Numerical Simulation And Experimental Research On Welding Process Of The Spiral Case-Stay Ring

In the hydropower station,the spiral case and stay ring are important components of turbine.The metal spiral case as the overflow structure is associated with the load-bearing structure stay ring by welding.The welding of the spiral case-stay ring is a complicated process.In the welding process,the temperature and phase transformation will change with the input of heat source.Due to the concentrated input of the high heat source,the weld zone will melt under the high temperature and then solidify with the decrease of temperature and the phase transformation.In the process of turbine operation,with the spiral case-stay ring running in high speed,it bears the complex loading and the welding defects is enlarged,which result in the cracking generation at the welding joint of spiral case-stay ring.The efficiency and stability of hydraulic turbine depend directly on the performance of welding joint between spiral case-stay ring.In this paper,ANSYS software is used to study the welding process of spiral case-stay ring.Considering the temperature field for the stress field,the welding temperature field of the spiral case-stay ring joint is simulated.On this basis,the stress of the spiral case joint is further studied.Distribution of strain and its results are analyzed.At the same time,temperature measurement,metallographic analysis and residual stress measurement are analyzed.The results are showed that:(1)In the welding process,the heat diffuses from the weld zone in the surrounding area.The temperature has the greatest influence on the near weld zone,while the temperature far from the weld zone is less impact.With the movement of heat source,the temperature gradient changes greatly at the beginning of welding,and the welding temperature rises rapidly in the short time,and then gradually cools.The temperature peak in the center of the weld is the largest,and the area around the weld is 10 mm away from the center of the weld.The temperature on both sides of the weld is 20 mm away from the center of the weld,and the temperature near the stay ring is higher than that near the spiral case.(2)According to the simulation result of welding stress field,the stress of metal inthe welding pool is zero.In the heating stage,there are large stresses region around the welding pool,and the larger stress areas mainly appear on the completed welds.The magnitude of stress values depends on the distance from the center of the welding pool.The stress value near the weld zone is larger than that far away from the weld zone.And the stress value gradually decreases with the increasing distance.In the cooling stage,the change is consistent with the heating stage.The difference from the heating stage is that the overall stress value is greater than the stress value of the heating stage.In the welding process,whether residual stress or transverse residual stress,there is a long tensile stress in the middle zone,and the fusion zone of the spiral case side weld is larger.The longitudinal residual stress on the volute side is larger than that on the ring side of the same seat,while transverse stress shows the opposite changes.The principal stress of welding residual stress is the largest in the weld zone,and decreases with the increase of the distance from the center of the weld.(3)According to the experiment,it can be concluded that the simulation result of the welding process temperature and residual stress is almost same with the test.The metallographic structure test shows that the surface structure of the sample is a mixture of fine pre-eutectoid ferrite and pearlite precipitated,which result from the flattened primary austenite grain interface,and the distribution of the structure presents a certain orientation.While the middle structure of the sample is a mixture of equiaxed ferrite and pearlite,and the surface hardness value is small.