Effects Of High Temperature Aging Treatment On Microstructure And Mechanical Properties Of 316L Weld Metal

316L austenitic stainless steels are widely used in nuclear power plants due to their excellent mechanical properties,corrosion resistance and.However,there is 5%-20%ferrite in the austenitic stainless steel weld.When the stainless steel containing austenitic and ferrite stays in the high temperature(>550℃),a large number of secondary phases will be precipitated in the ferrite,which may significantly reduce the toughness of the materials and threaten the safety of nuclear power plants.This work studied the aging treatment of 316 L stainless steel weld welded by argon atmosphere cold wire TIG welding at high temperature to explore the relationship between microstructure and mechanical properties of 316 L stainless steel weld and aging temperature.Optical microscope,scanning electron microscope(SEM),transmission electron microscopy(TEM)and electron back scattering were used to analyze the transformation of microstructure before and after aging.Charpy V type impact test and Vickers hardness tester were used to test the mechanical properties before and after aging.The precipitation behavior of ferrite in weld at different aging temperatures and its effect on impact toughness were analyzed.The original weld was treated with solution treatment at 1100℃.It was found that solution treatment could change the morphology of ferrite in the original microstructure of austenitic stainless steel weld.Aging treatment could not change the morphology of ferrite in weld.In the weld specimen aged at 800℃ for 10 min,it could be observed that granularχ phase and σ phase precipitated at the austenite/ferrite interface.Then,with the increase of aging time,σ phase grew up,and χ phase transformed to σ phase gradually.When the aging duration increased to 12 h,only σ phase was observed in the ferrite region.The contents of Cr and Mo in ferrite increased significantly after 12 h aging compared with before aging.The impact toughness decreases sharply after aging treatment,and the decrease is the most dramatic during 0-20 min,mainly because the ferrite area became brittle due to the rapid precipitation and growth of σ and χ phase.The effect of σ on the relative toughness is greater than that of χ phase.After aging treatment,the microhardness of the weld increases first and then decreases.The fracture of the weld after aging for 20 min or later showed the character of brittle fracture.With the increase of aging time,the weld fracture began to show the characteristics of "ridge" and "valley" and has direction.The ferrite zone exhibits brittle fracture characteristics,while the austenite zone tears in a plastic manner.The brittle ferrite was represented as a "valley" and the austenite region as a "ridge".After aging at 700℃ and 600℃,χ phase precipitated first at the ferrite/austenite interface.In addition,χ phase precipitated in ferrite at 600℃.Meanwhile the content of Mo in ferrite decreased and the content of Cr increased.With the increase of aging time,σ phase began to precipitate in part of ferrite,and all of the ferrite transformed to σphase.When σ phase precipitated in ferrite,the content of Mo in ferrite region increased.In addition,when the aging duration increased to 100 h,fine σ and χ phases began to precipitate at the austenite grain boundary.This was precisely why the plasticity of the austenite region was less affected,so that it was torn in a plastic manner.After aging at600℃,the impact energy of the samples decreased significantly,and the fastest decrease occurred in the first 100 h.The microhardness of the samples increased after aging.It could be observed from the fracture morphology that the specimens aged at600℃ all have a certain plastic deformation.When the aging duration increased to 60 h,the fracture began to show directivity.With the increase of aging time,the directivity becomes more obvious.According to EDS analysis of the fracture,it could be found that the brittle cracking area on the fracture is the ferrite region。By comparing the microstructure and mechanical properties of the weld after aging at different temperatures,it was found that the nucleation and growth rate of the precipitated phase increased with the aging temperature in the temperature range of600℃-800℃.The microhardness increased with the increase of aging temperature.The higher the aging temperature was,the faster the impact energy decreases with aging time.The higher the aging temperature was,the smaller the impact energy would be.Therefore,the post-weld heat treatment process of the weld should consider the aging precipitation behavior at high temperature,and the ferrite content in the weld should be strictly controlled.