| Control rods are the key parts of nuclear reactor, which controls the speed of chain reaction of reactor by driving device to adjust the depth of control rod into the fuel assembly, and then to control the output power. The control rod is made by loading Ag-In-Cd pellets into thin-wall stainless steel cladding(0Cr18Ni10Ti) and welding the top and bottom plug. Since control rods will run 10 to 15 years in reactor. The quality of control rods has a significant influence on safety and economical efficiency of the reactor. It is very common to cause crack while welding between thin-wall stainless steel cladding and end plug during manufacturing control rod. Cracks could bring seriously potential safety hazard to the operation of control rod in reactor, for example, cause reactor to stop accidentally or a large number of fuel assemblies to damage.Therefore, no cracks are allowed in weld specified clearly in control rod product specification.TIG welding equipment with flowing Argon is used to ensure the quality of control rod assembly here. Analyzed the factors influencing on weld crack, such as gap between cladding and plug, characteristic of welding equipment, heat treatment before and after welding, welding process parameters, oxides and impurities in weld and form factor of the weld by performing Macro-structure analysis, metallographic test and SEM analysis to austenitic stainless steel welding joints. And discussed on weld cracks, especially, discussed on the formation mechanism of the liquefied crack and solidification crack. It is effective to avoid cracks by using general control measures for Welding process.Research shows, it will reduce the welding cracks to some extent by changing the fitting dimensions to be a clearance fit between the cladding and the end plug. Changes in welding heat input can also reduce the welding cracks.It cannot decrease the cracks to preheat before welding and keep heat afterwelding.Grinded and polished the tube ends, raised the peak value and reduced the base value of the welding current, raised the rotating speed, decreased the taper angle of electrode and grinded electrode tip to cone shape, changed the gap between the cladding and the end plug and welded on different welding equipments. The result indicates that these normal improvements on welding process can reduce occurred probability of crack, but all of those improvements cannot eliminate the welding crack completely. As the result of deep analysis, it shows that the fundamental cause of crack is because weld penetration is too big and fusion zone is beyond the fitting surface of end plug, welding metal in this fitting surface to form hot crack because of fragility of interface under the stress between cladding and end plug, and cause the cold crack.Base on the analytical results on formation mechanism of welding crack,the following process control methods are got to avoid crack: control welding current reasonably, matched the welding current with electrode taper angle and taper dimension, and use clearance fit between cladding and end plug. Weld penetration is controlled effectively and hot crack is avoided by using these control methods. As the result, this theory is applied to product control rod successfully. |
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