Microstructure and Properties of 304L Stainless Steel Local Dry Underwater Laser Welding Joint with Filler Wire
At the 75th United Nations General Assembly,China announced that its carbon dioxide emissions would peak before 2030,and the nation will achieve carbon neutrality before 2060.In this context,nuclear power,as a clean,safe,efficient,and stable green and low-carbon energy source that can be developed on a large scale,can play a significant role in promoting green development and helping to achieve the"dual carbon"goals.During the long-term service of nuclear power plants,the stainless steel cladding of the spent fuel pool experiences aging effects,and its failure mechanisms primarily include mechanical impact,uniform corrosion,stress corrosion cracking,and pitting corrosion.Reliable underwater maintenance technology is crucial to ensure the safe operation and smooth life extension of nuclear power plants.Underwater laser welding,a relatively efficient in-situ repair technology,has the advantages of accurate heat input,low residual stress,high welding quality,and fully automated welding process,in addition to being less affected by water pressure.It has received widespread attention for underwater operations.Currently,there are few reports on the repair of crack defects at the L-shaped corner position at the junction between the bottom and wall of spent fuel pools in nuclear power plants.Therefore,a local dry underwater comer welding drainage cover was independently designed,and a local dry underwater laser wire-filling comer welding test was conducted on the 304 L stainless steel used for second-generation spent fuel pool cladding in nuclear power plants.The microstructures of the multi-layer and multi-pass fillet welds were analyzed using an optical microscope.Penetration testing was performed on the surface and cross-section of the weld using a DPT-5 dye penetrant.The micro-hardness distribution in different areas was tested using a micro-hardness tester.The polarization curves and Nyquist spectra of different regions in a 3.5％NaCl solution were measured using a VersaSTAT 3F electrochemical workstation.The results demonstrate that the forming quality of the fillet weld is high,and the metallurgical bonding is tight,with no obvious defects at both macro and micro levels.Owing to the protection of the pure argon gas environment,the surface of the fillet weld inside the underwater drainage hood presents a silver white fish scale as a delicate ripple.The penetration results revealed no obvious defects on the cross-section and surface of the weld seam.The microstructure is mainly composed of austenite and ferrite,and there is a clear boundary between the layers in the weld seam area.The heat-affected zone is not clearly displayed owing to the rapid cooling effect of water.The center structure of the fillet weld is mainly composed of vermicular ferrite,γ Austenite,and lath ferrite,and the crystal morphology is mainly equiaxed crystal.The fusion zone is mainly composed of vermicular ferrite and γ Austenite,with a small amount of feathery ferrite.The crystalline form is mainly columnar crystals,which grow perpendicular to the fusion line toward the center of the weld seam.The overlapping area is mainly composed of vermicular ferrite and γ Austenite.Owing to multiple heat accumulations in the overlap zone,it is equivalent to solid solution treatment on the surface of the weld,resulting in a decrease in the ferrite content and an increase in the austenite grains.The average micro-hardness values of BM,HAZ,and WM for multi-layer and multi-pass comer welding joints are 209 HV,226 HV,and 234 HV,respectively,with uneven distribution and an approximate M-shaped distribution.The polarization curve and Nyquist spectrum results indicate that the order of resistance to electrochemical corrosion in different regions is BM＞WM＞HAZ.Underwater laser wire-filling corner welding repair experiments were conducted using a self-designed comer welding drainage cover.A good process performance of the corner welding joints was obtained,which can provide a technical reference for the crack repair of spent fuel pools.
local dry underwater laser weldingspent fuel poolcorner repairmicrostructureelectrochemical corrosion
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