Investigation Of Fatigue Creep Behavior And Life Estimation Of Pressure Vessel Steel 1.25Cr0.5Mo At High Temperature

With the development of petrochemical enterprises in China, the service environment which typical pressure-bearing equipment such as pressure vessel etc. faces becomes more and more aggressive, particularly high temperature, high pressure, overtime serviced equipment etc., once fails, will bring a consequence one can not bear think about. As the typical failure mode under high-temperature and high-pressure environment, fatigue, creep and their interaction are always a puzzle in the face of scholars of every country. With respect to the extensively used 1.25Cr0.5Mo steel in high-temperature equipments of petrochemical enterprises, based on enormous amount of fatigue creep tests and fracture analysis on them, the major factors of their exhaustion creep behavior and their service life are analyzed, four kinds of fracture characteristic charts that can reflect the relationship for mutual conversion among fatigue damage, creep damage and fatigue-creep interactive damage, the judgment condition for exemption from creep failure analysis is proposed and fatigue creep life prediction method and damage evaluation method are established in this paper. Major research contents are as follows:Monotonic tensile mechanical property of 1.25Cr0.5Mo steel at different temperatures and tensile mechanical property under high temperature cycle are measured, the expression for cyclic stress vs strain curve of this steel is obtained, meanwhile it is found this material has slight cyclic hardening characteristics under 500-550℃.According to the characteristics of high temperature pressure-bearing equipment of petrochemical enterprises and service conditions of 1.25Cr0.5Mo steel, the control mode for fatigue creep, load waveform, test temperature and maximum stress adopted in test are determined.Through tests with various combination of temperature, maximum stress, stress amplitude and mean stress, the influence law of such factors on fatigue and creep of material and their interaction are analyzed, four kinds of fatigue creep fracture characteristic charts are established, the judgment condition for exemption from creep failure analysis are proposed preliminarily, meanwhile it is found mean strain rate is the major factor that affects cycle life.SEM analysis and energy spectrum analysis are conducted on the fracture under fatigue creep interaction under various loading environments, the effect of such factors as stress amplitude, mean stress etc. on fractographic appearance is studied, the fractographic appearance and fracture mechanism in the fatigue area (F), creep area (C) and fatigue creep interaction area (FC) under stress control mode are discussed in detail, and the evidences that fatigue promotes growth of dimples and creep voids and creep promotes dropout of inclusions or matrix are found, thus confirming the test fact that the service of life of material sharply declines when there happens violent fatigue creep interaction, meanwhile providing the microscopic basis for taking stress amplitude being bigger than mean stress as the judgment condition for exemption from creep failure analysis.Based on adequate recognition of fatigue, creep and their interaction behavior and major influential factors, three fatigue creep life prediction methods are proposed. One isstarting from thermodynamic fundamental law and based on engine theorem, a new model for fatigue creep life prediction is proposed, this model has certain theoretical basis with rigorous derivation, its capability for life prediction is better than FS and SEFS techniques;the other is by taking mean strain rate at half value period as control parameter and based on the concept of effective stress of damage mechanics, a simple life prediction model is deduced, this model can convert the complex relationships between fatigue creep service life and stress amplitude, mean stress, maximum stress and temperature to simple linear relationship between service life and mean strain rate, thus obtaining a principal curve, its life prediction is better;the third one is based on exhaustion ductility theory a fatigue creep life prediction method is proposed, this method can be used for stress control mode and can comprehensively reflect the effect of stress ratio, loading speed, tensile load maintaining time and mean strain rate, the accuracy of life prediction by this method is apparently higher than other methods.Starting from the angle of damage mechanics, a damage mechanic model for fatigue and creep and their interaction is established that can comprehensively reflect the effect of maximum stress and stress amplitude, the change in non-elastic strain energy density is selected as damage parameter to describe damage, a unified expression for damage evolution of 1.25Cr0.5Mo steel at different temperatures and under different loading conditions is obtained, which is some helpful exploration made by us for application of damage mechanics in fatigue creep interaction.The innovation points in this paper:(1) The law of effect of temperature, maximum stress, stress amplitude and mean stress on fatigue, creep and their interactive behavior of 1.25Cr0.5Mo steel is systematically studied, four fatigue creep charts are established and judgment condition for exemption from creep failure analysis is proposed.(2) The test fact is verified that material service life will sharp decline when there happens violent fatigue creep interaction from the microscopic fractographic appearance of 1.25Cr0.5Mo under fatigue and creep and their interaction in stress control mode, the microscopic basis for taking stress amplitude being bigger than mean stress as the judgment condition for exemption from creep failure analysis;(3) Starting from thermodynamic fundamental law, damage mechanics and exhaustion ductility theory, three new prediction models for fatigue creep service life under stress control mode are established with better effect of fatigue creep service life.(4) Starting from the angle of damage mechanics, damage mechanic model for fatigue, creep and their interaction is established that can comprehensively reflect effect of temperature, maximum stress and stress amplitude, and by selecting the change in non-elastic strain energy density as damage parameter, the damage mechanic model for fatigue creep interaction is established that can comprehensively reflect the effect of temperature, maximum stress and stress amplitude.