| In recent years,pipeline steel-as a tool for long-distance transmission of oil and natural gas-shows a flourish trend.In order to further improve the efficiency of oil and gas transportation,the pipeline construction is developed in the direction of large diameter and high pressure,and the safe operation of high pressure gas pipeline is the key of the pipeline construction.With the increase of the level of the pipeline and the wall thickness,the quality control of girth weld,especially the difficulty of weld fracture toughness control,has become one of the most important problems in the development of the pipeline.Even for X80 pipeline,which is relatively mature,weld seam impact toughness always fluctuate due to the composition change in tube material.Welding melt,the elements in base metal alloy dissolved in the molten pool may influence the phase transformation in the process of solidification and cooling in a certain degree.So it is very important to study the influence of the change of the composition in the raw materials of pipeline steel in microstructure and mechanical properties of weld that self-protection flux cored wire semi-automatic.In this paper,the actual welding process is affected by different temperature of the thermal cycle through welding thermal simulation experiment.Using scanning electron microscopy(SEM),transmission electron microscopy(TEM)and impact experiments,the different temperature of the thermal cycle on the protection of the influence of the organization and the impact toughness of weld were also researched.And the relationship between self shielded weld seam impact toughness and weld the amorphous regions was determined.The dilution rate of the alloy elements in the weld seam was determined by changing the content of Nb and Mo in X80 pipeline steel alloy element.And by means of optical microscope,SEM,impact test and CCT curve,the effect of alloying elements Nb and Mo alloying elements on the microstructure and impact toughness of the self shielded welding seam was determined and the alloy composition was optimized too.The results show that:1)The microstructure of X80 pipeline steel weld is mainly bainite ferrite,granular bainite and M/A element.The reason of weld seam toughness fluctuations is that there are a large number of massive M/A.In terms of "location-thermal cycling temperature microstructure and toughness",the toughness of coarse grain zone and fine grain zone is the best,and the two-phase zone and high-temperature tempering region are brittle,which accounts for the highest proportion.The toughness of the weld can be improved by increasing the toughness of the brittle region and decreasing the amount of M/A.2)The proportion of different alloying elements in the weld(dilution rate)is different.The dilution rate of Nb is 13%~25%and the average dilution rate is 20%.The dilution rate of Mo is about 15%.The increase of Nb content in alloying elements promoted the transformation of granular bainite,expanded granular bainite and ferrite phase region,increased the number of M/A,and increased the size,thus reducing the impact toughness of the weld.Therefore,it is suggested that the content of alloying element Nb in X80 pipeline steel should be controlled below 0.08%.3)The microstructure of weld in steel plate is related to the content of Mo.The higher content of Mo,the more granular bainite in the weld microstructure,the less the bainite ferrite structure,the more the M/A quantity,and the larger the size,the more dense.And with the increase of Mo content,the impact toughness of the semi automatic welding seam of the self shielded flux cored wire is gradually decreased,and the impact power dispersion is increased.It was suggested that the Mo content in X80 pipeline steel should be controlled below 0.25 %. |
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