| In nuclear power plants of pressurized water reactor(PWR), the joining between the dissimilar metals is an important technology with the characteristics of the complex processing, difficult implementation and high quality. So, it has still been a welding problem of manufacturing in nuclear power plants so far. Laser welding is a high quality, efficient and new welding method developed in recent decades. As the merits of deep penetration, high cooling speed and narrow heat-affected zone, it has become a focus in the field of laser processing and is been applied in automotive, shipbuilding, aerospace, nuclear power plants and other fields. For Alloy 52M/316 L dissimilar joints in the key component of the first loop of nuclear power plants, a new method of narrow gap laser welding with filler wire was proposed in this paper. The characteristics and microstructural transformation of laser welded dissimilar metals with filler wire were investigated, for providing technical supports of laser processing technology applied in nuclear engineering.The investigation on the welding processing and bead formation of laser welded Alloy 52M/316 L dissimilar joints with filler wire were carried out in this paper firstly. The results indicated that the laser welded joints were with distinct keyhole morphology and a deep overlap was present between two layers. The variation laws of weld height and width were revealed with laser power, welding speed and wire feed speed. The defects of incomplete fusion at the sidewall and porosity in the weld metal were illuminated. Microstructure and compositional distributions, the precipitation behavior of the secondary phases in the fusion zone were analyzed using OM, SEM and TEM. The microstructure and the precipitation laws of secondary phases at the interface of dissimilar welds were also researched. The interface microstructure of weld joints with electroslag welded(ESW) overlays and GTAW overlays were comparatively analyzed. Many small cellulars were found in the zones near the fusion line. With solidification preceded, cellulars turned into columnar dendrites. These columnar dendrites met in the center of the weld seam in symmetrical morphology at last. The equiaxed fine grains were formed in the remelted zones of multi-layer welds. In the interdendritic regions of fusion zone, Nb-rich precipitates, spherical composite oxides of Ti O2 and Al2O3 gathered together were revealed. It was found that Nb, Si, Cr and Fe element were enriched in the regions of ESW52M/WM interface and the Nb-rich phases were partially segregated in chains or combinations of particles. The secondary phases were precipitated and aggregated in ESW52M/WM interface at the solidification stage due to Nb, Si, Cr and Fe element enriched at the grain boundaries. Meanwhile, it was also found that due to the microstructural differences between GTAW and ESW52 M overlays, Nb-rich carbides or Laves phase did not appear at the GTAW52M/WM interface. Microstructure of WM/316 L interface was composed by tiny cellular with approximately 30-40μm width. The weld metal exhibited good compatibility with 316 L stainless steel, reflecting good interface binding. It was found that Cr and Mo element were enriched in the regions near WM/316 L interface, and the reason of σ phase precipitated in chains along the grain boundaries was revealed.Based on the morphologies, chemical composition and fractograph analysis of hot cracking in the fusion zone of Alloy 52M/316 L dissimilar joints, it was found that the metallurgical factors of solidification cracking were Nb-rich low melted eutectic phases and the interfacial oxides. The significantly regular dendrites were on the fracture surfaces with the characteristics of blunt and smooth grains and no torn edges. From the analysis of liquid flow and fillback in the formation process of solidification cracking, the liquid fillback speed vlx was derived. It was found that white precipitates in chain were distributed with in the zones of liquation cracking with the characteristics of low melted film. The grains on the fracture surfaces of the specimen containing liquation cracking were pebble-like and some liquid membrane appeared at grain boundaries. Chemical analysis shows that S-rich and Si-rich low melted eutectic phases and Nb-rich phases were precipitated on the edge of grains. The reasons of liquation film were induced by S-rich low melted eutectic phases and Nb-rich phases, and the mechanism of liquation cracking of Alloy 52M/316 L dissimilar welds were also illuminated in the views of the reasons of liquation film, constitutional liquation of Nb-rich carbides and liquation film migrations. The ductility dip cracking in the fusion zone of Alloy 52M/316 L dissimilar joints was initiated along the direction of columnar dendrite growth but could be deviated from the direction of dendrite growth. It was found that ductility dip cracking was mostly initiated at the trigeminal grain boundaries. The formation process of ductility dip cracking along grain boundaries was also considered to be the process of void nucleation, growth and coalescence at grain boundaries, and grain boundary sliding directly affected the void nucleation, growth and coalescence. The mechanism of ductility dip cracking in Alloy 52M/316 L dissimilar welds was explained in the perspective of the formation condition, the initiation and propagation.The results of mechanical properties indicated that microhardness of Alloy 52M/316 L dissimilar welds showed a downward trend across Alloy 52 M to 316 L stainless steel, and the average value of microhardness in the weld metal was 180 HV. The weak region of impact toughness was at ESW52M/WM interface. The reason of the toughness decrease was contributed to the presence of Nb-rich carbides or brittle Laves, oxide inclusions at ESW/WM interface. Fractograph analysis of the tensile specimens for ESW52M/316 L and GTAW52M/316 L dissimilar joints showed that the fracture mechanism of these two kind joints was both ductile fracture. However, it was also found that there were a lot of granular precipitates in the ESW52M/316 L joints, resulting in that ESW52M/WM interface was the weak zone in the welded joint. From the tensile test at the temperature of 350℃, it was revealed that the weak zone in the welded joint was on 316 L base metal and many voids were on the tensile fracture surfaces. This proved that the fracture mechanism was void aggregated ductile fracture. Electrochemical studies from polarization curves of the weld metal, in the corrosion solution containing Cl- and the mixture solution containing Cl- and S2O32-, showed corrosion features with multiple passivations, and the corrosion morphologies were also different. The laws of S2O32- promoting the anodic dissolution weld metal, delaying or impeding its passivation and reducing stability of surface passivation film were investigated. The corrosion mechanisms of Alloy 52M/316 L dissimilar weld metal in the two corrosion solutions containing Cl- and the mixed media containing Cl- and S2O32- were revealed, and the influences of microstructure, precipitates, and corrosion medias on the corrosion resistance of weld metal were studied. |
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