Study On The Toughening Mechanism Of 400 MPa Grade Low Alloy High Strength Steel Welding Materials

With the goal of carbon peaking and carbon neutrality,China is constantly trying energy solutions with lower carbon emissions.As a clean,low-carbon,safe and efficient energy source,nuclear power plays a pivotal role in the construction of Chinese energy system.Compared with developed countries,the proportion of nuclear energy in Chinese energy structure is relatively low,so it has great potential for development.The rapid development of nuclear power promotes the development of nuclear reactor pressure vessel（Reactor Pressure Vessel,referred to as RPV）towards large-scale and integration,so it also puts forward higher requirements for its supporting structure,of which the main connection method of support structure and RPV is welding,and the generation of welding defects will cause great hidden dangers to the safe service of equipment,so it is also necessary to ensure that the welded joints have good quality.In this paper,the effects of different W_Mn/W_Ni and W_Mn/W_Si on the microstructure and toughness of the deposited metal were studied by regulating the Mn,Si and Ni content and ratio,the changes in the microstructure and mechanical properties of the welding rod melted before and after heat treatment were compared,the effects of different W_Mn/W_Ni and W_Mn/W_Si on the microstructure and toughness of the deposited metal were studied,and the influence of chemical composition,microstructure on the strength and toughness of the deposited metal and the influence of heat treatment on the embrittlement of the melted metal were analyzed.The results show that:Three kinds of welding rods developed by independent design and development were used for welding tests,and three kinds of deposited metals were obtained,and their microstructure compositions were mainly a large number of acicular ferrite（AF）+quasi-polygon ferrite（QF）and a small amount of proeutectoid ferrite（PF）+ferrite side plate（FSP）.The results show that different W_Mn/W_Ni and W_Mn/W_Si mainly affect the proportion of AF+QF tissue,which in turn determines the low-temperature impact toughness of the deposited metal.When W_Mn/W_Si and W_Mn/W_Ni are 3.76 and 0.84,AF accounts for 39%,QF accounts for 34%,PF+FSP accounts for 27%,and the yield strength and impact force of the deposited metal are 581MPa and 128J,respectively,with the best strength and toughness matching and the best comprehensive performance.Due to the obstruction of small grain size grains and large grain boundaries,the crack propagation path of welded deposited metal is more tortuous and the impact energy is higher.After heat treatment,the deposited metal tissue recovers,the grain size becomes larger,and the number of high-angle grain boundaries decreases,which accelerates the expansion of cracks;At the same time,the decomposition of M-A constituents increases the precipitation of carbides on the grain boundary,and the embrittlement of the grain boundary and the reduction of the grain boundary binding force are the direct causes of the deterioration of toughness;In addition,during heat treatment,the dynamic dislocation is more likely to slip to the grain boundary,and it becomes tangled with the carbide,resulting in stress concentration;After heat treatment,the inclusions are more likely to fall off to form micro-voids,which may become crack sources or form microcracks during the fracture process.In the subsequent crack propagation,the microcracks are interconnected with each other and merge with the main crack to accelerate the expansion,and the impact toughness of the heat treatment state decreases significantly under the combined action of the above factors.The 2#welding rod is used to carry out docking test on 400MPa grade nuclear low-alloy high-strength steel and SA-738Gr.B steel,and the joint with good shape and no defects is obtained,and the bending resistance and hardness distribution of the joint are good,and the impact performance at different notch positions meets the requirements of the index.Further tensile test,the results showed that with the increase of the test temperature（25°C～300°C）,the tensile strength,post-break elongation and cross-section shrinkage of the joint before and after the heat treatment show a trend of first decreasing and then rising,and the yield strength and flexion ratio are gradually decreasing,and this change law is more obvious for the welded joint.Through the fracture toughness test,it is found that the tough brittle transition temperature at the center of the weld is-30.06°C,and the tough brittle transition temperature in the heat affected zone is-43.36°C,both of which are lower than the toughness transition temperature,reflecting good impact toughness,but the transition temperature range of the heat affected zone（9.57）is much smaller than that of the center of the weld（21.38）,so the heat affected zone is more likely to change from plastic to brittle.