Effect of heat input on microstructure and toughness of CGHAZ of deep-sea X70

In order to solve the embrittlement problem of coarse grain zone (CGHAZ) of deep-sea X70 pipeline steel in actual welding,the thermal simulation study of X70 pipeline steel was carried out under different thermal cycle processes.X70 pipeline steel CGHAZ was simulated by Gleeble-3800 thermal simulator to study the microstructure and toughness of CGHAZ under different heat input (HI) conditions in the range of 10~60 kJ/cm.The microstructure and toughness of CGHAZ were characterized by means of optical microscope (OM),scanning electron microscope (SEM) and Charpy impact test.The results show that the microstructure of the test steel under different heat input is mainly composed of granular bainite (GB),bainite ferrite (BF) and martensite-austenite component(M-A component).When HI increases continuously,the proportion of BF decreases,the proportion of GB increases,the M-A component coarsens,and the impact absorption energy first increases and then decreases.In the case of 20 kJ/cm,the excellent combination of BF and GB is obtained,the fracture is ductile fracture,and the impact absorption energy reaches [BF]173.8[BFQ] J;when HI is greater than 20 kJ/cm,the fracture is dissociated and fractured,and the impact absorption energy decreases obviously,with the lowest of [BF]18.8[BFQ] J.Therefore,lower heat input can improve the toughness of CGHAZ,make X70 pipeline steel have high strength,high toughness and good weldability,and provide some theoretical guidance for the optimization of welding process.[HQ