Effect Of Heat Input On The Microstructure And Properties Of Welding Heat Affected Zone Of Low Carbon Mo-V-N-Ti-B Steel

The large heat input welding steel has always been a hot spot in the field of steel,but in recent years,with the improvement of weldment requirements,the development of new steel systems has also encountered bottlenecks.In this paper,the Gleeble-3500 thermal simulation test machine was used to study the relationship between microstructure and properties of coarse-grained heat affected zone(CGHAZ)of Mo-V-N-Ti-B test steel under different welding processes,so as to provide a reference for the development of new type of high-input welding steel.The microstructure of Mo-V-N-Ti-B test steel under continuous cooling conditions was studied by Gleeble-3500 thermal simulator.The SHCCT curve was established.With the change of heat input,the structure of CGHAZ changes significantly.When the heat input is low,it is mainly composed of lath bainite(LB)and granular bainite(GB).When the heat input is comparatively large,it is mainly composed of polygonal ferrite(PF)and acicular ferrite(AF).And it is mainly composed of polygonal ferrite and pearlite(P)at the time of large heat input.According to the Johnson-Mehl-Avrami-Kolmogorov(JMAK)model,the phase transition kinetics model of the test steel under continuous cooling conditions was established.It is found that CGHAZ mainly diffuses controlled growth phase transition and interface controlled growth phase transition during phase transformation.With the decrease of heat input,the activation energy of diffusion-controlled growth phase transition increases,the phase transition rate slows down,and interface control The activation energy of the type growth phase transition is reduced,and the phase transformation rate is accelerated.By studying the influence of different heat input on the microstructure and properties of CGHAZ,it was found that when the heat input was at 25 kJ/cm,the austenite grains were not growing up due to the faster cooling rate,and only a small amount of particles were precipitated,mainly including TiN,Ti(C,N).There were fewer large-angle grain boundaries and more small-angle grain boundaries,resulting in a higher hardness of CGHAZ,so the impact toughness was the worst.With the increase of heat input,when the heat input was 75 kJ/cm,the number of precipitated particles increased,including TiN,(Ti,V)(C,N),BN,etc.,and TiN particles can be pinned at the austenite grain boundary,blocking austenite grain growth,and(Ti,V)(C,N),BN and other particles can promote intragranular polygonal ferrite(IGPF)and intragranular acicular ferrite(IGAF)nucleation.At this time,the large-angle grain boundary increased,and the crack was deflected during the expansion,so that the impact toughness was optimal under the heat input.When the heat input is 150 kJ/cm,the precipitated particles grow longer due to the longer temperature stay,and the pinning effect is weakened.Although the proportion of the large angle grain boundaries increases,the grain size is large,resulting in a decrease in impact toughness.