Study On The Microstructures And Corrosion Resistance Of Welding Heat Affected Zone For High-pressure Thick-wall X70 Submarine Pipeline Steel

High-pressure thick-wall X70 pipeline steel feasures high strength,high toughness,good weldability and good crushing resistance and is the highest strength grade of submarine pipeline steel in China,which has been successfully applied to the deep sea oil and gas development of the South China Sea.The submarine pipeline is subjected to the poor service condition of low temperature,high pressure,active corrosion.And corrosion of welding area is one of the most important forms of pipeline failure,which can lead to pipe leakage and blowout,thus causing serious economic loss and environmental pollution.Few reference about corrosion resistance of high-pressure thick-wall X70 pipeline steel has been reported nowadays.So it is necessary to study the microstructure and corrosion resistance of welding heat affected zone of highpressure thick-wall X70 pipeline steel systematically.In this paper,the microstructure and corrosion resistance of welded joint of X70 pipeline steel with high heat input were studied and the weak zone of corrosion resistance were found.The influence of high temperature holding time with high heat input and cooling time on the microstructure and corrosion resistance of coarse grain heat affected zone were studied by thermal simulation to obtain the better heat input.The main results and conclusions of this paper are as follows:(1)The microstructure and corrosion resistance of different zones in welded joint of X70 pipeline steel with high heat input in the industrial condition were studied.Results showed that the main microstructure of welded metal is fine grain acicular ferrite,the main microstructure of coarse grain heat affected zone is granular bainite and polygonal ferrite and fine grain heat affected zone and that of fine grain heat affected zone is polygonal ferrite.The corrosion tendency of heat affected zone is higher than that of the welded metal and the base metal,which is the weak zone of the corrosion resistance of the welded joint.(2)The microstructures and corrosion resistance of different heat affeceted zones of X70 pipeline steel with high heat input were studied.Results showed that CGHAZ consisted of polygon ferrite and granular bainite with coarse grain;FGHAZ mainly consisted of polygon ferrite;Intercritical heat affected zone(ICHAZ)mainly consisted of rough acicular ferrite and partial polygonal ferrite;There was no distinction between mictructures of subcritical heat affected zone(SCHAZ)and base metal.Results of corrosion resistance of different HAZs of X70 pipeline steel in the 3.5wt.%NaCl solution were FGHAZ>SCHAZ>CGHAZ>ICHAZ in turn.(3)The effect of holding time at high temperature on microstructure and corrosion resistance of coarse grain heat affected zone for X70 pipeline steel were studied.Results showed that the microstructure consisted of granular bainite and quasi-polygonal ferrite.The grain size of original austenite increased with the increment of holding time.The corrosion resistance of coarse grain heat affected zone in the 3.5wt.% NaCl solution became worse as increasing holding time.(4)The effect of cooling time on microstructure and corrosion resistance of coarse grain heat affected zone for X70 pipeline steel were studied.Results showed that the microstructure transferred from lath bainite to granular bainite and began to form ferrite as increasing cooling time;meanwhile,M/A group gradually coarsed.Ununiform microstructure and grain coarsed resulted in poor corrosion resistance of coarse grain heat affected zone.Thus,optimal parameters of welding heat input were 82.90~89.83 kJ/cm,in which the corrosion resistance of coarse grain heat affect zone was better.