| 316H which services in high temperature,corrosion and irradiation environment is main structural material used in construction of the sodium cooled fast breeder reactor in china,therefore,its welding material requires more strigent standards and acceptance criteria than other industries.Carbon plays an important role in 316 weld metal,maintaining its reasonable content can not only realize the weld metal and base metal strength match,but also guarantee absence of hot crack in weld metal during welding,therefore,studying influence of the carbon content on microstructure evolution and properties of nuclear grade 316 austenitic stainless steel weld metal is crucial in determining the chemical content of the welding wires.The thesis focused on 316 austenitic stainless steel weld metal,systematically studied the influence of carbon content on microstructure evolution,room and high temperature mechanical properties,intergranular corrosion resistance properties.The main research contents and conclusions of the thesis are summarized as follows:Three carbon contents 316 austenitic stainless steel welding wires were designed and fabricated according to available standards and calculation results by Thermo-Calc software.Three kinds of weld metals with different carbon contents were preparated by multi-layer and multi-pass TIG welding method.The microstructure evolution,mechanical properties and intergranular corrosion sensitivity of three kinds of weld metals during aging at 550? and 600? were characterized and analyzed.For the as-welded weld metal,with increasing C content,the yield and tensile strengths increased,while the elongation decreased owing to the increase of C solid solution strengthening effect.Moreover,both the high-ferrite content in low C weld metal and the precipitated M23C6 carbide in high C weld metal due to thermal cycles of subsequent beads deteriorated the impact energy obviously.The microstructure evolution of the three kinds of 316 stainless steel weld metals were studied by ? ferrite measurement,TEM observation and XRD residue analysis.The results indicated that during the aging process the rapid precipitation of M23C6 carbide occurred in ?-ferrite firstly owing to the high difusion rate of C.Once the carbon is depleted by precipitation of M23C6,the slow formation of phase occurred through eutectoid transformation(???+?)depending on the diffusion of Cr and Mo.Moreover,increasing C content promoted the formation of M23C6 carbides and inhibited the formation of a phase.Therefore,increasing C content accelerated the transformation of ?-ferrite in weld metal during aging process.Furthermore,after a long enough aging time,a transformation from M23C6 to a occurred.The variations of mechanical properties with aging conditions depended to a large extent on the microstructures at different aging conditions.For the low C weld metal aged at 550?,with the increase of the aging time,fine M23C6 first precipitated,then coarsened,after that a phase formed,which caused that the yield and tensile strengths first increased,then decreased,and finally increased slightly again.For the medium C weld metal,as the aging time increased,first the depletion of the solid solution C as a result the M23C6 precipitation deteriorated the strength,and then the formation of a phase improved the strength.For the high C weld metal,with the increase of the aging time,the depletion of the solid solution C and the coarsening of the M23C6 precipitates deteriorated the strength.Furthermore,with increasing aging time,both the precipitation and coarsening of M23C6 and increasing a phase content deteriorated the elongation and impact energy.For the low C weld metal aged at 600?,as the aging time increased,the increasing a phase content improved the strength obviously.For the medium and high C weld metals,as the aging time increased,first the depletion of the solid solution C as a result the M23C6 precipitation deteriorated the strength,then the formation of ? phase improved the strength.Furthermore,with the increasing of the aging time,the precipitation of M23C6 and a phase deteriorated the elongation and impact energy.Three-carbon-level 316H weld metals were tensile tested at 450?,550? and 650?.Yield strength and ultimate tensile strength of the as-welded deposits were proportional to carbon content,Elongation was inversely proportional to carbon content.The weld metals aged at 550? for 500h and 600? for 3000h were tensile tested at the same temperatures.Aging treatments led to yield strength of the low carbon weld metal tested at 550? was lower than that of the as-welded state,ultimate tensile strength of the low carbon weld metal was lower than the as-welded state over entire test temperature range,meanwhile,aging treatments led to yield strength and ultimate tensile strength of the high carbon weld metal were lower than that of the as-welded state over entire test temperature range.different changes of yield strength and ultimate tensile strength over the test temperature range between the low C and high C weld metals due to depletion of carbon and strain hardening effect caused by precipitation of M23C6 and ? phase.The aging treatments caused decrease of elongation in both the low C and high C deposits.Changes of 550? tensile properties of the low C and high C deposits with aging time at 550? were studied in more details.The creep rupture strength and creep ductility of the low carbon and the high C weld metals have been measured at 550? over the stress range of 290-316MPa and at 600? over 230-265MPa.The creep rupture strength of the high C weld metal was higher than that of the low C weld metal at both 550? and 600?.At 550?,as the decrease in the applied stress,the difference of the creep-rupture life between the two weld metals diminished due to the higher depletion rate of carbon by precipitation of M23C6 in the high C weld metal,while at 600?,the difference enlarged due to the massive precipitation of ? phase and extensive crack formation and propagation along?/austenite boundaries in the low C weld metal.For both the low C and high C weld metal,the decrease in ductility was adverse with the transformation percentage and related to products of the ?-ferrite transformation.The weld metals with different carbon contents were tested by H2SO4-CUSO4 solution corrosion method and double loop electrochemical potentiokinetic reactivation measurement method,the results indicated that at the aging temperature of 550?,the transformation of ?-ferrite in high carbon weld metal reached the stable state,therefore,M23C6 precipitated at the grain boundaries due to remaining carbon content in the austenitic matrix,while for the low carbon weld metal,M23C6 only precipitated around the ?-ferrite.The intergranular corrosion sensitivity of high C is more severe than the low C weld metal,because Cr depleted zone near ? ferrite is easier to recovery through the diffusion of Cr in the ? ferrite.At the aging temperature of 600?,the ?-ferrite in high carbon weld metal transformed completely and thus consumed more carbon in the austenitic matrix,which inhibited the precipitation at grain boundaries,while the larger volume of ?-ferrite in the low C weld metal completely transfored into ? phase leading to more severe intergranular corrosion sensitivity. |
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