Effect Of Pulsed Laser On Microstructure And Properties Of B950CF High Strength Steel Hybrid Welding Joint

In recent years,the construction of pumped-storage power stations at home and abroad has continued to develop at a high speed.In order to meet the requirements of hydropower stations and further reduce the weight of penstocks,1000 MPa steel plates have been used in the construction of hydropower stations.However,with the continuous improvement of the strength of high-strength steel,problems such as hot and cold cracks,softening of the heat-affected zone,and welding pores are more likely to occur in welded joints.In addition,penstocks need to bear large water pressure and impact pressure for a long time,and the fatigue performance of welded joints is very important to ensure the safety of high-pressure water delivery.Therefore,in this paper,aiming at the above problems,a systematic study of 8 mm thick B950CF high-strength steel is carried out by using pulsed laser-MAG hybrid welding.First,the surfacing test was used to study the influence of pulsed laser process parameters on weld formation,hardness distribution and pore defects.The influence of the addition of pulsed laser on the droplet transfer and joint residual stress in the hybrid welding process was also analyzed.The results show that with the increase of laser peak power,pulse frequency and pulse width,weld penetration and weld porosity gradually increase.The laser peak power and pulse width are the key process parameters that affect the hardness distribution of the joint.The peak power of the bottom layer laser is 5.5 k W,the pulse frequency is 40 Hz,and the pulse width is 10 ms;The laser peak power of the cover layer is 3 k W,the pulse frequency is20 Hz,and the pulse width is 7.5 ms.Welding of 8 mm thick B950CF high-strength steel can obtain beautifully formed welded joints without obvious pore defects.And the residual stress level is lower than the laser-MAG hybrid welding joint.The addition of pulsed laser changes the flight path of the droplet,and the axis of the droplet transition is better.Then,the optimized pulsed laser-MAG hybrid welding and laser-MAG hybrid welding joints were compared and analyzed for the microstructure,hardness distribution,tensile and impact properties,and the corresponding relationship between the microstructure and mechanical properties of the joints was established.The study find that the structure of the pulsed laser-MAG hybrid welding weld is mainly composed of martensite and bainite,the coarse-grained region is coarse lath martensite,and the fine-grained region is fine granular bainite and martensite.The structure of the incomplete transformation zone is composed of granular bainite,ferrite and a small amount of martensite.Compared with the former,laser-MAG hybrid welding has no obvious difference in structure but coarser crystal grains.The hardness of pulsed laser-MAG hybrid welding joints is generally higher than that of laser-MAG hybrid welding joints.The fine-grained area is the area with the highest hardness of the joint,and the incomplete phase transition area has been significantly softened.Both tensile specimens of joints fractured in the base material,and the fracture showed typical ductile fracture characteristics.Compared with pulsed laser-MAG hybrid welding,the ductile-brittle transition temperature of the weld zone of laser-MAG hybrid welding is higher,the size of the dimples in the fiber zone of the impact fracture is smaller and shallower,and the morphology of the radiation zone is smoother,which is more effective for cracks.The hindering ability is weak,and the low temperature fracture toughness is poor.Finally,the S-N curves of the two welded joints were obtained by the group method and thelifting method,and the reasons for the improvement of the fatigue performance of the pulsed laser-MAG composite welded joint were discussed by comparing the fatigue fracture morphology,the main crack growth behavior and the statistical analysis of the fracture porosity.Studies show that the average fatigue strength(N_f=10~7)of the pulsed laser-MAG hybrid welding smooth joint is 315 MPa,which is an increase of 23MPa,about 7.9%,compared with the 292 MPa of the laser-MAG hybrid welding joint.The fatigue source pores of the fatigue specimens of pulsed laser-MAG hybrid welded joints are smaller,and there are more fatigue bands and secondary cracks in the crack propagation stage,which are beneficial to improve the fatigue crack initiation life and crack propagation life,respectively.Less fatigue source pores and small size are the main reasons why the fatigue performance of B950CF high-strength steel pulsed laser-MAG hybrid welding joints is better than that of laser-MAG hybrid welding.