Influence Of Welding Line Energy On Microstructure And Properties Of CGHAZ (Coarse Grain Heat Affected Zone) Of X100Pipeline Steel

The development trend of oil and gas pipeline is the use of large-diameter,high-pressure oil and gas transportation, and the use of high intensity level pipeline.Currently, X100pipeline steel has been included in the standard of API-5L-2007. Itsapplication has huge economic benefits and can save the cost by5%to12%, mainly inmaterial savings and improving delivery pressure, reducing the amount of construction,lower maintenance costs, and optimization of the overall programs. With the economicdevelopment, the demand of energy is growing and the research and development ofX100pipeline steel has been paid attention to. Because the CGHAZ (Coarse Grain HeatAffected Zone) is the weak part of welded joint, it has a very important practicalsignificance that the affection on microstructure and properties of CGHAZ of line energyis studied.In this paper, the change of microstructure and properties of CGHAZ of X100pipeline steel was studied by welding thermal simulation method, microstructureobservation, mechanics performance testing. The relationship between the line energy,microstructure and properties was discussed as well. And the following conclusions weregot.The microstructure of thermal simulation CGHAZ of X100pipeline steel mainly isgranular bainite, lath bainite and polygonal ferrite, and former austenite grain boundariesare clearly visible. With the increase of the line energy, grain size coarses, granularbainite increases, lath bainite decreases and bainite ferrite coarses. There are a largenumber of M-A component in CGHAZ, and the shape, number and size of M-Acomponent have a significant impact on the properties of welded joints. With theincrease of line energy, the shape of M-A component in the grain boundaries andgranular bainite changes from granular or short rod to strip, chunk, the bundles alsothickens. With the increase of the line energy, the hardness and strength of CGHAZ descend.In addition to high yield ratio of CGHAZ, the hardness, strength and plasticity are quitegood. With the increase of the line energy, the impact toughness at room temperature andlow temperature descend because of the impact of grain size and the shape, number andsize of M-A component. But the impact toughness at low temperature is lower than atroom temperature. Especially when the line energy is34.5kJ/cm, the impact toughnesssudden drops because of cold crisp. With the increase of the line energy, the proportionof brittle fracture in impact fracture morphology is becoming increasingly larger by scananalysis. There are S, Ca, Al, Mg and other composite inclusions in dimples by EDSanalysis. Good toughness organization can be got by selecting the appropriate lineenergy. In particular, the line energy of pipeline steel which is used at low temperatureshould be strictly controlled. When the line energy is between16to27kJ/cm, theproperties of pipeline steel is quite good. The circumstances under which the number ofwire and wire distance of double submerged arc are certain, the line energy between16to27kJ/cm is recommended as welding energy.