Study on Microstructure and Fracture Toughness of Welded Joint of Large Thickness Steel Plate
The welded joints of ZG370-580H + Q460ND large-thickness steel plates were observed with macroscopic low-power microscope and microscopic metallographic microscope,and the defects after welding,microstructure,and formation mechanism were analyzed.The mechanical properties of the welded joint were studied after hardness measurement and crack tip opening displacement(CTOD)test on the base metal(BM),heat affected zone(HAZ),and welds(WZ).The results show that the welded joint is in good shape and no obvious defects due to welding are found.The Vickers hardness of the heat-affected zone on both sides of the joint is higher than that of the weld and the base metal,and the hardness of the heat-affected zone on the Q460ND side is the highest.The microstructure of HAZ on both sides is the same.The superheated zone is mainly composed of lath ferrite(LF),acicular ferrite(AF),and a small amount of pearlite(P).The normalized zone is mainly composed of evenly distributed fine pearlite(P)and ferrite(F).The equiaxed crystal zone and columnar crystal zone do exist in WZ,which is determined by the welding procedure of multi-layer and multi-pass welding.The maximum CTOD value of the welded joint was 0.7621mm,and the minimum CTOD value was 0.3037mm.The average CTOD values of different positions of the joint sort from largest to smallest as weld>ZG370-580H side heat affected zone>Q460ND side heat affected zone.The CTOD values of all specimens tested were higher than the minimum values specified in relevant standards.
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