Research On The Hot Rolling Interface Evolution Mechanism And The Properties Of Nickel/Nickel Alloy-Stainless Steel Bimetal Clad Plate

With the development of the economy and the increase in the number of people,clean energy will become a key factor in determining the future development of mankind.Among them,nuclear energy is considered an important part of clean energy.As a candidate for the fourth-generation advanced nuclear energy system,the thoriumbased molten salt reactor has the advantages of high inherent safety,low nuclear waste,and low proliferation risk.However,due to the high operating temperature and the strong corrosiveness of the heat transfer medium,the performance of its structural materials is facing severe challenges.Although UNS N10003 nickel-based alloy can meet the performance requirements of molten salt reactor structural materials,its cost is too high.Domestic and foreign studies have shown that bimetal composite panels can have the advantages of corrosion resistance,high temperature resistance and low cost.Therefore,the development of a bimetal composite plate for molten salt reactors has important theoretical and engineering significance for solving the economic problems of molten salt reactor structural materials.In this study,316 H stainless steel with excellent heat resistance was selected as the base material of the composite plate,and UNS N10003 nickel-based alloy and N4(pure nickel)were selected as the composite material.In the stage of exploring the hot rolling process,N4 was first used as the cladding,and the N4-316 H composite plate rolling experiment was carried out.The rolling process was designed by gradually increasing the reduction amount.The evolution of interface microstructure morphology,element diffusion mechanism and interface oxide generation mechanism during the rolling process have been studied.Then the effect of heat treatment process parameters on the mechanical properties of the composite plate was studied.On this basis,a composite plate rolling experiment with UNS N10003 alloy as the cladding was carried out to try to obtain a bimetal composite plate that can meet the application of hightemperature molten salt environment.First of all,this paper studied the basic oxidation property of pure nickel(N4)to determine its assembly pattern.The results showed that compared with stainless steel,pure nickel(N4)had poorer oxidation resistance and lower high-temperature strength.In order to protect the cladding during the rolling process,a symmetrical assembly pattern was used,which the N4 plate was placed on the inside and 316 H plate was placed on the outside.UNS N10003 contains about 6% of Cr and has high oxidation resistance.Therefore,when the alloy was rolled with 316 H,they could be placed on the inside or outside.In the hot rolling process test stage,firstly,N4 was used for composite rolling experiment,and the rolling process was designed by gradually increasing the reduction amount.The rolling was terminated when the plates were rolled to the 3rd pass,the 5th pass,and the 7th pass,respectively.And the plates were water cooled to room temperature.The evolution of the interface microstructure morphology,the element diffusion mechanism and the interface oxide generation mechanism were studied during the rolling process by sampling and analysis of every plates;what's more,the effect of heat treatment process parameters on the mechanical properties of clad plates was studied in a targeted manner.Finally,the rolling experiment of UNS N10003-316 H clad plate was carried out,in order to obtain a bimetal clad plate with strong corrosion resistance and excellent mechanical properties.The rolling process of N4-316 H clad plates was designed for 7 passes,the initial rolling temperature was 1200 ?,the total rolling reduction rate is 67%,and the rolling was terminated at 3,5,and 7 passes for sampling research.The study found that when rolling to 3 passes,the metals were bonded at close distances,the microstructure was slightly distorted,the plates on both sides of the interface were equiaxed grains,and the element diffusion was not obvious.When rolled to 5 passes,the grains of 316 H were elongated by the rolling effect,and lattice distortion appeared,and obvious diffusion was found near the interface.When rolled to 7 passes,the 316 H layer was a large number of elongated distorted grains,but the fine grain structure was mainly crushed at the interface,and the N4 layer was coarse grains.Ni,Fe and Cr near the interface Full diffusion,weak diffusion of Mo element led to enrichment at the 316 H side interface.The N4-316 H clad plate was in the physical contact stage and the physicochemical stage between 3 passes and 5 passes.The physical combination was the main function,and there was no obvious visible diffusion and metallurgical combination characteristics.By the end of the physical and chemical stage of rolling for 7 passes,diffusion began,and the main elements completed mutual diffusion at this stage,and finally formed a metallurgical bonding state.During the rolling process,Mn oxides were generated at the interface.Initially,due to the weak rolling reduction effect,oxides were distributed in the form of black particles along the interface direction;as the rolling progresses,the amount of oxides gradually increased.Due to the reduction effect,the oxides were distributed along the interface in long strips;in the later stage of rolling,the O element was exhausted and new oxides were no longer produced.However,due to the continuous reduction,the oxides were broken into fine oxides and distributed along the interface.The calculation of the Gibbs free energy of oxide formation showed that under low oxygen and high temperature conditions,Si and Mn were more susceptible to oxidation,but the content of Si in the material was very low,and the final result is Mn oxide.Three methods of stainless steel solid solution treatment,pure nickel annealing treatment,and stainless steel stress relief treatment were selected to heat the clad plates,and its influence on tensile properties and shear properties was studied.From the perspective of the metallographic structure after heat treatment,the solution treatment at 1060 ? could restore the 316 H stainless steel layer to its stable equiaxed austenite grains,eliminated work hardening and increased elongation,but the structure of the N4 layer became abnormally coarse.From the tensile test and shear test of the clad plates,the three typical heat treatment processes could meet the requirements of the national standard for the tensile strength,shear strength and elongation of the clad plates.Among them,the annealing treatment of pure nickel took into account the heat treatment effects of the two materials.After the heat treatment,the strength was the highest and the plastic toughness was also better;if the N4 layer only played a role in corrosion resistance,from the perspective of the mechanical properties and stability of the matrix,the stainless steel The solution treatment process also had its advantages.Therefore,the specific heat treatment process used in actual production should be subject to specific working conditions.After obtaining preliminary conclusions through the preparation of N4-316 H clad plates,rolling experiments were carried out on UNS N10003-316 H clad plates,and the effects of its interface characteristics,heat treatment process,aging and thermal cycles on the interface were studied.After the UNS N10003-316 H clad plate had undergone basic solid solution treatment,the main characteristics of the interface were that the two metals were far away from the interface where carbides were precipitated along the grain boundaries,there were a large number of complex structures with fine grains at the interface,and a large number of carbides were formed at the interface.Chains precipitated along the grain boundaries.The aging treatment released the stress of the clad plates,so that the diffusion effected at the interface was more thorough,the metallurgical bonding was stronger,and the carbide was larger;The samples processed by the cooling and heating cycle caused more carbides to precipitate at the interface.And the chain carbides precipitated along the grain boundary at the interface are finer.But far away from the interface,there are not as many carbides as the formers.In the end,the UNS N10003-316 H clad plate was treated under three actual working conditions,and its resistance to molten salt corrosion was tested and compared.The results showed that the clad plates were the most resistant to molten salt corrosion after long-term aging treatment;the resistance to molten salt corrosion was poor after cooling and heating cycle treatment.Molten salt corrosion experiments had been done for simulated heat treatment,and the results showed that the corrosion resistance of UNS N10003-316 H after damage was average,and the corrosion of the cold and heat cycle treatment group was the most serious.