Microstructures And Mechanical Properties Of Austenitic Stainless Steel Welds By Narrow-gap Gas Metal Arc Welding

In order to solve the problems of the thick plate Austenitic stainless steel weld structure for ITER, an ultra-narrow gap metal arc welding equipment is designed and manufactured independent.The thickness plate Austenitic stainless steel is welded by the automatic narrow-gap MAG technology in this research. The results showed that the large thickness plate316LN which belongs to the classification known as Austenitic stainless steel had been welded successfully using the automatic ultra-narrow gap MAG technology with low welding heat-input. Comprehensive mechanical properties of welded joints are ensured with good fusion and forming. Meanwhile, the high efficiency welding was achieved.And based on the relevant national standards, the thick plate Austenitic stainless steel weld joints are analyzed by the tensile tests, bending and impact tests. The results of bending tests showed that the narrow-gap MAG weld joints are ensured with good fusion. Compared to the traditional MAG weld method, the narrow-gap MAG weld joints with higher ultimate strength can be obtained by the narrow-gap MAG method. The results of low temperature impact tests showed that the impact value of the narrow-gap MAG weld joints are higher than the traditional MAG weld joints. With the increased of thickness from20mm to50mm, the impact value of traditional MAG weld joints decreased from105J to41.2J, low-temperature toughness of the weld reduced63.8%. With the increased of thickness, the narrow-gap MAG weld joints showed no trends of decreased. Even the impact value of two samples increased1.25J and17J.The solidification modes of weld metal are observed by the optical microscope and scanning electron microscope. The solidification mode of traditional MAG welds are mainly FA solidification mode, and one group is formed by F solidification mode. While the solidification modes of narrow-gap MAG welds center are maily A sodilification mode, the solidification modes of narrow-gap MAG welds fusion line are mainly AF and FA solidification mode. The SSGB SGB and MGB can be observed in the narrow-gap MAG weld metal.The microstructure of weld metal are observed by the optical microscope and scanning electron microscope. The columnar grains are in the area of weld joints near fusion line, the grains of area beside the columnar grains area are mainly large dendritic grains. While the grains of welds centre are mainly the small dendritic grains. And with the width of the columnar grains increased, the impact value of the traditional MAG welds decreased. The area of narrow-gap MAG weld joints near fusion line are columnar grains, but the range of this area is rather tiny. The grains in the area of weld center are mainly equiaxed grain.The microstructures of weld metal are observed by SEM. The results showed that the segregation manganese and molybdenum are severe in the traditional MAG weld metal. However, this phenomenon is not discovered in the narrow-gap MAG weld metal.The fracture mode of traditional MAG welds are brittle fracture, while the fracture mode of the narrow-gap MAG welds are ductile fracture, which indicated that the ductility of the narrow-gap MAG weld joints are much better than the traditional MAG weld joints.The traditional MAG welding method can be replaced by the narrow-gap MAG method. The weld joints of thick plate austenitic stainless steel with great mechanical properties can be obtained by the narrow-gap MAG method. A great influence on the product of thick stainless steel weld structure has been showed by the narrow-gap MAG.