Effect Of Welding Wires On The Mechanical Properties Of T91/316L Welding Joints And Corrosion Behavior Of The Weld Seam In Liquid LBE

T91 martensitic steel and 316 L austenitic steel are usually selected as structural materials of lead-cooled fast reactors(LFR)and accelerator-driven subcritical systems(ADS)due to their excellent high-temperature mechanical properties and excellent corrosion resistance.In order to meet the use requirements in actual production,different components are usually manufactured by different kinds of materials,such as the T91 linking pipes and the 316 L valves,inevitably involving a lot of dissimilar welding in production.Therefore,obtaining T91/316 L welding joints with high quality is a key process for the manufacturing of nuclear power components.As for the welding between T91 martensitic steel and 316 L austenitic steel,besides their big difference in chemical composition,their thermal conductivity and linear coefficient of thermal expansion are also quite different.These differences could lead to the rise of welding difficulty between T91 and 316 L.On the other hand,the coarsening tendency of martensitic grains during welding is obvious.Although some researches have been conducted about the TIG welding in the connection of martensitic steel and austenitic steel,there is few study about the selection of welding wires.However,for the dissimilar welding,the selection of welding wires could greatly affect the quality of welding joints.Suitable welding wire could inhibit the diffusion of carbon in the fusion zone and reduce the thermal stress in the dissimilar welding.So,in this paper,the microstructure and mechanical properties of T91/316 L welding joints using three different kinds of welding wires(ER309L,ER316 L and ERNiCr-3)were firstly investigated.And then,in order to study the corrosion behavior and investigate the corrosion mechanism,corrosion test in static lead bismuth eutectic(LBE)and corrosion test in flowing liquid LBE(relative flowing velocity: 2.98 m/s)of these three T91/316 L welding seams at 550? for 300 h,600 h,and 900 h were performed by using a self-designed rotating corrosion test device on the basis of previous research,which can provide useful data for the practical application of LFR and ADS in the future.The results showed that qualified T91/316 L welded joints can be obtained by using these three welding wires with appropriate welding parameters.In the as-welded condition,the tensile specimens were all brittle-fractured at the T91 side coarse grain heat affected zone(CGHAZ).After the post-weld heat treatment(PWHT),the fracture location of the tensile specimens was transferred to the 316 L base metal and the fracture mode was changed to toughness fracture with the increase of tensile strength being observed.The microstructure of welding metal was dendritic crystals with direction perpendicular to the fusion line.In the as-welded condition,the tensile strength of the welding joints was NiCr-3,309 L and 316 L in descending order.The tensile strength of all specimens after heat treatment was significantly increased,which was very close to the tensile strength of 316 L parent material.Before PWHT,the highest hardness was found in the fusion zone at T91 side,while the bump nearly disappeared after PWHT.In addition,in the static corrosion test,the surface features of the weld seam obtained with ER309 L,ER316L,and ERNiCr-3 wires in 550? oxygen-saturated liquid LBE for 300 h,600 h,and 900 h were mainly corrosion points and corrosion pits,while the corrosion surface was obviously directional in flowing liquid LBE.The EDS results of the cross-section of weld seam showed that a typical double-layer oxide layer was formed on the surface of all welds both in the static corrosion and the corrosion in flowing liquid LBE.The outer oxide layer was mainly contained by Fe and O elements with a small amount of Pb and Bi elements penetrating;the inner oxide layer was denser,mainly containing Cr,Ni,and O elements.Moreover,the corrosion mechanisms under these two corrosion conditions were similar,and they all showed the dissolution of matrix elements(Fe,Cr,and Ni)in liquid LBE and the diffusion of Pb,Bi,and O elements in the matrix at the solid-liquid interface.However,the high-speed LBE could promote elemental migration and accelerate the formation of oxide layers,thereby protecting the matrix from further corrosion.But on the other hand,the friction and wear caused by flowing liquid LBE could promote the corrosion of the samples.This study showed that the thickness of the inner oxide layer on all weld surfaces increased with time.In addition,by comparing the thickness of the inner oxide layers,the 309 L welding wire showed the best corrosion resistance to both static liquid LBE and flowing liquid LBE while the NiCr-3 welding wire was the worst,and PWHT could improve the compatibility of the weld seam with the liquid LBE.