【作者】 范志超；

【导师】 蒋家羚；

【作者基本信息】 浙江大学 ， 化工过程机械， 2003， 博士

【摘要】 压力容器经常在高温、高压、具有腐蚀性的环境下工作，一旦发生事故后果不堪设想。本文以压力容器用钢16MnR为研究对象，对其进行了一系列的中温环境下的应力控制的低周疲劳试验和抗硝酸盐应力腐蚀试验，对其中温环境下的低周疲劳行为、环境温度对疲劳性能的影响以及抗硝酸盐应力腐蚀性能有了较为全面的认识，建立了典型中温环境下16MnR钢的低周疲劳寿命预测方法、低周疲劳设计曲线及低周疲劳损伤力学模型，主要研究内容如下： 详细分析总结了16MnR钢在300℃、420℃下的应力疲劳行为，包括循环硬化软化规律、应变范围及塑性应变范围的变化规律、应力控制下的循环蠕变现象和材料的Masing特性等等，为进一步提出16MnR钢的寿命预测方法和疲劳损伤评估方法提供试验数据和分析基础。 通过不同环境温度下的简单拉伸试验和应力控制下的低周疲劳试验，找出了环境温度对应力控制下的疲劳行为和疲劳强度的影响。发现当温度处于250～375℃之间时，材料在动态应变时效的作用下，疲劳强度有了较大的提高，出现了所谓的“篮脆”现象，这时材料表现为：较低的循环蠕变速率、循环相关硬化特性；当温度超出这个范围时，材料表现为：较高的循环蠕变速率、循环软化特性。 对考虑平均应力影响的疲劳寿命预测方法进行了分析总结，对总应变能密度疲劳寿命预测模型进行了修正，给出了不同应力比下的总应变能密度计算公式，给出了300℃、420℃两个温度环境下的疲劳寿命预测模型，预测结果与试验结果吻合较好。 建立了300℃、420℃两个温度环境下的应力寿命曲线；经平均应力的修正，并考虑各种因素对疲劳寿命的影响之后，与ASME低周疲劳设计曲线进行比较，建立了两种温度环境下的低周疲劳设计曲线；针对16MnR钢250～375℃之间存在的“篮脆”现象，以250℃、375℃作为材料疲劳强度的临界分界点，分别建立了250～375℃、375～420℃两个温度范围的低周疲劳设计曲线。 从损伤力学的角度出发，对X.H.Yang和王勇的耗散势模型进行适当改进，使其能综合反映应力水平、环境温度对疲劳损伤进程的影响；以等效模量来定义损伤变量，新损伤变量具有明确的物理意义，测量方法简单；在适当假设的基础上，得到了“篮脆”温度范围之外不同应力水平、不同环境温度下的损伤演化模型，为损伤力学应用到压力容器的疲劳分析和寿命预测做了一些有益探索。 进行了不同温度环境下的疲劳断口扫描电镜分析，对相同应力水平、不同温度下的疲劳断口形貌有了初步的认识。浙江大学博士学位论文 另外，通过16MnR钢抗硝酸盐应力腐蚀试验，初步认识了16MnR钢的抗硝酸盐应力腐蚀性能，测定了母材及焊缝在硝酸盐环境下的临界应力强度因子Klscr，和应力腐蚀裂纹扩展速率da/dt;同时对比研究了缓蚀剂、材质、溶液温度对16MnR钢硝酸盐应力腐蚀开裂的影响。 本文创新点为:对16MnR钢中温环境下应力控制的低周疲劳行为有了较为充分的研究，发现16MnR钢的“篮脆”现象，揭示了中温条件下的循环硬化软化规律、循环蠕变规律及Masing特性等;修正了能反映平均应力影响的总应变能密度疲劳寿命预测模型，并给出了16MnR钢300℃、420℃时的疲劳寿命预测方法;建立了基于中温应力控制低周疲劳研究基础上的16MnR钢250一375℃、375一420℃两个温度范围的低周疲劳设计曲线;以等效模量定义损伤变量，提出了1 6MnR钢“篮脆”温度范围之外不同环境温度、不同应力水平下的一种损伤演化模型。更多还原

【Abstract】 Pressure vessel components and structures are often subjected to high temperature, high pressure, and corrosion medium. It must be a disaster once accident occurs. As to 16MnR steel used extensively in pressure vessel and piping, a series of fatigue experiments under stress control at elevated temperature and SCC experiments in nitrate solution were conducted to investigate its low-cycle fatigue behaviors at elevated temperature, effects of temperature on fatigue performance and SCC performance in nitrate solution. Life prediction methods, design curves and damage evaluation models of low-cycle fatigue are obtained. All studies can be concluded as follows.Stress fatigue behaviors of 16MnR at 300℃ and 420℃ are analyzed carefully. Laws of cyclic hardening and softening, transformation laws of strain amplitude, plastic amplitude, laws of cyclic creep caused by mean stress under stress control, Masing behavior of material, etc, are examined in detail. Experimental datum and analytical results are given in order to further investigate life prediction methods, design curves and damage evaluation models of low-cycle fatigue.Simple tensile tests and low-cycle fatigue tests under stress control at different temperature were conducted to investigate the effects of temperature on fatigue behavior and fatigue strength. It is found that when temperature lies between 250℃ and 375℃, "blue brittle" phenomenon happens, fatigue strength of material is enhanced greatly which can be rationalized by dynamic strain aging effect, and material exhibits lower velocities of cyclic creep and cyclic hardening character. When temperature exceeds this range, material exhibits higher velocities of cyclic creep and cyclic softening character.Life prediction methods of low-cycle fatigue, which consider effect of mean stress, are summarized, and a previously proposed energy-based life prediction criterion is modified in order to promote its convenience. Life prediction models of 16MnR at 300℃ and 420℃ are also given. Comparison between theoretical predictions with test results is found to be quite satisfactory.S-N curves of 16MnR steel at 300℃ and420℃ are obtained. After amendment of mean stress and considering effects of various factors on fatigue life, experimental datum are compared with the ASME Code, and low-cycle fatigue design curves of16MnR steel at 300℃ and 420℃ are established. Due to existence of "blue brittle" phenomenon, 250℃ and 375℃ can be regarded as critical temperatures of fatigue strength of 16MnR steel, and low-cycle fatigue design curves with temperature that ranged from 250℃ to375℃ and from 375℃ to420℃ are also established.Models of exhaustion potential of X. H. Yang and Y. Wang are ameliorated to consider effects of maximum stresses and temperature on low-cycle fatigue damage evolution. Equivalent modulus is selected to define damage variable, which has a definite physical meaning and can be measured by a simple procedure. Models of low-cycle fatigue damage evolution under various loading conditions with different maximum stresses and temperature are derived based on several hypotheses when temperature exceeds the "blue brittle" range. These are elementary explorations for the application of damage mechanics to fatigue analysis and life prediction of pressure vessel.SEM analyses of fracture surfaces were conducted, and preliminary understanding of fracture surfaces under various loading conditions with same maximum stresses and different temperature is obtained.SCC performance of 16MnR steel and welded joints are studied by the method of stress corrosion test for pro-cracked wedge-open loading WOL specimen in nitrate solution. Critical stress intensity factors and spreading rates of SCC of the parent material and welded joints were measured. Meanwhile, effects of rust inhibition, microscopic organization of metal, temperature, and solution concentration on SCC performance of 16MnR steel and welded joints are also investigated.In conclusion, detailed understanding of low-cycl更多还原