【作者】 程向梅；

【导师】 李萍；

【作者基本信息】 大连理工大学 ， 材料无损检测与评价， 2012， 硕士

【摘要】 304不锈钢(即OCr18Ni9)以良好的机械性能、低温强度、耐热性和耐蚀性被广泛应用于冶金机械、航空航海、石油化工和仪器仪表等领域。但是当其长期在高温高压并且临氢环境下工作时,304不锈钢同样会产生氢损伤；另外,当其暴露在腐蚀性环境中或经历不适当温度区间(400-850℃)加热时还可能出现敏化现象,使材料的晶间腐蚀倾向明显增加,这会严重影响工业部件的使用寿命,甚至导致灾难性事故发生。所以迫切需要研究一种无损检测方法来评价304不锈钢的氢损伤和敏化程度,以达到既不影响工业部件正常运作又能实时监测、预防重大事故发生的目的。本项研究基于超声信号与材料组织结构之间的相互作用,测量304不锈钢的横、纵波声速以及衰减系数并运用频谱分析方法分析一次底面回波信号。结合金相分析、电子探针分析所得的显微组织变化以及硬度测试结果,探讨了无损评价304奥氏体不锈钢氢损伤程度、固溶处理产物晶粒尺寸以及敏化程度的可行性与表征参数。研究结果如下：氢损伤严重影响奥氏体不锈钢部件的使用寿命,甚至会引发安全事故。实验中采用电解充氢的方法制备304不锈钢氢损伤样品,探讨了电解充氢过程中,微观组织结构转变对超声波声速、衰减以及频域特性的影响。研究结果表明：随充氢时间延长,304不锈钢声速呈现升高一降低一升高的趋势,衰减系数呈降低一升高一降低的变化趋势,这是因为：充氢会使材料微观组织结构的弹性模量、位错运动阻力以及应力状态发生改变。晶粒尺寸是影响材料力学性能的一个关键参数。采用超声波检测方法研究304不锈钢固溶处理产物晶粒尺寸的无损表征参数。发现：随固溶时间延长,304不锈钢晶粒尺寸不断增大,横、纵波声速均呈现递减的趋势,衰减系数却不断增大。分析认为：声速、衰减系数的改变归因于固溶处理对材料的晶粒尺寸作用以及晶粒尺寸对超声波传播的影响作用。基于最小二乘法拟合得到：横、纵波声速ν1、ν1与晶粒尺寸d线性相关：vt=-0.49254d+3188.40138,v1=-0.07632d+5772.88964；纵波衰减系数a与d符合如下规律：α=1.2605×10-15d6.5012+0.0995.发生敏化的奥氏体不锈钢产生晶间腐蚀概率大大增加,故采用超声波检测方法研究304不锈钢敏化程度的无损评价表征参数。结果表明：随敏化时间的延长,304不锈钢声速基本不变,但是纵波衰减系数会不断增大。原因归结为敏化使得304不锈钢晶界上出现大量铬的碳化物对超声波散射作用增强所致。幅度谱中的峰值频率随敏化时间的延长会向高频方向偏移,其峰值幅度也会明显上升。分析认为：延长敏化时间使得304不锈钢晶界上铬的碳化物颗粒逐渐变得密集且细小,从而引起较高频率子波的干涉增强。更多还原

【Abstract】 304stainless steels (0Cr18Ni9) with good mechanical properties, strength at low temperature, heat resistance and corrosion resistance are widely used in metallurgical machinery, aviation and navigation, petroleum chemical industry and instrument fields, etc. But when they work long time in high temperature, high pressure and the hydrogen conditions,304stainless steels will also generate hydrogen damage. In addition, when they expose to the corrosive environment or experience improper temperature range (400～850℃) heating, they maybe emerge sensitization phenomenon, which make materials’intergranular corrosion tendency increase obviously. That would seriously affect the service life of industrial parts or even lead to catastrophic accidents. Therefore, it’s urgent to study a nondestructive testing method to evaluate hydrogen damage and the sensitization degree in304stainless steel, in order to achieve an aim:which not only doesn’t affect the normal operation of industrial components but also can realize real-time monitoring, preventing serious accidents.Based on the interaction between ultrasonic signals and materials’organization structure, this study measured transverse wave velocities, longitudinal wave velocities and the attenuation coefficient of304stainless steel and used spectrum analysis method analyse echo signal. Combined with metallographic analysis, electronic probe analysis of the microstructure and hardness test results, this paper discussed the feasibility and ultrasonic nondestructive characterization parameters for hydrogen damage degree, solid solution processing product grain size and sensitization degree in304austenitic stainless steels. The results were as follows:Hydrogen damage will influence the service life of austenitic stainless steel parts seriously and even cause accidents. During our experiment, we used electrolysis hydrogen charging method to prepare304stainless steel hydrogen damage samples, and discussed the influences of ultrasonic attenuation,sound velocity, and frequency domain properties due to micro structure change in hydrogen charging process. The results showed that:with hydrogen charging time prolonged, the ultrasonic velocity of304stainless steels appeared the trend of "increase-reduce-increase", while ultrasonic attenuation appeared "reduce-increase-reduce" trend. The above results were due to that hydrogen charging will change materials’elastic modulus, dislocation motion resistance and stress state. The grain size is a critical parameter for materials because it affects materials’ mechanical properties. Using ultrasonic testing detect304stainless steels to study ultrasonic nondestructive characterization parameters for average grain size. It showed that:with solid solution time prolonging,304stainless steel grain size increased, transverse wave velocity, longitudinal wave velocity were decreasing, and attenuation coefficients are growing continuously. The above studies are mainly attributed to the effects of the solution treatment to the grain size and the effects of the grain size to the propagation of ultrasonic wave through materials. Based on the least square method, the linear relationships between the transverse wave velocities, the longitudinal wave velocity and the average grain size were obtained: vt=-0.49254d+3188.40138,vt=-0.07632d+5772.88964; meanwhile, the relevance of the attenuation coefficient to the average grain size was also described in the studied specimens: α=1.2605×10-15d6.5012+0.0995Austenitic stainless steels which have emerged sensitization will generate intergranular corrosion more likely. So we used ultrasonic detecting method to study ultrasonic nondestructive characterization parameters for the sensitization degree in304stainless steel. The results were as follows:ultrasonic velocities did not provide significant changes when ultrasound propagated in sensitized specimens, but the attenuation increased apparently with increasing sensitization treatment time. The attenuation increase was mainly attributed to the continuous distribution and possible coalescence of the carbides along the grain boundaries and the increased scattering effects of precipitated carbides to the propagation of ultrasonic wave through materials. The spectral analysis exhibited a shifting of the peak frequency to high frequency direction and an obvious increase of the peak amplitude in the amplitude spectrum with prolonging sensitization treatment time. The increase of the peak frequency and the peak amplitude was due to that the carbides along the grain boundaries became intensive and tiny with increasing sensitization treatment time, which increased the interference of the higher frequency wave.更多还原