Research On Fracture Mechanical Properties Of Electron Beam Welded Joint For Thick Staless Steel

Heavy thickness austenitic stainless steels(ASS) are extensively used in industries such as national defense, nuclear power, and chemical engineering duo to their superior performance such as excellent corrosion resistance and heat resistance. Due to the high energy density, rapid welding speed and small deformation of electron beam welding(EBW), it is capable of forming large depth-to-width ratio weld and narrow heat affected zone(HAZ). Hence, EBW is known as the most suitable welding method for welding the thick plate. However, it is more difficult to weld and easer to form weld defects, leading to forming high gradient residual stress and microstructural and mechanical heterogeneity with the increase of steel thickness, which affect fracture behavior of the overall thick plate stainless steel welded structures. Even it is prone to fracture accident in the process of welding construction. Therefore, it is very meaningful to study mechanical property, impact toughness and fracture toughness of thick electron beam welded joint on the influence of structural integrity and fracture damage.The material used for this investigation was 304 grades of austenitic stainless steel plate(50mm thickness). Microstructure, mechanical property, impact toughness and fracture toughness of thick electron beam welded 304 stainless steel joint were analyzed, and the effect of microstructural, mechanical heterogeneity on fracture behavior were also discussed. It is hoped to obtain the factor that influence fracture damage of thick plate electron beam welded joint, which can provide reference for reasonable structure design of thick austenitic stainless steel welded in nuclear power and defense industries. The main contents and conclusions are as follows.(1) The microstructures along the thick direction of thick plate ASS electron beam welded joint were detailed analyzed. It is found that microstructure of the thick electron beam welded 304 ASS joint are heterogeneous along weld thick direction. The microstructures of EBW fusion zone consisted of austenite and ferrite. It was clearly observed that the cross-section morphologies of δ-ferrite along weld thick direction of fusion zone center were net, skeletal and dendritic shape. In the bottom layer center of the plane section microstructure, there are about 50-80μm width ferrite strip along welding position. Solidification mode of weld middle and top layer is FA mode while that of bottom layer is AF mode. The microstructures near the fusion line consisted of austenite and banded ferrite. The microstructures of weld consisted of columnar crystal and equiaxed crystal from the weld edge to the center direction. When the solidification is FA, the ferrite content is between 8% and 10%.(2) Mechanical properties of the thick electron beam welded 304 ASS joint are heterogeneous along weld thick direction. The microhardness increased along FZ center line from the top to bottom of weld, but the tensile strength and elongation showed a trend of decrease along the weld thickness direction. The tensile properties reflect large areas of the average mechanical properties, while the hardness test captures the performance of the local weld center. Tensile samples were all broken in the parent metal.(3) This paper measured the impact and fracture toughness with different loading direction along the weld thickness direction. The effect of microstructural and mechanical heterogeneity on toughness was studied. Meanwhile, the impact and three point bending samples’ fracture surfaces were also observed to reveal fracture feature of the welded joint. The change rule of different fracture toughness on microstructure, mechanical index and different position was got. It is found that impact and fracture toughness of weld are greater than the base metal. The impact toughness of weld decreased gradually along the weld thickness, the bottom impact toughness which gap is along the welding position even lower than the base metal. Impact toughness of HAZ and base mental have no obvious change. There was no evident difference of weld fracture toughness along the weld thickness direction. The fracture toughness values in weld are higher than that of HAZ and base metal.