Effect Of Nb And Heat Input On Microstructure And Low Temperature Toughness Of CGHAZ Welded V-Ti-N Steel

In this paper,a series of heat input welding thermal simulation tests of V-Ti-N steel and Nb-V-Ti–N steel were carried out on V-Ti-N steel and Nb-V-Ti-N steel by Gleeble-3500testing machine,and the continuous cooling transition curve（SHCCT）of CGHAZ of the two test steels under different heat input conditions was constructed,revealing the microstructure evolution law of CGHAZ of the two steels.Exploiting scanning electron microscopy（SEM）,electron backscatter diffraction（EBSD）,transmission electron microscopy（TEM）,energy spectrometer（EDS）and other characterization tools,matched with Thermo-Calc thermodynamic calculation,impact toughness and hardness test,the microstructure transition and toughness alteration of V-Ti-N steel in the case of serial line energy were systematically analyzed,and the connection between heat input,microstructure evolution and impact property was clarified.The phase transition behavior of V-Ti-N steel and Nb-V-Ti-N steel CGHAZ in the continuous cooling process was studied,and the results showed that with the continuous improvement of heat input,the CGHAZ structure of V-Ti-N test steel gradually moved from Lath bainite+Granular bainite tissue（LB+GB）to the structure of Polygonal ferrite and Acicular ferrite（PF+AF）,and the CGHAZ structure of Nb-V-Ti-N test steel gradually changed from LB+GB to developed GB and coarse Polygonal ferrite（PF）Shift;As the heat input increased,The phase transition temperature of test steel CGHAZ rose gradually,and the hardness manifested a falling tendency.The addition of Nb element caused the SHCCT curve of the test steel CGHAZ to shift to the lower right,inhibiting the ferrite transition and expanding the bainite transition interval.The analysis results showed that as the welding heat input increaseing from 35 k J/cm to 100 k J/cm,the average size of the original austenite grain of CGHAZ elevated,the area fraction and average size of the hard phase M/A component increased,the main microstructure changes from LB+GB to PF+AF.this trend of hard phase M/A components caused the critical fracture stress to decrease with the increase of heat input,which reduced the crack initiation work.However,the phenomenon that elevated heat input promoted heterogeneous nucleation of intragranular ferrite on（Ti,V）（C,N）particles significantly refined the average grain size(MED_MTA≥15°)defined by the orientation difference angle is greater than or equal to 15°,and the proportion of large angle grain boundaries（HAGBs）were increased,which increased the density of HAGBs and the probability of cracks encountering HAGBs during the propagation process,which improved the crack propagation work.The combined influence of M/A components and large angle grain boundaries led to the monotonic increase of the impact toughness of V-Ti-N microalloyed CGHAZ as the heat input went up.