| Main pipe is the core component of the nuclear reactor cooling system,and its quality is an important factor affecting nuclear power safety.Due to the increased installed capacity of the CAP1400 nuclear power system,the shape and size of the main pipe have changed greatly,which poses great challenges to manufacturing technology.In this paper,the entire forging process of the large scale CAP1400 nuclear power main pipe is studied from open forging to final forging and post forging heat treatment,so that to provide a theoretical basis and technical support for the forging process of the large nuclear power main pipe.The high temperature rheological behavior of as-cast 316LN austenitic stainless steel with coarse dendrite was studied by means of thermal simulation compression experiment.Based on the stress-strain curve,the constitutive model of the material based on strain compensation was established.The microstructure evolution behavior of the material under different deformation conditions was studied.The results show that the dynamic recrystallization percentage of as-cast 316LN steel during high temperature deformation shows a positively relationship to strain rate.The dynamic recrystallization kinetic model and the grain size evolution model were established according to the dynamic recrystalization behavior.And the process conditions for obtaining the complete transformation of the material from the as-cast coarse dendrite structure to the fine homogenized forged structure are that the forging temperature is>1100?and the strain rate is?1s-1.The dynamic recrystallization nucleation mechanism of the material was studied by EBSD analysis of typical structure.Based on the thermal simulation tensile experiment,the tensile fracture behavior of as-cast 316LN steel under different process parameters was studied.The results show that the tensile plasticity of the material increases with the increase of deformation temperature and strain rate.Through the combination of physical simulation with numerical simulation,the accuracy of six classical fracture criterion to predict the fracture behavior of as-cast 316LN steel was compared,and it is concluded that the accuracy of the Normalized Cockcroft&Latham fracture criterion is higher.Based on the criterion,the critical damage value of cracks initiation in as-cast 316LN steel during various deformation conditions was determined,and the mathematical model between the value and deformation parameters was established.The results show that with the increase of temperature and the decrease of strain rate,the critical damage value of as-cast 316LN steel decreases gradually,and the crack is more easily produced.Single-pass and double-pass high-temperature compression experiments were used to study the high-temperature rheological behavior,dynamic recrystallization behavior,static recrystallization behavior,and meta-dynamic recrystallization behavior of as-forged 316LN austenitic stainless steel.Through the analysis of microstructure,the grain evolution behavior of as-forged 316LN during deformation and deformation gap was studied,and the relevant dynamic kinetic model and grain size model were established.Based on the EBSD and TEM analysis of sample compresed microstructure,the dynamic and static recrystallization nucleation mechanism of forged 316LN steel and the role of twins in these process are revealed.The effect of heating temperature and time on grain growth was studied by solution treatment on samples with different initial grain size,and the corresponding grain growth model was established.Based on the change of the grain size and the corresponding properties of the sample after solution treatment,the optimal solution treatment process is obtained,which temperature is 1030??1050?,and the solution time should not exceed 5hours.For the grain size of each part of the finished product of the main pipe with the grain size?grade 2 after solution treatment,the maximum tolerance of the grain size before solution treatment,i.e.after forging is grade 2.5.Through the secondary development of DEFORM software material database,a complete microstructure evolution model of as-cast and as-forged 316LN steel was established.With the aim of fine grain size,sufficient recrystallization and no cracks on the surface of forgings,a method of determining reasonable technological parameters in the process of upsetting forging is proposed.It is concluded that when the strain rate is 0.1s-1,the forging temperature should be controlled in the range of 1100??1160?,and the strain should be controlled in the range of 26.6%?31%.Based on the requirements of the final grain size of the main pipe after final forging,the rounding process of the main pipe body and the local forming process of the nozzle are optimized.The optimal rounding process is the double press revolving mode,and the angle of V anvil is controlled to 120°,the amount of anvil connection length is 20%?30%.And a new method of forming the nozzle is put forward. |
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