| Inconel690 alloy has excellent high temperature performance,corrosion resistance,oxidation resistance and other characteristics,which makes it widely used in key structural materials of nuclear power equipment,such as reactor containment,pressure vessel,heat dissipation evaporator,etc.These key equipments were under severe service conditions such as high temperature,high pressure,and severe corrosion,which put forward higher requirements for their welded joints.However,Inconel690 alloy and its matching FM-52 series welding consumables were very sensitive to Ductility-dip cracking(DDC),and DDC was a microscopic crack that is difficult to find,which will lead to serious safety hazards during service.In this paper,in order to improve the DDC sensitivity of the weld,on the one hand,the grain refinement was achieved by applying an external auxiliary field,such as the introduction of arc pulse and vibration auxiliary field on the basis of conventional TIG welding;On the other hand,by introducing in-situ second-phase particles as a nucleation matrix,the nucleation rate could be improved,the grains could be refined,and the migrating grain boundaries could be "pinned" to hinder the propagation of DDC cracks.The pulse parameters such as peak current,base current,pulse frequency and the ratio of base current to peak current were studied.It was found that the stirring effect on the molten pool was enhanced under large peak and base current.The penetration depth and width increased obviously,the area of equiaxed crystal zone increased,the dendrite arm spacing decreased,and the average grain size decreased.Compared with the medium frequency pulse(> 20Hz),the penetration depth and width,microhardness and strength under low frequency pulse(≤ 20Hz)were larger,which has a greater impact on the weld.If the frequency was too high,the pulse energy per unit time would be reduced,and the stirring effect of pulse arc on molten pool would be weakened.When the pulse frequency was 8Hz,the dendrite arm spacing and average grain size were small,the microhardness was large,188 HV,and the maximum tensile strength was 794 MPa.At this pulse frequency,increasing the base current and peak value could significantly increase the heating and melting effect of arc on base metal and greatly improve the efficiency of TIG welding.The vertical mechanical vibration parameters such as vibration acceleration and vibration frequency were studied under different welding line energy.Compared with the condition of small linear energy(1152J/cm),the mechanical vibration under large linear energy(1500J/cm)had a more significant effect on the weld formation,microstructure and properties.The large linear energy and small vibration acceleration(30.4mm/s2)significantly increased the penetration depth and the penetration width,and the penetration depth increased from 1.28 mm to 1.95 mm,an increase of 52.3%;the microhardness increased from 145 HV to 181 HV,an increase of 24.3%;the yield strength from 314 MPa to 370 MPa,an increase of 17.8%.When the vibration frequency was too large,the convection stroke caused by the vibration was reduced,and the scouring effect on the base metal was weakened,which was not conducive to the increase of the penetration depth and the broken dendrites.The large linear energy and small vibration acceleration(30.4mm/s2)significantly increased the penetration depth and the penetration width,and the penetration depth increased from 1.28 mm to 1.95 mm,an increase of 52.3%;the microhardness increased from 145 HV to181HV,an increase of 24.3%;the yield strength From 314 MPa to 370 MPa,an increase of17.8%.When the vibration frequency was too large,the convection stroke caused by the vibration was reduced,and the scouring effect on the base metal was weakened,which was not conducive to the increase of the penetration depth and the broken dendrites.Then,an in-situ Ti C second phase with a particle size of 4 μm and a volume ratio of 1.0%was tried to be introduced into the weld.The introduction of in-situ Ti C particles increased the tortuosity of the grain boundaries and refined the grains.The average grain size in the weld was reduced from 224.24 μm to 145.88 μm.The average grain size was reduced by 34%.Through the STF test,it was found that the critical strain of DDC increased after the introduction of Ti C particles in the range of 900 °C to 1200 °C.At the most sensitive temperature(1050 °C),the critical strain was increased from 2.4% to 3.5%(close to the performance of FM-82).The weld resistance to DDC is improved.Through the STF test,it was found that at the most sensitive temperature of DDC(1050℃),vertical mechanical vibration could significantly reduce the DDC crack sensitivity of the weld,and the critical strain range was increased from 2.0%-2.5% to 4.5%-5.0%.After mechanical vibration was applied,the grains of the weld were refined,and the DDC crack was more likely to end at the intersection of the three grain boundaries,and the DDC length was shorter under larger strain than under DC welding.But the pulse parameter had no obvious effect on it.At the most sensitive temperature of DDC(1050°C),vertical mechanical vibration could significantly reduce the DDC crack sensitivity of the weld,and the strain range was increased from 2.0%-2.5% to 4.5%-5.0%,which was nearly doubled.After mechanical vibration was applied,the grains of the weld were refined,and the DDC crack was more likely to end at the intersection of the three grain boundaries.The DDC crack was shorter under larger strain than under DC welding.But the pulse parameter had no obvious effect on it. |
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