Microstructure And Mechanical Properties Research Of Welded Joint Of Composite Plate Based On Small Punch Test

Austenitic stainless steel composite plate has good weldability and machinability which is widely used in the petroleum and chemical industry.The 304 SS/Q345R composite plate is mainly based on Q345R and the 304 stainless steel composite layer is applied on the surface by explosive welding.It not only has good welding performance and high strength of carbon steel,but also has the characteristics of corrosion resistance,wear resistance,high thermal conductivity of stainless steel and greatly saves the cost.However,due to the complex structure of austenitic stainless steel composite plate,the relationship between thermo mechanical behavior and microstructure during welding are not clear,the distribution law of micro structure and residual stress after welding are still unknown,so further research is needed.This paper is based on theory of elastoplasticity,it uses a combination of test and simulation to study the 304 SS/Q345R composite plate.The small punch test is used to obtain the mechanical properties of the welded joints and transition layers.The thermal and physical properties of the transition layer were obtained based on data fitting,and the structure was optimized on the basis of the original model.The temperature field distribution and residual stress distribution at the welded joint are studied.The influence of the welding temperature field on the mechanical properties of the welded joint of the composite plate is discussed.Electron Backscattered Diffraction and Energy Dispersive Spectrometerare used to investigate the micro structure evolution law of welded joints.This paper aims to provide a theoretical basis for the welding process design and optimization of stainless steel/low alloy steel and other composite plates.In order to investigate the influence of the stress discontinuity between the base layer and the clad layer on the mechanical properties of the composite plate,the load-displacement curve of the welded joint was obtained by using Small Punch Test(SPT).The results show that the yield strength and tensile strength of the material at the junction between the base layer and the clad layer are smaller than those at other locations.The yield strength obtained from the small punch test and the relevant data of the corresponding displacement are fitted to find the stress Discontinuities are mainly concentrated on both sides of the boundary layer.Using ABAQUSto create a sequential coupling program of welding temperature field and stress field,the existing model was optimized based on the fitting data obtained from the small punch test.Using FORTRAN language to control the loading and movement of the heat source during welding,and obtains the welding thermal cycle curve and residual stress distribution at the joint.By comparing the numerical simulation results before and after optimization,the results show that the maximum stress of the welded joint is mainly concentrated in the arc starting position of the base weld,and the maximum is 321 MPa.The transition from the weld and the heat affected zone to the base metal zone During the process,the residual stress gradually decreased from the maximum value to 0 and tended to be stable.Due to the multiple welding of the multiple layers,the residual stress caused bending deformation to the base plate.There are stress discontinuities at the interface between the two materials,which can easily lead to the failure of the welded joint.By optimizing the model,the stress discontinuity can be effectively improved.The microstructure,morphology and element content of the welded joint were analyzed using a super depth-of-field micions are formed in the shape of slabs and needles,and the austenite near the fusion line is mainly distributed in columnar grains,and the average grain size is smaller than that in other regions.During the welding of the base layer,the welding heat of the base layer will affect the bevel of the double layer.The closer the bevel position is to the weld,the more uniform the austenite distribution and the smaller the grain size.During the welding process,the C element of the clad layer increased significantly at the weld,while the Cr and Ni content of the clad alloy element remained basically unchanged.This conclusion also shows that the composition of the base metal and the weld zone is very close,which has a positive effect on improving the quality of the weld.The EBSD electron backscatter diffraction technique was used to find that the grain boundaries in the weld region were mostly small-angle grain boundaries and heavy grain boundaries.The small-angle grain boundaries and heavy grain boundaries contributed to the occurrence of constant cracks.