| Nuclear pump is the power transmission center of nuclear power plant,which has high requirements for sealing,stability,safety in operation.Nuclear pump shell,which generally made by austenitic stainless steel casting,is one of the key components to ensure these,and it is bulky and withstands many severe service conditions,such as high tension,vibration,corrosion and pitting corrosion of circulating water,etc.However,large steel castings are usually easy to form crack,porosity,inclusion,shrinkage,porosity and so on,and repair welding is an important and indispensable technology to guarantee the performance and quality requirements castings for.For researching on the microstructure and properties of the repaired welding layer,and optimizing repair welding technology of nuclear pump casting shell,the 304 austenitic stainless steel plates,which is same as the nuclear pump in materials,have been welded to simulate the nuclear pump casing repair welding in this paper.By analyzing the weld joints of 304 austenitic stainless steel plates,the basic rules of microstructure and properties of the repaired welding layer are obtained to provide technical basis for making welding process of nuclear pump casting shell.In order to study the effect of different parameters and methods on repair welding,U type,V type,and X type grooves have been set up on stainless steel plates to study the shape and size of the welding seam and the arrangement of solder layer on the influence of repair welding in this paper.At the same time,shielded metal arc welding(SMAW)and gas tungsten arc welding(GTAW)have been used to weld stainless steel plates for studying the effect of welding methods on the microstructure and properties of the welding layer and its interface.Furthermore,some plates have been cleaned and the other haven’t been cleaned before welding to study the effect of cleaning on the structure and properties.Besides that,some plates have been treated by solution treatment and the other haven’t been treated by solution treatment after welding to study the effect of solution treatment on the structure and properties.The results showed that,relative to the samples have been cleaned before welding,the fusion zone and the solder layer concentrate impurity phase,ferrite content is slightly higher,and the impact toughness,the tensile properties and the bending properties of weld joint are poor in the samples,which haven’t been cleaned before welding.Compared with the samples not treated by solution treatment,ferrite content of weld zone and heat affected zone decrease,precipitated carbides dissolve,the ferrite is dispersively distributed in austenite matrix in a skeleton,the composition is distributed more even,the difference of hardness between the weld zone and the heat affected zone is smaller,the tensile properties,bending properties and impact properties are significantly increased,and the intergranular corrosion resistance is obviously enhanced in the solid solution sample.Compared with SMAW welding,the cooling rate of the weld pool area is faster,the ferrite tends to be lath shaped,the hardness and impact properties of the fusion zone are better,and the content of ferrite is lower in the GTAW.In addition,the wider and shallower of the U groove,the impact toughness is better;and the deeper and deeper of the groove,the impact toughness is worse.The accuracy of measuring ferrite content by metallographic method in the weld zone of cast austenitic stainless steel is worst at 100 times and best at 500 times,while the accuracy of measuring ferrite content in the heat affected zone is best at 100 times and worst at 500 times.Compared with the secant method,the accuracy of computer image processing method is higher in most cases. |
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