Welding Process And Microstructure And Properties Of High Grade Pipeline Steel Laser-MAG Welded Joints

Oil and gas pipeline engineering is a large-scale welding and long-distance welding installation project.The quality of the welded joints of the pipeline steel determines the safety and service life of the pipeline during pipe forming and laying.The traditional welded joints of pipeline steel(submerged arc welding,gas shielded welding,etc.)cannot meet the requirements which have high mechanical performance because of the serious problems of softening and embrittlement with the development of pipeline steel in the direction of large diameter,high pressure and high level.Laser/laser-MAG hybrid welding has the advantages of low heat input,narrow heat affected zone,etc.It is expected to have the advantage of welding with pipeline steel.This paper is based on the National Natural Science Foundation project "Microstructure Control and Toughening Mechanism of Low-carbon Bainite Pipeline Laser-MAG Hybrid Welding Joints".The influence of low-heat input which is in accordance with laser/laser-MAG hybrid welding in single and double-pass thermal cycles on the microstructure and properties of coarse-grained zone in pipeline steel was studied.A laser-MAG hybrid welding experiment was conducted to investigate the effect of heat input(arc current intensity)on the microstructure and properties of the welded joint.Explore the effect of magnetic field on the microstructure and properties of laser welded joints.The main research results of the paper are as follows:1)The transformation rule of microstructure and properties of X100 pipeline steel in welding coarse-grain zone under low heat input(3.3 kJ/cm?4.6 kJ/cm)and high heat input(7.3 kJ/cm?25.3 kJ/cm)was studied by using a thermal simulation machine.The result shows that with the increase of heat input(3.3 kJ/cm?25.3 kJ/cm)the microstructure transformation process in the coarse-grained zone is lath martensite ? lath bainite?granular bainite.The microhardness(389.9 HV-260.9 HV)and the impact energy(low heat input 25.7 J?21.3 J,high heat input 157.4 J?12.2 J)is gradually decreased.The impact energy reached 75%of the base metal at low heat input(3.3 kJ/cm).With the increase of heat input,the increase of the prior austenite grain size(22.2 ?m?192.9?m)resulted in the decrease of the high-angle grain boundary density(56.5%?23.2%)and the gradually increase of the M-A constituent content(14.5%?67.1%).These are the two causes of the drop in the impact energy of the coarse grain zone.2)The influence of the microstructure and properties of the X100 pipeline steel in the critical coarse-grain zone was studied by using the thermal simulation machine through the double-pass thermal cycles experiment.It is considered that when the secondary peak temperature is in the critical region the chain M-A constituent or martensite is formed at the prior austenite grain boundary.And the impact energy of the sample is significantly reduced at this time.In the critical coarse-grained region,as the secondary peak temperature increases,the impact energy of the sample gradually increases,and the M-A constituent along the prior austenite gradually becomes sloppy.And the distribution position gradually changes from grain-boundary distribution to grain center.The morphology gradually changes from massive M-A to lath martensite.These precipitated second-phase structures will form crack sources when fractured and cause cracks to propagate along the prior austenite grain boundaries.So they can reduce the impact energy of the sample.3)The effect of niobium content on the microstructure performance of pipeline steel welding heat affected zone was studied by using welding heat simulation technology.The results showed that NNb and NbC is gradually precipitated out in the cooling process as the heat input increases in a single heat cycle.High temperature and retention time is short and coarse grain zone organization contained NNb and NbC failed to precipitate under the low heat input.So the two original austenite grain size in steel were similar and the two kinds of experimental steel group performance is basically the same.NNb and NbC are precipitated in the cooling process in the high heat input range.And more precipitates are precipitated in the high niobium steel due to the high niobium content.The influence of Nb content on the original austenite grain size increase.The effect of Nb element on impact energy is more obvious under high heat input.Both the reduction of heat input and the increase of Nb content can increase the impact energy of the critical coarse grain zone in the double-pass heat cycle.The impact energy of the critical coarse-grained zone in high-tempered steel reaches the base metal level after the double pass thermal cycle when the heat input is 4.6 kJ/cm.4)Laser-MAG hybrid welding experiments were performed on X100 pipeline steel using the laser-MAG hybrid welding system of YLS-6000 laser and MAG arc welding machine to study the effect of heat input(arc current intensity)on the microstructure and properties of hybrid welded joints.According to the analysis,the residual height of the welded joint and the width of the upper part of the welded joint gradually increase and the weld width at the lower part of the weld seam does not change much with the increase of heat input(3.75 kJ/cm?5.37 kJ/cm).The microstructure of the weld zone and coarse grain zone changes from lath martensite to granular bainite.The prior austenite grain size of the coarse grain zone increases from 13.65?m to 27.3?m.The microhardness of the coarse-grained zone(348 HV?315 HV?304 HV)and the joint(337 HV?307 HV?298 HV)decreases gradually with the increasing of heat input.The microhardness of the joint is lower than that of the coarse grain zone with the "low-strength match" wire.The microhardness of the joint is higher than the base metal at three different heat inputs.Under three kinds of heat input conditions,the strength of welded joints is equal to that of the base metal and the impact energy of the welded joints is lower than that of the base metal.The impact energy of the welds is 33 J?63 J.The impact energy in the heat affected zone is 53 J?99 J.The main reason for the above problems is the large number of pores in the joint.5)The effect of applied magnetic field current on the microstructure and properties of laser welded joints was investigated by using the YLS-6000 laser to perform external magnetic field assisted welding experiments on pipeline steels.The macroscopic appearance of the laser welded joint is V-shaped in the absence of a magnetic field.The macroscopic appearance of the weld becomes an X-shape under the effect of a magnetic field.The applied magnetic field had no obvious effect on the microhardness,tensile strength and impact energy of the welded joints.