304不锈钢晶间腐蚀敏感性的非线性超声表征

发布时间：2018-04-15 12:12

本文选题：奥氏体不锈钢 + 敏化　；参考：《大连理工大学》2016年硕士论文

【摘要】：奥氏体不锈钢因其优良的综合性能被广泛用于各行各业中,但敏化后的不锈钢极易发生晶间腐蚀进而引起构件失效事故,因此实现对奥氏体不锈钢晶间腐蚀敏感性的快速无损检测具有非常重要的意义。本文研究不同敏化程度的304奥氏体不锈钢对超声非线性特性的影响,为实现奥氏体不锈钢晶间腐蚀敏感性的无损表征与评价提供理论和试验依据。以CSK-I A标准试块为研究对象,本文探讨了信号调制模式、增益、衰减等系统参数对非线性超声传播特性的影响,据此提出合理的参数选择方案。实验结果表明：在试样厚度允许的情况下,设置较多的脉冲串周期个数并采用汉宁窗进行信号调制可以有效提高实验精度；衰减和输出电压级别不会影响非线性系数,增益在一定范围内以N-2N原则进行变化也不会影响非线性系数,但是增益调节过高会使信号产生谐波失真进而严重影响实验结果；脉冲串周期个数较少时,中频带宽设置过小会使信号失真进而严重影响实验结果。本文对不同敏化状态的304不锈钢进行非线性超声检测,辅以电化学动电位再活化测试,讨论了非线性超声表征不锈钢晶间腐蚀敏感性的可行性,并通过金相显微组织观察、X射线衍射等方法对其进行了分析,研究结果如下：1.不同敏化状态试样的声速和衰减系数并无明显区别,传统超声检测并不能有效地实现304不锈钢晶间腐蚀敏感性表征。2.不同敏化状态试样的非线性参量发生显著变化：非线性参量β/β0随敏化温度升高、敏化时间延长呈现增大的趋势。3.皮尔森相关系数计算结果表明非线性参量与敏化度之间存在很好的相关性,两者关系曲线亦表明非线性参量随敏化度升高而增大,因此分析认为非线性超声表征304不锈钢晶间腐蚀敏感性是可行的。4.金相显微组织观察与X射线衍射结果表明：经敏化处理,试样晶界析出了碳化铬(Cr23C6),并且碳化物析出随敏化温度提高、敏化时间延长而增多。因此分析认为：304不锈钢敏化过程中析出的碳化铬与奥氏体基体产生错配引发局部应变场,内应力的增加使超声波发生畸变滋生高次谐波,而碳化物析出越多超声波畸变越严重,因此非线性参量随敏化度升高而增大。
[Abstract]:Austenitic stainless steel is widely used in various industries because of its excellent comprehensive properties. However, the sensitized stainless steel is prone to intergranular corrosion, which leads to the failure of components.Therefore, it is of great significance to realize fast nondestructive testing for the sensitivity of austenitic stainless steel to intergranular corrosion.In this paper, the influence of 304 austenitic stainless steel with different sensitized degree on ultrasonic nonlinearity is studied, which provides theoretical and experimental basis for the nondestructive characterization and evaluation of intergranular corrosion sensitivity of austenitic stainless steel.In this paper, the effects of signal modulation mode, gain and attenuation on the characteristics of nonlinear ultrasonic propagation are discussed, and a reasonable parameter selection scheme is proposed.The experimental results show that when the thickness of the sample is allowed, the experimental accuracy can be improved effectively by setting a large number of pulse series periods and using the Hanning window to modulate the signal, and the attenuation and output voltage level will not affect the nonlinear coefficient.The change of gain in a certain range by N-2N principle will not affect the nonlinear coefficient, but too high gain regulation will make the signal produce harmonic distortion and seriously affect the experimental results.If the if bandwidth is too small, the signal will be distorted and the experimental results will be seriously affected.In this paper, the nonlinear ultrasonic detection of 304 stainless steel in different sensitized states and electrochemical potentiodynamic reactivation test are carried out. The feasibility of nonlinear ultrasonic in characterizing the sensitivity of stainless steel to intergranular corrosion is discussed.The X-ray diffraction method was used to analyze the microstructures. The results are as follows: 1: 1.There is no significant difference in sound velocity and attenuation coefficient between different sensitized samples. Traditional ultrasonic measurement can not effectively characterize the sensitivity of 304 stainless steel to intergranular corrosion.The nonlinear parameters of different sensitized samples changed significantly: the nonlinear parameter 尾 / 尾 0 increased with the sensitizing temperature, and the sensitization time increased with the increase of the sensitizing temperature.The calculation results of Pearson correlation coefficient show that there is a good correlation between nonlinear parameters and sensitivities, and the curve of the relationship also shows that the nonlinear parameters increase with the increase of sensitivities.Therefore, it is considered that it is feasible to characterize the intergranular corrosion sensitivity of 304 stainless steel by nonlinear ultrasound.The results of metallographic microstructure observation and X-ray diffraction show that Cr23C6C _ (6) carbide precipitates from the grain boundaries of the samples after sensitizing treatment, and the precipitation of carbides increases with the increase of sensitizing temperature and sensitization time.Therefore, it is considered that the mismatch between chromium carbide and austenite matrix in the sensitized process of stainless steel: 304 leads to the local strain field. The higher the ultrasonic distortion, the higher the ultrasonic distortion, and the more carbides precipitate, the more the ultrasonic distortion is.Therefore, the nonlinear parameter increases with the increase of sensitization.
【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG142.71

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