Study On Microstructure And Fracture Toughness Of Welding HAZ Of EH47 High-strength Ship Plate Steel

EH47 high-strength ship plate steel is mainly used to build large container ships.In the process of construction,the existence of welding heat affected zone often leads to the degradation of performance in local area and affects the safe service of container ships.In this paper,the microstructure and low temperature fracture toughness of coarse grain heat-affected zone(CGHAZ)for EH47 ship plate steel with different heat input are studied by means of welding thermal simulation technology,and the internal influencing factors are analyzed.In addition,EH47high-strength ship plate steel was welded,and the microstructure and fracture toughness of each sub region of the joint were studied.The results show that the microstructure of simulated CGHAZ of EH47 ship plate steel is mainly composed of lath bainite(LB)and granular bainite(GB);With the increase of heat input from 10k J/cm to 100 k J/cm,the content of GB increases,the content of LB decreases,the grain size increases,the total area fraction of M-A constituent increases,the coarse-stringer and massive M-A constituent increases,the island and fine-stringer M-A constituent decreases.The microstructure of fusion zone(FZ)of EH47 ship plate steel multi-layer and multi-pass welding joint is composed of a large amount of interlaced acicular ferrite(AF),massive quasi-polygonal ferrite(QPF)and a small amount of fine LB and GB.The content of large angle grain boundary is the highest and the Kernel Average Misorientation(KAM)value is the lowest;The microstructure of CGHAZ is composed of GB and LB,with the largest grain size,the lowest content of large angle grain boundary and the highest average KAM value;The microstructure of fine grain heat-affected zone(FGHAZ)is composed of many fine polygonal ferrite(PF),QPF and a small amount of fine LB.The grain size of this region is the smallest and the distribution of microstructure is uniform;The microstructure of critical heat-affected zone(ICHAZ)and sub-critical heat-affected zone(SCHAZ)is composed of QPF,PF and LB.The experimental results of crack tip opening displacement(CTOD)show that with the increase of heat input,the CTOD value(-20?)of CGHAZ increases first and then decreases.When the heat input is 30 k J/cm,the CTOD value of CGHAZ is the highest(0.12mm),and the CTOD value of CGHAZ is the lowest(0.01mm)at 50k J/cm.The CTOD value of CGHAZ is the lowest(0.20mm)in all sub-region of the actual welding joint,while that of FGHAZ is the highest(0.67mm).The results show that the high content of LB,QPF,PF and AF in microstructure are favorable to fracture toughness,while the high content of GB and M-A constituent is unfavorable to fracture toughness;Small grain size and LB packet size,high content of large angle grain boundary are favorable to fracture toughness;High average KAM value and Geometrically Necessary Dislocations(GND)density are unfavorable to fracture toughness;The island M-A constituents(d_max?1.7?m,area?1.5?m²)can arrest microcracks,which is beneficial to fracture toughness.And the fine-stringer ones(d_max?12.1?m,area?3.6?m²)can deform without excessive damage to fracture toughness.The coarse-stringer(d_max?7.1?m,area?6.7?m²)and massive M-A constituent would crack and debond from the matrix,which seriously decreases the fracture toughness.