The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment.During the isothermal aging stage,the amount of a-phase was significant...The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment.During the isothermal aging stage,the amount of a-phase was significantly increased at beginning and gradually became constant.The aging time only slightly affected the size and morphology of the σ-phase.Conversely,during the isochronal aging stage,the amount ofσ-phase grew rapidly with the increase in the aging temperature.The σ-phase with a large amount of stacking faults was prone to nucleate around the（Ti,Mo）C particles or at the grain boundaries.The（Ti,Mo）C particles can act as effective nucleation sites,where the σ-phase directly precipitates from the austenitic matrix.From this work,the growth of σ-phase is found to be influenced by the aging temperature,and a new mechanism of σ-phase precipitation from austenite has been proposed.展开更多
The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels（lean DSS）were examined after aging the materials at 800 ℃.Owing to the instability of ferrite,all DSS are known to...The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels（lean DSS）were examined after aging the materials at 800 ℃.Owing to the instability of ferrite,all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 ℃ and different secondary phases may form,depending on composition and microstructure.The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS,but only information on the equilibrium microstructures were achieved.Therefore,the materials were aged at various times,in order to verify the simulations and determine the precipitation kinetics.The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements,determining phase type,composition and volumetric fraction.At 800 ℃,grade 2101 was found to be only affected by Cr_2N nitrides precipitation,whereas a significant amount of σ-phase was found to form in LDX 2404 for treatment longer than 1 h,almost totally replacing ferrite after 50 h.Up to now,the intermetallic σ-phase has been observed only in the high alloyed DSS,and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.展开更多
The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ agin...The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ aging on the α'-martensitic transformation of the as-welded and the SHT weld metals were investigated.The results showed that the weld metal had poor thermal stability of austenite.The precipitation of NbC during the 850℃ SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α'-martensite.The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α'-martensite at the early stage of 550℃ aging.The homogenized diffusion of C and Cr in the matrix during 550℃ aging led to the restoration of austenitic thermal stability and the decrease of α'-martensite content.The C-depleted α'-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix,while the Cr-depleted α'-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.展开更多
文摘The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment.During the isothermal aging stage,the amount of a-phase was significantly increased at beginning and gradually became constant.The aging time only slightly affected the size and morphology of the σ-phase.Conversely,during the isochronal aging stage,the amount ofσ-phase grew rapidly with the increase in the aging temperature.The σ-phase with a large amount of stacking faults was prone to nucleate around the（Ti,Mo）C particles or at the grain boundaries.The（Ti,Mo）C particles can act as effective nucleation sites,where the σ-phase directly precipitates from the austenitic matrix.From this work,the growth of σ-phase is found to be influenced by the aging temperature,and a new mechanism of σ-phase precipitation from austenite has been proposed.
文摘The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels（lean DSS）were examined after aging the materials at 800 ℃.Owing to the instability of ferrite,all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 ℃ and different secondary phases may form,depending on composition and microstructure.The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS,but only information on the equilibrium microstructures were achieved.Therefore,the materials were aged at various times,in order to verify the simulations and determine the precipitation kinetics.The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements,determining phase type,composition and volumetric fraction.At 800 ℃,grade 2101 was found to be only affected by Cr_2N nitrides precipitation,whereas a significant amount of σ-phase was found to form in LDX 2404 for treatment longer than 1 h,almost totally replacing ferrite after 50 h.Up to now,the intermetallic σ-phase has been observed only in the high alloyed DSS,and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.
基金supported by the National Key Research and Development Program of China(2018YFA0702902)the Innovation Project of Shenyang National Laboratory for Materials Science(SYNL-2022)the China Institute of Atomic Energy(E141L803J1).
文摘The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ aging on the α'-martensitic transformation of the as-welded and the SHT weld metals were investigated.The results showed that the weld metal had poor thermal stability of austenite.The precipitation of NbC during the 850℃ SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α'-martensite.The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α'-martensite at the early stage of 550℃ aging.The homogenized diffusion of C and Cr in the matrix during 550℃ aging led to the restoration of austenitic thermal stability and the decrease of α'-martensite content.The C-depleted α'-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix,while the Cr-depleted α'-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.
