of Mn and Ni on the microstructure of weld and low temperature performance is systematically discussed.The results shows that the interaction of Mn and Ni effectively reduces the ductile-brittle transition temperature of the weld and promotes the formation of acicular ferrite in the weld.Under this test condition,the best combination of low temperature toughness and strength of X80 grade pipeline steel weld is obtained when 1.9%Mn in the weld matches 2.2～3.2%Ni,and the weld performance is far better than X80 grade pipeline steel Standard requirements for oil and gas pipelines,including low temperature toughness(123)≤Akv-45℃≤128.3J)yield strength(605MPa≤Rt0.5≤613.6MPaa).However,if the content of Mn and Ni in the weld metal exceeds this range,it will promote high-temperature phase transformation products such as proeutectoid ferrite(PF),side lath ferrite(FS)and even lath bainite,and it will severely reduces the low temperature toughness of the weld.Further study shows that when the density of grain boundary(>15°)and the proportion of effective grain with size less than 2 μm increase,volume fraction and inclusion size of M-A components decrease,the low-temperature toughness of submerged arc weld increases significantly.The acicular ferrite in the weld is multi-dimensional nucleated with 0.2～0.8μm composite inclusions as nucleation particles,preferentially growing along the conventional austenite habit plane.Cross-distributed acicular ferrite and granular bainite and fine M-A components divide and refine the weld structure,precipitation strengthening and fine grain strengthening.The welding seam undergoes a recovery and recrystallization process after quenching and tempering,and the M-A component decomposes and precipitates more carbides along the grain boundaries,resulting in a lower degree of low temperature impact toughness and hardness of the weld.The M-A components and fine carbides dispersed in the weld after normalizing at 920℃ normalizing 660℃ tempering effectively slows down the low temperature toughness of the weld.Through in-depth analysis of the composition of X80 grade pipeline steel submerged arc welding wire and the quenching and tempering heat treatment process,the dependence relationship between the composition of submerged arc welding wire and the low temperature performance of the weld seam is established,and the strength and toughness mechanism of X80 grade pipeline steel submerged arc welding joint is revealed,and effectively reduces the low temperature toughness fluctuation of submerged arc welds.This ensures the smooth implementation of the Sino-Russian East Line natural gas pipeline and at the same time provides a theoretical basis and technical support for the further promotion and application of X80 grade pipeline steel welded pipes and the development of welding materials in complex and harsh low temperature environments. |
