Study On Local Mechanical Properties Of Nickel Base Welded Joint Based On DIC And Small Punch Test

Nickel based welding materials have exceptional strength and high temperature resistance.Considering repair welding strength or carbon migration of dissimilar joints,nickel based welding materials can be used in significant positions of equipment and repair engineering of welding cracks.Therefore,it is of great significance to study the mechanical properties distribution of welded joints using nickel based welding materials.In this paper,the distribution of elements and microstructure,tensile behavior and fracture mechanism of micro region at room and high temperature of15Cr Mo R welded joint with ENi Cr Mo-3 were systematically studied by means of digital image correlation(DIC)and small punch test(SPT)combined with microscopic analysis.The main research work and conclusions are as following:(1)Distribution of elements and microstructure of welded joints of nickel-based welding consumablesMetallographic observation and scanning electric mirror scanning(EDS)were carried out on each area of the welded joint.The results show that Fe and Cr are the main elements in the base metal(BM)and heat affected zone(HAZ)without segregation;Cr,Ni and Fe are the main elements in the weld metal(WM),with a certain amount of Nb and Mo and segregation;the secondary phase precipitation was observed by scanning electron microscope(SEM)on the weld metal surface.The difference of Nb/Mo content leads to different morphology of precipitation phase.The microstructure of BM is ferrite and pearlite,and that of fine grain heat affected zone(FGHAZ)is fine ferrite and pearlite,the coarse grain heat affected zone(CGHAZ)is an overheated structure,i.e.retained austenite+martensite,with black network troostite distributed around the coarse austenite.The main structure of WM is dendrite,and its size is related to the temperature gradient and Nb/Mo content.(2)Evaluation of local tensile properties of welded joints based on DIC technologyThe local mechanical properties of nickel-based welding joint were studied by DIC technology.The full-field strain of the welded joint was obtained through the DIC test and transformed into a local stress-strain relationship,and the incomplete local stress-strain relationship was extrapolated using the Considère criterion to obtain the tensile strength and complete stress-strain curve of each region of the welded joint.The results show that the yield strength?_yof the welded joint increases from BM to WM,with some dispersion in the WM position.Using the correlation between hardness and yield strength?_yand tensile strength?_us,the hardness data were correlated with the yield strength as well as the tensile strength to obtain an empirical equation for hardness and yield strength and tensile strength.(3)Evaluation of local tensile properties and failure mechanism of the welded joint at room temperature based on small punch testBased on the small punch test at room temperature,the distribution of yield load P_y,correlation parameter P_muof tensile strength?_us,maximum axial displacement u_mand fracture energy E_spwin different regions of welded joint were studied,and the correlation between P_yand yield strength?_yin different regions of welded joint was established.According to the relationship between the small punch test parameters and the conventional test results,it is found that:HAZ has higher?_yand?_usthan BM,BM and HAZ have similar plasticity and fracture toughness.The strength parameter of WM is higher than that of BM and HAZ,but the toughness parameter is lower than that of BM and HAZ.In addition,the failure mode and fracture mechanism of each region of the welded joint were studied by means of macro fracture observation and metallographic observation of small punch test samples.The results show that:the failure of BM is due to a large amount of plastic deformation leading to the formation of holes in the slip band and the formation of crack propagation,which eventually leads to ductile fracture;WM specimens undergo less necking deformation prior to fracture,with cracks crossing the primary dendritic tissue to form fracture failure,which takes the form of a mixture of through-crystal disintegration and ductile fracture.(4)Evaluation of local high temperature tensile properties and failure mechanism of welded joints based on small punch testBased on the small punch test at 400?and 500?,the distribution of P_y,P_mu,u_mand E_spwin different regions of welded joint and their evolution with temperature were studied.Combined with the relationship between small punch test parameters and conventional test results,it was found that:at 400°C,HAZ has higher?_y,?_us,plasticity and fracture toughness than BM.The plasticity,toughness and yield strength of WM are lower than those of other regions,and the tensile strength of WM is between that of BM and HAZ;at 500°C,HAZ has better?_y,plasticity and fracture toughness than BM,and?_usis similar to BM.The yield strength and plasticity parameters of WM are lower than those of BM and HAZ,the toughness parameter is between that of BM and HAZ,and the tensile strength parameter is higher than that of BM and HAZ;In addition,with the increase of temperature,the?_yof each region decreases,the plasticity of each region increases,the?_usof BM and WM decreases,the?_usof HAZ increases first and then decreases,the fracture toughness of WM increases,and the fracture toughness of BM and HAZ increases first and then decreases.By comparing the macro fracture of small punch test at room temperature,400?and 500?and combining with the micro fracture of SEM at 500?,it is found that BM and HAZ always maintain the macro characteristics of toughness from room temperature to 500?,and WM changes to the macro morphology of toughness;at 500?,the micro cavities of BM and HAZ samples expand and connect with the increase of plastic deformation,resulting in ductile fracture;at high temperature,the plasticity and toughness of WM samples were improved,a large number of dimples,a small number of cleavage platforms and cleavage cracks throughout the specimen's thickness direction are observed,showing a ductile brittle mixed fracture.Through this study,the distribution of elements and microstructure of15Cr Mo R welded joint with ENi Cr Mo-3 as welding material was revealed,and the methods for evaluating local mechanical properties of welded joint by DIC technology and SPT were established.The distribution of mechanical properties,the evolution of mechanical properties and failure modes with temperature from room temperature to 500?in different regions were systematically studied by means of DIC,SPT and macroscopic and microscopic fracture observation.The research work provides a theoretical basis for the design and safety assessment of Cr-Mo steel welded joints using nickel based welding materials.