Study On Creep Behavior Of Steel Used In Ultra-high Pressure Tubular Reactor

With the continuous growth of service time of ultra-high pressure tubular reactor,more and more researches pay attention to the safe operation of tubular reactor.During the operation of the reactor,the short time overheating reaction caused by thermal decomposition will cause mechanical damage and thermal shocks damage to the tubular reactor.According to the statistics,the creep and damage of reactor’s material caused by short time overheating are the major factors which influence the safety of polyethylene reactor.Therefore,in order to accurately evaluate the life of the polyethylene tubular reactor and ensure its secure and efficient operation,it is particularly important to study the creep performance and damage of the tubular reactor material under the action of short time overheating.In this paper,the microstructure,hardness,fracture morphology and creep damage behavior of 30CrNiMo8 steel used in ultra-high pressure polyethylene tubular reactor were studied,under different temperature,time and stress.The creep lifetime prediction model and damage decision method suitable for the material were established.The specific work of the paper is as follows:Firstly,the tensile properties,material composition and hardness test of 30CrNiMo8 steel were carried out to understand the basic properties of the material.The results show that,the mechanical properties of 30CrNiMo8 steel are similar to that of AISI 4340 steel.Secondly,the standard uniaxial creep experiments under different temperatures and stress conditions were completed,and the creep results were analyzed,the creep deformation mechanism of the material were explored,and the creep fracture mechanism and the microstructure change of the specimens was analyzed from the microscopic point of view.The results show that,At 550℃,the creep deformation mechanism is dislocation climb.At 600 ℃,the creep deformation mechanism is mainly dislocation slip.The mechanism of creep fracture is ductile fracture.Thirdly,the Theta method was used to predict the creep lifetime for the material,also,a modified Theta method for steel creep life prediction were established.Then,the creep damage model for the steel was established from the view of continuous damage,and the material constants of the model were calculated.The results show that,the modified Theta method has a higher accuracy of life prediction,and the creep damage model has certain applicability.Finally,according to the service conditions of the reaction tube during the thermal decomposition of polyethylene,a short-time overheating experiment for 30CrNiMo8 steel was designed.By observing the microstructure and hardness of the material after short-time overheating,the damage degree of the steel at different temperatures and times was obtained,and the effect of the short time overheating on the creep property of the steel was investigated.The results show that,when the temperature is below Acl temperature,the cementite will be coarsened at the temperature of the material.When the temperature is above Acl,the deterioration of the material is mainly due to the phase transition of the material,and the over-heating will reduce the creep life of the material.