Fabrication And Properties Of 15-15Ti-Y Austenitic Stainless Steel In Lead-Cooled Fast Reactor

Lead-cooled fast reactors using liquid lead-bismuth coolant have attracted increasing attention worldwide due to their outstanding advantages in terms of economics,safety,and sustainability.However,the high temperature and irradiation dose combined with the strong corrosiveness,high density,and fast flow rate of the coolant in the lead-cooled fast reactor can cause severe corrosion of the cladding material under service conditions,resulting in the degradation of material performance and threatening the safe operation of the reactor.Therefore,the development of high-temperature,lead-bismuth corrosion-resistant,and radiation-tolerant cladding materials through composition design and microstructure control has been a research hotspot.In this paper,yttrium was introduced into a 15Cr-15Ni-Ti（1 =5-15Ti）austenitic stainless steel.A 50 kg 15-15Ti-Y steel was prepared by vacuum induction melting with fast pouring technology,hot forging and hot rolling.The microstructure was further adjusted by solid solution treatment,cold rolling,and annealing to achieve the uniformity and grain refinement.15-15Ti-Y steels with Y addition amounts of 0,0.25 wt.%,and 0.50 wt.%were named 15-15Ti00Y,15-15Ti-25Y,and 15-15Ti-50Y,respectively.The high-temperature mechanical properties,high-temperature stability,and lead-bismuth corrosion resistance of 15-15Ti-Y steels were evaluated by tests of high-temperature tensile,creep,aging,and lead-bismuth corrosion,and we further revealed the mechanism of the yttrium influence on the above key properties of 1515Ti steel.The research content and main conclusions are as follows:（1）Microstructure control and Y content optimization of 15-15Ti-Y.A microstructure control process of 1200℃ solid solution treatment for 1 h,80%deformation cold rolling,and 1000℃ annealing for 1 h was established for 15-15Ti-Y steel.The 15-15Ti-25Y steel had higher tensile properties at room temperature（RT）than 15-15Ti-00Y and 15-15Ti-50Y steels,with ultimate tensile strength,yield strength,and elongation of 651,340 MPa,and 52.0%,respectively.This may be related to the presence of smaller second phases in its matrix.For example,the number density of TiC in hot-rolled 15-15Ti-00Y steel was 3.10 × 10²⁰ m^-3,while the total number density of TiC and Y-O particles in 15-15Ti-25Y steel was 9.40 × 10²⁰ m^-3,which is nearly three times that of the former.The 15-15Ti-50Y steel,with banded TiC and coarse Y-O particles,had poorer tensile properties than 15-15Ti-25Y steel.Based on the microstructure and mechanical properties,the optimal grade selected was 15-15Ti-25Y steel.（2）High-temperature mechanical properties of 15-15Ti-25Y.The yield strength of 15-15Ti-25Y steel is consistently higher than that of 15-15Ti-00Y steel from 200-700℃.Both steels exhibit relatively low elongation and area reduction at 400℃ and 600℃,mainly due to dynamic strain aging（DSA）phenomenon.The calculated activation energies for dislocation motion in 15-15Ti-00Y and 15-15Ti-25Y steels at 600℃ are 184.4 and 225.9 kJ·mol^-1,respectively,indicating that more energy is required for dislocation motion in 15-15Ti-25Y steel.This may be related to the nearly three times higher density of second-phase particles in the matrix of 15-15Ti-25Y steel compared to 15-15Ti-00Y steel.The second-phase contribution to the yield strength of 15-15Ti-25Y steel at 600℃,calculated based on the Orowan strengthening equation,is approximately 105 MPa.The minimum creep rates of 15-15Ti-25Y steel are 9×10⁸s^-1 and 4×10^-7s^-1 at 650℃/250MPa and 700℃/200MPa,respectively,which is quite low.The stress exponents of 15-15Ti-25Y steel are 4.2,5.2,and 5.4 at 550℃,650℃,and 700℃,respectively,indicating that the creep deformation is dominated by dislocation creep mechanism.（3）Aging performance of 15-15Ti-25Y.The mechanism of Y addition on the aging performance of 15-15Ti steel mainly includes:（Ⅰ）inhibiting the recrystallization process and subsequent grain growth process.After aging at 500℃ for 1000 hours,15-15Ti-00Y steel had completely recrystallized,while 15-15Ti-25Y steel was still undergoing recrystallization at 600℃ and 700℃ after aging for 1000 hours,（Ⅱ）promoting the precipitation of second phases within the grains,which simultaneously decreased their size and increased their number density.After aging at 500℃,15-15Ti-25 Y steel had additional Chib precipitation compared to 15-15Ti00Y steel,and after aging at 600-700℃,the sizes of Ti（C,N）,Chib,and Chir in 15-15Ti-25Y steel were smaller than those in 15-15Ti-00Y steel.After aging at 500,600,and 700℃ for 1000 hours,the number densities of precipitates in 15-15Ti-00Y steel were 1.6×10¹³,6.7×10¹³,and 9.7×10¹³ m-2,respectively,while those in 15-15Ti-25Y steel were 2.2×10¹³,9.0×10¹³,and 20×10¹³ m-2,respectively,（Ⅲ）inhibiting the chain-like precipitation of M23C6 at grain boundaries.Almost no M23C6 was observed at grain boundaries in both steels after aging at 500℃.After aging at 600-700℃ for 1000 hours,the area fractions of M23C6 at grain boundaries in 15-15Ti-00Y steel were 6.0%and 13.6%,respectively,while those in 15-15Ti-25Y steel were 2.4%and 9.7%,respectively.The calculation results showed that the concentration of Y enrichment at grain boundaries was significantly higher than that of Mo,Ni,Cr,and Ti elements,indicating that Y was a strong grain boundary segregation element,which was beneficial to inhibit the formation and coarsening of intergranular M23C6 phase.The above advantages of Y addition led to the higher hardness,impact toughness,and uniform elongation of 15-15Ti-25Y steel than 15-15Ti-00Y steel after aging at 500-700℃ for 1000 hours.（4）Corrosion resistance of 15-15Ti-25Y against lead-bismuth eutectic（LBE）.Both 15-15Ti-00Y and 15-1 5Ti-25Y steels exhibited a dual-layer oxide structure,consisting of an outer magnetite layer and an inner Fe-Cr spinel layer.The thickness of the Fe-Cr spinel layer increased with increasing corrosion temperature and oxygen concentration,due to enhanced inward diffusion of O atoms.Compared to 15-15Ti-00Y steel,15-15Ti-25Y steel showed stronger corrosion resistance against LBE.This can be attributed to the lower oxidation rate and the more continuous and intact oxide layer formed on the surface of 15-15Ti-25Y steel,which was only 5.3 μm thick and consisted of a denser FeCr₂O₄ spinel layer with the addition of Y.The formation energies of the spinel oxide layers were calculated to be -18.70 kJ·mol^-1,but decreased to -38.56 kJ·mol^-1 with Y-doped,suggesting that Y addition promotes the formation of a denser spinel oxide layer.