| CAP 1400 is the third-generation passive pressurized water reactor nuclear power plants that developed by china.Its reactor coolant pump(the main pump)has two schemes:the canned-motor pump and wet winding motor pump.The shielding of the canned-motor pump is made of Hastelloy C-276 plates in 0.4-0.7 mm thickness.And it is one of the critical components for the main pump safety.Researches have proved that Hastelloy C-276 weldments by autogenous laser welding have good performance,and laser welding is an appropriate technique for welding such thin sheets.However,autogenous laser welding has high demands for preparation and clamping.The weld sag defect is a serious problem in long butt joints of thin sheets,and it restricts the application of laser welding in main pump shielding manufacturing.In the present work,laser welding with filler wire was applied in welding of Hastelloy C-276 thin sheets to eliminate weld sag defect.The theoretical groundwork of precision welding of Hastelloy C-276 canned-motor pump shielding was provided by studying the weld seam formation mechanism,analyzing the molten pool flow characteristic and evaluating the cavitation erosion resistance of the weld joint.The main contents and results are as follows:(1)The influence of pulsed parameters and butt gap on the weld formation were studied.A 3D transient model with molten pool free surface captured was built to analysis the molten pool characteristics.The results showed that pulse frequency and pulse width had similar effect on weld geometry due to they had same effect on duty-ratio.The non-destructive flaw detection results indicated that the weld seam met the ASTM standards.And the addition of filler metal increased butt gap tolerance significantly,the weld sag defect was eliminated.The simulation results indicated that the molten pool flow pattern was dominated by surface tension,and also affected by filler metal.The droplets reinforced the downward flow in the molten pool,so that the flow field and temperature field were changed.The downward flow developed more fully with wider pulse duration or higher pulse frequency which resulted in deeper penetration.And the heat transition from molten pool center to edge at lower surface was strengthened by the outward flow and resulted a wider lower surface than upper surface.The welding process was recorded by a high-speed camera.And it was found that within one laser pulse,the metal vapor plume expanded rapidly at first,then decreased to a small volume.After that the metal vapor plume would expand again until a near steady state was achieved.The filler metal was melted into the molten pool at front edge.(2)The influence of pulsed parameters on the weld fusion zone microstructure was studied.The microstructure evolution mechanism was further analyzed by molten pool melting-solidification simulation.The results show that the microstructure in the fusion zone was finer than that of base metal.There is no obvious HAZ near the fusion line.The grain in the weld coarsened with the increase of pulse duration and pulse frequency,but the distribution characters of mircrostructure remained unchanged.The grain boundaries of base metal near the fusion line might partially melted with large duty-ratio,for the low melting-point solute or impurity elements that segregated in the weld metal grain boundaries could be transported down the boundary pipeline into the base metal grain boundaries.The simulation results showed that the melting-solidification pattern might change with the increase of duty-ratio.The cooling rate changed with pulse duration or pulse frequency.The cooling rate changed from 10~4 K/s to 103 K/s with the increase of pulse duration or pulse frequency in this research and resulted in the coarsen of the grain in the weld.The solidification rate R was more sensitive to laser pulse parameters compared to temperature gradient G,and it was the main factor of the fusion zone microstructure evolution.The Mo element microsegregation and size of the precipitates both decreased with lower pulse duration or frequency for the higher cooling rate.So,both the amount of precipitates and the forming tendency of chain form precipitates were suppressed with lower pulse duration or frequency.(3)A vibratory cavitation erosion apparatus was built according to ASTM G32-10 standard,the cavitation erosion behaviors of the weldment were studied,and the cavitation erosion(CE)resistance of the weld metal and base metal were compared.The results indicated the tested weldment exhibited two stages:the incubation stage and the maximum erosion rate stage.There was no significant mass loss during the incubation stage,and the incubation time is about 2 hrs.The mass loss rate remained unchanged during the maximum erosion rate stage.The tested surface exhibited plastic deformation at first during incubation stage,the plastic deformation led to grain boundary extrusion.Then crack nucleated along the grain boundaries due to high stress concentration.Finally,the crack propagation resulted in mass loss and thus the CE damage occurred.Morphology like fatigue striations can be found on the damaged surface,indicated that the CE of C-276 had fatigue failure forms.For the weld metal,due to the microstructure inhomogeneity,regions showed the highest damage intensity located where the columnar dendrites grown towards to weld center interacted with the columnar dendtrites grown along the welding direction.The CE resistance of the weld metal was better than that of the base metal.This was mainly because the finer microstructure of weld metal and the large amount low-angle boundaries in weld metal resulted in miner grain boundary extrusion than that of base metal,and the CE damage was delayed.The twin-boundaries of C-276 base metal were more susceptible to CE due to the higher stress concentration level at twin-boundaries of FCC structure grains than other area at the early stage of CE. |
PDF Full Text Request