【作者】 张颜艳；

【导师】 邬冠华；

【作者基本信息】 南昌航空大学 ， 精密仪器及机械， 2010， 硕士

【摘要】 奥氏体不锈钢是以铬、镍为主要合金元素,碳在γ相中的固溶体,它具有良好的耐腐蚀性和低温韧性、强抗高温蠕变能力、不存在脆性转变温度等优点,广泛用于制作要求良好综合性能的设备和机件。在高温环境中使用的部件,晶粒会不断长大,出现蠕变损伤,强度降低;在腐蚀环境中使用的部件,腐蚀介质会导致部件产生腐蚀。所以为了保证设备安全,必须进行晶粒度测量和壁厚测量。金相法是常规测量晶粒尺寸的一种破坏性方法,工序多、周期长,不适合在役设备检测。本论文针对奥氏体不锈钢的单一奥氏体相特点,将任何声速的变化归因于晶粒度的变化,利用传播速度和衰减这两个材料超声表征可测参量,研究奥氏体不锈钢平均晶粒尺寸与超声声速和衰减之间的关系,实现奥氏体不锈钢组织的超声无损评定,为在役奥氏体不锈钢制件监测提供一种无损、快速、全面的检测手段。本论文先从材料学和超声学的角度对试样材质和尺寸进行选择和设计,然后对0Cr18Ni9不锈钢试样进行固溶处理(分空冷和水淬两种冷却情况)和金相试验,采用500mm测量网格截点法测量试样的平均晶粒尺寸,最后利用超声声速法、相对衰减法和频谱分析法对试样进行晶粒尺寸超声评价研究,并结合金相法建立奥氏体不锈钢组织与超声信号的关系。结果表明:(1)固溶温度升高,或者同一固溶温度下,增加保温时间,晶粒尺寸增大;在其他条件一定时,水淬条件下晶粒尺寸长大速度比空冷条件下的快。(2)晶粒尺寸增大,声速减小。水淬条件下奥氏体不锈钢平均晶粒尺寸与超声声速呈一次指数衰减关系,空冷条件下呈线性关系。(3)对于不同晶粒尺寸的奥氏体不锈钢, f一定时,晶粒愈粗大,组织愈不均匀,声波衰减也愈大,且衰减系数和平均晶粒尺寸之间的关系符合三次多项式。(4)奥氏体晶粒尺寸不同时,相邻晶粒之间的声阻抗不同,对超声波的散射也不同。晶粒越大,散射越严重,回波频谱的频率随晶粒尺寸的增大而降低,结构噪声频谱的频率随晶粒尺寸的增大而增大。能谱点分析发现固溶温度升高,或者同一固溶温度下,增加保温时间,固溶度增加,溶质原子溶入溶剂使溶剂的晶格常数改变,从而导致晶格发生畸变,降低了原子之间的结合力,导致弹性模量E降低。位错、孪晶等晶体缺陷的阻尼效应也降低了声波的传播速度。更多还原

【Abstract】 Austenitic stainless steel is the solid solution that carbon is in theγphase, chromium and nickel as the main alloying elements. With good corrosion resistance and low temperature toughness, strong high-temperature creep and without brittle transition temperature, etc, Austenitic stainless steel has been widely used to make equipment and parts that are required good overall performance. However, the grain will continue to grow in high-temperature environment, resulting in creep damage and reducing strength .Additionally, corrosive media will lead to corrosion of parts and reduce its’strength in corrosive environment. Therefore, in order to ensure the safety of equipment, we have to measure the grain size and thickness. Although metallographic method is the conventional method of measuring grain size, it is not suitable for detection of in-service equipment because of destructive, many processes and long period. On the basis of the feature that the austenitic stainless steel is a single austenite phase after solution treatment and any change in the speed of sound can be attributed to change in grain size of this feature, we use the propagation speed and attenuation to study the relationship between average grain size and ultrasonic velocity and attenuation, which provides a quick, comprehensive and non-destructive monitoring method for in-service austenitic stainless steel parts.Firstly, we select the specimen material and design specimen size in view of materials science and ultrasonics. Then we carry out metallographic analysis on 0Cr18Ni9 stainless steel specimens after solution (air cooling and water quenching) with using 500mm gauge network lattice cut-off point method to measure the average grain size. Finally, we study ultrasonic evaluation of the average grain size in three ways (ultrasonic velocity, the relative attenuation and spectrum analysis) and establish the ultrasound evaluation model of austenitic stainless steel combined with the metallographic test. It shows that: (1)If increasing solution temperature or extending holding time in the same solution temperature, grain size will increase: the grain under water quenching grows up faster than that under air-cooled. (2) Ultrasonic velocity is reducing as the grain growth: the relationship between average grain size and ultrasonic speed under water quenching shows an exponential decay trend, however that under air cooling shows a linear trend. (3) For austenite stainless steel with different grain sizes , when the frequency is constant , the more coarse the grains are, and the more uneven the organization is, the more heavier the acoustic attenuation is . The relationship between average grain size and attenuation coefficient under water quenching shows a cubic polynomial curve growth trend. (4) The ultrasonic scattering is also different, as the acoustic impedance in the adjacent grains is different. The larger the grains are, the more serious the scattering is. Echo frequency spectrum decreases and structure noise frequency spectrum increases as the grain grows up. The EDS point analysis shows that if increasing solution temperature or extending holding time in the same solution temperature, solid solubility will increase. Because the solute atoms dissolve to the solvent to change the lattice constant of the sovent, which results in the lattice distortion, the binding force between atoms and elastic modulus E will reduce. The damping effect from dislocation, twin crystal or other crystal defects also reduces ultrasonic speed.更多还原