Research On Creep Failure Behavior Of Full-size Pipe Elbow Of P92 Steel

With the rapid development of China's economy,power demand has risen sharply,but in the new situation,environmental protection requirements are netting nore strict.Generators with low elliciency and small capacity are increasingly unable to adapt to the requirelents of the times.Therefore,large-capacity,high-etticiency super(super)critical units are continuously built and put into production in China.As the main steel for the main steam pipeline of the super(super)critical unit.P92 steel is subjected to the harsh service environment such as high temperature,high pressure and corrosion.Once it is damaged,it will bring great economic losses and even casualties.Creep is one of the main failure modes in the service of P92 steel pipe elbow.Therefore,it is very important to study the failure behavior of P92 steel pipe elbow.In this paper,the study of creep behavior of P92 steel full-size pipe elbow was carried out.The experiment was interrupted 3 times for leasuring the hardness value and wall thickness value.Also,metallographic complexation of the surface were measured to study the microstructure changes during the creep process.The creep behavior has entered the second stage of creep.During the creep process,the creep of the inner arc is the largest,indicating that the inner arc is most concentrated.In the early stage of the creep experiment,the hardness value of the elbow rises and the hardness decreases again in the later stage,because of the retained austenite returning to martensite under the experimental conditions.At the back arc of the elbow,the wall thickness is reduced,and the wall thickness at the inner arc is increased.This is because the inner arc of the elbow is subjected to the tension and the inner arc is pressed during the thrust of the elbow.Analysis of the surface microstructure shows that at the beginning of creep,fine carbides are uniformly dispersed in the grain boundaries,and the grain boundaries are relatively clean.As creep progresses,the size of carbides in the grain boundaries gradually increases,but the number continues to decrease and eventually disappears.At the grain boundary,the carbides gradually increase and the size also increases.Based on the improved Norton-Bailey creep model,the finite element simulation of the full-scale experiment of P92 steel was earned out by ANSYS.The simulated creep strain curve was basically consistent with the experimental data,and the accuracy of the model was verified.At the same time,the development of creep damage by Von Mises stress and multiaxiality is analyzed by finite element calculation.The results show that the Von Mises stress has more influence on the damage than the multi-axis,but when the Von Mises stress level is equal,the more the multi-axis is,the faster the damage develops.The maximum position of the damage is the inner arc outer wall,which is consistent with the maximum creep strain position.With the proven creep model,the finite element analysis of the pipe elbow subjected to internal pressure shows that under the same internal pressure,the stress level of the elbow is greater than that of the straight pipe,and the smaller the diameter-thickness ratio,the higher elbow stress level.Ellipticity has a great influence on the stress distribution of the elbow.Because of the ellipticity,the maximum force point of the elbow changes from the inner wall of the inner arc to the inner wall of the side arc.Considering these effects,a creep life assessment scheme of the internal pressure elbow is proposed based on the new method,and a mathematical model of the stress level of the internal pressure elbow is given.