The Effect Of CO₂ Shielding Gas On Microstructure And Properties Of X100 Pipleline Steel Laser Weld

Oil and natural gas are important strategic resources in China,which have a great impact on the economic life of the nation.In order to reduce the cost of oil and gas transportation,using pipeline transportation is the most economical way.In the process of manufacture and laying of pipeline steel,welding is the most common connection method,and the weld seam is the weak zone of the mechanical properties of the welded joint because it undergoes the process of remelting and solidification.Using the traditional welding methods often leads to weld softening and grain coarsening,resulting in insufficient strength and toughness of the weld,which seriously affects the reliability of welded joints.With the rapid development of high-power laser,laser welding is expected to be used in the welding of pipeline steel due to the advantages of low heat input and high welding efficiency.In this study,taking X100 pipeline steel as the research object,the butt-welding test of X100 pipeline steel was carried out by using high power laser.The effects of shielding gas type,CO₂ content and welding heat input on the microstructure and properties of laser weld of X100 pipeline steel were studied.The main research contents and results are as follows:（1）The effects of Ar and CO₂ shielding gases on the microstructure and properties of laser welds of X100 pipeline steel were studied.The results showed that the microstructure of Ar shielding laser weld is mainly granular bainite（67%）,and the type of inclusion is Al₂O₃-Ti₂O₃ composite inclusion,while the microstructure of CO₂shielding laser weld is mainly acicular ferrite（91%）,and the type of inclusion is Ti₂O₃-Al₂O₃-Si O₂-Mn O composite inclusion.The thermodynamic calculation results showed that the molar volume fraction of Ti₂O₃ in CO₂ shielding laser welds is twice as much as that in Ar shielding laser welds.Inclusions with higher Ti₂O₃ molar fraction can promote the formation of acicular ferrite with high-volume fraction through the mechanism of Mn-depletion zone（MDZ）.Further analysis shows that the high angle grain boundary fraction（72.6%）of CO₂ shielding welds caused by high-volume fraction of acicular ferrite is about twice that of Ar shielding welds（35.9%）.The impact energy of CO₂ shielding weld is 1 and 7 times higher than that of Ar shielding weld at-40℃and-60℃,respectively.The main reason is that the high-volume fraction of acicular ferrite in CO₂ shielding weld can effectively improve the low temperature impact toughness of the weld.At-40℃and-60℃,Ar shielding welds showed ductile-brittle mixed fracture and brittle fracture respectively,while CO₂ shielding welds show ductile fracture and ductile-brittle mixed fracture respectively.（2）The effect of CO₂ content in shielding gas on the microstructure and properties of laser weld of X100 pipeline steel was studied.When the CO₂ content gradually increases from 0%to 100%,the oxygen content in the laser weld increases gradually from 24 ppm to 133 ppm.With the increase of CO₂ content,the volume fraction of granular bainite in laser weld decreased gradually,while the volume fraction of acicular ferrite increased at first and then decreased.The maximum volume fraction of granular bainite is 41%at 0%CO₂ and the maximum volume fraction of acicular ferrite is 75%at 60%CO₂.The type of inclusion in 0%CO₂ weld is Al₂O₃-Ti₂O₃ composite inclusion,while the type of inclusion in 20%-60%CO₂ weld is Ti₂O₃-Al₂O₃-Si O₂-Mn O composite inclusion.The fracture positions of the tensile specimens are all located at the base metal.At-20℃,the impact energies of 0%CO₂,20%CO₂,60%CO₂ and 100%CO₂welds are 64 J,54 J,52 J and 50 J,respectively,reaching 58.7%,50%,47.7%and 46%of the base metal,respectively.At this time,all weld fractures show ductile fracture.The impact energy of 0%CO₂ weld decreases significantly with the further decrease of temperature to-40℃and-60℃.At-60℃,the impact energies of 0%CO₂,20%CO₂,60%CO₂ and 100%CO₂ welds are 27 J,41 J,37 J and 35 J,respectively,reaching 24.3%,33.3%and 31.5%of the base metal,respectively.At this time,all the weld fractures show ductile-brittle mixed fracture.When the content of CO₂ is 20%,the weld has good impact toughness and tensile properties.（3）The effect of different heat input（2.5 k J/cm～4.2 k J/cm）on the microstructure and properties of laser weld of X100 pipeline steel was studied.When the heat input increases from 2.5 k J/cm to 4.2 k J/cm,the oxygen content in the laser weld increases gradually from 109 ppm to 177 ppm.With the increase of heat input,the volume fraction of acicular ferrite and granular bainite in laser weld increases gradually,while the volume fraction of lath bainite decreases gradually.The type of inclusions in laser welds with different heat input is Ti₂O₃-Al₂O₃-Si O₂-Mn O composite inclusion.The tensile results show that the fracture of 2.5 k J/cm and 3.0 k J/cm welding samples occurs at the base metal,while the fracture position of 4.2 k J/cm weld is located in the weld,which is consistent with the changing trend of weld microhardness.There is little difference in weld impact energy between 2.5 k J/cm and 3.0 k J/cm at-20℃and-40℃.When the temperature is further reduced to-60℃,the impact energy of the weld of 3.0k J/cm has better impact toughness.At this time,the weld fracture shows ductile-brittle mixed fracture.