Study On The Microstructure And Properties Of CGHAZ Of Welding Thermal Simulation In X80 Pipeline Steel

X80 pipeline spiral submerged arc welded pipes have been applied to the second line of west-east gas successfully.It marked basic application research and product application development of X80 pipeline steel have basic mature.But some problems need to be solved in order to further strengthen the security of X80 pipe.So basic on a X80 pipeline,its microstructure and performance of HAZ(Heat affected zone)was performed in this paper,especially the embrittlement of IC CGHAZ(Intercritical reheated coarse-grained HAZ)have been analysis deeply.Three thermal simulation methods have been developed,according to the chemical composition and properties of X80 pipeline steel and the actual process of SSAW and SMAW.The microstructure and performance of different zones with HAZ was studied in different welding thermal cycle by Gleeble-3500 thermal simulated test machine.The results showed that Charpy impact energy of thermal simulated sample with SSAW was better than SMAW.But impact toughness has declined markedly and micro-hardness has increased significantly when T_p=800?.The welding heat affected zone critical coarse grain zone(IC-CGHAZ)appeared local brittleness.When the peak temperature of the second thermal was at(?+?)two-phase regions,incomplete austenitizing results in the appeared of carbon-rich M-A component and in the form of chain structure distribution around the austenite grain boundary under the action of thermal cycle.It have affected seriously the impact toughness in the region.When T_p=1200?,grain sizes and low-temperature impact toughness decreased in this region.The microstructure of deposit was acicular ferrite and polygonal ferrite structure,and the content of ferrite increased and the impact toughness decreased with increasing heat input when T_p=1000?.When T_p=600?,impact performance in this region was relatively good result from the short time tempering of CGHAZ.The dyed microstructure of HAZ was investigated by optical microscopy(OM)and scanning electron microscopy(SEM).The results showed that M-A contents and length-width ratio of IC-CGHAZ for large heat input SAW were largest.The form of acicular M-A was distributed in ground original austenite grain boundary.M-A islands of IC-CGHAZ for SMAW were block distribution,whose contents were least.M-A contents of IC-CGHAZ for small heat input SAW were between two parties in the form of acicular and block distribution.The decline of IC-CGHAZ toughness was cause by the precipitation of M-A islands at the original austenite grain boundary probably.M-A islands was brittle second phase,crack was easy to happen and propagate,so leading to a decline in toughness.The microstructure of HAZ and impact fracture were analyzed for the influences of microstructure to the impact performance with a systematic study by OM and SEM.The results showed that the radial region of SCR-CGHAZ and IC-CGHAZ were small cleavage fracture.The grain size might have affected Charpy V-notch toughness less.But M-A islands place that distributed at grain boundary came into tearing ridge.The generation of cracks because of stress concentration near M-A islands.