| Thermal aging of nuclear stainless steels may occur during long term service in high-temperature and high-pressure water environment of 288～343℃,which will change the mechanical properties and stress corrosion cracking(SCC)properties and thus affect the safe operation of nuclear power plants.At present,a lot of researches on thermal aging has been done,but there are still many gaps in them.Firstly,the current researches mainly focus on the commonly used austenitic stainless steel rather than ferritic and martensitic stainless steel.Secondly,most studies mainly concern the mechanical properties of nuclear stainless steel while there is few data on the effect of thermal aging on SCC behavior of stainless steel in simulated nuclear power plant environment and the research needs to be further improved.For the above reasons,the 410S ferritic stainless steel,17-4PH precipitating-hardened martensitic stainless steel and 40Cr15Mo2VN high-nitrogen martensitic bearing stainless steel,as candidates for control rod drive mechanism(CRDM)material used in nuclear power plants,were subjected to accelerated thermal aging at 400℃ in air up to an exposure period of 3000 h and 10000 h.The effect of thermal aging on the microstructure,mechanical properties and SCC behavior of the three materials was studied,the thermal aging stability of the mechanical properties and SCC properties of them was comapared and discussed,and some guiding opinions were put forward for the application or optimization of them.The effect of thermal aging on the microstructure,mechanical properties and SCC behavior of the 410S was studied.The M23C6 carbides were stable in the early stage(i.e.,≤3000 h)and Oswald ripening of M23C6 carbides occurred in the later stage(i.e.,3000～10000 h).The Cr-rich α’ phases initially formed near the grain boundary and then spread to the interior of the grain with the increase of thermal aging time.At a shorter thermal aging time(i.e.,≤3000 h),the hardness and strength were basically unchanged due to the low proportion of Cr-rich α’ phase near the grain boundary while the impact absorbing energy and elongation decreased due to the local embrittlement of Cr-rich α’ phase.At a longer thermal aging time(i.e.,10000 h),the hardness and strength increased slightly owing to the hardening effect of Cr-rich α’phase forming in the grain,but the impact absorbing energy and elongation continued to decrease due to the combining effects of the growth of Cr-rich α’ phase and the Oswald rippening of carbides.Compared the unaged specimen,thermal aging of 10000 h promoted the SCC sensitivity of 410S by increasing the SCC crack growth rate(CGR)by 60%～90%,drifting the threshold stress intensity factor KISCC from 31.9～37.2 MPa(?)down to 21.5～26.9 MPa(?)and increasing the area fraction of intergranular SCC fracture from 30%to 43%.The Oswald ripening of carbides,the formation of Cr-rich α’ phase and the increase of yield strength were the main factors for the increase of SCC sensitivity.The effect of thermal aging on the microstructure,mechanical properties and SCC behavior of the 17-4PH was studied.Before thermal aging,the Cu-rich phase with a size of about 4 nm had the coherent relationship with the matrix and NbC.After 10000 h of thermal aging,the Cu-rich phase grew to 20～30 nm,and had the K-S orientation relationship(OR)with the matrix and the semi-coherent relationship with NbC;The G phases were found at the martensite lath boundary,lath interior,prior austenite grain boundary,δ-ferrite and the reversed austinite and had strict ORs with Cu-rich phase;Reversed austinites formed at the martensitic lath boundary and had the K-S,N-W or H-B ORs with the martensite matrix;M23C6 was found in local martensite matrix.The strengthening effect caused by spinodal decomposition,the precipitation of G phases and M23C6 was greater than the softening effect caused by the coarsening of Cu-rich phase and the formation of reversed austenite,leading to the increase of hardness and strength while the decrease of the impact absorbing energy and elongation with the increase of thermal aging time.The SCC test showed that the density of secondary cracks in the necking region and the average CGR of 10000 h specimen were 5.2 times and 28 times than that of unaged specimen respectively.The growth of Cu-rich phase,the precipitation of G phases and M23C6,the spinodal decomposition,and the increase of yield strength were the main reasons for the increase of 17-4PH SCC sensitivity.The effect of thermal aging on the microstructure,mechanical properties and SCC behavior of the 40Cr15Mo2VN was studied.In the early stage(i.e.,≤3000 h),the carbides and nitrides dissolved and the area fraction of them decreased by 14%.The Oswald ripening of carbides and nitrides and the enrichment of Cr and depletion of Fe in the martensite lath boundary occurred in the later stage(i.e.,3000～10000 h).The banded characteristics along the axial direction and distribution along the prior austenite grain boundary of carbides and nitrides did not change significantly during the whole process of thermal aging.In the first 3000 h,the area fraction of the carbides and nitrides decreased,and the decrease of the secondary phase strengthening was more than that increase of the solid solution strengthening,leading to decrease in strength.From 3000 h to 10000 h,the strength continued to decrease due to the dissolution of the small size carbides and nitrides and the reduction of Cr content in the matrix.Both the carbides and nitrides had Cr-depletion zone of about 30 nm at the interface with matrix.The content of carbides and nitrides was the main reason affecting the SCC resistance.The SCC sensitivity of unaged specimen was the highest due to the highest content of carbides and nitrides.After 3000 h thermal aging,the SCC sensitivity decreased due to the decrease of the content of carbides and nitrides.From 3000 h to 10000 h,the proportion of large size carbides and nitrides increased and had a higher splitting effect on the matrix,leading to a slightly increase of SCC sensitivity.In all samples,the secondary cracks propagated macroscopically along the banded carbides and nitrides in the L direction and microscopically along the prior austenitic grain boundary where the carbides and nitrides were distributed.Finally,the thermal aging stability of the mechanical properties and SCC properties of nuclear stainless steels was compared and discussed and some guiding opinions were put forword.For stainless steels with a lower interstitial atomic content(C+N≤0.09 wt.%),the higher Cr content was,the lower thermal aging stability of mechanical properties(hardness)was,namely 410S>17-4PH>AISI444>austenitic or duplex stainless steel.The mechanical properties of stainless steels with a higher interstitial atomic content(C+N>0.1 wt.%)had no significant relation to Cr content due to suppression of formation of Cr-rich α’ phase by interstitial atoms(i.e.,C and N).The mechanical properties of 40Cr15Mo2VN had the highest thermal aging stability due to the competition of the solid solution strengthening of interstitial atoms and the secondary phase strengthening of carbides and nitrides.Apart from the impact absorbing energy,410S had high thermal aging stability of mechanical properties.Due to the high content of Cr and the universality of microscopic changes during thermal aging,the 17-4PH had very low thermal aging stability.As a result,the thermal aging stability of the mechanical properties for the three stainless steels from high to low was:40Cr15Mo2VN>410S>17-4PH.According to the sensitivity coefficient of SCC and the ratio of CGR before and after thermal aging,the thermal aging stability of SCC resistance from high to low was:410S>40Cr15Mo2VN>17-4PH.The 410S should avoid violent impact during service;40Cr15Mo2VN could be kept in an environment below 450℃ for a period of time before service to improve the microstructure and property;the content of Ni,Cr and other elements of 17-4PH could be adjusted to optimize the performance. |
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