Research On Microstructure And Corrosion Resistance For Welding Joint Of X80 Pipeline Steel

Pipeline transportation steel is facing a series of security issues caused by corrosion of various geological environments in the long distance pipeline transportation. Because of different microstructure and impurities,the weld joints of pipeline steel are easiest appear crack caused by corrosion during the actual service process. Moreover,corrosion cracking caused by crack propagation with welding stress which will lead to serious consequences. So this research has studied systematically on microstructure,mechanical property and corrosion resistance for the weld joints of X80 pipeline steel which can provide basis for its large-scale promotion and application in the future.This research analyzes the weld thermal cycle curve influence on welding seam,HAZ and matrix metal by metallographic microscope and scanning electron microscopy. The results show that when peak temperature reach 1350℃ with heat input increasing,the bainite and the ferrite gradually decreases, but the granular bainite increases. As the peak temperature decreases,the bainite,the ferrite and the granular bainite gradually turn into massive ferrite.The hardness characteristics of welding HAZ of X80 pipeline steel are investigated under different welding heat input conditions. The results show that the microhardness of CGHAZ of X80 pipeline steel decreases when the welding heat input increases. But the microhardness of HAZ increases when the peak temperature increases.The corrosion resistance for matrix metal,HAZ and welding seam of X80 pipeline steel are investigated in 0.5 mol/LNa2CO3 +1 mol/L NaHCO3 and 5%NaCl+0.5%COOH solution. The results show that the effect of heat input on CGHAZ is obvious. As the heat input increases,the corrosion rate decreases,and the corrosion resistance of CGHAZ becomes better;At the peak temperature of 1350℃ high or 800℃ low,the corrosion rate is very high. The corrosion tendency of the welding seam and HAZ is much larger than the matrix metal,and their corrosion resistance are worse. The corrosion resistance in acid corrosion solution for the welding joint of X80 pipeline steel is worse than that in alkali. they can produce passive film both. But the passive film is not stable in acid corrosion solution.