Study On Microstructure And Mechanical Properties Of Welding Heat Affected Zone Of EH420 Shipbuilding Steel

In recent years,in order to improve the efficiency of shipbuilding construction and reduce production costs,high heat input welding technology is often used in welding large steel plates.Foreign countries have used oxide metallurgy technology for a long time to successfully develop shipbuilding steel that can withstand large heat input welding,which has greatly enhanced the profitability of the shipbuilding industry and the competitiveness in the international market.However,foreign steel companies keep this technology strictly confidential.Therefore,the development of large heat input welding shipbuilding steel with independent intellectual property rights is still a focus at present.In this study,welding thermal simulation experiments with different heat input were carried out on EH420 shipbuilding steel independently produced in China,and the failure forms and toughening mechanism of welding heat affected zone were discussed.The phase transformation during continuous cooling process was observed in situ,and the microstructure evolution and phase transformation mechanism were analyzed by combining characterization methods.The main results of this thesis are as follows:(1)The welding thermal simulations show that the microstructure and mechanical properties of the simulated coarse grain heat affected zone(CGHAZ)of EH420 shipbuilding steel were matched with the base metal under a medium heat input condition of 120kJ/cm.The microstructures consisted of a small amount of bulky grain boundary ferrite,intragranular granular bainite and pearlite.The refinement of grain size and the increase of high angle grain boundary(HAGB)fraction led to the increase of impact energy,and the fracture surface exhibited as a mixed form of fine and uniform equiaxed small dimples and large and deep dimples.The softening of the matrix was helpful to the closure of micropores and inhibited the propagation of main cracks.The reduction of the average size and the improvement of morphology of M-A constituent(martensite-austenite constituent),and the improvement of ferrite matrix morphology were beneficial to the increase of toughness.However,single-phase bainite lath sheaves,coarse granular bainite and coarse polygonal ferrite led to linear crack propagation.(2)By using confocal laser scanning microscope,it is noted that the bainite sheaves which were produced via primary nucleation and sympathetic nucleation formed fine-grained bainite structure through hard impingement and intersection.Low-angle grain boundaries(LAGB)existed between bainite sheaf laths and between sympathetic nucleation bainite and original bainite.HAGBs separated bainite laths of different variants and different bainite sheaves.Bainite laths of different variants had different growth orientations,and variant selection played an important role.(3)The growth rates of acicular ferrite in nucleation growth stage and stable growth stage which were measured by in-situ observation were 4.5?m/s and 5.7?m/s respectively.In the temperature range of 696.9-692.2?,acicular ferrite lath divided coarse austenite grains into many fine and relatively independent regions,and acicular ferrite and bainite lath formed in the temperature range of 692.2-659.1? were restrained within these regions to obtain fine grain mixed structure.(4)The positions of Widmanst?tten formation included austenite grain boundaries,grain boundary ferrite and early formed Widmanstatten(formed by sympathetic nucleation of face-to-face).Widmanst?tten had a higher growth rate by fine-tuning its growth direction.The Widmanst?tten lath formed interlocking shape through hard impingement.The growth direction of Widmanst?tten lath deflected or remained constant after hard impingement with inclusions.However,grain boundary ferrite restricted the growth of Widmanst?tten lath.The decresing the cooling rate resulted in a decrease in the growth rate of Widmanst?tten,while rapid cooling rate led to a lack of nucleation stage in the growth of Widmanst?tten lath.EBSD analysis results show that there was a 12° orientation difference between the secondary Widmanst?tten lath and the grain boundary ferrite.