Welding Hot Cracks And Cold Bending Performance Of X80 Pipeline Steel

At present,production technique for X80 pipeline steels has matured both domestic and abroad.The usage of high pressure,large diameter and high strength pipes becomes inevitable trends for the oil and natural gas transporting pipeline.Yet hot cracks and cold bending problems arise in X80 pipeline steel's manufacture and application.This dissertation is to research the mechanism of weld cracks and the phenomenon of cold bending in the production process for X80 pipeline steel,and analyze the factors from element segregation,weld forming factor and line energy through optical microscope and environmental scanning analysis.The field test was carried out in Hebei Iron & Steel Co.,Ltd.the main parameters of thermal cycle were obtained according to thermal cycling curves of inside and outside of submerged arc welding and the t_8/5 were estimated based on the empirical formula and submerged arc welding cooling curve.Finally,thermal simulation test system of CGHAZ was also proposed combined the field test resulted the theoretical results.In order to research the relations between performance and microstructure for cold bending cracking and coarse grain zone,thermal simulation tests at different t_8/5 were performed by Gleeble thermal simulation device.The consistence of microstructures between the actual thermal simulation CGHAZ and spiral and longitudinal welded pipe characteristics indicates that thermal simulation can reflect the actual micristructres and performances of coarse grain area.The results through thermal simulation and performance analysis show that MA component at ? grain boundaries increases and coarsens with the introduction of longer t_8/5.MA component between ferrite bundles changes from strip to massive shape,mean chord length increases,increasing the volume fraction occupied.When the average chord length is more than the critical crack size,the toughness of HAZ coarse grain region as the brittle crack source is decreased.The hardness,toughness and tensile and impact test results for thermal simulation sample shows that hardness at coarse grain zone decreased with the increase of t_8/5.?b and ?s are both decreased,reducing the impact and fracture toughness.The reasons are due to the segregation of S and P elements at the base metal,weld forming factor less than 1 as well as too high welding line energy.