| The new generation fast neutron reactors are designed to be operated under more serious conditions,including higher loop operating temperature,stronger container shields,and more corrosive liquid metals,which demond for higher performance of structural mateirals.Due to the good combination of mechanical properties,corrosion resistance,and excellent hot and cold working abilities,316H austenitic stainless steels have become one of the most important structural materials for reactor construction.Although the alloy should be a single-phase structure according to theoretical calculation,δ-ferrite could always be found in the industrial casting billet,and its content would increase with the increase of the size of the cast.In this study,the effect of δ-ferrite on the related properties of 316H austenitic stainless steels are systematically investigated by different analysis technology,such as scanning electron microscopy,electron backscatter diffraction,transmission electron microscopy,and three-dimensional atom probe.The following research results and conclusions could be drawn:(1)The formation mechanism of δ-ferrite in heavy-section cast and its effect on bending deformation behaviors and dynamic recrystallization behaviors have been studied.It is found that the δ-ferrite fraction shows a gradient distribution across the thickness of the～180 mm cast.The cellular shaped δ-ferrite with the faction of less than 3%is observed at the surface position of the cast,while skeletal shaped δ-ferrite with the faction of up to 8%is present at the center position.Macro-composition segregation across the thickness of cast changes the local Creq./Nieq.values,and finally results in the transition of solidification modes at different positions.The cooling rate has negliable effect on the formation of δ-ferrite at the surface position,while the slow cooling rate at the center position could effectively facilitate the reaction.During the cooling process after solidification,δ-ferrite could further decompose to σ phase,including direct transition(δ→σ)and eutectoid transition(δ→σ+γ2).When the plastic deformation takes place,stress concentrations near the σ phase are more likely to induce the initiation of microcracks,and the δ-ferrite at center position of the cast with continuous skeletal morphology could further accelerate the crack propagation,leading to the cracking phenomenon.In the hot working process,different from the discontinuous dynamic recrystallization based on the "grain boundary buldging"mechanism in austenite,δ-ferrite can effectively promote the dynamic recrystallization of austenite near the δ/γ phase interface,and the mechanism is continuous dynamic recrystallization.Thus,the dynamic recrystallization degree is significantly increased by the coupling effect of the two dynamic recrystallization mechanisms in the sample from center position of the cast,compared to that from the surface position.(2)The stabilities of δ-ferrite and austenite in plate during thermal aging at 550～950℃ are studied,and the coupling effects of temperature and stress on the microstructural evolution are further investigated.Different from the precipitation mechanisms near the grain boundary in austenite,δ-ferrite is dominated by the decomposition mechanisms,whose rate is usually accelerated with the increase of temperature.At 550℃,plenty of needle shaped Laves phase preferentially formed within the δ-ferrite due to the high supersaturation of Mo.With the increase of aging time,M23C6 carbides nucleate and grow around the Laves phase,finally results in the mixed structure of M23C6 and residual δ-ferrite,and do not change even after～10000 h.At 750℃,the element supersaturation in δ-ferrite decreases,accompanied by the increase of diffusion rate,and M23C6 carbide is preferentially formed at the δ/γ interface.After aging for 1000h,δ-ferrite finally decomposed into σ+γ2 structure.At 950℃,the diffusion rate of elements is further accelerated,and δ-ferrite could rapidly decompose into σ+γ2 structure after aging for 2 h.The decomposition behavior of δ-ferrite under loading at 550℃ shows that the dislocation density in δ-ferrite increases with the increase of stress level,which effectively promotes the precipiatation of intragranular Laves phase.Moreover,the precipitation of M23C6 carbide at δ/γ phase interface and austenite grain boundary is also obviously promoted in sample with higher stress level(～345 MPa),which is mainly attributed to the high diffusion rate of C atoms with the help of higher density dislocation in austenite.(3)The effect of δ-ferrite in solution-treated state and decomposition state on tensile properties,impact toughness,creep rupture life and fatigue properties of the alloy are investigated.In the tensile tests,it is found that the strength of the alloy is slightly increased by 4%δ-ferrite in solution-treated state(～20 MPa)due to its higher hardness,while the plastic is decreased.After thermal aging,decomposed δ-ferrite has no obvious effect on the tensile strength compared to the solution-treated state.The decrease of elongation is mainly attributed to the intergranular cracking of carbides at grain boundaries of austenite.In Charpy impact tests,δ-ferrite in the solution treated sample exhibits good plastic deformation ability and has little influence on the impact energy.However,during the therml aging process,the appearance of precipitates increases the hardness of decomposition of δ-ferrite,and the rapid initiation of microcracks around these brittle precipitates results in a siginificant decrease of the impact energy(from 351 J at solution treated state to 217 J after aged at 550℃ for 20 h,for example).With further increase of aging time,the impact energy tends to be stable.Moreover,the presence of δ-ferrite in solution-treated sample could increase the proportion of intergranular cracking in the creep tests,and the rupture life of the alloy is thus significantly reduced from 1838.4 h to 218.3 h.When δ-ferrite decomposes after aging treatment,the rupture life of samples could be further decreased to 104.6 h,which is mainly due to the preferential cracking at the decomposed δ-ferrite.In addition,during the fatigue crack propogation tests,the main crack could be deflected at the δ/γinterface,showing the high fatigue crack propagation resistance.However,in the sample aged at 750℃ for 10 h,the crack near the decomposed δ-ferrite changes to the flat path morphology because of the rapid initiation of microcracks near M23C6 carbides,and the fatigue crack propagation rate is thus increased.(4)The effect of δ-ferrite on the corrosion behavior of alloy both in solution treated state and thermal aged state in lead-bismuth eutectic with saturation oxygen at 550℃ has been investigated.The results show that 4%δ-ferrite has little effect on the oxidation rate of austenite,which only affects the local oxidation behavior,compared to that without δ-ferrite.In the soluton-treated state,due to the sufficient Cr content inδ-ferrite and the low formation energy of Cr2O3,a dense protective film of Cr2O3 can be formed at the δ/γ phase interface,which could block the outer diffusion of solute atoms and the inner diffusion of O.The resistance of δ-ferrite to lead-bismuth corrosion is significantly higher than that of austenite.When the δ-ferrite is decomposed after aging process,the presence of M23C6 consumes the Cr in the residual δ-ferrite,and the formation of Cr2O3 oxide layer is thus hindered.Furthermore,the interface between M23C6 and δ-ferrite could further provide a viable path for the internal diffusion of O.Both the oxidation of residual δ-ferrite and M23C6 accelerates the oxidation rate of decomposed δ-ferrite,however,the oxidation resistance of decomposed δ-ferrite is still better than austenite. |
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