| Austenite stainless steel is widely used in nuclear power construction due to its high thermal strength,dielectric corrosion resistance and good weldability,however,due to long-term service at high temperatures,austenitic stainless steel would produce thermal aging,and its welds would also change accordingly.E316L stainless steel electrode is mainly applied to the first circuit system of the nuclear power plant,all the welding joints on the main pipe are running in welded state,the main equipment has a small area where the weld will be heat-treated at about 600?.This type of weld metal will appear thermal aging with the prolonged operation of the first circuit water environment at 310?,as the degree of thermal aging increases,it will threaten the safe operation of nuclear power plants.In order to provide reference for the normal operation of nuclear power plants,it is important to study the thermal aging of austenitic stainless steel welds.This paper selected the welds made by domestic E316L-16 electrode and imported ESAB company's E316L-15 electrode,the accelerated thermal aging test was performed on the welded state and 610?×16h heat treated state at 400? for 400h,2000h and 3500h.The changes of ferrite morphology and content,type and distribution of precipitated phases,and intergranular corrosion resistance were analyzed and studied by OM,SEM,TEM,EBSD and EPR test methods.The difference between welded and heat treated welds,between E316L-16 and E316L-15 welds was compared.The main results are as follows:(1)The solidification mode of the 316L stainless steel weld is FA.Two different forms of ferrite were formed in the weld,one is skeleton,which appeared in the previous weld layer,the other is strip,which appeared in the last layer.(2)Irrespective of the kind of electrode,the ferrite content in the weld changed from less to more and then less with the increase of the accelerated heat aging time;Morphological changes are:the strip ferrite from flat grain boundaries ? part non-flat grain boundary ? all non-flat grain boundary,the skeleton ferrite from intermittent ? more continuous ?intermittent;The change of the resistance to intergranular corrosion of the weld is from lower to higher and then lower.(3)Under the same heat aging conditions,compared with welded weld,the content of ferrite and every kind of precipitated phase in the heat treated weld are lower,and the intergranular corrosion susceptibility and corrosion degree are higher.(4)In the same processing state,the ferrite content in the weld of the E316L-15 electrode is slightly higher than that of the E316L-16 electrode,and the ferrite is coarser and the resistance to intergranular corrosion is better.(5)During the accelerated thermal aging test,the main precipitated phases in nuclear grade 316L stainless steel welds include Cr3C2,Cr7C3,Cr23C6,and ? phases.Cr3C2 and Cr7C3 precipitated mainly in austenite and diffusely distribute,and their content continuously increases;Cr23C6 and ? phases precipitated and aggregated at the grain boundary of ferrite-austenite firstly,then precipitated and grew up in ferrite grains,Cr23C6 increased to a certain extent and then slightly decreased,the content of ? phase increased first and then decreased.;the total content of precipitated phases has an increasing trend.The intergranular precipitation of Cr7C3,Cr23C6,and a phases caused intergranular corrosion of the weld. |
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