Study On Microstructure And Properties Of Low-carbon Bainitic High-strength Steel Welded Joint

NiCrMoV series of low-carbon bainitic high strength steel, not only has high strength and crack resistance, and its plasticity and toughness are also excellent. Therefore, it has important theoretical and engineering value to study the microstructure and properties of NiCrMoV series of low-carbon high strength steel.In this paper, the microstructure analysis and mechanical properties have been carefully studies for the NiCrMoV series of low-carbon bainitic high strength steel and its welded joints of DM4-1, DM4-5, T5-20, T5-21, M100 and T100 obtained with six different welding methods.In the experiment, the microsturctures of the welded joints were studied by the main analysis methods of SEM, TEM, and EBSD. Moreover, the mechanical properties were studied by the tensile test, in situ tensile and the charpy impact test.The microstrures of the low-carbon bainitic high strength steel and its welded metal are composed of the bainite, ferrite, martensite and austenite. The ferrite and bainite are combined into lath.A small amount of film shape austenitic and martensite–austenite (M-A) constituents are distributed in the bainite grain boundary. TEM observations showed that high-strength low-carbon bainitic steel lath structure is mainly composed of ferrite, bainite and the film shape MA constituents in the bainitic lath boundaries.The microsturcture of DM4-1 and DM4-5 welded joints are composed of lath bainite and MA constituents. The width of bainite is about 400nm. Local block MA phases are mainly distributed in the lath bainite boundaries, the size of part block MA-island is about 300nm×1000nm, which is acrossed the inside of the lath bainite. The width of lath bainite in the T5-20 and T5-21 welded joints is narrow as same as the base metal.The retained austenite was also distributed in the grain boundary with the same direction as the lath bainite. The welded joint metal of M100 and T100 is composited of the lath ferrite and bainite. The lath size of the joints is vary larger, the minimum strip width is about 30nm, while the wide plate is about 1000nm. It is also found that the lath bainite has a smaller grain size of MA within the needle-like precipitates.The needle-like precipitates in the T100 joint is thicker than the M100 joint. The EBSD test showed that the retained austenite is less than 1% in different weld joints. In welded joints, the amout of retained austenite in TIG welded joints were less than the MAG welded joints.EBSD analysis showed that grains in the base metal with less than 10°misorientation angle is 62.2%, the angle from 10°to 50°range is 15.5%, and the angle from 50°to 60°range is about 22.3%. A large proportion of small-angle grains induces good strength and toughness for the low-carbon bainitic steel. Tensile mechanical properties test and Charpy impact test was designed at different experimental temperature conditions including 20℃, -50℃, -110℃and -196℃.The results show that the tensile strength of high-strength low-carbon bainitic steel was 1016MPa and the percentage of area reduction was 73% at 20℃. At -196℃the high-strength low-carbon bainitic steel still has a good strength and toughness, especially the percentage of area reduction was still as high as 62%.The fracture section shrinkage rate of MAG weld joints decreased rapidly at low temperature is 15%, while the percentage of area reduction of TIG weld joints at low temperature is still about 60%, so the TIG welded joints has good low temperature toughness than the MAG welded joints. Charpy impact tests show that the base material's impact toughness of 175J is obtained at room temperature( 20℃), the impact toughness of 44J is obtained at -196℃. At the low temperature -196℃,the Charpy impact toughness are less than 6J in MAG welded joints, while Charpy impact toughness of 20 J are obtained in the TIG welded joints. In situ SEM tensile tests show that the specimens under tensile loading a large number of internal dislocations and slip lines, and micro-cracks, crack growth, blocked, passivation are produced and then expanded until the entire specimen fractured.The cross-section observations impact fracture tests at -196℃showed that crack propagation is hindered by the grain boundaries and bainite colonies and the boundaries of bainite lath , especially the thin lath bainite. The block MA constituents had less resistance to crack initiation and propagation.