【作者】 杨铁成；

【导师】 蒋家羚；

【作者基本信息】 浙江大学 ， 化工过程机械， 2006， 博士

【摘要】 随着我国石化企业的发展,压力容器等典型承压设备面临的环境越来越苛刻,尤其是高温、高压、超期服役等,这些设备一旦发生失效后果将不堪设想。作为高温、高压环境下典型的失效模式,疲劳、蠕变及其交互作用始终是摆在各国学者面前的难题。本文针对石化企业高温设备中广泛使用的1.25Cr0.5Mo钢,在对其进行了大量的疲劳蠕变试验和断口分析的基础上,分析了影响疲劳蠕变行为及其寿命的主要因素,建立能反映疲劳损伤、蠕变损伤及疲劳蠕变交互作用损伤相互转换关系的四种断裂特征图,提出了免于蠕变失效分析的判定条件,建立了疲劳蠕变寿命预测方法和损伤评估方法。主要研究内容如下: 实测了1.25Cr0.5Mo钢在不同温度下的单调拉伸力学性能和高温循环拉伸力学性能,得到了该钢的循环应力应变曲线表达式,同时发现材料在500～550℃下具有轻微的循环硬化特性。 根据石化企业高温承压设备的特点和1.25Cr0.5Mo钢的使用情况,确定了疲劳蠕变试验的控制方式、加载波形、试验温度、试验采用的最大应力。 通过温度、最大应力、应力幅、平均应力的不同组合试验,分析了这几种因素对材料疲劳、蠕变及其交互作用行为的影响规律,建立了四种疲劳蠕变断裂特征图,初步提出了免于蠕变失效分析的判定条件,同时发现平均应变速率是影响循环寿命的主要因素。 对各种加载条件下的疲劳蠕变交互作用断口形貌进行扫描电境分析和能谱分析,研究应力幅、平均应力等因素对断口形貌的影响,对应力控制模式下疲劳区(F)、蠕变区(C)及疲劳蠕变交互作用区(FC)的断口形貌和断裂机理进行了详细讨论,找到了疲劳促进韧窝和蠕变孔洞长大、蠕变促进夹杂物或基体脱落的证据,从微观形貌上证实了当发生激烈的疲劳蠕变交互作用时材料寿命急剧下降的试验事实,同时也提供了把应力幅大于平均应力作为免于蠕变失效分析判定条件的微观依据。 在充分认识疲劳、蠕变及其交互作用行为和主要影响因素的基础上,提出了三种疲劳蠕变寿命预测方法。一是从热力学基本定律出发,在能量法则的基础上提出了一种新的疲劳蠕变寿命预测模型,该模型具有一定的理论基础,推导严密,其寿命预测能力优于FS、SEFS方法;二是以半寿命处的平均应变速率作为控制参量,在损伤力学有效应力概念的基础上,推导出一种简单的寿命预测模型,该模型能将疲劳蠕变寿命与应力幅、平均应力、最大应力、温度之间的复杂关系转变为寿命与平均应变速率更多还原

【Abstract】 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.更多还原